Re: [CAD_CAM_EDM_DRO] DRO Encoders

>
> As pointed out to me the other day by Jon Elson USDigital do not make a 1000 line encoder disk let alone a 2000, it is 500 lines per rev in quatrature. The next point is the value for Pi. What is yours? When I was at school in the dim and distant past Pi was 22 / 7, which in decimal is 3.142857143 my calculator does not have any digits. What I'm saying is that the normal measure we use for Pi is 3.142 so clearly there is an error here. Then there is the problem of how the disk is mounted on the machine, can you honestly say that you can machine to 0.001" as measured on every micrometer. Your own, yes maybe but if I came alone and measured any piece you made I would probably get a different number. I am not demeaning your skills, what I am saying is that homeshop machinists we do not usually have the very high accuracy measuring equipment that industry can afford. We machine to a fit and use measurements as a guide to achieve that fit.
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> So what we get when all the errors are added together, with the inexact Pi, the small disk with its error in diameter traveling a long way, in relative terms and all this additive error! A DRO which is harder to impliment than a linear scale.
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> As to point two. I have a machine that has an X axis that is a little longer than 29".

Please fix you mail program.
Make it add line feeds, it is VERY ugly and hard to follow.

Second, I Didn't use an abacus or even a slide rule in school, just
fingers, and I for the most part they had moved away from listing most
figures as fractions to decimal notation before I learned what Pi Was.

I learned it as 3.14149.

In the middle ages they just called it 4 and called it good.
Same goes for many feeds & Speeds formula 3,4 whatever it takes...

My Calculator lists pi as 3.14159265359
or so say Mr. Hewlett & Mr. Packard.

Which for any real purpose agrees with his numbers.

.318 Times pi = .999026463842 I won't complain about billionths of an inch.

I don't know this is the best solution, and wonder about the error in the system,
but the math as it stands is right, IMHO.

One issue at hand, is the fact the wheel will not turn at exactly that rate
the cable will compress in the inside edge, and stretch on the outside, and
the actual effective diam of the shaft would be larger than .318.

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>
> I've been following the discussion of linear encoders for a DRO. And have a
> couple of questions again.....
>
> (1). Why not use rotary encoders? They are very available and cheap. If my
> math memory is correct Diameter * Pi = Circumfrence of a circle. So, a .318
> diameter wheel gives a linear distance of .9990264786... per rotation. Using
> a 1000 count per rev encoder results in a resolution of .0009990264..... If
> that's not good enough, use a 2000 counts per rev. That's only another $7 or
> so from US Digital. It's not that difficult to mount the rotary encoder on
> the non moving base of each axis's slide.

Marc