Ballscrew Sizing Options


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    Default Ballscrew Sizing Options

    Hi All,

    I've been doing some math for my CNC build. This is a steel frame machine with moving gantry. I'm at the point where I really need to order my ballscrews. I've been looking at the effects of using different Nema 34 motors, different pitch sizes of ballscrews, and even possibly gear reduction. The way my machine is designed, belt reduction is not an option. I already have nice SYK Nema 34 mounts, so if I use a gear reducer, it will have to be in the form of something that fits a Nema 34 frame. I'd prefer not to do this due to the added cost and potential for backlash.

    I'm also looking at what I might do in the future to upgrade with servos, but this is not in the budget at this time.

    For my budget, I have been thinking all along of using rolled Chinese ballscrews with double nuts. I have 15mm ID size bearings for the end machining. I'm not sure if I can use 20mm screws, I don't think the end machining will work for 15mm bearings.

    For the 25mm rolled ballscrews, it looks to me like these are my only options:

    2505 (nope, not using this one)
    2510
    2525 (does not appear to have a double nut option, but I could use this effectively with 1:3 gear reduction to give better performance than 2510)

    Resolution isn't my main concern. Good acceleration, low backlash, and sufficient force for aggressive cuts in aluminum are. Also, it would be nice to have a top speed of 400 IPM or even a bit more. I've done the math on this using inertial effects and the motors I want to use initially, and it is hard to find an "ideal" pitch.

    Are there any other sizing options in the less expensive ballscrew category that other people have found? I'd like to find a 2515 ballscrew, either that or an affordable half inch pitch ballscrew (so that would be 12.5mm pitch) but from the pages I've looked at from manufacturers of inch size ballscrews, they don't even list the prices, and I'm guessing it's an arm and a leg. 20mm pitch is pushing it without some form of reduction, and I believe it only comes in 20mm OD size.

    So if you know of any Chinese or other suppliers for different pitches of ballscrews (around 11-16mm), that are considered to be in the lower cost bracket, please let me know.

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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by NIC 77 View Post
    Hi All,

    I've been doing some math for my CNC build. This is a steel frame machine with moving gantry. I'm at the point where I really need to order my ballscrews. I've been looking at the effects of using different Nema 34 motors, different pitch sizes of ballscrews, and even possibly gear reduction. The way my machine is designed, belt reduction is not an option. I already have nice SYK Nema 34 mounts, so if I use a gear reducer, it will have to be in the form of something that fits a Nema 34 frame. I'd prefer not to do this due to the added cost and potential for backlash.

    I'm also looking at what I might do in the future to upgrade with servos, but this is not in the budget at this time.

    For my budget, I have been thinking all along of using rolled Chinese ballscrews with double nuts. I have 15mm ID size bearings for the end machining. I'm not sure if I can use 20mm screws, I don't think the end machining will work for 15mm bearings.

    For the 25mm rolled ballscrews, it looks to me like these are my only options:

    2505 (nope, not using this one)
    2510
    2525 (does not appear to have a double nut option, but I could use this effectively with 1:3 gear reduction to give better performance than 2510)

    Resolution isn't my main concern. Good acceleration, low backlash, and sufficient force for aggressive cuts in aluminum are. Also, it would be nice to have a top speed of 400 IPM or even a bit more. I've done the math on this using inertial effects and the motors I want to use initially, and it is hard to find an "ideal" pitch.

    Are there any other sizing options in the less expensive ballscrew category that other people have found? I'd like to find a 2515 ballscrew, either that or an affordable half inch pitch ballscrew (so that would be 12.5mm pitch) but from the pages I've looked at from manufacturers of inch size ballscrews, they don't even list the prices, and I'm guessing it's an arm and a leg. 20mm pitch is pushing it without some form of reduction, and I believe it only comes in 20mm OD size.

    So if you know of any Chinese or other suppliers for different pitches of ballscrews (around 11-16mm), that are considered to be in the lower cost bracket, please let me know.
    I don't understand the case against 2505? You wouldn't have a need for a gearbox, and spinning it around 2000rpm should be no problem with a servo. Plus it's readily available with a double nut assembly. A little bewilderment on my part why you'd buy the bearings first; I believe 25mm ballscrews typically take 17mm bearings as a standard. Since there's a spring seal at both ends, you'd have to turn a longer bearing journal on the driven end so you can use one of the supplied spacers which would slip inside the seal (hopefully you got 2 spacers). Otherwise you'll have to turn another shoulder so that slips inside the seal and seats against the inner race of the AC bearing. The problem with this is, when it comes time to replace said screws, they'd have to be custom-machined again.

