Re: ball screw preferences

Mark,

Thank you for the reply.

As I suspected the machine you're talking about does NOT
have "plain" ballscrew and ballnuts; it is a high end, high dollar
solution.

The .002-.005 of a ballscrew withOUT an anti-backlash compensated
nut is something that many folks can comment upon. It is VERY normal.
And that IS sloppy for many CNC machine uses.

So you either add oversize balls--to retain the use of a single nut;
OR you use multiple nuts in tandem. Both of these ARE well-proven
anti-backlash solutions; and each will attain MUCH better than .002-
.005 backlas AT THE NUT. (End mount slop is another source of
backlash that the original poster didn't comment upon...)

Your description of your mechanical components clearly show that you
are working with GROUND screws and nut WITH anti-backlash features.

When you have GROUND ballscrews/nuts; ther are acoupe other ways to
achieve anto-backlash that are not immediately obvious. That is
because in each of these methods, there is a slight shift int he
grinding of at least one of the ball nut tracks; and THIS is what
preloads the assembly. You'd never "see it" enless you were looking
forit, and know HOW to look for it, AND had the tools to measure it.
You might be "aware" that "something" is making this screw "stiff"
(motion control meaning od the word stiff here; i.e., No backlash.)

Anyways, thank you for the clarification of what you were talking
about. I try to keep beginners in CNC from being led towards high
end solutions for reasons of specsmanship, as most do NOT need
the "numbers" that sometimes are handed down here--and at other CNC
group sites!

Ballendo


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mark Vaughan" <mark@...>
wrote:

>
> Hi Balendo
>
>
>
> My APTC hand mill is 5 or 6 though backlash on Acme screws.
>
> My recently acquired CNC mill is an Avon Circa 1989, built for an

aerospace

> group, with low hours on it. The ball screws that were several

thousand

> pounds each, the servo motors are 6KW, and fetch over £1000 second

hand, the

> servo cards are over £1200 each, it's a big professional machine,

and was

> very expensive in it's day and weighs several tons.
>
> I bought it cheap with a faulty CNC controller and moved it over

to mach3,

> and a G100.
>
> Without having stripped the screws, I had about 0.5 (yes 5/10) of

a though

> of backlash, on one axis, and something over a though on the other

axis.

>
> I worked on resetting the end thrust races and got that down to

less than

> 0.5 though, about 0.3 most of the time, this varies a little along

the

> length of the screw.
>
> This measurement is taken using a 1/10 of a thou per division

large clock

> face DTI, (1 thou is over an inch span on the dial), nudging up

against a

> block, then nudging off until the dti moves.
>
> You can push and pull on the bed, and you get about the same

result, though

> a hard wack on the bed will bounce the dti about 1 though.
>
>
>
> Now you say you normally find 2 to 3 though backlash. To me this

seems a bit

> large for a ball screw, an acme screw isn't much more, and a

Bridgeport

> screws spec says better than half a thou, which makes my findings

relative

> to a Bridgeport.
>
>
>
> Last night I rebuilt these preloaded anti backlash ball screws, I

can place

> a mag mount on the screw, up against the nut assembly, and if

there is

> movement on the dti as I push and pull on the nut, it is less than

1/10 of a

> thou, and that may have been a little rotary action in the nut.

The ball

> nuts are not quite as I stated, now they are all cleaned, the

tapers on the

> C washer do not come up against the taper on the nuts, these are

just there

> to allow the C clip to spring open past the screw without damaging

the

> screw. The C washer is ground so that the two ball screws nip up

against

> each other when the impact on the C washer, then a connecting key

joins the

> two ball nuts. The same as you do to build an anti backlash kit on

an acme

> screw. The ball nuts do move on the screw, but they are not what

free, there

> is some resistance. The firm that made them say this is exactly

normal, it

> does raise slight concern whether this all gets tighter as things

warm up,

> but apparently thermal expansion has been allowed for in the

allows of the

> design. The only play I can find on this screw now is the end

float on the

> thrust races.
>
> I looked at the other screw I have not stripped on my X axis, and

this is

> about the same.
>
>
>
> I do agree that thermal expansion along the bed is going to move

all this

> out quite a bit.
>
> These are the numbers and measurements that fell out, they don't

seem out of

> the ordinary to me, but 2 to 3 thou which you consider normal is

to me very

> sloppy.
>
>
>
> I also work on spark eroders, and wire machines where we have

repeatedly cut

> carbide and cover to 2 to 3 microns. These machines use

antibacklash ball

> screws, and are heated to control thermal expansion. I just put the
> increased accuracy down to the fact the there is no machining

