Hi guys, I converted my Enco type milling machine to CNC about a year ago, but now it is time for another upgrade. I need advice on some problems I am having with the Z axis EXTREME backlash and the backlash in the X and Y axis.

The Z axis is rack and pinion type the has a lever to lock it in place when performing milling operations. I usually keep it locked and my step motors power through when it is locked. The problem is that when it is locked The column will "stick" and the stepper motor will turn, but the axis will not move until the rack and pinion mesh. This causes about .030 error. What should I do to correct this?

The X and Y axis are lead screws and they are really taking a beating since I converted it to CNC. The backlash in the X axis is about .0135 and the Y axis is .018. I want to remove the lead screws and replace them with ballscrews that have ZERO backlash. Can any of you recommend any companies that can supply these? I do not want to pay several hundred per axis, but will spend a couple hundred. There seem to be a whole bunch on Ebay right now. Can anybody recommend one that might work for me?

The X lead screw is about 26", the Y is about 12"

I will post pics of the machine if needed.

possible lead screw candidates

http://cgi.ebay.ca/UNUSED-BALL-SCREW-WITH-29-1-4-OF-THREADS_W0QQitemZ250021693255QQihZ015QQcategoryZ25290QQrdZ1QQcmdZViewItem
http://cgi.ebay.ca/NSK-BALLSCREW-16-06-PRECISION-C1-870mm-BALL-SCREW-CNC_W0QQitemZ170021698933QQihZ007QQcategoryZ55826QQrdZ1QQcmdZViewItem


I appreciate any suggestions you guys can give me. Thanks for your time (in advance), Ryan

MY $.02

If you are going with 1/2 inch 5tpi ball screws, McMaster Carr ($1.25 /inch and 1/2" ball nuts are $23 each and you would have to preload with a 2nd ball nut.
Ron

Are McMaster Carr's ball screws ground or rolled? The website does not say. I do not want rolled since they are not much better then some leadscrews.

They are rolled, but they may be what is in your budget of a couple of hundred for the 3 axis. I suspect that percission ground ball screws will be very expensive.

As you may know, the rolled ball screws are about .004 accuracy per foot. If you are milling a peice which 3 inches in length, the ballscrew (assuming you use preloaded ballnuts set to give zero backlash) could contribute .001 inch error and still be in spec.

Now, the machine using the rolled ball screws still would have excellent repeatability.

I guess you will need to look at your requirements and determine how much accuracy you need and that will drive how much you will need to budget.

Ron

Does anyone know if any of the low cost controllers (EMC, Mach1-3, TurboCNC) support mapping of the screws?

If one does, then the rolled screws can be used to closer tolerance and the lead error would realistically only contribute to backlash.

Aaron

Mach3, has a setup screen for ScrewMapping

I haven't delved enough into it. But this might be what your talking about.

See Attached Picture.

Homebrew,
To answer your origional question, I used roton rolled screws and made my own pre-loaded nut setup. The biggest thing you can do to take up the slack in the Z is to lossen the worm gear housing and push the pinion closer to the rack. The other thing I did was trash that stupid spring, put on a thrust bearing in its place to keep the shaft in one place and counter weighted the quill with about 30# of iron.
You can see some of the details on my web page as noted in the tag line.

Bubba

Homebrew,
To answer your origional question, I used roton rolled screws and made my own pre-loaded nut setup. The biggest thing you can do to take up the slack in the Z is to lossen the worm gear housing and push the pinion closer to the rack. The other thing I did was trash that stupid spring, put on a thrust bearing in its place to keep the shaft in one place and counter weighted the quill with about 30# of iron.
You can see some of the details on my web page as noted in the tag line.

Bubba


Bubba, do you have a detailed write up on the double ball nut you made. Or a better picture. I think I understand how it works, but the picture is a little small.

Also, do you know what the price tag/specs of the roton ballnuts/screws are/were.

I forgot about them as a supplier. If I remember their prices were pretty good.

Deviant,
From my spreadsheet:
Roton www.roton.com $135.53

59321-60 Ball Screws -- , Thread Size: 5/8 x .200 x 60, Hand: RH $43.25 1 $43.25
19193 Ball Nuts -- , Thread Size: 5/8 x .200, : - , Hand: RH , Type: 2, Style: Square, Thread Size: .938 - 16 $19.40 4 $77.60
shipping 1 $14.68
tax 1 $0.00

Hope this comes out ok in final format:}) I also ordered two "sets" of wipers, but come to find out they sell them each and they were about $10 each. They also have/had a $40 minimum.

