Ball vs. Anti Backlash Acme Screws

[eman5oh]

I am in the process of building a cnc router for sale and was wonder what you prefer and why. I have built machines with both and know the pros and cons of each system. I am giving two choices that are close in price. They are as follows, single nut rolled ball screws or precsion rolled acme screws with antibacklash wear compensating nuts. So what do you like?

[gar]

060112-1952 EST USA

eman5oh:

You need to define displacement vs force for each excluding thrust bearings, and whether there is any backlash.

Also you need to define static breakaway and dynamic running torque for the nut alone, and with a maximum realistic load force.

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[eman5oh]

Originally Posted by gar 060112-1952 EST USA eman5oh: You need to define displacement vs force for each excluding thrust bearings, and whether there is any backlash. Also you need to define static breakaway and dynamic running torque for the nut alone, and with a maximum realistic load force. .

I did not want to get into the design details, just looking for what people are looking for in say a 3 ft x 3 ft light duty cnc router. I know that the ball screw has higher load rating as well as much greater effecancy. The draw backs are that it will have a little backlash and is more effected by getting dust inside the nut as well as being a little noisy while running. The Acme be quite running and have little to no backlash. It also handles dirt and dust well. Its main draw back is that it has lower load rating and with too much force you will overcome the backlash spring. You also need more motor to turn them do to lower effecancy ratings.Taking these things into consideration what would one want on the same machine know the above.

[carlnpa]

I'd go to precision acme with zero backlash nuts. I have a K2 machine that had nonprecision ballscrew and nuts, accuracy was not good until upgraded to the precision parts. Precision acme should also be much less costly than any ballscrew. I also have a machine with 1/2-10 acme on it with 200oz direct drive motors, it will readily snap a 1/4 inch mill without even slowing down. I doubt your machine will even get close to the load capacity of a properly sized acme screw. I have talked to people using the delrin zero backlash nuts on metal cutting heavy machines and they are still not getting backlash at heavy loads. Acme may need more force to turn but it needs less force to hold.

[eman5oh]

Originally Posted by carlnpa I'd go to precision acme with zero backlash nuts. I have a K2 machine that had nonprecision ballscrew and nuts, accuracy was not good until upgraded to the precision parts. Precision acme should also be much less costly than any ballscrew. I also have a machine with 1/2-10 acme on it with 200oz direct drive motors, it will readily snap a 1/4 inch mill without even slowing down. I doubt your machine will even get close to the load capacity of a properly sized acme screw. I have talked to people using the delrin zero backlash nuts on metal cutting heavy machines and they are still not getting backlash at heavy loads. Acme may need more force to turn but it needs less force to hold.

Did you go with precision ball screws or acme screw the second time around? You may be surprised but the price difffernce is not that much and in some cases the acme screws can cost more. This is some what offset by the fact the acme screws are not a hard and are easier to machine.

[ger21]

You can probably cut your cost by at least half going with a slighty lower precision screw, such as the Keystone that MSC sells. Imo, even the slightly less accurate screw with anti-backlash nuts are preferable to ballscrews with backlash.

Also, if you use multiple start acme screws, the efficiency can increase up to 50%.

[eman5oh]

I agree with you on that one. Lead error can be delt with in software by screw mapping, much better than software backlash comp.

[gar]

060113-1013 EST USA

Why the assumption that a single nut ball screw has to have backlash?

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[eman5oh]

Taking a low cost Thomson or equivalant 5/8-5 ball nut and screw off the self it will have .001 -.004 backlash in it. At least that has been my experiance with them. I know that you can reload the nuts with bigger balls and what not, but I am just looking at off the self parts.

[carlnpa]

I'm building another machine and will use Acme with zero backlash nuts. My K2 machine had Nook standard ballscrew, the ball nut had 0.007 backlash (which was within their standards) and 0.005 to 0.007 in 12" on the screw (also within standards). These standard ballscrews are not sold as suitable for cnc use, and they are not, they are motion grade ballscrews. My standard grade, super cheap Enco acme with zero backlash nuts give me less than 0.005 in 12" for 5% of the cost of the precision ballnut/ballscrew assembly from Nook anyway. IMHO
A single ballnut can have very low backlash, but either you or the factory is going to match the ball bearings to achieve it at a much greater cost than a simple acme zero backlash nut (ie dumpster sells these nuts for like $19.00). Nook wants $100 to load a single ballnut, $175 will buy a double ballnut for zero backlash.

[eman5oh]

I ended up ordering some 1/2-8 2 start Keystone Acme screw class 2C and nuts from Dumpster. I will let you all know how these work out. For me retrofit mill I used Thompson ball screws .004 lead accuacy, with a single ball nut. I reloaded them with new balls and they ended up with .0005 to .0025 depending where you checked them. It works pretty well, but I would like to find some bigger balls for it. Any have a sorce for these. The ones I used were .125 +/- .0001.

[phantomcow2]

Well i will be using rolled ballscrews with a single nut for my CNC router. One thing that is appreciated, is that the backlash is constistent. THis means i can use software compensation. Or I might just buy another nut and make it antibacklash.
I know the accuracy is not nearly as great as some of the acme out there, but i think a lot of it comes down to what you want to do.
For my router, i am looking for speed and ruler accuracy, so the .004" per foot is fine