Re: Alternative Stepper Motor mountings, etc.

Thanks, well writen.. I could see the angles as you were writing.

opps, 200 Rev on 10 TPI is 0.0005" per inch I left out a '0'

My tolerance for overall length and what I'm hoping for are
different. I'm aiming to get 0.005" but my requirements are to be
wihin about a 1/16" foot to foot, and 1/8" over the 8 foot part
length.

regardng ACME rod, I found my local industrial hardware supplier can
get a 10 ft length for about $95.00 my 1/2-13 in 12 ft cost $4.00.
I'm sure that once I get that far, some phone time will yield a
philadelphia supplier. Shipping from California will be $$ Thanks
for the explanation on architechual acme vs. machine acme. I always
wondered how Harbor freight could offer vices so cheap.

Regarding the axial and lateral forces with 60° thread vs acme, at
this point in the project the 60° will suffice as there is no real
load. The tray moves, locks, drills, unlock, move...
I haven't calculated how may inch pounds, but any multiplied force
from a thread will never have any problems.

Once I can prove the unit will work, then I'll need to get the Acme.
I almost went with 1/4-20 for the mathematically similar thread
spacing. But it was just too light and flimsy.


Dave

--- In CAD_CAM_EDM_DRO@y..., ballendo@y... wrote:
> 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