Re: [CAD_CAM_EDM_DRO] Re: What makes a machine CNC ready?

The problem with ACME screws in a CNC application is backlash. CNC machines travel faster and much further than their manual counterparts. Adjust all of the backlash out of an acme screw and the rotational friction increases. Then when the middle of the screw wears slightly and that backlash is removed, the screw will lock up on the ends of the travel.

Many Manufacturers are over-zealous in their ratings. The worst ballscrew is better than the best ACME screw any day of the week for CNC use. If you have to move something, would you rather slide it or roll it? Now over a long period of time and as quickly as possible..

Ron




ballendo <ballendo@...> wrote:
Tony,

The efficiency of a HIGH lead acme screw can get pretty close to a
ballscrew...
And its sliding contact has other advantages where dust and swarf
are a factor. Adding to this, it does not suffer from repetitive
short cycles of distance moved the way ballnuts do.
(Interestingly, the vee roller bearings Fred uses on his machine DO
suffer from this; sop perhaps it all ends up "a wash"<G>)

You can look at several MFR. sites to see this. (I see Fred has
already posted a few.)

At low leads, I agree with your assessment. Also Fred is using Kerk
screws/nuts I believe, and these are teflon coated which does
improve the efficiency as well; but at a prohibitively high price
IME for onesie twosie hobby use purchases. (Kerk is really set up to
provide for OEMs IMO/E.)

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tony Jeffree <tony@...>
wrote:

>
> At 18:38 27/06/2006, you wrote:
> >This is an article of mine that appeared in the Digital Machinist,
> >special edition of Home Shop Machinist.
>
> Fred -
>
> Nice article, but I think you exaggerate the efficiency of the

typical Acme

> screw by a factor of around 2. The figures I have seen quoted are

in the

> 20-40% range, not 60-70% as you state. Googling on "acme lead

screw

> efficiency" turned up these references:
>
> http://www.roton.com/index.php?section=6#9
>
>

http://www.reedlink.com/ProductInfo~Productid~77865~ProductName~Lead-
Screws.html

>
> http://www.desktopcnc.com/articles/leadscrews.htm
>
> I have seen claims for Acme screws that are almost as efficient as
> ballscrews (85%), but these are very much the exception & would

likely cost

> more than a "typical" ballscrew with a similar performance.
>
> So in reality, I don't think you will see a lot of difference in

efficiency

> between "typical" V-form and Acme thread leadscrews - and

certainly not a

> factor of 2.
>
> Regards,
> Tony
>

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