    You can try Roton for ballscrews, they have them in inch sizes... 20mm screw I believe uses 15mm bearing supports.



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    Default Re: Ballscrew Sizing Options

    If you want to stay steppers, go on eBay and find some surplus Parker E-AC drives and HV-342 or HV-343 steppers. Capable of 3000rpm with torque. Plus you can go 110V direct to the drive, no PSU needed. I use the NEMA23 steppers with the E-AC on my mini-mill.



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    Default Re: Ballscrew Sizing Options

    My moving table router has a 1 inch x 1 inch lead ball screw on the X with a 1:1 timing belt drive, Y axis (table) is a 4040 ball screw with a 1.5:1 timing belt reduction, the table weighs about 700 lbs. Both driven by a NEMA34 1280 oz/in stepper running at 80 volts. 600 IPM rapids at slightly reduced acceleration, 400 IPM cutting at normal cutting acceleration.

    The motor RPM on the Y is about 572 at 600 IPM, and on the X it's 600 rpm at 600 IPM I have the microstepping set to about 1 micron of axis travel/step on both motors. If you use the low rpm torque of the steppers they work pretty well, but they drop off rapidly at higher RPMs

    Why the mix of metric and inch ball screws? I have no idea, I didn't design the damned thing.



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    Default Re: Ballscrew Sizing Options

    Hey guys, sorry it took so long to get back to you. I've been fighting a really bad cold, sick as a dog for the past few days.

    Quote Originally Posted by louieatienza View Post
    I don't understand the case against 2505?
    Please have a look at the attached graph.

    Ballscrew Sizing Options-ballscrew-comparison-2-jpg

    I calculated a total inertia for the system, including motor inertia, ball screw inertia, and load inertia, then calculated the amount of torque required to accelerate at a given "G". Then looked at the motor torque vs speed equation, and subtracted the torque required for acceleration at different speeds. The leftover torque was used to calculate how much cutting force was available while accelerating using the standard equation. Both sides of the equation assume 90% efficiency and do not include things such as bearing friction or ballscrew preload.

    As you can see from the graph, 5mm pitch is not a good choice for my application. If you look at a maximum cutting force of 200 to 300 lbs taking very heavy cuts from an 8 HP Spindle, you can see that the 15mm pitch still gives me the available cutting force I want at slower speeds while also adding about 100 IPM to my top speed when compared to 10mm pitch.

    Quote Originally Posted by louieatienza View Post
    You wouldn't have a need for a gearbox, and spinning it around 2000rpm should be no problem with a servo. Plus it's readily available with a double nut assembly.
    One of my ballscrews will need to be quite long. Around 1470mm. If you assume a root diameter of 22.2mm, the Nook critical speed calculator says 1277 RPM is the max. So with a 5mm pitch, that would be 251 IPM maximum, even if I were using a servo, which I can't afford at this juncture.

    Quote Originally Posted by louieatienza View Post
    A little bewilderment on my part why you'd buy the bearings first;
    What came first the chicken or the egg? 12mm is too small and 20mm wouldn't give me good machining for a 25mm OD ball screw. That left 15mm.

    Quote Originally Posted by louieatienza View Post
    I believe 25mm ballscrews typically take 17mm bearings as a standard.
    Some of the more expensive brands of ball screw had a 17mm ID bearing option, which for them is the standard to use for 25mm ball screws. Not all manufacturers make this size. When you are using SYK mounts for Nema 34, your options are to use FK15's or FK20's. I believe that 15mm bearings are the most popular size that are used with Chinese rolled 25 series ball screws.

    I had a look at Roton. That would triple the cost. Yikes. But I do appreciate the suggestion .

    As far as using 20mm OD rolled ballscrews from China, unless it has a lip machined in it like the more expensive brands have, right now I can't see using this size....they take a long screw, cut it and machine it, so there isn't a lip for the spacer to sit against if you go too small.

    Take this add for example:

    Anti-backlashed RM2010--500 mm Ballscrew & Standard Machine End | eBay

    It just doesn't look right to me.

    Quote Originally Posted by Jim Dawson View Post
    My moving table router has a 1 inch x 1 inch lead ball screw on the X with a 1:1 timing belt drive, Y axis (table) is a 4040 ball screw with a 1.5:1 timing belt reduction, the table weighs about 700 lbs. Both driven by a NEMA34 1280 oz/in stepper running at 80 volts. 600 IPM rapids at slightly reduced acceleration, 400 IPM cutting at normal cutting acceleration.
    The only torque vs speed graph I could find for those motors shows them running at 60 Volts. I would like to find out what happens with more volts. Also with the 1600 OZ In motors. The 60Volt performance is worse than for the 1280 Oz In motors, but what happens if you use a 100V power supply? Oh wait, I bet the driver wouldn't take it. Yes, my G201x's are limited to 80V.

