Re: What makes a machine CNC ready?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Steve Blackmore <steve@...>
wrote:

> 0.001 per foot is not that precise, it's grade 3. That's 25.4um per
> foot, the screws I use are 0.052um per foot (grade 7;) and are

the ones

> priced here.
>
> As an example..
>
> 16x4mm is $32 per foot, $91 for the screw and $18 for the seals.

Here is an example of a typical ball screw that a home shop user
might use( many do as they are available and prices are posted):

http://www.homeshopcnc.com/page3.html

A 6 foot screw (5/8 - .2) with 2 nuts would be $140

In an equivalent 1/2-10, 2 start acme thread, with a single anti-
backlash, wear compensating nut ($32), the total would be $85.

With a little light oil and good alignment, the acme nuts will hit
close to the efficiencies quoted in the kerk motion web site.

Both screws are priced at inflection points, and both will handle
the loading of a typical Xylotex or HobbyCNC driver, or an IMService
servo.

Now, in addition most home users will not have equipment or contacts
to machine the ends of a hardened ball screw, so add machining cost
to the ball screw ( $40 at least), and it must be sealed in a
flexible cover to prevent contamination. Probably another $75 per
axis.

The acme screw on the other hand is made from soft steel and the
ends can be turned on most any of the import lathes, even your
buddy's.

Net difference for the same utility lead screw, 2 axis table:

Ball screw ($140 + $40 + $75) * 2 = $510
Acme Lead Screw $85 * 2 = $170

Savings $340 The ball screw costs 3 times what the acme lead screw
costs.

With heavy home use, the ball screw will probably fail in 5 years.
The lead screw will require a nut replacement in 2-3 years($64).

Fred Smith - IMService
http://www.cadcamcadcam.com/hobby