ACME ( was Re: leadscrew, acme screw, threaded rod, ballscrew

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Chris L <datac@l...> wrote:

> Nobody is commenting on the screws mcmaster calls: "Precision
> Modified-Acme Lead Screws and Nuts" Currently, these are on page

967

> on-line. These really are not "Acme" screws so it is a little
> misleading. They are rolled precision "Lead Screws". I was told

that

> Mcmasters screws come directly from Kerk. Unfortunately, McMaster

only

> sells the lighter duty self adjusting antibacklash nut, but do not

rule

> it out. If used them on Z axis applications and they will run day

in and

> day out for a year maybe two all the while maintaining no backlash.

I

> have also ordered the Screws from Mcmaster, (its cheaper) and then
> ordered Kerks VHD nuts.
>
> So instead of this .006-.009" per foot slop you have to wrestle out
> using conventional Acme screws, these offer .0006 per inch.
>
> I consider this style leadscrew the only option short of ball

screws and

> in some, no, - many - cases much better than using ball screws.
>
> Of course they are priced somewhere between the cheap stuff and the
> expensive stuff.....
>
> Chris L

I commented on presision acme as being better than plain leadscrew
acme.

McMaster has a writeup on leadscrews on page 962 and there they
mention 0.003" per foot.

you commented that this is slop, but slop typically implies
hysteresys or backlash.

for the newbies, error comes in specifics. backlash we assume you
know. repetitiveness, the ability to come back the the same point is
another. lose a few steps on a stepper and you lose repetitivness.

distance from theroretical is another. Motorola calls this End Point
Straight Line Fit. it refers to a straight line from zero to the
other end. if your leadscrew is rated for +/-0.01" per foot it
could be off -0.1 the first foot and +0.01 the second. overall it
would be perfect. the EPSLF would be 0.01" This would be bad if you
are making circles as they would not be round. but for circuit
boards, it would not make much difference as they do not have to fit
another assembly.

Linearity Error, the error may be inclusive so a 10 foot section
really has the threads lining up at 120.030" or 119.070" of course,
it could be 120.00 but at 5 feet +/- the 0.15"

This type of error has no real meaning in a lot of the wood routers
and other hobby stuff so a really poor screw with 0.015"/inch would
be repeatable.

we don't often talk about temperature and humidity as inducing
errors, but a cold shop, 50 degrees will have different
charasteristics from a hot shop 90 degrees. this is another place
hysteresis comes into play. some nuts may become more elastic at
higher temperatures and oils may become thicker at colder
temperatures. these will be errors that cannot be adjusted out.

like you mentioned Precision modified-ACME have better than 0.003"
and have a smoother finish to reduce friction. These are along the
lines of the best ACME are better than the worst ballscrew comment I
made.

I agree that an ACME or modified-ACME is mcuh better suited in some
applications. ball screws cannot shed chips like a screw.

Bottom line is to use what you REQUIRE. don't buy linear rails and
anti-backlash ball nuts if you are making wooden signs or home
circuit boards.

but don't waste your time with allthread and chains on a Bridgeport
retrofit.

My advice stays the same. DESIGN with the ultimate in mind (ie:
leave the space for it), but TEST with cheap crap(cheap allthread),
and INSTALL the best cost/function you can get away with.

As a note here, I used some cheap all-thread on a table and found
mistakes in the table that would have prevented me from using the
ball screws I so desired. the $10.00 for the screw was well worth it.

Dave

>
>
> turbulatordude wrote:
>
> >Lead screw is a generic term or how you are transfering power.
> >others are rack and pinion and there is a way to fix a chain or
> >timing belt at each end of a base and use that as the means of
> >movement.
> >
> >ACME rod is a relativly square edge.
> >
> >http://www.williamsform.com/Threaded_Rods/All-Thread_Acme_Rod/all-
> >thread_acme_rod.html
> >
> >ACME thread is the stuff you see on vices and manual lathes and C
> >clamps.
> >
> >it comes in a contractor grade. that would be used to clamp

things

> >together. no real tolerance or quality.
> >
> >then there is a commerical grade that is slighly better but has a
> >error over the length that makes it unsuitable for anything that
> >approximates precision.
> >
> >there is also an archtechual ACME, this is usually the chrome

plated

> >stuff or otherwise finished to look good. again, no machine value.
> >
> >then there is the mechanical ACME we usually think about.

designed

> >for machines and comes with some notation of accuaracy. often

0.006

> >inche per foot or worse. This is usually called leadscrew and may

or

> >may not be termed ACME too.
> >
> >then there is presision that is about 0.003" per foot or better.
> >
> >regardless of which style you buy, it transfers power better than
> >angled threads. the higher precision, the more you pay and the
> >better the stuff.
> >
> >the precision stuff will also be the straight and ground. that

adds

> >to the efficiency in power transference (and cost).
> >
> >like anything in this hobby, the presision stuff just costs $$$.
> >
> >all-thread angled rod is also available in most of the above style
> >except leadscrew or precision. the best you'll get is ground or
> >rolled threads which move more smoothly, but the straightness and
> >tolerances are not there. no reason for it.
> >
> >The cheapest ball screws are light years better than all-thread,

but

> >the best ACME is better than cheap ball screws, but not much.
> >
> >for power transfer, ball screws beat the others hands down. they

are

> >just way too efficient.
> >
> >Check out McMaster for the ACME stuff and they should have some

write-

> >ups on the different types.
> >
> >Dave
> >
> >
> >
> >Addresses:
> >FAQ: http://www.ktmarketing.com/faq.html
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