Best diy z-axis designs?


Page 1 of 3 123 LastLast
Results 1 to 20 of 48

Thread: Best diy z-axis designs?

  1. #1
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Best diy z-axis designs?

    What do you guys think is the best Z-axis design for a diy machine? By "best" I mean a z-axis that is optimal in terms of strength, stiffness and accuracy for a machine that will cut hard woods and aluminum.

    I have gone through a number of design changes since I started my project and I am not sure if my Z-axis is going to be good enough. My long axis runs parallel to the (fixed) gantry beam which means that my Z-axis rails are going to be taking a lot of the force from the side. It's making me wonder if I bought thick enough rails...

    I bought two of these for my Z-axis (I.e. 2 rails plus 2 bearings):





    The rails are 20mm wide with a double wide bearing on each. At the time (when I bought them), I thought it would be ok to use two double wide bearings instead of four smaller ones. Is this a bad idea? There will be 8" of travel.

    My spindle is over 35lb and I was planning to mount it on a 1" thick x 6" x 8" steel plate with the rails mounted on another steel plate 1" x 6" x 12". This is a lot of combined weight...

    Many of the high-end commercial machines seem to use long square or rectangular columns for the Z-axis but most of the diy machines and smaller commercial units use a couple of thin plates. Should I be looking to make a Z-axis column or would this be over-kill for a benchtop aluminum milling machine?

    What about adding a pair of gas springs (or equivalent)? I haven't seen many diy machines with springs on the Z but they are used on most milling machines. Is there any value in adding springs to the Z axis on a CNC router?

    For reasons I can't remember, I bought 2-start 1/2" leadscrews instead of ball screws for all three axis on my machine. The "anti-backlash nuts" that came with them are made of deralin. The ballnuts used on ball-screws all seem to be steel but I can't find any steel lead screw nuts for my set-up. It looks like a lot of weight is going to be resting on the deralin nut on my Z-axis. Should I be concerned about this? Why don't they make steel leadscrew nuts?

    Similar Threads:
    Attached Thumbnails Attached Thumbnails Best diy z-axis designs?-img_3969-jpg   Best diy z-axis designs?-img_3971-jpg  


  2. #2
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Why don't they make steel leadscrew nuts?
    Because the steel would wear out the screw. They do make brass nuts.

    Where did you get the delrin nuts from? Many are only rated for about 25lbs of force.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  3. #3
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by ger21 View Post
    Because the steel would wear out the screw. They do make brass nuts.

    Where did you get the delrin nuts from? Many are only rated for about 25lbs of force.
    I got them used off eBay as a set that included the nut, bearings, mounting block etc. They came off some kind of commercial CNC machine but I don't know how heavy duty it was. I can go back and ask the seller probably but I don't know if he would know.

    I wasn't too concerned about it when I bought them because I wasn't planning on buying a 35lb spindle...

    Should I ditch the leadscrew for the Z axis and but a ballscrew instead or should I search for a brass nut? Is brass any better than deralin? I always thought of brass as one of the softer metals and certainly not a "heavy-duty" material.

    I am less worried about the deralin nuts for the other axis because they won't be holding all the weight. I still wish I would have purchased ball-screws instead. I hate flimsy parts...

    What would be the equivalent spec of a 2-start lead screw in a ball-screw? The ball screws I have seen advertised don't state "1-start" or "2-start" so I assume their speed is stated in different type of metric?



  4. #4
    Member
    Join Date
    Mar 2004
    Location
    Wisconsin
    Posts
    413
    Downloads
    31
    Uploads
    1

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by Goemon View Post
    Should I ditch the leadscrew for the Z axis and but a ball screw instead or should I search for a brass nut? Is brass any better than deralin? I always thought of brass as one of the softer metals and certainly not a "heavy-duty" material.
    Ball screws are not the best fit for all machines. The tricks people pull to remove backlash instead of running double nuts, like that of installing larger balls do not really provide miracle results... more like a patch to make a ball screw nut kind of work in limited space.

