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  1. #21
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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Dock - The proof is in the pudding so the theory is incorrect at this point. Good to know. Be careful with saying things like I'll go from 16mm to 20mm down the road. 20mm stack ht is different to the 16mm and there will be many things to change associated with that part. Do the best you can with what you have or bite the bullet and do things now. Think the path through...

    1200rpm gives you a max velocity of 1200x5mm= 6000mm/min (238in/min). Depending on what you do this could be a fast rapid or a slow one. Its going to have lots of grunt so thats the plus. So if your going to cut heavier material, aluminium, hogging heavy timber then its the correct choice. I don't think 6m/min is slow for a personal machine. Yes its slow for a commercial machine that has to make money and meet a thru put.

    Now's the time to balance all the parts (and costs) and check they are all reasonable for the machine utility you want. Being a second machine aim at something that is reasonable and sellable. Your 3rd machine will be the really good one as you have been thru the learning curve and your needs/knowledge will change. No good chasing top end speed when it will be used infrequently if it costs a bunch.

    So perhaps write down a specification for the machine as you need it then see if its on track. Helps the paralysis to get it onto paper...

    Peter

    for instance to go back to the beginning you say its a 3060 gantry on a 4ft wide gantry. That's quite a small one for the grunt the 1605 screws will have. Time to review the structure... You have made a basic mistake of buying things before you have sorted the entire puzzle. Once you put a part in place you have frozen many things and every bit of a machine is connected to the rest of the machine in some way. Machine No3, sort the entire machine on paper before you buy anything.... It actually happens faster that way.

    Last edited by peteeng; 02-27-2020 at 04:41 PM.


  2. #22
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    Default Re: Thoughts on this stepper/lead screw combination

    Hi,
    to be honest I agree with peteeng, if you are thinking about 2010 screws do it now. Trying to retrofit larger screws is likely to be a nightmare.

    My mini-mill has 2005 C5 screws and while it might be slower than I might achieve with 2010's I sure do like the thrust that comes with the finer pitch.

    My new mill build has double-nut THK C5 3205 screws, they are really nice. I'll be running them direct coupled to Delta 750W servos, 3000 rpm rated,
    for a max G1 of 15m/min. I can run the servos up to 5000 rpm in 'field weakening mode' for G0's of 25m/min. If I needed more thrust I'd use a planetary reduction,
    and seriously even G1's of 15m/min are ANY amount fast enough for a hobby machine, I sure as hell don't need MORE speed. Trying to contain the
    acceleration and cutting forces is going to be the determinant (of overall machine performance).....not the servo speed, nor the screw pitch


    When all said and done provided you match the pitch of the screw to the torque/speed of the motor who cares? I seriously doubt you'll get anywhere near
    the whip speed of 16mm screws....although I'll defer to peteeng's analysis on that score, thus with a fine pitch screw you choose a motor of higher speed but
    commensurately lower torque whereas with a coarse pitch you would go for higher torque at lesser speed. In the event both motors will have (near) equal power.

    It looks like you are very close to a set of decisions about both the screws and the steppers which will in large degree dictate the accelerations, thrusts and,
    as a first guess, cutting forces. You should use those numbers to assess the rest of your design.

    Craig



  3. #23
    Member Dock7676's Avatar
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    Default Re: Thoughts on this stepper/lead screw combination

    So I am making progress on the build and figured I would at least share a few pics of the gantry coming together. It is probably going to be pushing 100 lbs, as I am at 70 lbs the way it is now with the z axis that is sitting under it. Still need to add the steppers, ball screws, spindle, etc. Do you guys think this will be too heavy for the stepper setup that we have come up with?

    I am still making changes to the gantry sides plates. Going to angle them back more to try and balance the gantry better. And obviously they won't be MDF, they will be 1/2 aluminum

    Also a picture of the base that I just picked up. Salvaged from my local scrap yard. It is an old weight scale. I was going to weld up a new table, but I came across this while selling some scrap the other day and it is just the size I needed - 48x72. It is perfectly square and true. Couple very minor indents in the diamond plate, but I am going to level the deck with self leveling epoxy, so it shouldn't matter. It's incredibly heavy - 450lbs.

