Timing belt questions

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Mattpeshal
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Timing belt questions

Post by Mattpeshal »

Hi everyone,
I'm building a really basic plasma table, it's going to be very simple and light weight.
My plan is to use a timing belt type system with 15mm open loop belts. I've seen many people using a single pulley to drive their X and Y axises. I have a bunch of 24 tooth pulleys and would like to use them. I'm having a hard time wrapping my head around how to calculate the ratio the pulleys will give me.
The picture below is the basic idea I'm basing my setup on.

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Francisco
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Re: Timing belt questions

Post by Francisco »

I guess you mean a 15 mm wide belt. You need to know the belt's pitch to calculate the travel per rev, but it's really simple: pitch x n, where "n" is the number of teeth on the driving pulley (in your case, 24). If you are using a 2 mm pitch system, you'll have 2 x 24 = 48 mm axis travel per pulley rev. If the pulley is mounted directly on the motor shaft, and you have, say, 180 steps per rev, you'll advance 48/180 = .266666 mm (.0105") per step.
msimpson99
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Re: Timing belt questions

Post by msimpson99 »

Some things to keep in mind with belt drive systems.

1. While they can work OK on small tables, they dont scale very well.
2. The belts will always have some wiggle so they are not good for the higher Gs needed for plasma cutting.
3. As you tension the belt the machine must be re-calibrated. Any fixed numbers given to you are only starting points.

I just got through with alot of experimenting with belts on a small 3' x 3' low end table. I was not happy with any of the results, so I am shifting to R&P.
Mattpeshal
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Re: Timing belt questions

Post by Mattpeshal »

I have my doubts about using timing belts. The design I came up with will be fairly light in weight and will be using very low drag bearings. I'm hoping this combination will give me decent results.
I'm sure I'll step it up to R&P somewhere down the road. I'm just building the table for something to do and pass time while drinking beers in my garage. I went with a belt system strictly for its low cost and no other reason.

I'll mostly be using the table to cut basic off-road parts for my Suzuki samurai....and a fire pit for my cottage.
I leaned a valuable lesson with the cnc router I built, I'm keeping the plasma table a secret from my neighbors. For months I was making cottage signs, rc parts and all types of pintrest crap for everyone one. It was a huge headache, oh and I think made one million cutting boards as well.
msimpson99
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Re: Timing belt questions

Post by msimpson99 »

Dont get me wrong. A belt system will certainly work. It just wont operate with a lot of acceleration without some wiggle. Without the acceleration, you will get some nasty things when changing directions.

The build I was playing around with worked great with low acceleration. I tested it out doing some routing and cut some great parts in plastic and wood. I would not use a belt based machine to cut aluminum.

Here are some tests I did at various stages of acceleration:

https://www.youtube.com/watch?v=Ly6BrAjuocI

https://www.youtube.com/watch?v=wz89pRICQdM

To see more info on the test and some of my early prototype tests you can use this link:
http://www.kronosrobotics.com/plasmabuild1

Now for a real comparison look at this video:
https://www.youtube.com/watch?v=bTc2v_qYARc

This machine is running at about .5 Gs and can do speed over 600IPM. Its extremely rigid and was design to be a production CNC router. It will easily cut Aluminum, plastic, wood and other materials. The router is a bit too fast (slowest speed is 5000 RPM) to cut steel. It does a great job as a plasma cuter since It has been tweaked to do so. I has a ohmic sensor and floating head.

Its my KRMx02 design which has 5 build sizes. This is the smallest, which is why I chose it. You can see more about it and my plasma conversion here:
http://www.kronosrobotics.com/krmx02-plasma/

It is currently using R&P drives with a 2.73-1 reduction. ITs an expensive build which is why I am experimenting with some other build options.
Wadzii
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Re: Timing belt questions

Post by Wadzii »

If you are building a 2x2 table the belts will work ok. on my 5x10 table the belts were absolutely useless on the long 10ft side and pretty much useless on the 5ft side.

Dont waste time or money building anything larger than 2x2 with belts. It'll just frustrate you when you waste a bunch of time and metal because the mess doesnt do what it should. Holes wont be round, corners wont be square. Acceleration has to be too low to provide good results from plasma.

I changed over to a ring and pinion and couldn't be happier. It took a 1/2 hour to dial in the ring and pinion to where it would do production vs weeks messing around with the belts and various tensioners and all that crap.
tcaudle
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Re: Timing belt questions

Post by tcaudle »

Mattpeshal wrote:Hi everyone,
I'm building a really basic plasma table, it's going to be very simple and light weight.
My plan is to use a timing belt type system with 15mm open loop belts. I've seen many people using a single pulley to drive their X and Y axises. I have a bunch of 24 tooth pulleys and would like to use them. I'm having a hard time wrapping my head around how to calculate the ratio the pulleys will give me.
The picture below is the basic idea I'm basing my setup on.
Figuring the steps per inch and rations is pretty easy . While knowing the tooth pitch and the number of teeth will give the a numbe,r an easier way is just to measure the pulley diameter at the teeth and multiply by our old friend PI (3.1416). That will tell you the amount of linear motion you can expect per one rev of the pulley.
Lets say the pulley diameter is 1.5 " . that times PI is about 3.76 (a 34 tooth pulley with a XL belt with .2 pitch would actually have 4.8 in of linear motion per rev.
Since its a direct drive to the pulley (no interim belt reduction) you can expect the following:
resolution using a 200 steps per rev motor (forget microstepping at this point) would be 3.76/200 or about .018. top speeds you be around 900 cutting and 1800 rapids BUT rather poor acceleration with smaller motors. Microstepping only helps with smoothness of rotation at lower RPM and becomes a non-factor at higher RPM because of inertial rotary forces in the motor. It cannot be counted on for increased resolution.

So what you have with a direct single pulley drive is a rather course, low resolution , low torque, high speed linear drive. All of the other problems mentioned are a reality although there are fixes for them. In the end a direct drive stepper to any pulley system and even R & P will have similar characteristics. You simply cannot use smaller diameter pulleys on a belt to gain torque and resolution because it reduces the number of engaged teeth and that causes some slippage and the forces ten to want to roll the belt off the pulley more.

Where you see belt drives is in machines that tha have an ultra light load (low inertia) like a printer head and shorter distances to travel. There are often load position sensors . As the load gets heaver the challenges mount to take out flex , slippage and centrifugal forces You have to go to wider thicker belts and bigger pulleys with more teeth and auto -tensioning mechanisms

I agree a 2 x 2 might be a good candidate for a simple belt drive BUT then again a 2X2 has a limited use profile for doing what plasma is best at: cutting sheet material.
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