I've been scouring the net for information about the best toothed belt drive for accuracy and minimum backlash. It would seem that the GT2 profile is favoured by many for this. But there are some very funny quirks in the system when it comes to actually getting the bits.
Our favourite engineering store (otherwise known as eBay) features toothed belts and pulleys of course. A few vendors for most every variety and size, but in general they are all expensive and charge lots of shipping. The shipping costs from USA to Australia (where I live) can easily be double or triple the cost of the goods themselves - all many USA vendors seem to know know about is Priority FedEx. Sigh.
However, China and Hong Kong to the rescue ... but not for everything. They are big on GT2 belts and pulleys in the 2 mm pitch - at least in some sizes. Digging into what's on offer, I find that if the parts are required for some Reprap designs they are widely available, at very good prices - but other parts or sizes are not. OK, some design cunning required at my end. There's a bit of a range of 2M GT2 6 mm wide and 10 mm wide belts available, so that's OK. But what about pulleys?
Well, a few pulley sizes are available, such as 20 tooth, 36T and 40 T, with 5, 6, 6.35, & 8 mm bores, but nothing larger. As I need a custom design for a larger pulley (100 - 120T), I will have to make it myself. Interestingly, I was able to find two versions of the GT2 profile on the web. There are very complex drawings in the original Gates Patent 4,515,577 , but there was also a simplified version which is most usable.
Attachment 263910
You have to start with the theoretical PCD rather than anything measurable - that's the PLD in the diagram. Roughly speaking, that's the level where the glass or steel tension members run. That's how it works. The big problem is that the cutter needed is very very thin: 1.11 mm diameter. I don't think you can machine around a blank to any great depth with that one! (Such cutters have a DoC of about 3 mm anyhow.)
It is tempting to try to cut the teeth in bulk by drilling a circle of 1.1 mm holes, then turning the rest, but that will leave very sharp edges at the tops of the teeth, and these edges will chew the hell out of the belt. So the teeth will have to be at least trimmed one by one on a rotary axis. And of course you would like flanges on the sides.
One way of doing this is to cut the teeth out on a rotary axis (in the usual manner) and then stick flanges on later. That's simple, except for the multiple passes needed to get that R=0.555 mm base to the tooth - which is actually critical to the zero-backlash part. So, off to eBay looking for ball-end cutters with a diameter of 1.1 mm. Unfortunately they don't seem to exist at any reasonable price.
Aha - but ... most carbide milling cutters are diamond-ground these days on a programmable CNC machine (often German), using a canned routine driven by parameters entered by the operator. And 1 mm ball-end mills often start with a 4 mm blank: plenty of room. Could someone make me some with a 1.1 mm diameter by tweaking the parameters? After all, if GT2 uses a 1.1 mm cutter, then there could be others interested in this size as well. So I asked a reasonable Chinese vendor I have dealt with before. The answer (within 6 hours) was YES. The price was just a little bit more than for the 1 mm ones: US$25 for 5 cutters. Coating is available on request.
Current status: we have agreed on price and quantity. Delivery will take a couple of weeks. I am getting 5 of them - I am sure to break one or two! When they turn up I will test them. If it takes 15 passes at 0.05 mm per pass per tooth - who cares? The machine will be doing the work, not me. I can cut at a reasonable speed with that DoC anyhow. All being well, I will report back with the vendor's eBay name.
Oh yes: OD and PLD info is readily available from a PDF on the SDP/SI web site. Most helpful.
Cheers
Roger