Gearing a machine for faster rapids help please

[Ranscon]

I am starting to build a second machine. I am thinking of 2:1 or 3:1 gearing. I am going for faster rapids on this one. Where is the best place to find gears for this purpose? What else do I need to know about gearing? How will this affect the torque of my steppers? Any other problems I may encounter? Thanks for any help on this, Jeff

[rock4xfab]

With any regualr stepper as speed goes up torque goes down.. I am sure there is a way to gear it correctly but I dont no the formula. And even after it is geared I realy dont know if it is linear. I would fear you would loose position over long distance as the speed ramped down if it does not get into the correct torque curve of the motor, would it loose position?? I am new to steppers as all my machines are Servo so rapids are not an issue. I am just starting with my first stepper system with the new plasma table I am building.

You may want to contact a MFG of the stepper you want to use and ask them where you need to be geared to get the best of both worlds.

[arizonavideo]

You might want to tell us the type of screws and the TPI.

Stepper size and driver and PS voltage would also help along with the size of the machine.

[Ranscon]

The machine will be 3' x 5' or 3' x 6' with 425 oz/in motors, 3/4-6 single start acme on the 6' run, 5/8-6 on the 3' run. Keling 4030 drivers. I have a 24v PS but am thinking about changing it to a 36 or possibly 48v one. What difference do the amps make? I don't want to fry anything. This setup may be fast enough with a bigger PS. I'm looking to get 100-120+ ipm with 150+ipm rapids. Please let me know if I need gearing or not. Best I can figure, running the motors at about 500rpm or so to keep them in their torque range will get me about 80-85ipm. What will more voltage do to my rpm's?Thanks

[ger21]

Doubling the voltage should roughly double your speed. As for amps, you can't supply more than the motor's rating or you'll damage it. If you reduce the amps, you reduce the torque.

As for reduction, there's no hard and fast rule, as all machines are different, and motors, drives and screws need to work as a matched set.

Guys using rack and pinion usually use about one motor revolution per inch of travel. However, your acme screws are probably only 40% efficient, so you only have half the power that the rack and pinions have, so you'll probably need a little more reduction then that.

[Ranscon]

Where is the best place to purchase a 2:1 or 3:1 pulley and belt setup? Thanks again, Jeff

[Torchhead]

The process of the design is a series of trade-offs. With a given motor you can only get so much RPM. It's a function of the volts applied and the motor characteristics. Roughly RPM is linear with applied voltage so with 48 volts you will get close to twice the RPM as 24. It's not totally linear because of IR losses but that's another story. So what you find is the top RPM under even a moderate load is going to be limited by the power supply voltage. All steppers exhibit a non-linear drop off of torque with RPM. If you chart the torque curve and the RPM line where they cross for a given torque requirement you get motor stall. The usable RPM of a stepper may well be less than it's top RPM with no load.

Most modern drives are "chopper" (PWM) output and charge a coil to the applied voltage and limit the peak to a preset. You can't overcurrent a motor unless you set the current limit on the driver too high. You need enough to drive all the motors moving under load at once but the higher the voltage the shorter the pulses (at the fixed current) so the average current drawn is less. Power is voltage times current time duty cycle. You can get by with about 50% of the total motor nameplate amps added together, especially on an unregulated linear supply that can handle short periods of major overload.

In the mechanics you are always trading speed for torque and resolution. What that means is if you do a 3:1 pulley step up you can expect 3 times the rapid speeds but you lose 2/3's of your torque AND (worse) 2/3's of your resolution. Since resolution (smallest distance you move with one step) is a factor in accuracy that number goes down. Right now you have (worst case) a resolution of 1/1200. With the 3:1 you are down to 1/400 (if you wonder where the numbers come from) it's the full step count of the motor or 200 times the leadscrew TPI or 6. You cannot count microstepping as part of the equation because as RPM increases the effect is lost. It's for smoothness not to increase resolution. The equation is always the same with linear speed on one side and torque/force & resolution on the opposite.


Since the amount of torque you can supply to the load defines how fast you can accelerate the load as motor torque goes down so does acceleration. To gain back acceleration you have to: lower the mass (weigth of the load), lower the frictional and cutting forces, or raise the amount of torque you can supply. Sometimes it requires more than one change.

The good news is that at cutting speeds your steppers are turning slower so they have more torque so you make up some of the torque you lose in the step up.

So: Up your voltage to at least 48VDC. Consider no more than a 2:1 step-up pulley
(effective 3 TPI) and keep the gantry weight down as much as possible and still have the rigid structure you need for routing.

TOM caudle
www.CandCNC.com
Totally Modular CNC Electronics

[lasik2025]

Where is the best place to purchase a 2:1 or 3:1 pulley and belt setup? Thanks again, Jeff

Jeff, Surplus Center has #25 chain and many sprockets to choose from at what seems like decent pricing. I bought a 10 ft length of chain, some links, and a half dozen of sprockets (some with bore the size of my motor shaft and some with the bore size to match my lead screw) for about $50 to experiment with. Regards, Dave

http://www.surpluscenter.com/sort.as...s&keyword=PC25

http://www.surpluscenter.com/sort.as...s&keyword=PH25

http://www.surpluscenter.com/sort.as...s&keyword=PL25

[ahren]

Ranscon,

If speed is your main concern, I would highly recommend our rack and pinion drive system over ACME rod -- most of our customers are able to easily get rapids around 600 IPM with this system, with motors similar to the one you have specified. Additonaly, there is no concern about whipping like you will have for a longer screw axis. The mechanical system is geared to take advantage of stepper motors low-end torque, but maintains sufficient resolution for accurate cutting. Feel free to check it out here:

http://www.cncrouterparts.com/produc...products_id=50

Best regards,

Ahren
www.cncrouterparts.com