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mícheál
11-14-2004, 09:14 PM
I am designing my own CNC router after two years of deliberation! Originally, I wanted to build one, then I wanted to buy one, and now I'm back to building one again. The reason is, for the large size bed I want, 48" x 24", the off-the-shelf prices are too high. But big bed sizes with z-axis travel of 6" need very rigid construction. HERE'S MY QUESTION!: I want to drive my x-axis on both sides, i.e. a screw drive on both x-axis guides. The obvious problem is keeping them in sync!! Any suggestions?

(My best idea to date is that both screw drives could be driven by a single shaft, perpendicular to both and coupled via helical gears. This shaft would then be driven by a single stepper motor.) - Mícheál

WoodSnarfer
11-14-2004, 10:08 PM
I had asked basically the same question..and was pointed to this design as a reference:

http://website.lineone.net/~rmtucker/

I think this solution (one servo, one belt driving two screws) is performance/cost optimal. The gear idea would probably work, but sounds very expensive (if you want to minimise backlash). Also, I'm sold on using servos -- especially for a larger axis. The flatter torque curve across a wider range of (higher) RPMs seems like a better solution.

Regards,
Chris

mícheál
11-14-2004, 10:42 PM
Wow! Amazing, thank you! Exactly what I wanted to see. This looks like a really good design. Do you think the two rollers are spring loaded to keep the belt taut? Have you designed your own machine? Are you going to drive the x-axis like this? Do belts introduce some backlash themselves? Do you know any other issues with belts I should consider? I hadn't thought about steppers v servos, but glad to hear servos could be an advantage for longer axes. I'm still concentrating on rigidity, low inertia and low friction. Is friction ever a big issue?

Regards, Mícheál.

WoodSnarfer
11-15-2004, 07:58 AM
I believe that someone mentioned that the motor mount itself was movable, to adjust the tension on the belt.

I'm basically as far along as you are -- just designing my router in my head :)

I do like the idea of driving the x-axis this way, so I am going to try to use this method, yes. I think that anything in the drivetrain has the potential to introduce backlash -- but (I am no expert on this), my thinking is that the softer composition of the belt (meaning that it is very slightly compressable), and the angular profile of the belt teeth mean that when matched with the correct gear, the potential for backlash is very close to zero. The only issue I can think of is keeping the tension on the belt correct, as the belt may stretch over time. Buying a high-quality belt and keeping the tension adjustable should take care of that, too.

Friction needs to be calculated as part of the sizing you do on your motor's torque requirements. Search the forums here to find other threads on this topic...but the short answer is that if your ball screw is 80% efficient, then you need to compensate by having your motor/driver sized to consider that.

-Chris

ger21
11-15-2004, 09:48 AM
but (I am no expert on this), my thinking is that the softer composition of the belt (meaning that it is very slightly compressable), and the angular profile of the belt teeth mean that when matched with the correct gear, the potential for backlash is very close to zero. The only issue I can think of is keeping the tension on the belt correct, as the belt may stretch over time. Buying a high-quality belt and keeping the tension adjustable should take care of that, too.

Go to http://www.sdp-si.com , they have a lot of great belt info. I believe that the GT2 profile belts thay have have the least backlash, practically zero. I believe most stretching will occur in the first day or so, and the belt will remain fairly constant after that. Actually, the XL type belts with the trapezoidal teeth, actually have a little bit of play when matched to the pulley. The GT2 belts have a curved profile that fits much more closely to the pulley.