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Old 03-14-2017, 11:22 PM   #21
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Plastic, metal rubber - as long it works. I'm using large plastic pulleys (80T) on my cnc mill conversion (have a small aluminum hub), been fine for years. The belt is rubber. Don't get caught up in flash and bling for something that doesn't need it.
Another way of thinking - if the rubber tooth can transmit the load why not a plastic tooth? There are better things to spend money on (like tooling!)
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Old 03-15-2017, 12:11 AM   #22
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im going for ali pulleys 'cos i like shiny things

all the bearings turned up today and i have been going through blogwitch's website he posted. need to give them a buzz tomorrow as they don't stock the 120t pulley you have to phone and ask for them. i have decided to go for 6:1 gearing as it seems a lot of other people use the ratio on there home build. even manufactured 4th axis (ebay ones, not HAAS) use 6:1.


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Old 03-15-2017, 02:09 AM   #23
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I should have added 'while you are prototyping'.
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Old 03-15-2017, 02:14 AM   #24
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One of the considerations you haven't mentioned is what you will be machining. This will drive the consideration of holding torque and moving torque.
You don't get a torque multiplication effect of 6:1 though with this setup unless you are machining at the very center of the table. Inside the diameter of the large pulley you'll have a torque multiplication effect to the stepper, the closer to center the closer to the 6:1 ratio. Outside it you'll be reducing the effective torque. At your above mentioned 4" radius/8" diam (~200mm diameter), the torque reduction will be very pronounced. 200mm working radius:77mm pulley = 2.6:1 effective torque reduction.
Those Aliexpress tables tend to advertise 'engraving', as machining hard materials or taking significant cuts would probably cause the steppers to loose steps.
A workaround is to use very small diameter cutter to keep the forces down - i.e. no hogging or taking big cuts with a facemill. You'll probably need to keep the cuts small anyways due to loss of rigidity with all the joints and suspended working platform.

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Old 03-15-2017, 03:02 AM   #25
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Quote:
Originally Posted by blighty View Post
call me a snob, but not to keen on plastic pulleys, but they do have selection of ali ones

I have mixed feelings about plastics myself but the newer resins can produce pulleys and gears as good as aluminum.
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Old 03-15-2017, 09:34 AM   #26
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Blighty, if you are in the UK, maybe also look at RS Components I've bought a couple of pulleys from them and they were reasonably priced. T5 is a metric size (5mm pitch). My stepper reduction drives are using a 5M belt which is similar to T5 except the grooves are rounded and are meant to have longer life and less torque loss. I'd never heard of them before until IO bought these reduction drives. I think it is a pretty new standard. I'm sure the belts will be hard to find when the time comes to replace them.
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Old 03-15-2017, 09:42 AM   #27
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Quote:
Originally Posted by ninefinger View Post
One of the considerations you haven't mentioned is what you will be machining. This will drive the consideration of holding torque and moving torque.
You don't get a torque multiplication effect of 6:1 though with this setup unless you are machining at the very center of the table. Inside the diameter of the large pulley you'll have a torque multiplication effect to the stepper, the closer to center the closer to the 6:1 ratio. Outside it you'll be reducing the effective torque. At your above mentioned 4" radius/8" diam (~200mm diameter), the torque reduction will be very pronounced. 200mm working radius:77mm pulley = 2.6:1 effective torque reduction.
Those Aliexpress tables tend to advertise 'engraving', as machining hard materials or taking significant cuts would probably cause the steppers to loose steps.
A workaround is to use very small diameter cutter to keep the forces down - i.e. no hogging or taking big cuts with a facemill. You'll probably need to keep the cuts small anyways due to loss of rigidity with all the joints and suspended working platform.
good points to bring up.

i tend to cut Ali and brass with the odd mild steel thrown in there from time to time. more so with the steel as i have just started a 2" Burrell build.

for the time being, cuts will be cut on the light side. any holes that need drilling away from the center will be done at 33mm/min and not the normal 140. at a later date I will be putting some form of disk brake on the axis's. controlled by a pneumatic piston.
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Old 03-24-2017, 08:42 PM   #28
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a quick up date.

after a week or so of trying to sort some pulleys out...... was trying to get 20 /120 tooth pulley. no one seemed to have the 120 in stock, so had to be made. one of the quotes i got was 195 each. so i finally went for 15/90tooth. price cam down to 70 odd quid all in. should turn up middle of next week.
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Old 04-17-2017, 08:43 PM   #29
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few weeks later................bits have turned up. not really what i was expecting. thought they would be Ali, but their made of steel and a bit on the heavy side.
the 90mm pulleys have cam locks, one of the 15 tooth has a 6mm hole in it and the other just has a pilot hole, but both need an 8mm.

i'll have a play over the next few days see what i can come up with. thinking about making the 4th axis first. with a means of fitting it to the 5th axis when needed.
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Old 04-25-2017, 09:00 AM   #30
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stupid ruddy cnc!!

started to machine up the top plate for the 4th axis. needed to cut 177mm x 223.35mm (stupid size i know) Y axis travel is 182mm minus the 12 mm cutter
just had enough to make the cut, but a bit on the risky side. so i changed to a 10mm cutter to give me a bit more room. still a bit risky. then thought i would change the size from 177mm to 170mm that should give me loads of room.....hit the button and off she goes. it does all the inner work fine, takes the first pass around the out side all fine, takes another pass, still looks ok. looks like a got away with it. meanwhile the swarf is piling up on the Y way cover so on the 3rd pass the Y jams up, stepper jumps about a million steps into the work........... C**K IT!!

so started with 177mm end up with 168mm

need a bigger mill.


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