I quite like the servo belt idea but I think it needs pretty accurate
clearance between the fixed and moving belts. Note that you can't clamp
the fixed belt. Clamps cover the teeth. If I was doing this I'd look
into polyurethane based adhesives. For instance the stuff they use to
bond in car windscreens sticks to just about anything and is very tough.
Devcon also make some impressive 2-part polyurethane rubber adhesives
that are very strong and adhere well to both rubber and metals.
An unsupported belt that length will have significant stretch. I used
some kevlar belt specifically designed for positioning on a CNC drill
about 10 years ago. It was about 4m long and I saw positioning errors of
up to 0.5mm under load. As the spec was +/- 0.5mm on that job I just got
away with it after using a little bit of software compensation. On the
plus side it has been running 10+ years in a production environment with
no detectable wear. About the only issue I ever had with it was a pulley
that came loose.
If you go for rack, look for 'precision' rack. The cheaper racks are
pretty rough. They aren't designed for accurate positioning. You
mentioned putting the feedback encoder on the pinion. This is a bad
idea. Any backlash in your gear train will make it really hard to tune
the motors and the motors will tend to oscillate when stationary or
moving slowly. This will put a lot of strain on the gear train. Gears
don't like repetitive reversal like that. I have worked on a lot of rack
driven routers and they all used encoders on the motors. Most used
direct drive to the pinion with huge motors or single stage belt
reduction. Are you looking at brushless motors? If so they will have
built in encoders.
You are thinking of using 2 module 30 tooth pinions and your motors are
2.4Nm continuous. Looking at the specs of other 750W motors you have
about 7Nm peak. It's not generally a good idea to use all of that but
you can use say 4Nm for acceleration.
direct drive:
2.4Nm / 0.03 = 80N
4/0.03 = 133N
That's not really enough. Let's assume a single belt reduction. About
the most you can sensibly get with one stage of belt reduction is 5:1 so
you now have 400N continuous, 665 peak. Double that if you are using two
motors. It's not up to the sort of forces a large industrial router can
generate but it should be plenty for most applications. It is unlikely
you will use any where near that even taking friction losses into account.
What sort of work are you going to use this router for? If you are
processing sheet materials I would strongly recommend using a vacuum
bed. If you are using blanks, vacuum pods work well.
Les
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