    A "precision modified lead screw" such as those sold by Kerk (and Mcmaster) can be perfect for may machines and obtain zero backlash with less nut cavity space.

    If you want to handle weights, you need the right nuts. Kerk VHD nuts are the cats meow. Ones for 1/2" screws run somewhere around the $60-$75 mark direct from them. Mcmaster sells Kerk screws under the "precision modified lead screw" name, but they do not sell the VHD nuts unfortunately.... they only sell the three pronged nuts for that 25-30 lb application.

    Compare your nuts to this style below in the link, as there are other manufacturers of lead screw nuts that are similar in performance.

    VHD Anti-Backlash Nut | Lead Nuts | Haydon Kerk

    What is the turns per inch of travel on your screws ? Usually the 4 start screws in 1/2" diameter are 1/2" travel per turn. That is excellent for your average router as long as your screw isn't longer than say... 48". Longer than that and it is too thin and wants to whip too much at high rpm.

    Chris L


  5. #5
    Member
    Join Date
    May 2005
    Location
    USA
    Posts
    3920
    Downloads
    0
    Uploads
    0

    Default

    It is highly unlikely that the seller knows anything. Many just buy machines at auctions or private sales and part them out.

    Quote Originally Posted by Goemon View Post
    I got them used off eBay as a set that included the nut, bearings, mounting block etc. They came off some kind of commercial CNC machine but I don't know how heavy duty it was. I can go back and ask the seller probably but I don't know if he would know.
    At work we use all sorts of leadscrews and when properly sized the synthetic nuts can be very serviceable. However for what you are describing on your Z axis you probably want a ball screw and msybe a larger diameter.

    As an aside dont assume that ball screws are instantly more reliable. In the end if the parts arent sized correctly or abused they will fail just like a synthetic. Once you go beyond the well documented usages common in this forum it pays to run through the engineering calculation and design data the manufactures supply. You likely can grt away with Half inch leadscrews with your heavy Z ( maybe 60++ pounds when finished).
    I wasn't too concerned about it when I bought them because I wasn't planning on buying a 35lb spindle...

    Should I ditch the leadscrew for the Z axis and but a ballscrew instead or should I search for a brass nut? Is brass any better than deralin? I always thought of brass as one of the softer metals and certainly not a "heavy-duty" material.
    First off brass or in many cases bronze, covers a wide array of alloys. Some of the Ampco alloys are extremely wear resistant.

    As an aside here, when i first left high school i was working in a die cast foundery. A machine had sheared off a bronze guide way buggering the original mounting screw holes. So im the kid standing next to the old engineer and i say no problem i will drill ne holes in the bronze part, transder them to the machine and we will be good as new!!! How could we loose right. Well the engineer smirked at the idea but said OK drill those holes. So back to the shop to drill the holes. 45 minutes later the engineer comes by to see how it is going! I still had-not made it through with one hole much less two. Tried every trick i knew, made sure the bits drilled steel and aluminum fine but yet i couldnt get anywhere in the blick of bronze. The engineer then let me know that yhe material was Ampco 18 a very wear resistsnt alloy.

    That is a long aside but the point is i dont look at a shinny brass like part and assume that it is a soft easy to work material.

    I am less worried about the deralin nuts for the other axis because they won't be holding all the weight. I still wish I would have purchased ball-screws instead. I hate flimsy parts...
    It isnt the weight so much as it is the for es generated when accelerating.
    What would be the equivalent spec of a 2-start lead screw in a ball-screw? The ball screws I have seen advertised don't state "1-start" or "2-start" so I assume their speed is stated in different type of metric?
    Ball screws come in all sorts of configurations. Some are multi start others are not. You should be able to devine what a specific combination is from the manufactures web site. In the end you want the right movement per revolution for your machine.



  6. #6
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    My long axis runs parallel to the (fixed) gantry beam which means that my Z-axis rails are going to be taking a lot of the force from the side. It's making me wonder if I bought thick enough rails...
    The rails should not be considered as part of the structure, or be used to increase stiffness. Most 20mm rails can handle loads well over 2000lbs.

