Gt2 pulley Less Teeth = better?

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jkanzaki
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Gt2 pulley Less Teeth = better?

Post by jkanzaki » Tue Sep 03, 2013 9:39 am

Hello, all

Another upgrade I would like to do is the Gt2 Pulley, I was searching in the forum to see if i can find any thing, it didn't return the result I was looking for.

Anyhow, I was poking around http://www.robotdigg.com/product and they have Gt2 pulley for sales at a good price compare to other places.
Here's the part confuses me, they sell Gt2 pulley in 16, 20, 36 teeth in 5mm bore. Is there advantage picking 16 over 36 or 36 over 16?


http://www.robotdigg.com/product/59/Ros ... GT2-Pulley <---- this work for shapeoko?
http://www.robotdigg.com/product/10/Ope ... h-GT2-Belt


Thanks guys, I am so new to CNC I hope you don't mind my questions. :)
Shapeoko 1 - dissassemble
Shapeoko 2 - 1 x 1m expansion TinyG, superPid, Dw660, Aluminum extrusion bed, Nema 23, acme, Z axis upgrade, Dual Y
Shapeoko 3 - 0051 Stock Dw611, SuperPid, limit switch, 9mm belt

bjbsquared
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Re: Gt2 pulley Less Teeth = better?

Post by bjbsquared » Tue Sep 03, 2013 1:20 pm

I would be inclined to go with the diameter that is closest to the stock ShapeOko design. You will most likely have to adjust the steps/mm in the controller settings.

I'm not sure that I see a big benefits of going with a different belt and pulley flavor. Perhaps if you are going to do so high precision PCB routing or something of the sort. In that case the smaller tooth count might make since as the steps/mm would be higher. But any accuracy improvement would be lost if the tolerances of the machine are not tightened.
Last edited by bjbsquared on Tue Sep 03, 2013 5:15 pm, edited 1 time in total.
#1016 - 1M x 1M Y: Dual Motor Drive w/Looped Belts, X:Dual Rail with wlanfox blocks, Z: Stock, Spindle:DW660

akhlut
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Re: Gt2 pulley Less Teeth = better?

Post by akhlut » Tue Sep 03, 2013 2:41 pm

Speed vs accuracy.

Example:

You have a GT2-16T pulley and 200-step motors. Each full rotation will move the gantry ~ 32mm, or each step will move the gantry ~ 0.16mm

Compare that to a GT2-36T pulley. Each full rotation will move the gantry ~ 72.2mm, or each step will move the gantry ~ 0.36mm

So the 36T pulley must be driven at least twice the microstepping rate of the 15T to achieve nearly the same positional accuracy.

So you gain speed, but lose accuracy with the larger pulley. But that is not the whole story - a larger pulley can apply more torque to a belt as it has a larger contact area than a smaller pulley of the same pitch. Of course this is relative depending on how many teeth are engaged at a given time.

http://www.sdp-si.com/D265/PDF/D265T008.pdf

http://www.gatesmectrol.com/mectrol/dow ... r=brochure

cvoinescu
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Re: Gt2 pulley Less Teeth = better?

Post by cvoinescu » Wed Sep 04, 2013 12:24 am

akhlut wrote:So you gain speed, but lose accuracy with the larger pulley. But that is not the whole story - a larger pulley can apply more torque to a belt as it has a larger contact area than a smaller pulley of the same pitch. Of course this is relative depending on how many teeth are engaged at a given time.
With a larger pulley, your motor's torque works against a longer arm (radius of the pulley), so exerts less force on the belt. So while a larger pulley can better transmit force, there's less of it... To me, that's the main argument against using a larger pulley, rather than the loss of resolution.

For GT2 belt, a pulley with 20 teeth is fairly common and a good size; smaller gives you more force for the same torque, but contorts the belt more, so I assume it has more friction and wear. 16 teeth is the lowest I'd go.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

jkanzaki
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Re: Gt2 pulley Less Teeth = better?

Post by jkanzaki » Wed Sep 04, 2013 5:07 am

Thanks for the clarification guys.