load, no drag

> from long thick gib strips etc, but the ball screws are very

accurate, as

> proven by the work which was measured in the national physics lab.
>
>
>
> Dr. Mark Vaughan Ph'D. B.Eng. M0VAU
>
> Managing Director
>
> Vaughan Industries Ltd, reg in UK no 2561068
>
> Water Care Technology Ltd, reg in UK no 4129351
>
> Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
> Phone/Fax 44 1872 561288
>
> _____
>
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of ballendo
> Sent: 11 March 2007 10:21
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: ball screw preferences
>
>
>
> Hello,
>
> Snips, inserts follow...
>
> >In CCED, "Mark Vaughan" <mark@> wrote:
> <snip> on a mill a ballscrew without backlash adjustment is

normally

> >within about 0.5 thou of backlash, mine measure at about 0.2
> >including the endfloat on the main screw which isn't bad.
>
> If I'm understanding what you've written here, I find it a bit

hard

> to believe. You're saying that your NON anti-backlash
> ballscrews/nuts have 5 tenths BL? And that the entire backlash of
> this NON anti backlash axis is 2 tenths? No way.
>
> IMO/E Most NON-backlash compensated ballscrews will have .002-

.005"

> backlash. Ground screws may be less; and a BL compensated GROUND

BS

> assy. may be non-obvious (unless you had to buy it new!<G>)
>
> The good news is that if the original poster needs to use one nut;
> he probably CAN get by with one Bnut IF he reloads that nut with
> oversize balls. These may be had from Ball-tec in Los Angeles, CA,
> USA. I've spoken on this subject in this forum many times in the
> past; a search of the group archive should show up plenty of posts
> with details. AIRC, Dan Mauch has used a supplier in Chicago to
> reload BN's with oversize balls.
>
> I have always found it "interesting" how many folks have (or claim
> to have<G>) specs in the "tenths' (.0001"). Since this is a
> beginners forum; I'm also always trying to get folks to understand
> that specsmanship is often a path that need not be trod.
>
> Real-world commercial machines (like bridgeport vertical mills)

can

> be flexed with one finger enough to deviate 2 or 3 thou... And the
> simple thernal exapansion of the aluminum--or wood--that many DIY
> CNC machines are made from quickly exceeds tenths iduring

a "normal"

> shop day! Measuring to tenths is not something done willy-nilly,

the

> heat from your body can change parts dimensions that much!
>
> The really cool thing is that for a whole lot of REAL-world

projects

> and parts; such tight dimensional tolerances are NOT needed.
>
> Which is a good thing, since many cannot--or will not-- actually

be

> getting them...
>
> You CAN get relational fits that work, and +/- .005 between

features

> is a more realistic AND "normal" job shop accuracy
> requirement.
>
> >On a router though at least the wood ones, you aren't looking for
> >that kind of accuracy. I would try using one and measuring the
> >backlash.
>
> Probably .003" IMO/E. The double-nut preloads are often even
> sloppier than the ones designed as single nuts. Because they CAN

be,

> since the mfr. KNOWS they will be used in "opposing" pairs.
>
> BTW, as a former furniture mfr.; I can say that woodworking

machines

> DO need decent accuracy as well. Relational fits and surface

finish

> are the reasons.
>
> Summing up: It's FAR easier to have repeatability than Absolute
> dimensional accuracy. Don't confuse resolution with achievable
> precision/ location... And specsmanship is IMO a waste of time.
>
> >Make sure there is some sort of wiper on the end to keep the dirt
> >out. Mine don't have seal wipers between.
>
> Good advice.
>
> >If you building a small router for something like PCB work, then
> >backlash might be more critical.
>
> True.
>
> >Since one normally pushes against the other with a set preload, I
> >would have thought they may well be a lot more efficient (free)
> >with only one ball nut.
>
> Free, yes. AND full of backlash. UNLESS they're ground screws and
> that fact isn't immediately obvious to a surplus purchaser with no
> prior motion component experience.
>
> Hope this helps,
>
> Ballendo
>
> >>WJS wrote:
> >>Subject: [CAD_CAM_EDM_DRO] Re: ball screw preferences
> >>My question is can I take off one of the nuts without "screwing"
> >>things up? I need the extra travel. WJS
>
>
>
>
>
> [Non-text portions of this message have been removed]
>