    The other option I have is to find a bigger Nema34 and go right to 25mm pitch without reduction.

    What acceleration settings are you using? How long is the ball nut on your 1 inch pitch ball screw? Is there any backlash?



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by NIC 77 View Post
    The only torque vs speed graph I could find for those motors shows them running at 60 Volts. I would like to find out what happens with more volts. Also with the 1600 OZ In motors. The 60Volt performance is worse than for the 1280 Oz In motors, but what happens if you use a 100V power supply? Oh wait, I bet the driver wouldn't take it. Yes, my G201x's are limited to 80V.

    The other option I have is to find a bigger Nema34 and go right to 25mm pitch without reduction.

    What acceleration settings are you using? How long is the ball nut on your 1 inch pitch ball screw? Is there any backlash?
    I have the drives maxed out at 80V (actually 82V peak), running a 3KW unregulated power supply. They have lasted for about 4 years so far with no problems.

    Acceleration and deceleration is set for about 45 IPS^2 or 2700 IPM^2 for cutting, for long 600 IPM rapids, acceleration set at about 75% of that. For short rapids < 4 inches, it's set at 500 IPM at full acceleration. The software looks at the vector distance and adjusts the speed and acceleration accordingly.

    A bigger NEMA 34? I'm driving a 1x1 (~2525) direct with a 1280 ozin motor on my X axis, and the carriage weighs about 300 lbs. It's a dual Z axis system.

    My ball nut assembly is about 4 inches long. It's an adjustable double nut system. I haven't actually ever checked the backlash, I should do that one day. I've not had any problems cutting circles, they come out round so that would indicate any backlash is minimal.



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by Jim Dawson View Post
    Acceleration and deceleration is set for about 45 IPS^2 or 2700 IPM^2 for cutting, for long 600 IPM rapids, acceleration set at about 75% of that. For short rapids < 4 inches, it's set at 500 IPM at full acceleration. The software looks at the vector distance and adjusts the speed and acceleration accordingly.
    OK, so you're running at about 0.12G. Are you happy with that acceleration? What software are you using? I had a look at some different softwares a while ago to see which ones support a different acceleration value for rapids. Obviously you are not using Mach 3 software?

    Quote Originally Posted by Jim Dawson View Post
    A bigger NEMA 34? I'm driving a 1x1 (~2525) direct with a 1280 ozin motor on my X axis, and the carriage weighs about 300 lbs. It's a dual Z axis system.
    I meant bigger than the ones I currently have. Also, you are running at 80Volts, so who knows what kind of actual torque you are getting from them? More than you would with 60V, but how much more?

    I plan to cut aluminum aggressively with a powerful spindle with this build, so cutting forces are something to consider.

    Quote Originally Posted by Jim Dawson View Post
    My ball nut assembly is about 4 inches long. It's an adjustable double nut system. I haven't actually ever checked the backlash, I should do that one day. I've not had any problems cutting circles, they come out round so that would indicate any backlash is minimal.
    Yes, the Chinese rolled ballscrew 2525's have a small nut and no double nut option as far as I can tell. I think it might be an issue for me if I use them, but I don't really know.

    Thanks for your feedback!



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by NIC 77 View Post
    OK, so you're running at about 0.12G. Are you happy with that acceleration? What software are you using? I had a look at some different softwares a while ago to see which ones support a different acceleration value for rapids. Obviously you are not using Mach 3 software?
    The acceleration seems about right for this machine, it's snappy, but not crazy. I'm not in a huge hurry most of the time. Most of my runs are pretty long, 9 to12 hours, so I run it lights out most of the time.

    I wrote my own software, it has a few features you don't normally find in commercially available software.

    I meant bigger than the ones I currently have. Also, you are running at 80Volts, so who knows what kind of actual torque you are getting from them? More than you would with 60V, but how much more?

    I plan to cut aluminum aggressively with a powerful spindle with this build, so cutting forces are something to consider.
    Oh, sorry. The curves for my motors are for 70V, so I'm getting a little more out of them than the curves would indicate, but I have no idea just how much more. I do cut the idle current back to 20% so they don't get hot. You will be loading yours far beyond what I ever will, so more power might be needed. I have a customer that does a lot of aluminum on his router, it has 750W AC servos on it, dual rack & pinion drive on the Y axis and single motors on the X and Z About a 5KW spindle. Seems to have plenty of power when driving a 3/8 endmill pretty aggressively.

