Re: [CAD_CAM_EDM_DRO] Backlash on acme leadscrew

Russell Shaw wrote:

>
> I just subscribed to this group.

Welcome

>
> I was thinking of making a 2m x 1m area 3-axis cnc
> router for wood, plastic, and aluminium, using cheap bits.

More power to ya!

> Do the 'square-thread' leadscrews such as at
> http://www.cnckits.com have much backlash?

Acme threads are not actually square, but they do have
a flat top and flat valley which might give that impression
unless you look carefully. The flat top adds to the
thickness and strength of the threads, and the flat valley
minimizes stress concentrations there (compared to sharp
V-shaped valleys).

Do they have backlash? Yes, but it depends on the length
of the nut. I have found that 3" long steel nuts (on a
1" dia 5 TPI Acme thread) has very little (2 thou) backlash,
but these are new nuts on new leadscrews, and so that is
going to increase over time as the nut wears.

A 3" long nut threaded from UHMW is about the same. I am
currently experimenting with 6" thick threaded UHMW, and
there is no discernable backlash, and are self-lubricating
and quiet as a side benefit. There might be some backlash
as the forces are built up over hundreds and thousands of
pounds, but I haven't measured that yet. The tearout of
1" acme threads is 2000 lbs/linear inch, and so 6" would
be good for approx 12,000 lbs before failure, so the plastic
is not a concern there.

Acme (2G fit) nuts do have a noticable 'radial slop',
90 degrees perpendicular to the thread, and so should be
combined with linear motion components which offer constraint
in that direction. But in the linear fore-and-aft directions,
I can say using long nuts has essentially eliminated the backlash
for me.

The so-called 'acme anti-backlash' nuts I am not a fan of,
partly because of their expense. They are essentially two nuts
spaced apart on the leadscrew, but directionally compressed
together, or forced apart. Both have advantages and disadvantages.
However, they achieve their effect by causing more wear and friction.

> Does the 'spring between two nuts' trick work well to get
> rid of backlash on these?

Yes (causing more wear and friction), but the backlash
elimination effect goes away once the resistance against
the nut moving exceeds the pressure of the spring (holding
the nuts apart).

With or without anti-backlash nuts, acme leadscrews have
been around a long time and perform well, and will always
be more inexpensive than ballscrews. Surprisingly, acme
leadscrews on the Z axis have an advantage over ballscrews:
when motor power is removed from the vertical axis using
ballscrews, there is the likelyhood that gravity might
cause it to freefall (ie gravity 'backdrives' the screw).
Fine threaded acme's won't do that (ie over 2TPI).

Doug Fortune
http://www.cncKITS.com