    You said you were going to mount the rails to a 12" steel plate? Is that how long the rails are? You might find that you need longer rails.
    And you are only putting one bearing per rail? You really should have two. Using only one puts much more torque on the rail, and will not be as rigid.

    Many of the high-end commercial machines seem to use long square or rectangular columns for the Z-axis
    You only see this on 5 axis machine, where the Z axis is very long. I'm not sure I've ever seen a 3 axis machine using a tube for the Z axis.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  7. #7
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by ger21 View Post
    The rails should not be considered as part of the structure, or be used to increase stiffness. Most 20mm rails can handle loads well over 2000lbs.

    You said you were going to mount the rails to a 12" steel plate? Is that how long the rails are? You might find that you need longer rails.
    And you are only putting one bearing per rail? You really should have two. Using only one puts much more torque on the rail, and will not be as rigid.



    You only see this on 5 axis machine, where the Z axis is very long. I'm not sure I've ever seen a 3 axis machine using a tube for the Z axis.
    That is good to know on the rails. It makes me feel better about my "portal mill" design.

    The Z axis rails are 12" with the plan being to achieve 8" of travel. There is one bearing block per rail but they are double wide - i.e. They provide the same amount of coverage as two of the regular size bearing blocks. When I bought them, I was copying the design of one of the commercial Z-axis assemblies I had seen but I have never been comfortable with my decision. I should have asked here before buying them....

    I recently purchased some new 24" 20mm rails with 4 bearing blocks which I was going to use for the moving table. Maybe I should use them for my Z-axis instead. If double-wide bearings don't perform as well as two regular bearings then my current z-axis rails are useless to me. I wouldn't be able to get anywhere near 8" of travel with two of the double wide bearings on each. They are over 3" wide...

    I should be able to achieve at least 10" of travel with 24" 20mm rails without sacraficing too much stiffness right? I have a 2" x 6" x 24" x 1/4" wall stainless steel rectangle tube I could use to mount the z rails to. This would be closer in size to my spindle too. I haven't measured it but it looks to be at least 20" long...

    - - - Updated - - -

    Quote Originally Posted by ger21 View Post
    The rails should not be considered as part of the structure, or be used to increase stiffness. Most 20mm rails can handle loads well over 2000lbs.

    You said you were going to mount the rails to a 12" steel plate? Is that how long the rails are? You might find that you need longer rails.
    And you are only putting one bearing per rail? You really should have two. Using only one puts much more torque on the rail, and will not be as rigid.



    You only see this on 5 axis machine, where the Z axis is very long. I'm not sure I've ever seen a 3 axis machine using a tube for the Z axis.
    That is good to know on the rails. It makes me feel better about my "portal mill" design.

    The Z axis rails are 12" with the plan being to achieve 8" of travel. There is one bearing block per rail but they are double wide - i.e. They provide the same amount of coverage as two of the regular size bearing blocks. When I bought them, I was copying the design of one of the commercial Z-axis assemblies I had seen but I have never been comfortable with my decision. I should have asked here before buying them....

    I recently purchased some new 24" 20mm rails with 4 bearing blocks which I was going to use for the moving table. Maybe I should use them for my Z-axis instead. If double-wide bearings don't perform as well as two regular bearings then my current z-axis rails are useless to me. I wouldn't be able to get anywhere near 8" of travel with two of the double wide bearings on each. They are over 3" wide...

    I should be able to achieve at least 10" of travel with 24" 20mm rails without sacraficing too much stiffness right? I have a 2" x 6" x 24" x 1/4" wall stainless steel rectangle tube I could use to mount the z rails to. This would be closer in size to my spindle too. I haven't measured it but it looks to be at least 20" long...



  8. #8
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    The longer you make it, the less stiff it will be. Keep it as short as possible, for maximum stiffness.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  9. #9
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by wizard View Post
    It is highly unlikely that the seller knows anything. Many just buy machines at auctions or private sales and part them out.


    At work we use all sorts of leadscrews and when properly sized the synthetic nuts can be very serviceable. However for what you are describing on your Z axis you probably want a ball screw and msybe a larger diameter.