    Yes, the Chinese rolled ballscrew 2525's have a small nut and no double nut option as far as I can tell. I think it might be an issue for me if I use them, but I don't really know.

    Thanks for your feedback!
    You could always add another nut and make a double nut system. It really depends on what your target accuracy is.

    It's my pleasure to help out.



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by NIC 77 View Post
    OK, so you're running at about 0.12G. Are you happy with that acceleration? What software are you using? I had a look at some different softwares a while ago to see which ones support a different acceleration value for rapids. Obviously you are not using Mach 3 software?

    I meant bigger than the ones I currently have. Also, you are running at 80Volts, so who knows what kind of actual torque you are getting from them? More than you would with 60V, but how much more?

    I plan to cut aluminum aggressively with a powerful spindle with this build, so cutting forces are something to consider.

    Yes, the Chinese rolled ballscrew 2525's have a small nut and no double nut option as far as I can tell. I think it might be an issue for me if I use them, but I don't really know.

    Thanks for your feedback!
    When you're dealing with steppers, you can't get too aggressive with acceleration, because the potential for missed steps arises.

    A definition what you mean by aggressively would help, and what exactly are you cutting would help. Provided they can handle it, the higher voltage moves the torque available at a higher rpm, proportional to the voltage increase. The extra voltage allows the current to "flow" faster till it reaches the current set by the drive, before it "chops" the voltage. This is one reason why you can use a PSU rated 25X the rated voltage of the stepper, and possibly more. As far as "cutting forces" - I don't think the idea is to "shove" the tool through the material. Aluminum cuts pretty freely, more so with the beast of a spindle you plan to use. I think the force to move your axes around at the speeds you want is probably not more than the force needed to get your spindle through a cut. If you can get that 800lb. gantry to move at the speeds you want, the spindle is just coming along for the ride. Unless you have some fantasy about running a 6" shell mill at 2500rpm and 400IPM and .25" DoC full width.

    One issue with using a ballscrew with a very high lead is the easier potential for the nut to backdrive. I have my router rebuild in the works with high-lead ballscrews in conjunction with planetary gearheads and servos. I have heard of some issues with some "super speed" VMCs and the large lead ballscrews - which are precision ground and driven by high-power servos - though I have not experienced them personally. I've seen high-lead precision ground ballscrews with double nuts, but haven't seen a rolled high-lead ballscrew with same.



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by Jim Dawson View Post
    I wrote my own software, it has a few features you don't normally find in commercially available software.
    Well done, that's impressive! I wish some commercially available programs had more input parameters with regards to acceleration at different speeds!

    Quote Originally Posted by louieatienza View Post
    A definition what you mean by aggressively would help, and what exactly are you cutting would help.
    Aluminum alloys. If I look at HSMAdvisor, and I input a 0.75" 3 flute bit, and a 0.25" DOC, slotting 6061-T6, it gives me a feed of 119 IPM, a power of 6.73HP and a cutting force of 204 lbs.

    I'm not sure if my machine will be able to handle this, but if it can, I think that is around the maximum I would want to push it.

    I'm pretty new to this program, but it does give a cutting force, which is nice. My performance estimate doesn't include bearing friction or probably a few other things that don't work in my favor, and the motor torque vs speed graph, who knows how accurate that is, so I figure that if I have 300lbs of cutting force available between 0 and 100 IPM and 200lbs available between 100 and 200 IPM that I should be fine. I think an acceleration of 0.3G is what I'm aiming for. Is there an error in my logic?

    As always, a big thank you to everyone who has contributed to this thread!



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    Default Re: Ballscrew Sizing Options

    Aluminum alloys. If I look at HSMAdvisor, and I input a 0.75" 3 flute bit, and a 0.25" DOC, slotting 6061-T6, it gives me a feed of 119 IPM, a power of 6.73HP and a cutting force of 204 lbs.

    I'm not sure if my machine will be able to handle this, but if it can, I think that is around the maximum I would want to push it.

    I'm pretty new to this program, but it does give a cutting force, which is nice. My performance estimate doesn't include bearing friction or probably a few other things that don't work in my favor, and the motor torque vs speed graph, who knows how accurate that is, so I figure that if I have 300lbs of cutting force available between 0 and 100 IPM and 200lbs available between 100 and 200 IPM that I should be fine. I think an acceleration of 0.3G is what I'm aiming for. Is there an error in my logic?

    As always, a big thank you to everyone who has contributed to this thread!
    If I take your federate target of 119IPM, 3 flutes, and a chipload per tooth (from my LMT/Onsrud catalog) for a 3/4" endmill of .0102", that comes out to 3,889rpm (119/.0306). I don't think your spindle will output 6.7HP at that speed. Even if I used 1% tool diameter for chipload (which is what I use) that's still only 5,889rpm.