    I am adding rails to the y axis to spread the gantry sides to about 55" inside so that I can fit 54" wide sheets of HDPE up on the table, which is what I seem to be doing the most of right now.

    Attached Thumbnails Attached Thumbnails Thoughts on this stepper/lead screw combination-img_20200228_001003355-jpg   Thoughts on this stepper/lead screw combination-img_20200228_001015181-jpg   Thoughts on this stepper/lead screw combination-img_20200228_001044292-jpg  
    Last edited by Dock7676; 02-28-2020 at 01:08 AM.


  4. #24
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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Dock - what motors did you settle on? From the rest of the structure I'd double the gantry width and use 2 extrusions to take advantage of the grunt you will have. Peter



  5. #25
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    Default Re: Thoughts on this stepper/lead screw combination

    Pete, you are absolutely right, I had pulled the trigger on ordering parts far too early. Being a personal machine that I am running for a hobby and some side jobs, I was making an attempt to keep the cost down. This is not going to be a production machine that runs every day, just trying to make a stout machine that can handle the side work I have been doing. And I also kinda just like the process of building the machine itself!

    My first build was just a MPCNC to get my feet wet and learn. It has been a lot of fun, and as sillyy as it is, it had been quite capable. I have more hours of run time than I can count on that thing in the past year I have had it, and I am starting to take on a lot of paying projects that just require a full sheet table.

    You are right. Now that I have actually started designing one of my own, I will probably end up selling this one and making machine #3 in the future. Learning from my mistakes on this one.

    Craig, I think I am going to stick with the plan we have started with for now. 1605 screws are in hand, the steppers that we had talked about are ordered and the 77v power supply and Dm860t drives I am going to order tomorrow. I am sure this will still be a capable machine for the time being, be it slow or not. It should still be a whole hell of a lot more capable than my plastic CNC!

    Maybe I will make some sort of whip control device or something if need be.....I've seen a few ideas on YouTube. It will just give me something else to design and build! And I can always go to 1610 screws without changing the stack....

    Everyone here is super helpful. If I could buy you guys a beer over the intranets I would!

    -Andrew

    Forgot to add a picture of the base:

    Attached Thumbnails Attached Thumbnails Thoughts on this stepper/lead screw combination-img_20200223_155037312-jpg  


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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Andrew - what motor did you settle on? Peter



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    Default Re: Thoughts on this stepper/lead screw combination

    Pete,
    It is this one that Craig had suggested.

    Object reference not set to an instance of an object.

    Also the 3060 t-slot I went with is 3"x6", not 30mm x 60mm.....I wasn't very clear. Still too small?



  8. #28
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    Default Re: Thoughts on this stepper/lead screw combination

    Pete I didn't see your second to last post. I can certainly add to the gantry, I was just concerned about weight. But if you think that I should have more than enough torque to throw it around, I will beef it right up!



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Andrew - A 5mm pitch screw at 3.1Nm torque will shove 357kgf (790lb.f) so it will move quite a structure. Plus your Y has two of them (will move a small car) ...At cutting speeds of 2000mm/min say thats over 100kgf possible load on the gantry. If you are trying to hold a good tolerance at that load then yes you will need more then a 75mm wide beam. Especially a construction extrusion that is not very efficient for its size. Can't have too much stiffness... set up the gantry flat across horses or benches and hang 100kg on it if it deflects more then your required tolerance then double it up. if possible hang it eccentric at your max Z as well to include torque.

    Look up some tool force calculators to find out your cutting loads... this can be a beast..

    Peter

    Force = torque (Nm) x 2PI x 0.9/ (9.81 x pitch in mm ) in kgf where 0.9 is typical efficiency of ballscrew



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    Default Re: Thoughts on this stepper/lead screw combination



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    Default Re: Thoughts on this stepper/lead screw combination

    A 5mm pitch screw at 3.1Nm torque will shove 357kgf (790lb.f) so it will move quite a structure. Plus your Y has two of them (will move a small car) ...At cutting speeds of 2000mm/min say thats over 100kgf possible load on the gantry
    That 790 lbs of force is when the stepper is not spinning (holding torque). At 1000 rpm, you'll be lucky to have 1/3 of that. Which may still seem like quite a bit of force. The problem is that you have to accelerate all that weight, which takes a lot more force than just holding it still.
    And 1000 rpm is still very slow.

