[Rob37]

Hello All

First off let me say i love my cnc router, below is a picture of it. What I am wanting to do with it in the future is change the X and Y axis to belt drive. What i would like to do for her Y axis is to turn a drive shaft with two pulleys one on each end pulling her along two belts one on both sides of her similar to the chain drive systems that are out there. Also, I want to pull her X axis back and forth with a belt as well. The motors she currently has are 382 oz/inch and they run flawlessly at 30 IPM with direct drive on 10 tpi acme lead screws. I want to get rid of the leadscrews for distance, noise and maintenance reasons, I also want to speed her up to say 50 or 60 or a bit more IPM with belts but I don't want to lose torque. I am thinking of putting the motor on a drive shaft that gos the width of the machine and on each end of the shaft put a 10 tooth pulley to drive the belts on either side of the machine. I am not in a big crank to do this modification but I am pondering it and need input from all here. Belt drives seem to be Taboo on other sites and I wanted to turn here for more input.

Thanks
Rob37 PS- The machine has a 3' x 4' table

[ger21]

What kind of drives are you using to only get 30ipm?

I think you'd be better off with 1/2-8 2 start or 4 start acme.

The 2 start should get you 150ipm, and the 4 start even faster.

Just using long belts would give you a lot of springiness.

[Rob37]

Hey Gerry

using 10 tpi acme rod from Enco it was only $9.00 for 6 ft pieces. Plain white delrin drive nuts. If I try to increase the feedrate past 35 IPM I get inconsistent runs on dry cutting she will start to stall in spots. She sounds great at 50 IPM but stalling is still an issue. with the velocity at 30 and the acceleration at 5 she will run no problems. not sure if maybe I have a setting wrong in Mach3 or maybe the Motors just can't handle the speed. all of the drive screws move really well no hangups. maybe I need to just keep working on motor tuning in mach3.

Rob37

[Rob37]

Hey again Gerry

Motor tunings are fine used my surfacing program to run a dry cut and adjusted feed rates up until the stalling starts 30 is still the most reliable. with acceleration at 5

Rob37

[ger21]

What motor drives?

[harryn]

First, I would like to say how much I like your rail setup. It looks like those are angle iron and V groove bearings ? The angle iron I looked at had sharp edges and I wondered if it would work.

Also, how do you adjust them so they are aligned ?

OK, back to your questions on belt drives. The reason I am so interested, is that I am also working on a belt drive based setup, and I did a bunch of calculations on what it takes to make it work.

First - nearly everyone underestimates how strong of a belt is really required, which is why belt drives have a reputation of being springy. The belt drive calculators usually based their recommendation on what it takes to transmit POWER, which is limited by tooth strength. In this kind of design, you are transmitting MOTION, which is limted by belt stretch.

The advantage screw type drives have, is that the linear holding forces are really not transmitted back to the motor directly, but instead to the screw thead, and then to the shaft bearings. In a belt drive, 100% of the force is transmitted down that belt and has to be held back by the motor torque.

The other factor that is commonly underestimated is that you cannot assume that the load is distributed evenly to each belt on each side of the table, but instead, each belt must be capable of the full load when the router is on that side of the table.

The great thing about the belts though, is that done properly, it can be a fast, powerful, and virtually zero racking approach to movement.

As far as sizing, it will take at least 3 in wide, and ideally 4 inch wide belts on each side to run the router so that cutting forces and acceleration are non issues for belt stretch. That assumes you are using something like steel wire urethane belting such as brecoflex 100AT10 (100mm wide, metric 10 pitch) belt. (Edit) I looked again at the size machine you have, and a single 6 inch belt down the middle should do it.

Your motors will also need some beefing up to handle the increased torque load - probably closer to 800 oz in and a 3:1 reduction, which is pretty easy with a belt drive with a cross shaft like that.

Last, but not least, make sure that your motor drivers can supply real holding torque at zero rpm. It is common for drives to reduce the current at zero rpm, but with belts you can't do that. I am not an expert at all, but I think this is easier with stepper motors than servos.

[harryn]

Your motors might also just be having resonance. Consider searching on resonance dampers either here on on youtube.

[Rob37]

harryn Thanks for the compliment on the rail system. the vgroove bearings are steel and the rails are aluminum angle. The steel bearings wear the aluminum to a point eventually I can tighten the bearings with lower set bolts.

Rob37