    The power you'll need to move 800lbs at 200IPM is about 300W. From an online torque-speed chart for a 940oz-in stepper, using 72V, produces about 150W power between 480 and 720rpm. A 3:1 final reduction should give you some headroom. Roton actually makes ballscrews with .333" leads. For 5', with nut, would be about $260, less the flange for the ballnut. The closest you'd find in metric would be to source a surplus ground ballscrew with 8mm pitch. Usually they are 32mm diameter screws, which would be a good size for your span.



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    Default Re: Ballscrew Sizing Options

    Aluminum alloys. If I look at HSMAdvisor, and I input a 0.75" 3 flute bit, and a 0.25" DOC, slotting 6061-T6, it gives me a feed of 119 IPM, a power of 6.73HP and a cutting force of 204 lbs.

    I'm not sure if my machine will be able to handle this
    Have you posted any pictures of your machine design? Without knowing what you're building, I'd have to say no chance. And if the spindle you have is from a router, I don't think it'll last long attempting cuts like that. What rpm is that cut at?
    If you look at $10,000-$15,000 milling machines, designed for cutting metals, they don't have anywhere near that kind of capability. A $30,000 Haas Toolroom mill has a 7.5HP spindle that only spins at 4000 rpm.

    I think if you look at every build log here, it's unlikely that you'll find a machine that accelerates at 0.3G. Acceleration generally requires more power than cutting. And since acceleration is linear, you'll need to have enough power to accelerate up to your maximum speed. If you want to have 400ipm rapids. You'll need that acceleration force all the way up to 400ipm. Not sure if the cutting forces you mentioned are in addition to the acceleration force required?

    Gerry

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    Default Re: Ballscrew Sizing Options

    The power you'll need to move 800lbs at 200IPM is about 300W.
    Correct me if I'm wrong, but I think the 800lb number was someone else's machine.

    Gerry

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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by ger21 View Post
    Correct me if I'm wrong, but I think the 800lb number was someone else's machine.
    Guessing, based on the fact that NIC mentioned having a 9HP spindle that alone would probably weigh 100lbs.



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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by ger21 View Post
    And if the spindle you have is from a router, I don't think it'll last long attempting cuts like that. What rpm is that cut at?
    Around 5800 RPM according to HSMAdvisor if I remember correctly. A Router? Noooo. It's actually an 11kw ISO 30 ATC spindle, but I will be using up to 6.37 kW (8.5HP), which should peak at 6950 RPM and flatline after that. 18000 RPM is the max I would run it to.

    Of course, that's assuming I can get the darned thing to work.

    Quote Originally Posted by ger21 View Post
    If you look at $10,000-$15,000 milling machines, designed for cutting metals, they don't have anywhere near that kind of capability. A $30,000 Haas Toolroom mill has a 7.5HP spindle that only spins at 4000 rpm.
    Shame on Haas for having less power than a spindle from a wood working machine!

    Quote Originally Posted by ger21 View Post
    I think if you look at every build log here, it's unlikely that you'll find a machine that accelerates at 0.3G.
    After all the stuff we discussed on Ramsey's build, he's looking at around 1G I thought. And you mentioned 0.25G as a minimum. Was that only because he wanted to go to 1800 IPM? What would you recommend for me?

    I had a look at some of the pro top end VMC machines. One I looked at today have over 2G acceleration advertised. Others were over 1G. Even the video I like to post of that fast DIY machine was 0.5G. I thought 0.3G might be a good goal for me? At least 0.25, I don't know. Really, I don't know. It seemed reasonable?

    Quote Originally Posted by ger21 View Post
    Acceleration generally requires more power than cutting. And since acceleration is linear, you'll need to have enough power to accelerate up to your maximum speed. If you want to have 400ipm rapids. You'll need that acceleration force all the way up to 400ipm. Not sure if the cutting forces you mentioned are in addition to the acceleration force required?
    We have discussed this ad nausium in Ramsey's thread and in 1Jumper10's thread. Yes, I am aware. Unless you use one of the control softwares that has different inputs for acceleration for feed and rapids.

    If you look at the graph I posted in post #5 of this thread, those available cutting forces are in addition to the 0.25G acceleration. And that's not actually the worst axis, because there are two motors to supply the force once the acceleration torque has been dealt with, but it does illustrate what I was trying to say in regards to ball screw pitch. Could there be math errors? It's possible. I think it's good though.

    Quote Originally Posted by ger21 View Post
    Correct me if I'm wrong, but I think the 800lb number was someone else's machine.
    Yes. Ramsey said his gantry would be 800 lbs. And I said mine was around 650. Then I actually took the time to do a better and comprehensive weight estimate, and realized that mine was also 800 lbs. So there's the source of confusion.

