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View Full Version : To Ball Screw or Acme? (Sieg X2)



tjhj
05-13-2009, 01:00 PM
Hello All,

Thank you for taking the time to read this and to help if you do.

I am a new owner of a HF Mini Mill, and am much better with electronics than mechanical things so I purchased this unit with the intent of converting it to CNC operation.

I decided that because this is my first CNC machine build I would be better going with a pre built kit instead of making my own, and maybe the next one I will build from scratch, but this is where I have found an issue.

The two kits I have been considering are the CNC kit from Little Machine Shop.
http://littlemachineshop.com/product...ory=1687114045

And the ball screw kit from CNCFusion. Kit #4.
http://www.cncfusion.com/minimill1.html

I have been reading everything I can find and talking to everyone who will talk to me, and the general response I get always is Ball Screws have less backlash and cost.

If I am going to spend quite a bit of money converting a machine to CNC why not spend the little more and get Ball screws, If there is really no downside I would rather spend an extra $100 dollars up front to get better performance than 4-5x times that much later to make it right.

Now the big question price aside are ball screws really that much better?


There has to be a disadvantage to the use of ball screws or why doesn't everyone use ball screw kits?


Is the backlash reduction more consistent with a ball screw over a proper Acme thread with anti backlash nut?


I was told back drive can become an issue with ball screws, any truth to that?

Will this choice affect my ability to hand operate the machine, for quick touch up/ ect work?

One person told me that ball screws are more sensitive to cleanliness and can more easily become damaged by trash getting in/on them. Is this true? Or a major consideration?

I was also told that ball screws are less used due to their cost, in a manufacturing environment I can understand the cost of acme screws being advantageous a few dollars over thousands of units adds up. Is this where the choice stems from?

Am I over thinking this?

Thank you for your time and any help or advice you can share.

TJ

PS: I hope this is the correct forum, I thought this was more of a general question than a CNC specific machine specific question.

Ryobiguy
05-13-2009, 01:59 PM
While I can't answer all the specifics, I would vote for spending a few extra bucks up front and getting the ballscrews.

Kinda like this: you want to fly across the country quickly. Do you want the plane you ride in to be powered by a propeller, or by a jet engine?

Not sure how little backlash those particular ballscrews have (rolled, single circuit ballnut,) but they will have some.
If you can crank down the acme's anti-backlash nut hard enough to match the ballscrew's backlash, it probably would be very hard or impossible for the motors to turn it at all.

Ballscrews roll very nicely, and will not load the steppers/servos as much allowing much higher speeds. If you want to do some manual touch ups, just lock the axis that isn't being used, because with enough cutting force, yes, it will back-drive the table out of position. Not sure if you're going to see that much force on an X2, but I wouldn't consider backdriving a reason to avoid ballscrews for a machine you intend to CNC.

I don't know specifics about long term maintenance, but there probably is some truth that they are more sensitive to getting loaded with swarf, but in either case if swarf is getting to the leadscrew then something needs to get fixed.

Ballscrews used less in manufacturing due to their cost? Ever seen a production CNC machine that uses acme screws?

-Matt

Mark Hockett
05-13-2009, 02:20 PM
Ball screws are the best choice for CNC applications and acme screws are the best choice for manual machines for the reason Ryobiguy gave. If you don't have a servo or stepper motor to hold the screw position it can easily move due to the low resistance. I have a manual knee mill with acme screws and a CNC knee mill with ball screws. When there is no power to the CNC mill it is very easy to push the table by hand and get the table to move and the ball screw to spin. On the manual mill I can't push on the table and get it to move.

Ball screws have almost zero backlash where an acme screw will need some play to work smoothly.

sansbury
05-13-2009, 02:47 PM
Brief answer: if you're going to buy a kit buy ball screws, get the overloaded nuts (extra $), otherwise build it yourself.

More detailed answer:

1. Screw "goodness" is actually a factor of several things.

- Backlash: Less is better
- Lead deviation: How precise is the positioning ability of the screw, i.e., the pitch is nominally .200" per revolution, but in practice there is some deviation, usually expressed in .00X"/foot or what have you
- Efficiency: How much effort to turn the screw is lost to friction

Ball screws are the grail in many cases because they can have (effectively) zero backlash, very little pitch deviation, and very high efficiency, all much better than possible with acme screws.