What about the quality of the pulley, my friend told there are plastic, aluminum and steel/metal
plastic < aluminum < steel/metal my assumption correct?

http://www.robotdigg.com/product <---- these aluminum pulley offer in this does anyone know if they are good quality?
Shapeoko 1 - dissassemble
Shapeoko 2 - 1 x 1m expansion TinyG, superPid, Dw660, Aluminum extrusion bed, Nema 23, acme, Z axis upgrade, Dual Y
Shapeoko 3 - 0051 Stock Dw611, SuperPid, limit switch, 9mm belt

cvoinescu
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Re: Gt2 pulley Less Teeth = better?

Post by cvoinescu » Wed Sep 04, 2013 9:20 pm

That's not an accurate classification, nor do I agree with the ranking. Plastic pulleys, if well made, are robust and hard-wearing. There are all-plastic pulleys designed to be pressed on the motor shaft, and plastic pulleys with an aluminum insert, which have grub screws. Most aluminum pulleys are machined out of an extruded blank, with collars (for the smaller diameters) and flanges pressed on. Flanges can be aluminum or steel. I haven't looked into steel pulleys, so I can't comment about those (not sure they'd be available in these small sizes). I've seen a huge variation in quality, from excellent (e.g. Stock Drive Products, Misumi) to utter crap (a random Chinese supplier). Unfortunately, you can't tell from a picture whether aluminum pulleys will be machined true, or plastic ones made of the correct material; and I haven't sampled RobotDigg's stuff yet.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

Gadgetman!
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Re: Gt2 pulley Less Teeth = better?

Post by Gadgetman! » Wed Sep 04, 2013 9:27 pm

Plastic is always cast(or printed if you happen to have a 3D printer and enjoy failure) but the metal pulleys are machined.

There are basically two ways to machine a pulley;
1. Toss a lump of metal into a CnC machine and hope for the best...
2. Use a dedicated tooth-cutter to cut out the teeth.

Guess which one is the most accurate?

Also, 'cheap CnC machining' usually means burrs and sharp edges. Possibly also uneven spacing. In other words; they 'eat' belts...
Weird guy...
Shapeoko 2014F: 1000mm X/Y, 300W Quiet Cut spindle, Arduino Uno/G-shield and GRBL 0.9i
15x30 drag chains, custom spindle mount, 9mm belts, 8mm endplates, 6mm motor plates.

cvoinescu
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Re: Gt2 pulley Less Teeth = better?

Post by cvoinescu » Wed Sep 04, 2013 10:14 pm

Gadgetman! wrote:There are basically two ways to machine a pulley;
1. Toss a lump of metal into a CnC machine and hope for the best...
2. Use a dedicated tooth-cutter to cut out the teeth.
There's a third way, which may actually be more widespread. Many aluminum pulleys are made from extruded pulley bar stock. The bar gets the tooth profile during extrusion, so it has nice clean edges, smooth surfaces, and the profile can be very accurate. It's also cheaper than cutting the teeth. The bar stock is fed into a CNC machining center, where the ends get turned down, and the center is bored. Small pulleys may get an aluminum collar pressed on at one end; larger ones are already thick enough to take a grub screw. Holes for the grub screw are drilled and tapped, and finally one or two flanges are pressed on (ideally made of steel, so that they withstand the belt rubbing against them, and pressed into a curved profile that they don't damage the belt).
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

Gadgetman!
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Re: Gt2 pulley Less Teeth = better?

Post by Gadgetman! » Thu Sep 05, 2013 5:00 am

Yes, but that requires making a die. That doesn't happen unless they expect to make large numbers of aluminium pulleys of that exact size.

A lot of the stuff sold for use on 3D printers at least is of the CnC-machined variety.
Weird guy...
Shapeoko 2014F: 1000mm X/Y, 300W Quiet Cut spindle, Arduino Uno/G-shield and GRBL 0.9i
15x30 drag chains, custom spindle mount, 9mm belts, 8mm endplates, 6mm motor plates.

jkanzaki
Posts: 166
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Re: Gt2 pulley Less Teeth = better?

Post by jkanzaki » Thu Sep 05, 2013 8:09 am

I am just going to order from RobotDig and report back if they are good or not... once i get them.
Shapeoko 1 - dissassemble
Shapeoko 2 - 1 x 1m expansion TinyG, superPid, Dw660, Aluminum extrusion bed, Nema 23, acme, Z axis upgrade, Dual Y
Shapeoko 3 - 0051 Stock Dw611, SuperPid, limit switch, 9mm belt

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