    My main gantry is quite heavy duty, and I have done basic bending and torque calculations on it. Whether it will be good enough to do that cut, who knows, it's already mostly put together, so we will see when the time comes. I'm certainly not going to add more weight to it.

    Quote Originally Posted by louieatienza View Post
    Guessing, based on the fact that NIC mentioned having a 9HP spindle that alone would probably weigh 100lbs.
    It weighs over 90lbs with no tool holder or other accoutrements.

    I honestly don't know that my machine will be capable of cuts like that. But theoretically, if it is, I should plan to have 200 lbs available for cutting force when sizing my ballscrews and motors just in case. Unless I totally missed what HSMAdvisor was telling me...........

    Quote Originally Posted by louieatienza View Post
    The power you'll need to move 800lbs at 200IPM is about 300W. From an online torque-speed chart for a 940oz-in stepper, using 72V, produces about 150W power between 480 and 720rpm. A 3:1 final reduction should give you some headroom.
    There are two motors for the 800lbs. A reduction does give me some headroom, but I also need to increase the pitch of the ballscrew if I use a reduction (from 10mm to 20 or 25mm) OR I need to use a servo that is capable of higher speeds. A 3:1 reduction with a 400W servo? I should do the math on that. I think I know where I can get some 5:1 gear reducers. If the servo goes to 5000 RPM that could work with 10mm pitch and give me a half decent top speed for the size of my machine. You know, perhaps I should just buy the 10mm pitch and get it built.

    Quote Originally Posted by louieatienza View Post
    Roton actually makes ballscrews with .333" leads. For 5', with nut, would be about $260, less the flange for the ballnut. The closest you'd find in metric would be to source a surplus ground ballscrew with 8mm pitch. Usually they are 32mm diameter screws, which would be a good size for your span.
    If you look at my graph, 10mm is better than 8mm. That's what I'm going on. I have tried the math with 32mm diameter pitch screws. There is a substantial increase in inertia on the long axis when you go to 32mm diameter, which slows down the acceleration noticeably. If I were to increase the diameter I should either put more power in, or increase the pitch so the ball screw doesn't have to spin as fast.

    Really, I just made this thread to try and find out if there was a Chinese supplier of ball screws hiding out somewhere who had some 2515 screws. LOL. But discussion is good. I still wish they made a 2515 screw of the inexpensive variety.



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    Default Re: Ballscrew Sizing Options

    I guess I should have looked at the chart, where it says 800lb gantry.

    A Router? Noooo. It's actually an 11kw ISO 30 ATC spindle, but I will be using up to 6.37 kW (8.5HP), which should peak at 6950 RPM and flatline after that. 18000 RPM is the max I would run it to.
    By router, I meant woodworking spindle, which is what you have. Do you have a torque curve for it?


    After all the stuff we discussed on Ramsey's build, he's looking at around 1G I thought. And you mentioned 0.25G as a minimum. Was that only because he wanted to go to 1800 IPM? What would you recommend for me?

    I had a look at some of the pro top end VMC machines. One I looked at today have over 2G acceleration advertised. Others were over 1G. Even the video I like to post of that fast DIY machine was 0.5G. I thought 0.3G might be a good goal for me? At least 0.25, I don't know. Really, I don't know. It seemed reasonable?
    I think when I said 0.25G, that's what I would want. You might find it difficult to achieve when using steppers. You said servos were not an option at this time.

    Are you figuring any headroom in your calculations? So, at 0.3G, you need 150lbs/motor (800/2 *.3) + 200lbs cutting force, or 350lbs of force. If that's your requirement, you really should have 400-425lbs of force, or even more. Especially with steppers.


    And I've really only seen 25mm screws in the sizes you mentioned. I'd probably say that 2510 are a good compromise. A little lower speed, but will give you more force. What you're building is really outside the range of stepper power.

    Last edited by ger21; 06-11-2017 at 01:51 PM.
    Gerry

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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by ger21 View Post
    By router, I meant woodworking spindle, which is what you have. Do you have a torque curve for it?
    It's from a 2000 Biesse Protec Excel. Perhaps someone on this forum has an exact torque curve for it? To my knowledge, Biesse uses HSD spindles, and all of the HSD spindles that I've looked at in either their wood or aluminum and plastics line peak at 12000 RPM then go flat for a bit, then might drop off a bit to 24000 RPM if that model goes that fast.

    So for me to run it up to 6.37kW, my peak would occur at 6950 RPM on the V/f curve instead of following the line up to 11kW and 12000 RPM.

