Well, as you all know I have been machining up some pulley sheaves.

Before setting the top-slide I hunted around on the internet to find the correct flank angle. I was advised by the belt supplier that it was a 'Z' section belt with a 10mm width at the top and a radial thickness of 6mm. (The internet suggested that this type has an included angle of 40 degs. I therefore set the top slide (with a DI) to machine 20 degs.

I have checked the finished parts by several methods) and I have no doubt that I'm within 0.25 deg of the target of 20. Unfortunately neither the new or the old belt seems to be a particularly good fit with my 40 deg angle pulley.

Now, I had assumed that the pulley flank angle would be the same as that of the belt. I had previously measured the used and worn pulleys and decided that they were pretty close to 40, so 40 they must have been when they were new ! Unfortunately, now that I check them more carefully they look to be more like 34 degs!!!

Unfortunately this would seem to tie up with more information I have gleaned from the web suggesting that the pulley angles for a Z drive can be 34 or 38 degs depending on pulley diameter.

So is this really correct or am I confused and mislead by alternative facts in the web ??? Are drive pulleys really made to a different angle to the one on the belt ? Might this be something to do with the bending of the belt causing the inner zone to compress and get fatter ?

If you experts confirm this is the case it looks like I will have to start the machining all over again

Gerry