    As an aside dont assume that ball screws are instantly more reliable. In the end if the parts arent sized correctly or abused they will fail just like a synthetic. Once you go beyond the well documented usages common in this forum it pays to run through the engineering calculation and design data the manufactures supply. You likely can grt away with Half inch leadscrews with your heavy Z ( maybe 60++ pounds when finished).

    First off brass or in many cases bronze, covers a wide array of alloys. Some of the Ampco alloys are extremely wear resistant.

    As an aside here, when i first left high school i was working in a die cast foundery. A machine had sheared off a bronze guide way buggering the original mounting screw holes. So im the kid standing next to the old engineer and i say no problem i will drill ne holes in the bronze part, transder them to the machine and we will be good as new!!! How could we loose right. Well the engineer smirked at the idea but said OK drill those holes. So back to the shop to drill the holes. 45 minutes later the engineer comes by to see how it is going! I still had-not made it through with one hole much less two. Tried every trick i knew, made sure the bits drilled steel and aluminum fine but yet i couldnt get anywhere in the blick of bronze. The engineer then let me know that yhe material was Ampco 18 a very wear resistsnt alloy.

    That is a long aside but the point is i dont look at a shinny brass like part and assume that it is a soft easy to work material.

    It isnt the weight so much as it is the for es generated when accelerating.


    Ball screws come in all sorts of configurations. Some are multi start others are not. You should be able to devine what a specific combination is from the manufactures web site. In the end you want the right movement per revolution for your machine.

    I had no idea that there were brass alloys that were that hard. I spent the last 5 years criticizing air gun manufacturers for using brass for their fill probes. Time to put on those orthopedic shoes so I can stand corrected...

    I was assuming that ball screws were a more heavy duty choice because that is what I have seen on every mill I looked at. I don't really know much about them though. It seems logical that either a lead screw or a ball screw could be strong and precise if specified appropriately as they are both made of hardened steel. A steel ball screw nut vs a deralin "anti-backlash" lead screw nut is a different story. I look at the deralin nut and can't help wondering if it will snap with the spindle weight resting on it.

    I was kinda hoping you guys would say something like "don't worry, those deralin nuts are made to be as strong as steel" or "you could lift your house with one of those things".

    It's annoying because there seems to be a lot of good deals on used ball screws available now. It looks like it would cost more to buy a suitable lead screw nut than it would to buy a ball screw with ball nut plus pillow blocks.



  10. #10
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by ger21 View Post
    The longer you make it, the less stiff it will be. Keep it as short as possible, for maximum stiffness.
    If it was your build, what size plates and rails would you choose to achieve 8" of travel and how far apart would you space your bearing blocks?



  11. #11
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by datac View Post
    Ball screws are not the best fit for all machines. The tricks people pull to remove backlash instead of running double nuts, like that of installing larger balls do not really provide miracle results... more like a patch to make a ball screw nut kind of work in limited space.

    A "precision modified lead screw" such as those sold by Kerk (and Mcmaster) can be perfect for may machines and obtain zero backlash with less nut cavity space.

    If you want to handle weights, you need the right nuts. Kerk VHD nuts are the cats meow. Ones for 1/2" screws run somewhere around the $60-$75 mark direct from them. Mcmaster sells Kerk screws under the "precision modified lead screw" name, but they do not sell the VHD nuts unfortunately.... they only sell the three pronged nuts for that 25-30 lb application.

    Compare your nuts to this style below in the link, as there are other manufacturers of lead screw nuts that are similar in performance.

    VHD Anti-Backlash Nut | Lead Nuts | Haydon Kerk

    What is the turns per inch of travel on your screws ? Usually the 4 start screws in 1/2" diameter are 1/2" travel per turn. That is excellent for your average router as long as your screw isn't longer than say... 48". Longer than that and it is too thin and wants to whip too much at high rpm.

    Thanks, I'll check them out.

