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