    So I am making progress on the build and figured I would at least share a few pics of the gantry coming together. It is probably going to be pushing 100 lbs, as I am at 70 lbs the way it is now with the z axis that is sitting under it. Still need to add the steppers, ball screws, spindle, etc. Do you guys think this will be too heavy for the stepper setup that we have come up with?

    100 lbs is not heavy at all for a gantry.
    The proper way to size a motor, is to determine how fast you want the machine to go, and how quickly you want it to accelerate to that speed. The problem is that 99.5% of people building machines have no idea how fast they want the machine to be, so have no idea what motors they need. Most just go with a 300-400oz Nema 23, like everyone else uses.

    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)


  12. #32
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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Gerry - Andrew is doing the best he can with what he has. Sure one correct and best way is to complete the structural design and analysis so you know the weight & stiffness of everything. Then you do the inertial analysis to establish the motor sizes, plus this needs to be framed around "how fast do you need to go and what acceleration do you need as well" which needed to be on paper at the beginning. Then you find the design deficits, then you fix these. All that takes about 20 design rounds and a couple of months for someone that can do that. Hobbyists jump in and chew at it for as long as it takes to get it to work. Which with persistence Makers achieve. So all out there keep making.

    I appreciate that 3Nm is the stall figure but thats the figure we have to work with. Plus it will happen in a crash or a very slow crash event so the machine has to be able to cope with such a force. The torque drop off is about linear to 1200rpm as noted. Is 1000rpm (5000mm/min for 1605) slow for rapids or for cutting? Since a machine is intended for cutting not just moving air then if a compromise has to be made its best to err on the cutting side then the rapid side.

    This is the tradition issue with power design. Power=Force x speed. We have a fixed power so we can design for speed or force but not both. Would be good to have a fast gearbox so we could get the best of both for our machines. Servos I suppose answer that question if you have the $$$ . Has anyone tried to change usteps on the fly? a sort of electronic gearbox? Peter



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi Dock.
    On my machine im running x and y axis with genuine leadshine em806's and z axis has a gekco 201x. I have leadshine's latest nema 24 425oz motors on all 3 axis and using a 72v ps similar to anteks.
    I will say the leadshine drivers never heat up the motors they stay cool to the touch. The z motor with geko gets very warm almost too hot to touch. Im planning on replacing the geko with another em806.
    I only have 450 x450 x 200 travel and 15 and 12mm ballscrews but i can run them so fast that i reckon i could rip the z axis off the x if went flat out into the stops.
    Anyway if not going servos i recommend leadshine all the way.



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi V2T - what usteps are you using and what actual speed are you talking about and whats the pitch of the screws? Then we can figure some stuff out. Ta Peter



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi pete.
    Firstly the part about ripping my z axis off was me joking. But i can run my steppers scary fast especially with such small travel.
    My x and y are 10mm pitch, z is 5mm pitch.
    The leadshines are running 8 microstep the gecko 10. Im using uccnc and have x and y velocity set at 8000 units but when i was setting up had it up to 16000 units which was when it got scary. Off top of my head dont know what the speed was. I did work it out and its written dowm somewhere.
    But im really impressed with the leadshine gear. Very smooth runs cool hopefully lasts.
    Servos would be nice but so would be a real vmc. I can dream.



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi V2T - If you are using mm metric in UCCNC then the 8000 units is 8000mm/min. 8usteps is 1600 usteps to the rev. The accelerations will be set high to get up to 8m/min then down in 450mm. But with 10mm pitch you have lots of grunt with those motors. Whats your acceleration settling? Peter



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    Default Re: Thoughts on this stepper/lead screw combination

    Made the same 1605 mistake building my router. As a 'fix' I drive the screws 1:2 rpm ratio using timing pulleys. On a 3' (approx) screw rapids run 360"/min with nothing noticecably bad happening. Other than the screw spinning twice as fast as necessary I'm not sure what the other downsides are to this arrangement. Your router is larger so type of kludge may not work at all.

