Re: extreme accuracy for cnc jewelry

---I thought jewllery was an artistic craft, not requiring too much
accuracy. What type of jewellery would require high accuracy?
T. Ackland

In CAD_CAM_EDM_DRO@egroups.com, Doug Fortune <pentam@h...> wrote:

> thomasm923@a... wrote:
>
> > Hello-
> >
> > This is my first post to this facinating group. I am am jewelry

model-

> > maker and hope to build a cnc mill sometime in the future. I have
> > plenty of questions to ask but I'll keep this post brief.
> >
> > If I ever learn enough to attemp to build my own machine, I would
> > like the machine to have the following qualities: moderate speed,
> > high resolution, and accuracy. I have been reading about micro-
> > stepping as a way to getting higher resolution, however I am
> > concerned that the micro-steps are really not that accurate.
>
> Please consider the use of "differential threads" (see my original
> posting 10Aug2000 complete with C source code). This technique
> is impractical for large movements (ie over an inch or two) because
> of the huge mechanical advantage given. On the other hand, you
> can use tiny steppers directly coupled to the shafts.
>
> An exerpt:
>
> I've been playing with different mechanical leveraging ideas
> to be used both for direct movement of the table and as a
> way to increase the resolution of rotary encoders
> ( see 11 Apr 2000 "motion-multiplying pulleys: linear to rotary

encoder")

>
> .....
>
> Well I came across another VERY interesting page at:
> http://www.geocities.com/CapeCanaveral/Hall/4425/ Marvin W.

Klotz

>
> which has lots of (DOS) programs (source code & executables)
> to aid the machinist. It is well worth your time to look it over.
>
> One of the gems there is a description of "differential threads",
> wherein various combinations of course and fine threads acting
> together differentially, can create huge mechanical leverages
> (or counts/rev for encoders).
>
> This is great for us, because it may give us the capability of

having

> HUGE number of steps per inch of advancement. For example,
> using a 180 step/rev stepper with at 10 microstep driver like the
> Geckodrive (ie 1800 steps/rev), then using a 56 TPI course and a
> Metric 56.444 TPI (ie 0.45 mm/thread), then you'd get 12,801,539
> turns per inch, or an advance ratio of 7.81E-8 inch per step

(0.000
000 0781 ").

>
> A more usable example is using a 28 TPI with the above metric thread
> to get 100012.493134 steps/inch (or approx 0.000 01" per step).
>
> With such ratio's as these, there should be no requirement to use
> pulleys to increase leverage, so the motor can direct drive the

shaft.

>
> There are of course some interesting "details", which I leave up to
> the "sufficiently curious".
>
> By using some of Marvin Klotz's thread tables and differential

thread

> formula:
>
> pc = coarse pitch (tpi)
> pf = fine pitch (tpi)
> pe = effective pitch of differential thread (tpi)
>
> (1/pe) = (1/pc) - (1/pf)
>
> I whipped up a program to compute all the different ratios one could
> expect from different thread combinations. The idea is to sort the

output

> and discover the proper combination to use to achieve a ratio near

what

> you desire.
>
> Doug Fortune