Hello Guest it is April 21, 2019, 04:38:23 PM

Author Topic: Ballscrews - Ground or Rolled?  (Read 13170 times)

0 Members and 1 Guest are viewing this topic.

Offline BarryB

*
  •  267 267
    • View Profile
Ballscrews - Ground or Rolled?
« on: September 23, 2009, 01:08:20 AM »
Hey folks, I'm at the stage where I need to order a set of ballscrews, their endmounts and what not.  I see a couple places have precision ground and others have precision rolled ballscrews.  The rolled variety are much cheaper, is there a disadvantage to this kind?  Here's where I'm looking:

http://www.homeshopcnc.com/Ballscrews2.html

http://www.automation4less.com/store/categories.asp?cat=1242

You guidance is appreciated.  I can't wait to finish Milla.

Barry
Re: Ballscrews - Ground or Rolled?
« Reply #1 on: September 23, 2009, 01:19:38 AM »
I'll be waiting for the feedback on this one! 

Offline RICH

*
  • *
  •  7,313 7,313
    • View Profile
Re: Ballscrews - Ground or Rolled?
« Reply #2 on: September 23, 2009, 08:01:20 AM »
I think you need to look at the "specs" of any ball screw.

I just finished a lathe conversion were there are two different types of screws. One is ground and the other is rolled. In general the ground screw will maintain the pitch  more accurately and will be straighter over the length.
The associated ball nut for the screw is just as important as the screw since here is where backlash comes into play.
 On a rolled screw you will probably find that there there is a tolerance on the pitch, like .004" per foot and that it may be increase or decrease ie; it gains in it's travel such that in small moves the change in pitch is minor.

The only screw which has an adjustable nut is made by Techno and the nut is some $135 for a 5/8" screw.
For this lathe i have found that the ground verses the rolled is only better by  a half a thou in backlash, but
the backlash was is due to how much preload is applied.

Now i have a friend that uses just an acme threaded rod and does great work even though he has some .062" of backlash.

I got to go and reply will with more info, but  for now, I just thought that some practical info would be apropriate

RICH




Offline RICH

*
  • *
  •  7,313 7,313
    • View Profile
Re: Ballscrews - Ground or Rolled?
« Reply #3 on: September 23, 2009, 10:15:59 AM »
Barry,
Had a chance to look at the ball screws you had kinks for. Gee's only about $500 difference. From what i see in the pictures the ones from homeshop cnc are rolled screws and have preloaded nuts.

You can find a lot of info on ball screws via the web. Heck, just go to Nook or NSK sites and you have all the reading you could want!

I noticed that they offer machining of the ends. I took me 6 hours to machine the two ends on one of the ball screws ( grind and then machine).  Went thru 3 carbide cutters ( 3 sided insert). It's a PITA. SO you may want to
realy consider farming it out or using a favor or two up.

Back to the practical side of things:
1. Ball screws, as a system, ie; type, mounting, size, grade, etc act as a system which is application dependent.
   Since the screw is a major component of movement, you would idealy  like the best. Practicaly speaking ,
    as long as the screw satisfies the application ( which you have not defined ), then it may not make any sense to
    spend money for quality not required.
2. You can spend a small fortune on bearings and they play an important part since because they are part of the 
    system.
3. You can have the best system components, but, if the installation is lacking, then you will downgrade how the
    components will function.

So what are your thoughts now on the matter?

RICH
       

Offline Dan13

*
  •  1,244 1,244
    • View Profile
    • DY Engineering
Re: Ballscrews - Ground or Rolled?
« Reply #4 on: September 23, 2009, 11:01:30 AM »
Today, with the new technology and manufacturing processes, the precision of a rolled screw doesn't differ much from the precision of a ground screw. Several years ago this was not the case, if one wanted a precision ball screw, the only choice was a ground ball screw.

Today, many manufacturers offer precision rolled ball screws that don't fall much from their ground analogues, for much cheaper costs. However, it seems that as the screw diameter increases it becomes harder to maintain the high precisions of a rolled screw, so for bigger screws and sufficient precision you might need to go for ground screws. Another thing with bigger diameter precision rolled screws is the backlash being higher as the diameter increases. For precision rolled ball screws smaller than 12mm diameter, the practical backlash declared by the manufacturer can be as small as ZERO.

Precision rolled ball screw are available with precisions better than 0.023mm/300mm.

Daniel

Offline BarryB

*
  •  267 267
    • View Profile
Re: Ballscrews - Ground or Rolled?
« Reply #5 on: September 23, 2009, 11:48:11 AM »
A $500 difference adds up when you need 4 of them, that adds up to some serious cash.  My machine will mainly be cutting wood and sometimes aluminum and brass.  I of course want the best that is necessary.  Below is a link to the base machine frame that I have built.



Thanks for the info Dan13, I didn't know about backlash concerns for larger diameter rolled ballscrews.  I think I will need a larger diameter so ground is probably better.  I notice that lower grades are better too, so from one site they are C7 and cheaper, but the other site is C5 which is more expensive.  That site does do the machining for me though, so maybe the higher price will even out over all.

Thanks for your help.

Barry
Re: Ballscrews - Ground or Rolled?
« Reply #6 on: September 23, 2009, 12:06:01 PM »
That is one nice and serious looking framework!

Offline Dan13

*
  •  1,244 1,244
    • View Profile
    • DY Engineering
Re: Ballscrews - Ground or Rolled?
« Reply #7 on: September 23, 2009, 02:13:40 PM »

Thanks for the info Dan13, I didn't know about backlash concerns for larger diameter rolled ballscrews.  I think I will need a larger diameter so ground is probably better.  I notice that lower grades are better too, so from one site they are C7 and cheaper, but the other site is C5 which is more expensive.  That site does do the machining for me though, so maybe the higher price will even out over all.


The info is a common info and has to be confirmed with the particular ball screw manufacturer. My note about backlash is true for single nut with normal backlash or reduced backlash (preloaded). The trend now is to use a single preloaded nuts. The manufacturers are trying to avoid using double nut preloaded systems these days, as it's expensive and can only be supplied as a screw-nut preloaded and adjusted assembly.

Even if you need bigger diameter ball screws, I am not sure you have to go for ground ones, as we are talking of very small backlash rates for single nut. For a 40mm diameter ball screw the backlash (of single preloaded nut) would be 0.02mm. The standard backlash (no preload) would be about 0.04mm. I think these values can well suite your requirements, since you're speaking of C7 precision grade, which I think is about 0.05mm/300mm.

I think you can still adjust those nuts for ZERO backlash, but I believe the manufacturers wouldn't recommend doing this. Again, you need to check against the particular manufacturer regarding the particular product. As always, they can get you anything, depending on how much you're willing to pay.

Daniel

Offline RICH

*
  • *
  •  7,313 7,313
    • View Profile
Re: Ballscrews - Ground or Rolled?
« Reply #8 on: September 23, 2009, 04:02:58 PM »
Barry,
Nice enclosure.
You need to take a look at the manufactures info on the ball screws for recomended support, speed, etc of the ball screw. That will give guidance for diameter.
RICH
Re: Ballscrews - Ground or Rolled?
« Reply #9 on: September 24, 2009, 02:16:47 AM »