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Marty J. Roberts
10-17-2012, 05:36 PM
Hello folks, has anyone had any experience with doing a ball screw retrofit on a Bridgeport knee mill. Does anyone recommend a company that sells a good kit? In advance thanks for the info! Just kicking the idea around..

justanengineer
10-17-2012, 05:52 PM
Not to be an ass, but why?

duckman
10-17-2012, 06:05 PM
Marty I have done them usually for CNC conversion, if your doing it for a manual machine you will probably going to regret it, only because ball screws can be back driven even if your table and saddle locks seem to work like they should, one good grab and you've ruined the part that you've got way to many hours into. TJMHO

PaulT
10-17-2012, 07:01 PM
If you are on a tight budget, check out http://rockfordballscrew.com/

They make affordable kits for doing BP ballscrew upgrades. They aren't the most accurate ones out there, you can pay more and get better ones, but to be candid they are accurate enough for most jobs that are suitable for a BP type mill in the first place.

Paul T.
www.springtest.com

Marty J. Roberts
10-17-2012, 07:23 PM
I have two mills tore down getting the heads remaned and the ways are being regound and scraped and I am sitting here looking at wornout screws and was wondering if ball screws would be a more durable option..... I have decided to replace the screws and nuts with OEM... I was just wondering about ball screws......

JoeLee
10-17-2012, 10:34 PM
A friend of mine replaced his worn ACME screws with ball screws and wish he hadn't. They are way too loose (no drag) for a manual machine and just a slight vibration will cause them to move.

JL.....................

EVguru
10-18-2012, 04:40 AM
You don't need much friction to stop ballscrews from backfeeding. An unpowered steppr motor on a manual/cnc machine is enough and could be used as a power feed with no intention of going to machine control.

On my Acme equipped machine I tend to lock whichever axis I'm not moving as a matter of course anyway.

Damping grease might do the job too.

oldtiffie
10-18-2012, 05:01 AM
Taking the say "X" slide/table for example, if you grind say 0.020" from the male (bottom, on the cross-slide) the "X" nut axis will "stick up" 0.020" + wear above the table-end bracket bores for the lead-screw.

If say "Y" (female/bottom) the of the cross-slide has 0.020" ground from it the "Y" nut axis will "stick up" 0.020" + wear above the bottom of the "Y" cross-slide.

The acme nut will not be a problem as it will be removed for the nut on the ball-screw.

The "X ball screw height will need to be adjusted to suit the table end-bracket bore and the "Y" ball-screw height will need to be similarly adjusted to the level of the cross-slide and bracket bores as well.

As I understand it, ball-srcrews may nottake kindly to unwarranted lateral loads caused by screw-to table end bracket bore axis.

The key to "ball-screw back-feed" is the screws co-efficient of friction and its helix angle which are functions of the screw pitch diameter and its lead. ie the larger the ball-screw pitch diameter and the smaller its lead the smaller its helix angle will be and the least likely it will be to "back-feed".

justanengineer
10-18-2012, 08:55 AM
A friend of mine replaced his worn ACME screws with ball screws and wish he hadn't. They are way too loose (no drag) for a manual machine and just a slight vibration will cause them to move.


My other concern beside the low coefficient of friction involved would be price. Precision ball screws are significantly more pricey than a simple acme setup, which with a little creativity you may even be able to make yourself. Some will tell you that good deals abound on "precision" ball screws via ebay, but my experience has been that most are either low quality junk or worn out used parts.

MaxHeadRoom
10-18-2012, 10:38 AM
There are basically two types, precision ground and rolled, rolled being cheaper of course, but in either case, make sure the nut is pre-loaded, I have found the Hi-Win from Taiwan as being good quality ground type and very good price.
Max.

Rich Carlstedt
10-18-2012, 05:15 PM
My friends have got parts for Bridgeports from Hi Quality in Ohio and are very happy

http://hqtinc.com/

Rockford Ballscrew also make very good rolled ballscrews

Precision ground ballscrews are only of value in CNC applications with stepper drives or open loop control.
Closed loop CNC's with glass/magnetic scales do not need Precision screws for accuracy, but may need it for preload control

Having a DRO on a manual makes "precision" a non-event for leadscrews.

Retrofitting a ballscrew on manual mill works, if you have powerfeed engaged at all time, like a friend of mine has.
For manual movement while cutting-----it's a pain

Inline with oldtiffie remarks, the nuts definatly need allignment.
Another method which I have used is to remount the endcaps on the table and knee. They are pinned at the factory for nut allignment.
Remove the pins, and open up the holes/counterbores in the caps. Move the table to the far end and then lighly snug a few capscrews.
Repeat at the other side, and do the same. If thecapscrews have enough clearance, the caps will self allign. Do both again, and then drill new pin holes. Do not use the old ones !
Rich

oldtiffie
10-18-2012, 05:41 PM
There are basically two types, precision ground and rolled, rolled being cheaper of course, but in either case, make sure the nut is pre-loaded, I have found the Hi-Win from Taiwan as being good quality ground type and very good price.
Max.

