Re: [CAD_CAM_EDM_DRO] Wish to add DRO to tube bender using rotary encoder and timing belt

Q: How do you plan to interface it, and to what ?

Any quality timing belt solution will work.
All will have about 0.1 mm or much better accuracy - down to about 0.01 mm.

If you are using mach3, for example,
you can just buy belts and pulleys, and encoder, and fit all together.
You can then directly input the encoder reading into mach.

It will display it with no work.
With simple VB code, you can use it in a macro pump, to adjust your
work/bending.

Note-
for good results, you must tension the belt.
A few bearings on tensioner pulleys and the belt in tension about 10 kg,
for 3 mm timing belt, and there you are.

The encoder res you need depends on the diameter of pulley where it
contacts the belt.
You need 6 teeth min contact - its best to use 12 teeth pulley or more.
(Technically - it does not matter as this is a no load measuring
application. Practically, its hard to work with very small parts, shaft,
pulleys and teeth).

Circumference of circle is 2 x pi x r.
So for say 14 teeth pulley, with 5 mm htd belt.
D = 0.832 in ( 21.13 mm)
Pitch D is = 0.877 in ( 22.27 mm)
So belt circumference of this pulley is 2 x pi x pitch D / 2 = pi x pitch D,
= 3.14 x 22.27 / 2 mm = 69.92 mm circumference.

1 degree of angle moves the belt 1 / 360 degrees * circumference.
= 69.92 mm / 360 = 0.194 mm.
So a 1 degree or 360 quadrature count encoder resolves to this resolution.
So a 1 degree quadrature encoder, or 90 count (360 / 4 = 90 counts or
lines) would give you about 0.19 mm resolution.

To achieve a given accuracy, you should generally count and resolve to
about 4 x theoretical resolution.
Small res encoders from US digital, between 100 - 300 counts, have the
same cost.
So a 200 lines encoder will give you the needed accuracy.
Your speed and sampling rate is low - as bending will take seconds
(compared to resolving a servo running at 5000 rpm) therefore you do not
need to worry about the sampling rate.
As the cost is the same, and more accurate is better, get the 300 count
encoder.
The cheapest US digital encoder is more than adequate. About 20$.

A 14 teeth HTD pulley of 5 mm teeth, 9 mm wide will be easy to fit,
tension and will last for essentially forever in this application.
You can use a smaller one if you want (like xl or mxl).

Pulleys and belts from any of the usual us suppliers.
Good luck.


al.swift wrote:

>
> Need some advice. I want to add DRO to the length axis of our tube
> bender. I have seen something similar which had a long toothed timing
> belt approx 3m runing thru a rotary encoder with a couple of small
> wheels acting as tentioners. The encoder was fixed on a linear guide
> that allowed it to run up and down the timing belt. I' m looking for
> 0.5 mm acuracy / repeatability (I know they're different). A similar
> system can be used for the plain of bend angle but using a closed belt
> measureing perhaps 0.5 degree. Where shall i look first?
>
> __._,_.