    Anyone who says "It only goes together one way" has no imagination.


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    Default Re: Thoughts on this stepper/lead screw combination

    Hi CS - I don't think its a bad solution. You need to connect the motor to the screw somehow. So either a coupling or a belt drive is the usual. The belt allows ratio adjustment and making the drive length shorter if you have reflexed the motor vs in series. 360ipm or 9000mm/min is a good rapid in my books. Plus couplings that match the high torque are hard to get.

    So Dock you could gear your screw now or into the future if speed is needed. Time to review your specs? Peter



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    Default Re: Thoughts on this stepper/lead screw combination

    Hi,
    as I've posted before I use 5-phase Vexta steppers and low lash (2<arc mim) 10:1 planetaries. This was my first machine so the design process
    was not particularly logical.The stepper/gearbox combinations I bought, were because they were available second hand (Ebay).

    The consequence of a 10:1 reduction is very marked increase in thrust and thereby acceleration at the cost of G0 speeds. The Vexta steppers and
    AC input Vexta drivers allow effortless 2400 rpm from the steppers, which through the gearbox results in 240rpm at the output shaft which is direct
    coupled to a 2005 C5 ballscrew, for a G0 of 1200mm/min or 60inch/min. I accepted the decisions I've made have reduced traverse speed but the thrust
    is amazing (14kN stall) and I may set my acceleration as high as I like, limited only by the fact that the machine starts throwing itself around the shop.
    Without tying it to the wall I've set accel at 375mm/s2. I have had it as high as 5000mm/s2 but its so violent it scares me!
    375mm/s2 gives me very good toolpath following at an amiable pace.

    As it turns out given that my machine is so small (travels= 180 mm x 180 mm x 180 mm) I find the modest G0's not much of an issue but I sure do
    like the sparkling acceleration. Certainly toolpath following is much improved by high acceleration but contrary to 'legend' believe that its more
    important to cycle time than high speed. Most toolpaths spend more time accelerating than at max speed, therefore to minimise cycle times
    high acceleration is AS if not MORE significant than high speed.

    My advice to OP is go for the best acceleration and thrusts you can reasonably achieve but realize those goals may limit traverse speed. My experience
    is that traverse speed is secondary to acceleration and thrust for cutting.

    Craig



  20. #40
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    Default Re: Thoughts on this stepper/lead screw combination

    Sorry guys, i was away and left this thread hanging for a few days!

    Thanks for the encouraging words Pete! Funny story, I chucked the lead screws in my drill and ran them up on my bench again. I did not play much with lower rpm's when I was originally testing, as I assumed that the screw would whip more at higher rpm's. Low and behold at around 550-600rpm the shaft starts to whip, and then goes away as the rpm increases from there......so you were absolutely right when you found that the Hiwin formula stated shaft whip at 600rpm. If I am still running into this once the machine is built, I change to larger ballscrews on the y axis (and x axis if need be) without much effort. Also I could try something similar to this, which is a pretty neat way to get around shaft whip:



    V2 - Glad to hear you were happy with the Leadshine gear, I wound up ordering their steppers at the recommendation of Craig. Servos would certainly solve some problems, but cannot justify it for my uses.

    CS - Good idea using timing pulleys, it certainly would not be hard to implement in the future. Glad I am not the only one that thought 5mm ballscrews were a good idea lol. The great amazon deal on a rail/ballscrew package got me!

    Craig - Glad to hear that the high gear ratio is serving you well, and you are right about the acceleration probably being much more important than traverse speed. I am coming around to the idea of higher torque and acceleration at the expense of the traverse times, I just figured it would be more of a convenience when setting up jobs. Maybe I was overthinking this whole thing. I am going to get it built and go from there!



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Thoughts on this stepper/lead screw combination

Thoughts on this stepper/lead screw combination