Re: [CAD_CAM_EDM_DRO] Re: Long X axis Method of travel.

dex367 wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
>>Toothed belt drive may be your best option. Especially a belt that
>>
>>
>normally
>
>
>>lays on a flat surface, so it won't flap as the machine moves.
>>
>>Jon
>>
>>
>
>With a toothed belt drive what kind of repetability and resolution
>would I expect?
>Would a chain and sprocket be better than a tooth belt drive even if
>both were layed on a flat plastic surface?
>With this type of drive, would an automatic tensioner help?
>
>

I think you can get pretty good repeatability. The resolution is determined
by the stepper or servo encoder, not the belt. Unless you go to glass
scales and tons of temperature-controlled iron, you can't even think
of getting .001" accuracy over 8 feet, so you need to understand that
the larger the machine, the lower the accuracy over the entire working
volume. You might be able to get .010" accuracy over a foot range
if you are very careful in designing and setting up such a machine.

One disadvantage of toothed belts is that they wear out quickly if bent
backwards. That makes it tough to design a system where the belt lays
on a surface when not on the sprocket.

I can envision a system where a toothed belt is laid with the teeth up, and
a system of two idler pulleys lift the belt off the table and pay it onto a
slightly tilted drive sprocket. The idler pulleys keep the sides of the
belt from grinding against each other. I really don't know if such a
scheme world work, but if I wanted to build such a machine, it sounds
like something that might have a chance.

Chains need to be lubed, and that will attract sawdust, which will cause
lots
of problems. Also, chains don't like high speeds and have some
non-linearities
in the motion as each link comes off the sprocket teeth.

Aircraft cable can do all sorts of neat things like this, and good cable
with proper design can be smooth, accurate, repeatable, very efficient,
and still have good life. They make some plastic-coated vable for
applications
like this, I think.

An automatic tensioner is almost always used in these sorts of systems.

Jon