I think it may surprise some that some/many lathe and mill lead-screws are "rolled" and then straightened ready for use. ie not all are "machined and ground" - but they do surprisingly well.

I doubt that you could say that these lead-screws are "all-thread" quality but even they ("all-thread") are not too bad at times either.

Lead screws tend to get worn in the centres or in one position and so there will be pitch errors between slightly worn, worn, badly worn and not worn much areas of the screw at all.

Some might get quite a shock if they test a lead-screw against a good DRO or a good dial indicator and slip gauges - it won't all be good news - or bad either.

Ball-screws may not be much better than normal acme lead-screws if the ball-screw end-play is not zero and if the ends are not pre-loaded correctly.

If my lead-screws were too worn I'd opt for a new acme thread and nut combination/set - every time.

I can live with even a lot of lead-crew "end-play" or "back-lash" - easily.

I suggest that some check the specs for lead-screws/acme thread from McMaster-Carr and Boston Gear and the like.

gnm109
10-18-2012, 05:45 PM
Marty I have done them usually for CNC conversion, if your doing it for a manual machine you will probably going to regret it, only because ball screws can be back driven even if your table and saddle locks seem to work like they should, one good grab and you've ruined the part that you've got way to many hours into. TJMHO

My Webb Mill was formerly a CNC unit and was fitted with ball screws. When I was refurbishing it, I inquired what it would cost to change to standard screws and they wanted $1,200 for a kit so I left the ball screws in. I've been using it nearly every week for the past three years and have not encountered the problem that you describe. I lock the table down on both axes if not moving and otherwise I'm generally careful and have zero issues. I also have a servo on the X axis so perhaps that may help. I've heard lots of folks say that ball screws are no good on a manual, but they are sure smooth.

One other benefit is that there is zero slop on the "Z" axis and only about .004 on the X axis. That's not bad for balls screws that date from 1987.

oldtiffie
10-18-2012, 06:02 PM
Not all ball-screws have zero back-lash - even when new - as some are better then others - including the "zero" adjustment.

MaxHeadRoom
10-18-2012, 07:31 PM
This is why you should always opt for a double nut or preloaded nut, many, rolled types especially, are not pre-loaded.
Max.

Mcgyver
10-18-2012, 07:44 PM
come you guys, if we don't like billet how can we stand references to leadscrews on mills? there are no leadscrews on a mill. :)

its not something I've thought much about, but there was a guy at CNC who though highly opinionated was a veteran metal worker. Struck me as a senior guy who'd being doing it for a living for ages, knowledgeable etc. He was vehement that ball screws on a manual mill were an incredible upgrade; with anti backlash style ball screws and angular contact mounts you get zero backlash; you move the the handle, the table moves. Like i say, it hasn't struck me as a must have, but there's at least one experienced metal worker who thinks its well worth it

oldtiffie
10-18-2012, 08:40 PM
I can't see why a ball-screw and a common lead-screw can't be used as just a plain lead-screw. Some ball-screws have larger pitches than lead-screws so "fine setting" will be more of a problem.

oldtiffie
10-18-2012, 09:11 PM
The "most accurate" lead-screw it the one on a machine with a DRO on it - if the measurements are taken on/off the DRO - in which case lead-screw wear and inaccuracy are quite irrelevent - mostly - unless you are screw-cutting on a lathe where wear and accuracy matter as there is no compensating for them as the wear on the lead-screw and the half-nuts will show up on the job as pitch inconsistencies and errors.

But beware of the tolerance on DRO's - +/- 5um usually and the more expensice +/- 1um.

+/- 5um (5 micrometers aka microns) = +/- 0.00004 x 5 = +/- 0.0002" (ie +/- 2 tenths)

+/- 1um = +/- 0.00004 x 1 = +/- 0.00004" (ie +/- 0.4 tenths)

http://www.machineryhouse.com.au/Easson-Scales

lakeside53
10-19-2012, 12:47 PM
I have a BP mill with quality ball screws and an Acurite 3 axis cnc conversion. This conversion allows the use of the mill in manual mode (turning wheels). My "backlash" is "almost unmeasurable" (i.e, not material). I use it manually often, and do not experience any problem even when aggressively climb milling. YMMV.

MaxxLagg
10-19-2012, 06:59 PM
I have a BP mill with quality ball screws and an Acurite 3 axis cnc conversion. This conversion allows the use of the mill in manual mode (turning wheels). My "backlash" is "almost unmeasurable" (i.e, not material). I use it manually often, and do not experience any problem even when aggressively climb milling. YMMV.

We retrofitted 7 or 8 Bridgeports about 15-16 years ago with the Acurite CNC's and ballscrews. We run them in both CNC and manual mode all the time, day in, day out, and have no problem with them in either mode because of the ball screws. They have no appreciable backlash. People get into more trouble having their gibs out of adjustment than problems with the ball screws.