    Something like this is what I believe I have:

    http://www.hsdusa.com/bo/allegati/Fi...988a_13kw5.pdf

    Quote Originally Posted by ger21 View Post
    I think when I said 0.25G, that's what I would want.
    We're not so different, you and I.

    Quote Originally Posted by ger21 View Post
    You might find it difficult to achieve when using steppers. You said servos were not an option at this time.
    Right, but I need to select my screws so that when I do upgrade, I don't have to replace my screws to get better speed and acceleration.

    Quote Originally Posted by ger21 View Post
    Are you figuring any headroom in your calculations? So, at 0.3G, you need 150lbs/motor (800/2 *.3) + 200lbs cutting force, or 350lbs of force. If that's your requirement, you really should have 400-425lbs of force, or even more. Especially with steppers.
    So for a 15mm pitch screw and 2 motors using the basic force equation for a ballscrew with 90% efficiency, and the as tested CNCRP motor graph with 48V power and 10x microstepping, that gives 845.4lbs of thrust available at 60 RPM.

    Compare that with my graph in post #5 and you will see that I have 555.5 lbs left over at 60 RPM in my graph. 845.4-555.5 =289.9 lbs. These figures include both motors for the gantry.

    But (800 *.25) = 200 lbs. So, this is less than was needed. The 90lb difference is found in the inertial effects.

    Basically, I no longer calculate force for acceleration based on the basic force equation. Instead, I calculated a total inertia for the system. This is the inertia for the motor (in this case I guessed it was 0.00018 kg*m^2 because I couldn't find it published) + the inertia for the screw (I found a chart with inertia values for popular rolled ball screw sizes / length) + the inertia for the load. Then I could calculate how much torque was required to accelerate the system at a given G.

    Then I could subtract this from the available torque according to the motor graph, and see what was left over. Back to the basic equation to figure out how much force is available when you're not using that force to accelerate, and hence don't have the inertial effects.

    Equations are here:

    http://www.engr.sjsu.edu/bjfurman/co...iaformulas.pdf
    https://www.aerotech.com/media/85449...d%20sizing.pdf

    Basic equation (no inertia) is:

    Force = (Torque x 2 x pie x efficiency) / (Lead x 10^-3)

    Where lead is in mm, Force is in N, and Torque is in Nm.

    Of course this doesn't include bearing friction beyond 90% efficiency or deviations in my motor performance from the published motor graph or ball screw drag torque so yes, some headroom is still needed.

    If you look at my graph in post #5, there is plenty of headroom with 15mm pitch but not with 25mm pitch. And it's worse for the carriage graph because there is only one motor and a longer screw. I may use larger steppers, with around 5mH inductance, and 60 or 80V power supplies for the Carriage motor and the Z. I've looked at the graphs for these motors and they do have better performance when I put them in my spreadsheet. Or, I may try to find a stepper with a flatter curve but one that goes to a higher RPM and use a gear reduction and 25mm pitch.

    I actually almost bought some bigger steppers at auction this morning on EBay but someone snatched them from me with 3 minutes left in the bidding .

    If I could find a 15mm pitch screw, that would be best, but most likely I should just get on with it and use 10mm.



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    Default Re: Ballscrew Sizing Options

    So for a 15mm pitch screw and 2 motors using the basic force equation for a ballscrew with 90% efficiency, and the as tested CNCRP motor graph with 48V power and 10x microstepping, that gives 845.4lbs of thrust available at 60 RPM.

    Compare that with my graph in post #5 and you will see that I have 555.5 lbs left over at 60 RPM in my graph. 845.4-555.5 =289.9 lbs. These figures include both motors for the gantry.
    But you can't find affordable 2515 screws?
    And what do you plan on doing at 60rpm? Imo, you always calculate force at max rpm, which is where you need it.
    At 600rpm, you only have 48% of the torque that you have at 60rpm. That gives you about 350ipm. With 2510 screws, that drops to 235ipm, but gives you 50% more force.
    If you run those at 70V, you probably can pick up 20% more speed. As a general rule, speed is proportional to voltage. Going up to 70V flattens out the torque curve.

    Gerry

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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by ger21 View Post
    But you can't find affordable 2515 screws?
    And what do you plan on doing at 60rpm? Imo, you always calculate force at max rpm, which is where you need it.
    At 600rpm, you only have 48% of the torque that you have at 60rpm. That gives you about 350ipm. With 2510 screws, that drops to 235ipm, but gives you 50% more force.
    If you run those at 70V, you probably can pick up 20% more speed. As a general rule, speed is proportional to voltage. Going up to 70V flattens out the torque curve.
    I believe with rolled, you can get 16mm pitch.

