# increasing ballscrew speed using timing belt and pulleys



## jnrouter (Jun 15, 2014)

hey everyone, I finally got my cnc router moving thanks to jumps4 and all the electronic help he gave. now I have to speed things up, I'm using ballscrews with a 5 lead and 200 steps and the max speed I can get is about 90 ipm. A lot of the machines I've read about can move up to 800 ipm but that's with servo motors. I would like to rapid at about 200 to 300 ipm. If I use timing pulleys and belts I figure a 1 to 3 ratio will give me about 270 ipm. Am I figuring this right? and will I loose accuracy doing this. Any help or suggestions will be appreciated.


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## DMS (Jun 15, 2014)

Gearing up is not necessarily going to get you faster rapids, mostly because at some point you won't have enough torque at the screws to accelerate. If you could provide more info on your machine/motors, that would be helpful. 

Another question is, why do you want such high rapids? And do you do a lot of rapid moves in normal usage? By gearing up, you are likely going to be reducing overall torque, which means your max cutting speeds are likely going to go down. Another question is, what is your current acceleration? If your acceleration is not high enough, then you will not hit your max rapid speed on short movements, so you would not get anything out of the deal in most cases. The only way you are going to be able to see if you are going to get any improvements is to try it, or find the torque curves for the motors, and see what they would be capable of at the new speed/torque

Now, if you already have a belt drive system, then it would be pretty simple to swap out pulleys and a new belt, and re-configure mach to see if this will help you, and you can always go back. Pulleys can be pricey though...

- - - Updated - - -

To add, yes, you will lose some accuracy doing this (accuracy will be reduced by the amount you gear up, unless you have linear scales... in which case ignore my last comment, nothing will change). The question is, will that loss in accuracy matter. At 200 steps X 5 tpi, you are getting 1000steps per inch (0.001" per step). Not bad. Likely you are also using some sort of micro stepping, so assuming 10x micro stepping, that brings you to 10000 steps per inch (0.0001" per step). Do you need that? Probably not. Are the mechanics of the machine even capable of that repeatability and accuracy? Really unlikely. Another thing to think of is that with a simple parallel port setup, you will likely not be able to generate steps fast enough to reach those speeds, you would need something like a smoothstepper to get there (which, maybe you already have).


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## JimDawson (Jun 15, 2014)

I agree with DMS.  Gearing up the step motors is not the way to go, due to losing torque.

I would start playing with the system tuning.  In order to rapid at 300 IPM, the step motors would only have to turn 1500 RPM, this is in the range of most step motors.  That is only 5000 pulses/second if you are not micro stepping.  This is still in the range of Mach3, which will output about 20,000 PPS, more or less depending on the computer.  You may or may not have enough power supply or controller to pull this off.  

I would mechanically disconnect one of the axis motors and start playing with the controller to see what speed you can get out of it with no load.  At some point won't turn any faster and will probably decouple (and growl at you).  Normally they will run at maximum speed in full or half step mode, depending on the controller.  Then reconnect the load and tune the acceleration to what the motor will take.  You may find that under load the max speed will be drastically reduced or that the acceleration ramp distance will exceed your travel length.

With out knowing what hardware you have, it's pretty hard to make any educated guesses as to tuning.

To put this in perspective.   I retrofitted an industrial 4x8 router with steppers. It will rapid at 600 IPM, I did this in testing.  Normal rapid moves are limited to 300 IPM however.  In this case the steppers are actually geared down at 1.4 to 1 and running 4 pitch lead screws.  For this project I used NEMA 34 steppers, a 70V power supply, and crazy fast controls, capable of 6 million PPS, and micro stepping at 20,000 PPR.  Not inexpensive, but still less than servos.


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## DMS (Jun 15, 2014)

I have DC servo's on my mill. For comparables sized steppers, the overall price was not that much greater. Now if you are talking about AC servos.. then you start talking about taking a second mortgage...

Jim's comment did make me realize something; counter-intuitively you may be able to go faster if you gear things down instead of up. A lot of it depends on why you are limited at 90ipm. If you are hitting a torque limit, and the steppers and the rest of this system can still go faster... then gearing down might make sense. I still think you want to try to track down the torque curves for those motors and see where the sticking point is.


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## jnrouter (Jun 19, 2014)

thanks to everyone for their input, there is a lot of info to process. I think the first thing I should do is look at mach3 and see if all things are configured correctly. I'll be back!


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