View Full Version : Ballscrew Size and Pitch
10-01-2013, 10:33 AM
Advice required. If you were designing a steel framed router with rails 1800mm (X axis) and 1500mm (Y axis) and 620mm (Z axis) what size and pitch ballscrews would you use.
Because of the lengths of the X and Y axis they appear to be beyond the 1610 ballscrew because of whip, etc. I could use BK bearings at both ends but not sure what benefits that would provide.
I was thinking 1616, 2010 or 2020 for the X, maybe 1616 for the Y axis and 1605 for the Z axis although some of these seem expensive. Any suggestions?
10-01-2013, 12:53 PM
Although I have not checked myself, it is apparent that Chai on eBay now sells RM2010 ballscrews, so for the X and Y axes they are probably the cheapest option that will perform well. RM1610 may be acceptable for the Y axis - it depends on how fast you really need the machine to go. If you can find 1616 cheaply then that will actually allow higher feedrates than 2010, since the feedrate you get close to the critical speed is greater and the inertia is much lower. I've posted the calculations for this many times before, so you should be able to find them.
It does depend on what you're cutting to an extent. If you're cutting at low feedrates (e.g. aluminium, engraving) then clearly optimising for high speed with high lead screws (=low resolution) is not sensible. However, the length of the Z-axis you have chosen implies you will be cutting large pieces of wood or foam, in which case you do want high feedrates and resolution isn't such a big concern.
Putting angular contact bearings at both ends will increase the critical speed of the ballscrews, enabling a higher feedrate but only if the motors you select are capable.
1605 is a good choice for the Z-axis. 620mm is a rather large Z-axis which means it will be hard to make a strong machine.
10-01-2013, 02:34 PM
Thanks for the reply. The z is rather long but as im having an adjustsble bed I'm hoping to make it as versatile as possible.
I have the the profiled rails and they dictate the size of the machine but I wanted a cutting area of 4×4.
I have looked into the critical speeds and the lengths seem just beyond 1610.
11-01-2013, 08:26 AM
I can confirm that Chai does indeed sell both RM2005 and RM2010 ballscrews.
17-01-2013, 03:03 PM
I think the only practical solution for a 1800mm Axis is to fix the ballscrew and drive the nut.
This is rather expensive but you get a very fast and dynamic machine and a 20mm Ballscrew will be OK.
(I have a >2000mm Axis with a 16mm ballscrew - but this is a foam cutter with low requirements - the spindle bends quite a bit even though it is fixed at both ends and put under moderate tension)
If you drive the ballscrew as usual you'll need something about 30mm diameter to get any reasonable speed - but this will still be very very low dynamic due to the spindles high inertia.
17-01-2013, 03:31 PM
I can confirm that Chai does indeed sell both RM2005 and RM2010 ballscrews.So Can I as i have Two 2100mm 2010 ballscrews and nuts in my front room, waiting to go on my machine along with the rotating ballnut mounts made by Jonathan..
17-01-2013, 04:44 PM
So if I was to use 2 2010 ballscrews on th X axis with BK15 blocks at each end and a single 2010 ballscrew on the Y with 2 BK12 blocks would the criticalspeed be low?
From my sketches the screws would have to be 1600mm long to get the required travel.
18-01-2013, 07:01 AM
Jim get yourself some decent drives as well. They make a world of difference. The 70v chinese ebay one's are pretty shit compared to the digital drives. I can't see why anyone would use them after experiencing the digitals. Leadshine AM882 is a good starting point.
I've got 1.5m long screws, one a 1610 and the other is using dual 2010. With the AM882 and 75v I have no probs with 10m/min and high acceleration 1m/sec^2. Before with the cheap chinese 70v's I was lucky to get even 4m! Rubbish.
I'd go 2010 for the X but either 1610 or 2010 is fine for the Y.
18-01-2013, 07:50 AM
Thanks for the comments, I haven't really thought about the electronics but like the idea of the Leadshine AM882 drives but I'll wait until the machine is built or at least fully designed so I know what I'm trying to move around. The design keeps changing and developing but now that I've got the rails at least the size is fixed.
The rotating ballnut has intrested me but seems to add another layer of complexity that may not be needed if 10m/min is possible with BK blocks.
18-01-2013, 08:20 AM
Work out the rpm of the screw for your wanted travel speed and look at the info pdf for the BK15 bearing block to see if you are exceeding the max value. Why do you want to get travel speeds of 10m/min?
18-01-2013, 09:51 AM
I don't know if that speed will be needed but it was mentioned in a previous post and seemed like a good target.
What kind of rapids and acceleration are needed for wood and aluminium, and what do you all have and feel comfortable with?
Also it seems very difficult to get torque/performance profile curves for the various stepper motors so I'm not sure what RPM you can expect without losing torque.
Sorry if my terminology is wrong.
18-01-2013, 10:25 AM
Doesn't matter what rapids are since its irrelevant to cutting. On a larger machine such as yours its nice to have since it'll get from one side of the bed to the other quicker but other than that its not important.
As for cutting. Its dependant on material, machine rigidity, spindle power, end mill size, depth of cut etc. But very rough ballpark speeds for a typical DIY CNC cutting wood would be 3m-5m/min at 4-6mm DOC. Aluminium is often cut at 0.7-1m/min with 1-2mm DOC. No firm answer though so don't take that as gospel.
Acceleration is really about reducing cutting time with short moves such as 3D/2.5D. Higher it is, the quicker you accelerate after a direction change.
18-01-2013, 02:35 PM
Jim for this size machine to cut the materials your after cutting then you really need 2010 screws but it's not so much for the speed and more for putting the usable Torque in the place where it's needed.? When cutting.!
With correct voltage and decent drives Expect around 900-1100rpm for nema 23 motor before torque starts dropping off fast. So like has been said with 10mm pitch then 10mtr/min rapids is no problem really but that's a minor plus. Whats really important is that the bulk of the motors torque will be around 4-600rpm and that's where you'll be when cutting woods,plastics etc.
Now for Aluminium then 10mm pitch is waste and you'd be better with 5mm pitch for the increase in resolution and torque. Again 5mm pitch will give 5mtr rapids but when cutting the motors will be working much lower up the torque curve giving best torque where it matters while cutting.
You workout what you'll be doing most off and then decide what's important to you.? Resolution or speed.?
All I say is 10mm pitch will Cut Aluminium just at lower resolution which unless doing fine detail engraving won't be a problem and if you do plan on doing fine engraving then you've built the wrong machine anyway.!!
Forget rotating ballnut Idea it's not needed for this length machine and makes things overly complicated and an expense that's not needed.
The BK15 blocks will easily handle any RPM a stepper can reach so don't worry your self with that.!. . . It's a NON issue.
18-01-2013, 04:35 PM
So Can I as i have Two 2100mm 2010 ballscrews and nuts in my front room, waiting to go on my machine along with the rotating ballnut mounts made by Jonathan..
Er, I hope not as you said recently they were RM2510 so that's what I made the rotating ballnut mounts for, since at the time 2010 wasn't available. If they are RM2010 I will have to make new shafts to suit.
18-01-2013, 06:07 PM
Thanks Dean and hopefully I'll post a new thead in the build section but I have so many questions like us newbies.
I have sketches so hopefull I will get even more good advice.
18-01-2013, 06:41 PM
Er, I hope not as you said recently they were RM2510 so that's what I made the rotating ballnut mounts for, since at the time 2010 wasn't available. If they are RM2010 I will have to make new shafts to suit. Yep you are correct, that was my mistake, however i did enquire about 2010 when i purchased mine and Chai confirmed he did them..
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