Re: [CAD_CAM_EDM_DRO] B/S vs. acme was Re: What makes a machine CNC ready?

B,

Given the choice:

Threaded screw or ballscrew? Bushing or ball bearing? Every alledged "advantage" pointed out below could be applied for a sleeve bushing as well. Are sleeve bushings used on these million dollar machines with acme screws? Why put an expensive angular contact ball bearing on the end of an acme screw if the info below is true?

I'm not convinced that an acme screw is "ever" better in a CNC application. Not saying one cant be used, just that the ballscrew will always be a better option over it. For precision, high speeds, lower friction, lower stress on the electronics, service life, etc.

Ron


ballendo <ballendo@...> wrote:

>In CCED, R Rogers <rogersmach@...> wrote:
>/// What are the advantages of acme or 60 degree screws over
>ballscrews in a CNC application?

First, let me ask you a few questions:
If all cnc machines would be better using ballscrews, why aren't
they all using ballscrews???
Why do screw manufactureres sell both types?
Why do CNC machines come with both types?

Let's list a few reasons:
1) Acme screws have better load capability for their size.

2) Acme sliding surfaces are less likely to fail catastrophically.
(so where safety and maintennance are issues, you may find acme
instead of B/S)

3) Acme sliding surfaces require less in the way of dust/swarf
exclusion.

4) Acme screw/nuts do not rely on recirculation of the balls and
therefore do not need/expect "clearing moves" (means moves of long
enough travel often enough to prevent inaccuracies and wear caused
by short movements. I already mentioned this one a couple times now.

5) Acme sliding surfaces can provide a smoother result since the
effects of rolling balls--with rolled and other cheap B/S--do not
show up in the work surface.

6) Acme screws can be run without lubrication which is sometimes
necessary for cleanroom and vacuum environments.

7) Acme nuts may be cast inexpensively into shapes that save OEM's
money and assembly time, and also reduce maintennance costs.(#1
above is an additional factor here too!)

8) Potentially lower cost for many machine types (although as other
replies here show; this is not the only factor. Which is what I
already said!<G>)

9) Stub acme can have a higher column strength than a similar sized
B/S in certain sizes. This can allow faster speeds, cheaper
endmounts, and/or longer travels.

9A)The larger core in some sizes can also allow for larger end
bearings with higher load capability, for a given size. (This is
listed separately from #1 above because this load capability is not
the screw itself, but it's end bearings.)

10) Acme screws/nuts are quieter. (Not all CNC machines are
VMC's!)

Are ten plus enough, or should I keep going?<G>

Ballendo

>>B wrote:
>>A statement like this "worst B/S is better than best Acme" does
>>little to promote actual learning and truth about the differences
>>and advantages of EACH type of screw. "

>/// So if someone building a linear CNC device has the choice
>between the two, sometimes it's an "advantage" to use an ACME screw
>over a ballscrew? When? Other than cost and availability, I can't
>think of any.
>
> Ron

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