    My current lead screws are 1/2" / 2 start. I wasn't looking for maximum speed as my machine is small and will be mostly cutting hard materials. I am not worried about the diameter of the screws except for maybe they Z-axis. I am starting to understand the need for springs on milling machine Z-axis assemblies...

    I am trying to picture what would happen to my spindle if that Z axis lead screw nut snaps. In my current design, it would just fall off...



  12. #12
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    The dual spindle machine I'm building has 8" of travel.

    The rails are 15mm, and 15" long. The rails are 3" on center, and the bearings are about 4" on center from each other. I designed this to be as compact as possible. The screws are 1/2-8 2 start acme, and the nut is a very tight fit between the bearings.

    For a Z axis, imo, you can put the rails as close together as you can, allowing room for the nut and screw, and with 20mm rails, the bearings can be very close together. 20mm rails are much larger than the 15mm rails I'm using, so they'd probably need at least 5"-6" on center spacing.

    I used to use a machine that had the spindles mounted on single rails. But the rails used roller bearings, which are much more rigid. It was a 10HP HSD ATC spindle that weighed about 40 lbs. The two roller bearing carriages were mounted right to the spindle, with a small aluminum plate making the connection.

    Attached Thumbnails Attached Thumbnails Best diy z-axis designs?-spindles-jpg  
    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  13. #13
    Member
    Join Date
    Apr 2009
    Location
    USA
    Posts
    5516
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by Goemon View Post
    I had no idea that there were brass alloys that were that hard. I spent the last 5 years criticizing air gun manufacturers for using brass for their fill probes. Time to put on those orthopedic shoes so I can stand corrected...

    I was assuming that ball screws were a more heavy duty choice because that is what I have seen on every mill I looked at. I don't really know much about them though. It seems logical that either a lead screw or a ball screw could be strong and precise if specified appropriately as they are both made of hardened steel. A steel ball screw nut vs a deralin "anti-backlash" lead screw nut is a different story. I look at the deralin nut and can't help wondering if it will snap with the spindle weight resting on it.

    I was kinda hoping you guys would say something like "don't worry, those deralin nuts are made to be as strong as steel" or "you could lift your house with one of those things".

    It's annoying because there seems to be a lot of good deals on used ball screws available now. It looks like it would cost more to buy a suitable lead screw nut than it would to buy a ball screw with ball nut plus pillow blocks.
    If you're worried about load bearing of the AB nut, you can design your carriage such that it holds both an AB nut, and plain nut which can be of bronze

    ACME screws and rolled ballscrews are made by roll-forming steel rod. For the ACME screw, the roll-forming process work-hardens the steel sufficiently since the nuts are made usually of Delrin (or other plastic) or bronze. The ball races in a rolled ballscrew are induction-hardened (hopefully!).

    One should be careful when purchasing used ballscrews on eBay, and you would have to determine the condition of said screw. For ground ballscrews, it also helps to know how to decipher the precision grade, and preload properties of the screw, before you buy. Downloading catalogs from the major manufacturers and understanding the code charts can help you score some excellent screws for a bargain, even NOS. Having bought and used both ground and rolled ballscrews, I have to say I'm a bit spoiled, as there is no contest on a comparison. There are some rolled ballscrews that are rolled then ground, which can be very precise. Then again there is ground ACME screw as well, though the price can be just as much as a comparable ground ballscrew.



  14. #14
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by ger21 View Post
    The dual spindle machine I'm building has 8" of travel.

    The rails are 15mm, and 15" long. The rails are 3" on center, and the bearings are about 4" on center from each other. I designed this to be as compact as possible. The screws are 1/2-8 2 start acme, and the nut is a very tight fit between the bearings.

    For a Z axis, imo, you can put the rails as close together as you can, allowing room for the nut and screw, and with 20mm rails, the bearings can be very close together. 20mm rails are much larger than the 15mm rails I'm using, so they'd probably need at least 5"-6" on center spacing.