    While you only have 48% of the torque of the stepper at 600rpm vs. 60rpm, the power output is about 5-fold (about 150W vs. <30W). In fact the power output is pretty constant from 600-720rpm. This was extrapolating a torque curve chart for a 960oz-in stepper at 48V (CNCRP website...)

    Last edited by louieatienza; 06-11-2017 at 11:43 PM. Reason: corrected error


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    Default Re: Ballscrew Sizing Options

    Quote Originally Posted by NIC 77 View Post
    Around 5800 RPM according to HSMAdvisor if I remember correctly.
    So I was off by 89rpm...

    Shame on Haas for having less power than a spindle from a wood working machine!
    That's like comparing apples to pizza. It's a toolroom mill, not a VMC. Hass' VMCs have 30HP spindles that peak at around 40-45HP.

    I had a look at some of the pro top end VMC machines. One I looked at today have over 2G acceleration advertised. Others were over 1G. Even the video I like to post of that fast DIY machine was 0.5G. I thought 0.3G might be a good goal for me? At least 0.25, I don't know. Really, I don't know. It seemed reasonable?
    Maybe for servos, not sure for steppers - at least that and be reliable.

    If you look at the graph I posted in post #5 of this thread, those available cutting forces are in addition to the 0.25G acceleration. And that's not actually the worst axis, because there are two motors to supply the force once the acceleration torque has been dealt with, but it does illustrate what I was trying to say in regards to ball screw pitch. Could there be math errors? It's possible. I think it's good though.
    I think there may be something amiss in your graph. With the 5mm pitch, you show zero cutting forces at about 140IPM, or about 700rpm? That's at the peak of the power curve for the particular stepper. Using CNCRP's torque-speed chart and 48V PSU, at 720rpm there's about 283oz-in of torque. Plugging into Mariss' power equation, that's about 151W power. Or with a 5mm lead ballscrew about 690W at 141IPM. With a 15mm pitch, you'll be spinning at about 235rpm (to get 141IPM). Again, using CNCRP's chart, you have 566oz-in of torque, or about 98W power at 235rpm, or about 164W at 141IPM.

    Yes. Ramsey said his gantry would be 800 lbs. And I said mine was around 650. Then I actually took the time to do a better and comprehensive weight estimate, and realized that mine was also 800 lbs. So there's the source of confusion.

    My main gantry is quite heavy duty, and I have done basic bending and torque calculations on it. Whether it will be good enough to do that cut, who knows, it's already mostly put together, so we will see when the time comes. I'm certainly not going to add more weight to it.
    It weighs over 90lbs with no tool holder or other accoutrements.
    I feel like a mind-reader!

    I honestly don't know that my machine will be capable of cuts like that. But theoretically, if it is, I should plan to have 200 lbs available for cutting force when sizing my ballscrews and motors just in case. Unless I totally missed what HSMAdvisor was telling me...........
    I did this quick with an online milling HP calculator (from custompart.net):

    Ballscrew Sizing Options-spindlehp-jpg

    There are two motors for the 800lbs. A reduction does give me some headroom, but I also need to increase the pitch of the ballscrew if I use a reduction (from 10mm to 20 or 25mm) OR I need to use a servo that is capable of higher speeds. A 3:1 reduction with a 400W servo? I should do the math on that. I think I know where I can get some 5:1 gear reducers. If the servo goes to 5000 RPM that could work with 10mm pitch and give me a half decent top speed for the size of my machine. You know, perhaps I should just buy the 10mm pitch and get it built.
    If you look at my graph, 10mm is better than 8mm. That's what I'm going on. I have tried the math with 32mm diameter pitch screws. There is a substantial increase in inertia on the long axis when you go to 32mm diameter, which slows down the acceleration noticeably. If I were to increase the diameter I should either put more power in, or increase the pitch so the ball screw doesn't have to spin as fast.
    If you increase the lead of the screw, you'll need to increase the size of the motor to have sufficient torque. Again, I think something is amiss in your graph, or I am not understanding how you derived the force-speed curves. The graph at CNCRP's site tells a contradictory story.

    Really, I just made this thread to try and find out if there was a Chinese supplier of ball screws hiding out somewhere who had some 2515 screws. LOL. But discussion is good. I still wish they made a 2515 screw of the inexpensive variety.
    2516 is a common thread lead for rolled ballscrews. Again I do mention that I've read reports of some issues with larger "super speed" VMCs and high-lead ballscrews. Since you mentioned that accuracy (and likely finish) are not of major concern I suppose those concerns may be moot. Most of what I remember concern circular interpolation if I remember correctly.



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