Re: Alternative Stepper Motor mountings, etc.

Dave,

I'm surprised you haven't found 10 foot acme! Of course, precision
suppliers like Ball Screw and Actuators in San Jose, CA will have it,
but it should also be available from "standard" industrial suppliers
as well. Maybe you mean you haven't found inexpensive acme that is
accurate enough for our sorts of uses? Do keep in mind that there are
many uses for acme threads where the consistency of pitch/lead that
we rely on is NOT present... Thread FORM is 'their' need.

(insert RAB question here)

>How are you handling the backlash on the nut to the threaded rod.
> was thinking of using bog standard M6 threaded rod for it's 1mm
>pitch, is there a big advantage to ACME rod ?
>Rab

You may recall a post of mine recently about the 14-1/2 degree
pressure angle of older gears compared to the newer standard of 20
degrees (I was referring to rack/pinion drives then, but the concept
is the same) In a rack and pinion drive, the 20 degree gears will put
MORE force into "pushing themselves away from each other" than will
the 14-1/2 degree gear/ rack combination. Also IF they ARE out of
alignment, the 20 degree will be further out of mesh (and therefore
less accurate) for a given center distance variation.

A Vee thread form is in the vicinity of 60 degrees, for a "flank"
angle of 30 degrees. A "square" thread has a "flank" angle of 0
degrees, and an acme has something like 10-15 degrees. There are two
results from these differences. (besides the difficulty of machining
the "steeper" flanks of the acme and square thread)

1)The threads are stronger. Acme threads have fewer threads per inch
than other thread forms. This means that the material at the base of
the thread is 'wider'. It has been repeatedly shown that the first
thread carries a "substantial"(50-70%) portion of the total load
placed on the screw assembly, with the ENTIRE load often carried by
the first 4 threads... (ever wonder why standard nuts are so thin?)
There is more "meat in the acme thread form (and square) PER thread...

2)The steeper flank angle means that more of the force applied is
directed ALONG the screw LENGTH (axially), rather than being
reflected 'outward'.(refer to the rack/pinion example above) What I
mean is, when you turn a square thread nearly ALL the twisting force
is applied to the NUT in the axial direction; when you turn a vee-
thread a lot of the force is "used up" as the 30 degree flank vectors
(redirects) it outward from the screw center.

Vee threads are designed to tighten up and STAY tight, so this
tendency to redirect the load in a direction OTHER THAN AXIAL is a
good goal! Thread forms designed to transmit motion, on the other
hand, will have steeper flanks so as not to waste this force... We
WANT the nut to move! This is why Vises and clamps have acme or
square threads.

Acme threaded rod is often used (industrially, or architecturally)
where the first reason (stronger threads) IS THE reason! For this it
need have no precision, since we are only gonna thread it 4 or so
turns into a nut, and are using it for its resistance to a pull
(tension) force. Beware of 'standard' industrial acme rod for motion
control applications!

(back to Daves post)

> I am doing the prototype with 1/2-13 threaded rod, 10 feet long.
> with plans to use 1/2-10 ACME on the final unit. (still looking
> for 10 ft ACME rod)
>
> Using a 10 TPI for discussion, my stepper does 200 steps per
> revolution, and with 10 rotations per inch, each rotation is 0.1"
> travel. each step therefore is 1/200 of 0.1" or 0.005" per step.

This would be .0005, or 2000 (200 times 10) steps per inch.

> I am looking for an overall accuracy of 0.005" in the final
> unit, so all I really need to concentrate on is one connection
> that being the ACME rod to the nut.

I'm not sure here if you're still wanting .0005 or if you DO
mean .005? I HOPE you are looking for .005 OVERALL. So many people
are trying to do the imposssible; some just don't realise it. Even
holding an overall ACCURACY of .005 will take concentration in ALL
areas of the machine, IMO. Most job shops, working with machine tools
designed and constructed for precision, will use .005 as a "usual"
feature to feature tolerance.

>In my application, I am more interested in repeating a location,
>than overall accuracy from end to end.
> Dave

Single direction positioning (often G61) is the way many older cnc
machines (drilling, mostly) handled the "repeatability" problem. In
this way, the position is always taken from the same "side of the
slop".

Hope this helps.

Ballendo