    I used to use a machine that had the spindles mounted on single rails. But the rails used roller bearings, which are much more rigid. It was a 10HP HSD ATC spindle that weighed about 40 lbs. The two roller bearing carriages were mounted right to the spindle, with a small aluminum plate making the connection.
    It looks like your 2 spindle machine uses far lighter spindles that what I am using (if the ones in the render are representative of what you plan to use) but there are two of them. Are you using one lead screw for both or one each?

    What type of nut are you using?

    In your compact design, the spacing of your 4 bearings doesn't cover much more area than using 2 of the double wide ones. The ones I have are each 3.7" long by 3" wide. This pic shows them next to a regular size bearing block from a 15mm THK rail:



    Do I need more bearings / more coverage because I have a longer heavier spindle or is the bearing size irrelavent - I.e. Four is always better than two regardless of how much area they cover?

    Given the differences in spindle size and weight, do you think I can get away with copying your design? It looks like something I could replicate with parts I already have which is nice. If I have to buy additional rails and bearing blocks the 15mm THK rails are a lot cheaper too.

    My spindle is square and comes with pre-drilled threaded mounting holes so I can connect it to the bearings directly with a steel plate which would be similar to the HSD set-up you described. It would all be neat and simple.

    Attached Thumbnails Attached Thumbnails Best diy z-axis designs?-img_3995-jpg  


  15. #15
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by louieatienza View Post
    If you're worried about load bearing of the AB nut, you can design your carriage such that it holds both an AB nut, and plain nut which can be of bronze

    ACME screws and rolled ballscrews are made by roll-forming steel rod. For the ACME screw, the roll-forming process work-hardens the steel sufficiently since the nuts are made usually of Delrin (or other plastic) or bronze. The ball races in a rolled ballscrew are induction-hardened (hopefully!).

    One should be careful when purchasing used ballscrews on eBay, and you would have to determine the condition of said screw. For ground ballscrews, it also helps to know how to decipher the precision grade, and preload properties of the screw, before you buy. Downloading catalogs from the major manufacturers and understanding the code charts can help you score some excellent screws for a bargain, even NOS. Having bought and used both ground and rolled ballscrews, I have to say I'm a bit spoiled, as there is no contest on a comparison. There are some rolled ballscrews that are rolled then ground, which can be very precise. Then again there is ground ACME screw as well, though the price can be just as much as a comparable ground ballscrew.
    I think the advice on being careful with used screws is worth taking for all parts. I have had mostly good luck with used parts but there have been a few that I wish I would have asked more questions on. There is no way to avoid them though... if you want to build a quality machine on a smaller budget. In general, I have ended up with better quality parts than what I could have got for the same money new. CNC parts are so unreasonably expensive.

    I like the idea of adding a second nut, I just can't find any reasonably priced metal flanged nuts for 1/2" 2 start lead screws. I was looking into using gas springs to take some of the weight off them. Once it starts getting to $60 per nut, it starts to make more sense to look for used ball screws from large VMC's instead.

    I understand that ball screws come in grades like c7 and c5 or c3 which relates to the tolorances. I don't understand how lead screws are graded. Mine were sold as "acme precision" but I don't know if that has any actual meaning or if it's just marketing like "aircraft grade".

    With all my research, I was never able to determine how to choose the right grade aside from "buy the best you can afford". Nobody seems to be able to articulate what difference I would see in a finished part made on two otherwise identicle machines if one used c3 instead of c5 ball screws. Maybe it matters less on a smaller machine like mine.



  16. #16
    Member
    Join Date
    Apr 2009
    Location
    USA
    Posts
    5516
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by Goemon View Post
    I think the advice on being careful with used screws is worth taking for all parts. I have had mostly good luck with used parts but there have been a few that I wish I would have asked more questions on. There is no way to avoid them though... if you want to build a quality machine on a smaller budget. In general, I have ended up with better quality parts than what I could have got for the same money new. CNC parts are so unreasonably expensive.

    I like the idea of adding a second nut, I just can't find any reasonably priced metal flanged nuts for 1/2" 2 start lead screws. I was looking into using gas springs to take some of the weight off them. Once it starts getting to $60 per nut, it starts to make more sense to look for used ball screws from large VMC's instead.