I have in cad, how they were made, and it is kinda a working "sketch" but would be more than happy to share it with you if you like. Please PM me with and address and I will send it to you.

Bubba

Thanks you for the replies.

I opened up the worm gear housing and that seems to be where most of the backlash is. I tried loosening the housing, but that does not seem to solve the problem. I think the only solution is to completely rebuild the Z from the ground up, scraping the the rack and pinion and worm gear system. A worm gear driving a rack and pinion is not a very good design and just creates mega backlash.

Here are some pics so you can see what the machine design is.

Contact Nook, I am almost positive that they manufacture higher grade rolled ballscrews than the ones we readily buy from homeshopCNC and places of the like. They will be shiny, like hte ones Syil uses. Like I said, you may be able to ask for a quote.
Also, check Hiwin

I just ran a test on Rockford rolled ball screws and their double nuts, using my DRO over 28 inches on the Z axis of my lathe that is being converted to CNC. Just for fun we loaded a 19 inch chuck and a 8 inch kurt vise in the cross slide to possibly duplicate weight and some cutting pressure.Using a simple Z program, back and forth at 51 ipm 22 times, then to home and back to 28 inches, a recorded error ( maxium) of .00087 was noted. It always returned to home at .ooo,(?), the biggest error at the 28 inch stop.The ways and ball screw were well lubricated.Unless you are making a commercial machine that must repeat at .0001- 24 hrs a day, I would say that a well set up system,using rolled ball screws and double nuts, would be fine for your application.Most errors occure due to poor bearing mounts, to include every thing that NC cams has pointed out many times concerning the correct way to engineer the mounts and pre-load.You could spend thousands on ground ball screws, but have horrible errors with out the correct bearing set up.

Adobe Machine ( old as dirt)

I just ran a test on Rockford rolled ball screws and their double nuts, using my DRO over 28 inches on the Z axis of my lathe that is being converted to CNC. Just for fun we loaded a 19 inch chuck and a 8 inch kurt vise in the cross slide to possibly duplicate weight and some cutting pressure.Using a simple Z program, back and forth at 51 ipm 22 times, then to home and back to 28 inches, a recorded error ( maxium) of .00087 was noted. It always returned to home at .ooo,(?), the biggest error at the 28 inch stop.The ways and ball screw were well lubricated.Unless you are making a commercial machine that must repeat at .0001- 24 hrs a day, I would say that a well set up system,using rolled ball screws and double nuts, would be fine for your application.Most errors occure due to poor bearing mounts, to include every thing that NC cams has pointed out many times concerning the correct way to engineer the mounts and pre-load.You could spend thousands on ground ball screws, but have horrible errors with out the correct bearing set up.

Adobe Machine ( old as dirt)

This is a post I will refer to many times, well done! I think this clears up a lot of misconceptions about this screw, I always believed they were more accurate than the .004" per foot.

Hey All,

I have decided for the Z axis I will do what Jeff Davis did at homeCNC.info, which is to scrape the rack and pinion and replace the Z axis motion with a ball screw hidden in the head casting. The manual handwheel will be replaced with a 200 ppr encoder to control the up and down motion of the Z axis. If I use an encoder then I will not have to use the jog commands in Mach3 to zero the Z axis, It will also suit my needs for non CNC machining.

I still have not decided which company to go with for the ball screws, they all seem like reputable companies. Since I will be going with rolled ball screws I assume I will have to buy 2 ball nuts for each axis to eliminate backlash?

Should I buy the bearing blocks from one of these companies or should I make them? I have been reading poor bearing blocks will cause unwanted backlash, and I am no aerospace machinist :).

Thanks for you time and suggestions. They are welcomed

the homeshopcnc.com ball screws are a good buy.

I would very much advise you to : Use double ball nuts and buy the bearing set for at least the "Fixed end" which should include the pre-assembled housing with fixed angular bearings,the other end is just a simple radial support and you most likley could machine that ( even using 6061 alum.) Go to Rockford Ball screw web site @ www.rockfordballscrew.com.and download their selection chart&service manual ( even tells you how to reload ball screws if you have an "accidental unloading") All the specs are there, with instructions how to set the "preload" on their double nut systems..really pretty neat and readable info.You can use their calcs and formulas to come up with the correct ( and most economical ) size of ball screws.

Adobe Machine (old as dirt)

You should be able to make a nice backlash free setup using a bearing duplex. 15-20 bucks for each set of bearings from vxb.com