Ballscrew Size and Pitch

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?

Jim

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.

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.

I can confirm that Chai does indeed sell both RM2005 and RM2010 ballscrews.

Hi,

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.

Christian

Quote:

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Originally Posted by Mad Professor

I can confirm that Chai does indeed sell both RM2005 and RM2010 ballscrews.

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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..

Rick

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.

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.

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.

Jim

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?