    I understand that ball screws come in grades like c7 and c5 or c3 which relates to the tolorances. I don't understand how lead screws are graded. Mine were sold as "acme precision" but I don't know if that has any actual meaning or if it's just marketing like "aircraft grade".

    With all my research, I was never able to determine how to choose the right grade aside from "buy the best you can afford". Nobody seems to be able to articulate what difference I would see in a finished part made on two otherwise identicle machines if one used c3 instead of c5 ball screws. Maybe it matters less on a smaller machine like mine.
    Automation equipment is not expensive... if you're purchasing for an industrial corporation. We're purchasing industrial equipment for personal hobby use, or one-man business - expensive. Automation equipment are tested and guaranteed to certain tolerances and performances, that engineers can factor in and trust the manufacturer's specifications so it meets their needs. These aren't consumer products; they just happen to be what a growing cottage industry wants.

    Same thing for bronze leadnuts. Although, in that case, usually the leadnut is threaded outside to be received by a flange that is purchased separately. They're not cheap, but they are more compact than ballnuts, and likely far easier to maintain.

    Precision ACME usually means a tolerance of around .003"-.004"/ft. Regular ACME I believe can be .004"-.009"/ft (the cheap stuff that MSC sells). A C7 ballscrew is slightly more accurate than a very good precision ACME screw. The C7 (or T7) is what is considered "transport grade," which means suitable for conveying and other transport functions that do not rely on the lead accuracy of the screw for positioning. Fortunately for most of us, .002" is more than enough for our needs, and as long as an anti-backlash mechanism is in place, the repeatability would be very good - far more important than lead accuracy.

    Well, in general, the higher the precision (lower the number) of the screw, the tighter the tolerances are, meaning that even a single nut precision ground ballscrew will have anti-backlash properties simply by design. Although a lot of the machines that such screws operate in are in controlled environments, with probably (well likely) higher precision mounting surfaces than we can obtain at home. They'll also run smoother and quieter than a rolled screw, which should translate into a higher surface finish quality. I don't think any rifleman could detect a difference of .001" in the thickness of a rifle stock. If you're mounting something directly to you molded part, and expect to do so with no post processing, then maybe; but I would presume in your case those surfaces would need to be trued up after casting. The other thing is, the tolerances of the rest of your machine would have to be at the very least within the range of that of the screw, and preferably tighter. otherwise you lose much of the advantage of using the precision ground ballscrew in the first place. And that includes the radial BBs in the blocks, motor couplers, linear rails (which can come in three or more preload ratings and five accuracy ratings), drive motors, and everything they're mounted to.



  17. #17
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    It looks like your 2 spindle machine uses far lighter spindles that what I am using (if the ones in the render are representative of what you plan to use) but there are two of them. Are you using one lead screw for both or one each?

    What type of nut are you using?
    Yes, they are 12lb spindles. Each Z axis (spindle) has it's own screw.
    I'm using standard plastic flange nuts, modified for anti backlash by cutting slots in the end, and using an o-ring to compress the nut to the screw.
    I'm only using these for the Z axis.

    What size is that rail/bearing you have 30-35mm? If so, you can probably get away with one rail, but I'd still want two blocks on it. But two smaller rails and 4 blocks would still be better, as the loads are distributed better.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  18. #18
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by ger21 View Post
    Yes, they are 12lb spindles. Each Z axis (spindle) has it's own screw.
    I'm using standard plastic flange nuts, modified for anti backlash by cutting slots in the end, and using an o-ring to compress the nut to the screw.
    I'm only using these for the Z axis.

    What size is that rail/bearing you have 30-35mm? If so, you can probably get away with one rail, but I'd still want two blocks on it. But two smaller rails and 4 blocks would still be better, as the loads are distributed better.

    The rail is just 20mm. It's only the bearings that are oversized at 3.7" x 3". If I put both on the same rail then I would need longer rails to achieve 8" of travel. The rails and bearings are the same size as the set (2 rails with 4 bearings) I bought for my moving table. I was going to switch them but the table rails are too long.

