CNC plasma table

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CNC plasma table

Postby quadmasta » Fri Jun 24, 2011 11:21 pm

I'm working on designing a MakerSlide based CNC table kit and I've noticed most commercial units use rack and pinion system for moving the gantry/z carriage. Since they don't really need any torque resistance I'm confused by this. Would there be any reason other than "it won't burn up" to use a rack and pinion rather than a timing belt system?
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Re: CNC plasma table

Postby bdring » Fri Jun 24, 2011 11:58 pm

Belts get a little bouncy after about 3 feet. You could use non moving belt (like a rack). Some racks are surprisingly cheap though.
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Re: CNC plasma table

Postby BenJackson » Sat Jun 25, 2011 12:14 am

I've also seen designs using non-moving bike chain (as a rack).

It would be great to have a MakerSlide solution like:

http://www.cncrouterparts.com/rack-and- ... l?cPath=21
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Re: CNC plasma table

Postby BenJackson » Sat Jun 25, 2011 1:41 am

Example of fixed roller chain and fixed belt and looped belt all in one plasma:

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Re: CNC plasma table

Postby r691175002 » Sat Jun 25, 2011 3:42 am

Belts are actually more common than one might expect even on large cnc routers. I've read that even under extreme loads on 8' tables they can maintain +-0.001" as long as you choose the right belts and tension them properly.

There are also alternative systems that combine fixed and moving belts for greater accuracy and speed such as this (http://www.cnczone.com/forums/linear_ro ... er_if.html)

Belts are generally overlooked by most DIYers since many people assume that they stretch under load. If/when I build a cnc router I will be using belts because its much easier to ship them.

In terms of difficulty of mounting both solutions are similar, but if you want a belt with similar rigidity to a rack/pinion it will probably cost more (but shipping might make up the difference for long lengths). Lots of tradeoffs for particular situations.
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Re: CNC plasma table

Postby macona » Sat Jun 25, 2011 7:06 am

I used to work on commercial cnc machines when I used to work for airgas. Rack and pinion is used for several reasons. Its scaleable.You can add sections of track very easily. The rack is usually kept vertical which makes it hard for crud to build up in the rack. Encoders for feedback run off the same rack which makes design easier (No such thing as steppers on a commercial machine). If you need to move the gantry quickly you can disengage the motor to the rack with a little lever and push the gantry by hand. Gears are pretty cheap, so is the rack. If someone smashes the rack with a forklift loading material on/off the machine all you need to do is replace the damaged section.

Remember, most plasma cutting tables are really only intended for +/- .020, maybe more. It is not a usually a finishing operation so you can get away with a less accurate drive. For many operations it is good enough with a quick hit of a grinder to knock off the slag. New high def machines are a different story and I am not sure what they used. At the time I worked I only had one customer that had a high def machine.
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timing belt/pulley questions

Postby quadmasta » Sun Jun 26, 2011 7:59 pm

Thanks for all of the replies. Rack & pinion looks like it's going to be the way to go. It doesn't look like I'd have enough resolution directly driving the pinion with the stepper so I'd have to reduce it using a belt/pulley system.

Does anybody have a good source for metal timing belt pulleys of different sizes as well as a good source for smaller looped belts?
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Re: CNC plasma table

Postby bdring » Sun Jun 26, 2011 8:00 pm

Stock drive and/or McMaster will have everything you need.
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Re: CNC plasma table

Postby callcbm » Mon Jun 27, 2011 8:44 pm

http://www.polytechdesign.com/belts.php
I'm using these guys prices seem to be good and ship fast
Thanks Randy
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Re: CNC plasma table

Postby callcbm » Mon Jun 27, 2011 8:48 pm

I'm using a direct drive belt and I can .0019 resolution 5mm 20 tooth pulley with 2000 step give .0019 resolution
Thanks Randy
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