How big of a CNC stepper motor would I need to move a 300Lb 52"x 9' moving table on rack and pinion track? Possible?

Working on a custom CNC machine and I want my table to move instead of the gantry. My best case scenario would be having the table move 300 pounds and my worst case scenario, although acceptable, would be 120 pounds. I would assume the need to allow for the weight of the table itself, another 50 pounds. I'm sure that I could get the table to move but have concerns with: when the table is out eight feet from the router at a compete stop, pulling it back with that kind of weight is going to cause problems. I would like the table to use rack and pinion track and gears as well. I have an additional question as well: Obviously I'm going to need a sizable motor to move what I'm proposing but I would not need something so big to deal with the other two axes, is it okay to mix different size motors run from the same control panel? I would think yes, but want to be sure that I don't need to add anything special to help. I'm open to and appreciative of any and all suggestions and help. Thank you in advance for your assistance.

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frollard4 years ago
Is there any reason to not use a screw-drive to move the table? if using steppers, geared or not, a rack and pinion is tough to remove the slop, especially with straight tooth gears.

Also, power is a function of how much acceleration this table is going to do. You could use a tiny nema23 motor with huge gearing and it would move, just very slowly.
If you stepped it up to a much beefier motor, it would be able to throw the table around at dozens of mm/s.
shoby1 (author)  frollard4 years ago
I chose rack and pinion because of it's modular capabilities whereas screw-drive would be limited to whatever the screw could handle. I didn't think something in the nema23 category would work at all and was thinking about something in the nema42 category. As for power, I don't want it to be slow but it doesn't need to win the race either. Time is money and this machine will eventually be spitting out parts and pieces on a daily basis. If I can figure out the electronics portion of this that is.
frollard shoby14 years ago
Like I said - it all depends on gearing. given enough time a wristwatch escapement could build enough torque to move a mountain, rack and pinion will need a big motor and a big gear ratio which introduces slop.
You're outside the range of stepper functionality here. The inertia of the load+table is going to be total killer. If you HAVE to do it that way, you're in DC servo motor territory - eminently "doable" though.
shoby1 (author)  steveastrouk4 years ago
Would that be true even if I added multiple steppers? I'm not against servo's, from my reading, they seem to have some real benefits but cost is another story. And since the servo motors came up, can I mix servo motors and stepper motors together? Meaning, servo for the table and stepper for the other two axis's?
Thinking about this, if the Load is 300lbs, what does the CARRIAGE weigh ? To figure the drive size, we'll need to know that, as well as the required maximum slew speeds, and some handle on the cutter forces.
Yes, you can mix and match - its down to the amplifiers you use.

This is clearly a big project physically, from the information you've give, so I wouldn't "spoil the ship for an 'apporth of tar" as we say in my part of the UK !

Rack drive will be Ok if your intended precision is fairly low. Personally, I'd use an MTD belt and toothed pulleys - my real CNC machine shop's machines are both "belt driven" in places.
Why do you want the entire table to move?

It's much easier and cheaper to move the gantry up and down the length of the table then it is to move the table. You also have to deal with keeping the entire table steady and true while it's moving. Not to mention how much extra space you will need to allow the table top to travel. The large motor to move that huge table and extra weight will need a more powerful controller. You need to check with the specs of your controller to see what size motors it can handle. A large load like that may require 2 motors to help keep everything steady and true.
shoby1 (author)  mpilchfamily4 years ago
Thank you for your comment. I need the table to move because of size. My table will be removable, leaving me an approximate 60" x 30" frame. The table will also break down, I may not always need an 8' cutting area. Plus, the shop is somewhat small and this particular size fit's nicely into one area. With the table removed, it's easily stored. I was hoping to attach the motor to the frame itself and have the gear track mounted to the table. It would actually add weight in an area that I need weight. I'm not against 2 motors in any way either, if I needed 4 to make this happen, I would use 4. I'm using 80/20 materials for the table's frame and Open Rail as the linear v-groove system. (a little beefier than what they show in the kick-starter store but same design) The table is currently designed to always have approximately 6" in the machine body itself even at it's fullest extension. I have noticed a left and right wiggle when fully extended and my thought was to increase the length of the table, not the usable area but just the frame so that it would always be at least half way in the machine body itself at all times. I was thinking that the table would stay square under those conditions? I have not created a controller at this point, just a frame with moving parts. Electronics is where I get stumped, not completely but enough that I will need help. I've contacted Atlanta Drives because of a specific motor that I saw on their site, wanted to go to NJ and meet with someone but they said no! Lol! Evidently they don't do that. I also understand that what I'm proposing is not cost efficient but sometimes we need to spend money to make money. Easier is another story, that's why I'm looking for help. And I do appreciate all the responses and suggestions thus far. Thank you all.