    I am going to buy some new ones. Looks like there are a lot of good deals on the smaller rail sets.



  19. #19
    Member
    Join Date
    Mar 2017
    Location
    United States
    Posts
    926
    Downloads
    0
    Uploads
    0

    Default Re: Best diy z-axis designs?

    Quote Originally Posted by louieatienza View Post
    Automation equipment is not expensive... if you're purchasing for an industrial corporation. We're purchasing industrial equipment for personal hobby use, or one-man business - expensive. Automation equipment are tested and guaranteed to certain tolerances and performances, that engineers can factor in and trust the manufacturer's specifications so it meets their needs. These aren't consumer products; they just happen to be what a growing cottage industry wants.

    Same thing for bronze leadnuts. Although, in that case, usually the leadnut is threaded outside to be received by a flange that is purchased separately. They're not cheap, but they are more compact than ballnuts, and likely far easier to maintain.

    Precision ACME usually means a tolerance of around .003"-.004"/ft. Regular ACME I believe can be .004"-.009"/ft (the cheap stuff that MSC sells). A C7 ballscrew is slightly more accurate than a very good precision ACME screw. The C7 (or T7) is what is considered "transport grade," which means suitable for conveying and other transport functions that do not rely on the lead accuracy of the screw for positioning. Fortunately for most of us, .002" is more than enough for our needs, and as long as an anti-backlash mechanism is in place, the repeatability would be very good - far more important than lead accuracy.

    Well, in general, the higher the precision (lower the number) of the screw, the tighter the tolerances are, meaning that even a single nut precision ground ballscrew will have anti-backlash properties simply by design. Although a lot of the machines that such screws operate in are in controlled environments, with probably (well likely) higher precision mounting surfaces than we can obtain at home. They'll also run smoother and quieter than a rolled screw, which should translate into a higher surface finish quality. I don't think any rifleman could detect a difference of .001" in the thickness of a rifle stock. If you're mounting something directly to you molded part, and expect to do so with no post processing, then maybe; but I would presume in your case those surfaces would need to be trued up after casting. The other thing is, the tolerances of the rest of your machine would have to be at the very least within the range of that of the screw, and preferably tighter. otherwise you lose much of the advantage of using the precision ground ballscrew in the first place. And that includes the radial BBs in the blocks, motor couplers, linear rails (which can come in three or more preload ratings and five accuracy ratings), drive motors, and everything they're mounted to.

    Makes sense. I have been assuming that there is a limit to how much precision a man could benefit from on a homemade machine due to all the other limitations. Then again, the same applies to a lot of commercial machines based on what people here post about them. There are definitely companies selling $20,000 - $50,000 machines that fall short of their owners expectations of precision. I guess there is a reason why people pay $200,000 for a Datron CNC router.

    The level of accuracy I need is to be able to make a two part mold and have both sides fit together properly. I use precision molds for the inletting and won't be using my CNC for that part.

    I have ordered a c5 16mm ball screw with a 5mm pitch and 2 ball nuts for my Z-axis. I'm sticking with my lead screws for the X and Y axis.

    I am going to copy Ger21's Z-axis design for now. I think I am going to have to switch out my steel plates for "precision ground flat" aluminum ones though. The 1" thick steel plate I bought is so heavy that I can hardly lift it. A 1" thick aluminum plate with carbon fiber feels more manageable. It's a shame I wasted so much time hand scraping the steel flat.... what a boring process....



  20. #20
    Member ger21's Avatar
    Join Date
    Mar 2003
    Location
    Shelby Township
    Posts
    35538
    Downloads
    1
    Uploads
    0

    Default Re: Best diy z-axis designs?

    There are definitely companies selling $20,000 - $50,000 machines that fall short of their owners expectations of precision
    Depending on the size, and it's intended use, $50K could be considered a very low end machine. The machine we have for cutting cabinet parts is $150,000, and would not be suitable for what you want to do. But it's designed to cut cabinet parts, and it does that very well.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


Page 1 of 3 123 LastLast

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

Best diy z-axis designs?

Best diy z-axis designs?