In the real world, those are attainable only with ground ball screws, which are very expensive--$500/axis or more. So, the hobbyist kits all use rolled ball screws, which can be made pretty good through the use of oversized balls in the nut, without which the ballscrew will often have more backlash than the equivalent acme setup. CNCFusion offers this as an extra-cost option, IIRC.

Backlash in an acme screw can be reduced through the use of a split nut, and some people have modded the brass nuts on the X2 to do just this. There is a person on CNCZone named chukkie I think who did this and has posted more pictures of precision machined parts from his X2 than all but a few other posters. In theory, split nuts wear and require adjustment to keep the backlash down. In theory, ball screws do not wear as quickly and so do not require adjustment once set up.

In practice, the X2 is a box of parts held together by chance and good intentions, and I can't run mine more than an hour or two without something coming loose. As a result, I now do a "preflight" check each day before using it where I go through and check that a dozen or so things are still firmly attached, and this prevents most WTF!? incidents. I will say that the ball screws on mine have yet to require adjustment, but I can't say that adding them to the list of things to check would be that big a pain. If you want power-up-and-go reliability, you're unlikely to get it from a CNC X2.

I used ball screws but made the parts myself, save for paying a machinist to turn down the screws on a proper lathe, and spent about $300 for the mechanical parts. CNCFusion would cost about $600 for the same. I would not buy the LMS kit myself, not because it's not perfectly good, but because I would build the parts myself for a lot less money.

The thing I'd throw out there is that by the time you're done, a CNC X2 often costs as much as a turnkey CNC Taig, and is not far from the $3000 or so that the SIeg KX1 goes for.

sbmathias
05-13-2009, 03:46 PM
Here's an article from the latest issue of Machine Design magazine:

http://machinedesign.com/article/a-critical-look-at-acme-ball-and-roller-screws-for-linear-motion-0506

It may provide you with some useful information.

lazlo
05-13-2009, 04:53 PM
In practice, the X2 is a box of parts held together by chance and good intentions, and I can't run mine more than an hour or two without something coming loose.

Hilarious quote! :)

When you say "Overloaded nuts" -- do you mean preloaded ballnuts, or are you referring specifically to using oversized balls in the nuts?

mochinist
05-13-2009, 06:10 PM
You might look up Evans CNC build, I wouldn't have believed acme threads would work as well on a CNC build as his setup apparently does. Of course his wasn't an off the shelf setup, but his way may prove to be cheaper. If I remember correctly he used part of the acme shaft and made a tap out of it, to tap his threaded nuts. Again I'm just going off memory but I don't think he has much back play and when he does get some, making a new nut is really simple.

Your Old Dog
05-13-2009, 06:35 PM
No expert here but I do have two cents so here it is :D

I can see where the acme thread would be far stronger but that the ball thread in cnc cases does not need to be stronger as they take far more smaller bites. I don't think you'll be taking any hog cuts in cnc unless your math is way off :D Good luck and welcome to the forum. We have many here with cnc, some home brew. Yes you came to the right place. What's your view on world polotics and are you Republican or Democrate? You a tree hugger or chopper? LOL

aboard_epsilon
05-13-2009, 06:48 PM
I have a friend who has cnc'd his x2 ...

he says the ball screws are greased ..and no need to oil the lead screw ..

which i doubt ..

what do you guys say

all the best.markj

Mark Hockett
05-13-2009, 07:04 PM
I have a friend who has cnc'd his x2 ...

he says the ball screws are greased ..and no need to oil the lead screw ..

which i doubt ..

what do you guys say

all the best.markj
Mark,
On my Haas CNC lathe and Fadal VMC all the ball nuts are greased. The grease interval specified by the manufacturers is once every two weeks. On my Milltronics CNC mill the table feed nuts are lubed with the pressure feed oiler and the Z axis is greased. The ball nuts are sealed and Haas recommends only two pumps with the grease gun so the seals do not get damaged.

aboard_epsilon
05-13-2009, 07:12 PM
i haven't properly looked at it mark ..but suspect its not as sophisticated as your professional machines ..doupt it even has a nipple to grease .

all the best.markj

sansbury
05-13-2009, 10:02 PM
Hilarious quote! :)

When you say "Overloaded nuts" -- do you mean preloaded ballnuts, or are you referring specifically to using oversized balls in the nuts?

I mean oversized balls--a lot of people say "preloaded" but I understand this to mean something different--i.e. two nuts shimmed against each other.