Motor and Screw Size

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06 May 2011 02:52 #9624 by Highlander01
I am building a cnc for mostly cutting wood/maybe aluminum on occasion, aprox 60x36". What motor size and screw thread per inch have people been happy with for speed and force output. I am looking at screws with 1/2in per rev and starting out with 125 oz in stepper motors that I have extra of. I am guessing that I will be upgrading motors if I go with 1/2in per rev screws.

(I am interested in hearing about combinations that did not work well also. It should give a better understanding of too small and too big.)

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06 May 2011 18:57 - 06 May 2011 18:57 #9629 by BigJohnT
Replied by BigJohnT on topic Re:Motor and Screw Size
I would contact the ball screw manufacturer on something that big. I don't think you will have much luck with a 125oz in holding torque stepper on a machine that big.

On my 36" x 52" plasma table I use 276 OZ-IN, NEMA 23, TRIPLE STACK motors, Gecko 203v drives at 70 volts with timing belt drives. The gantry is extremely light and has NO cutting forces or side loads.

I'm guessing you thought you could use all-thread or something like that for your screws... if so you need to study commercial machines that size to see what they use and ignore the flea-bay el-cheapo mdf ones for study.

John
Last edit: 06 May 2011 18:57 by BigJohnT.

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11 May 2011 16:51 #9726 by andypugh
Replied by andypugh on topic Re:Motor and Screw Size
Highlander01 wrote:

I am looking at screws with 1/2in per rev and starting out with 125 oz in stepper motors that I have extra of.

125 oz.in can be seen as pushing round a 1" radius circle with a 125oz force. so that is 2 x pi inches distance.
In that time the 1/2" pitch screw moves the tool 1/2"

The ratio of 2" x pi to 1/2" is 12:1, so your absolute maximum cutting force (assuming 100% efficiency everywhere) is 1500oz (or 90lbs)

That sounds a little low to me for aluminium, but if it took that much force to work a handheld router then we would all need muscles like Popeye.

I would use a toothed-belt reduction stage though, at the very least that gives you the option of changing your speed/force trade off relatively inexpensively.

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