HTD3 timing belts and pulleys - how can they be this crap?

What now, fatty?

Bastard nightmare. Having finally assembled my precision ground ballscrew, back-to-back thrust bearings and servo motor, there is a shit load of backlash between the motor and the ballscrew, equivalent to around 100um of linear motion. Even a blind man could see this in the dark.

WTF?? Is it the belt or the pulleys? Almost certainly the pulleys you'd think, as the belt surely isn't going to be shipped like that. On the other hand, the pulley stock used in the production of these (mainly hobby) pulleys is probably flung out by the skip load in some shithole in China with little regard for quality.

So what should it look like then?

Let's find out what we should be seeing when we look at an HTD3 pulley:

https://www.pfeiferindustries.com/timing-belts/timing-belt-tooth-profiles-and-pitches

and 

https://www.pfeiferindustries.com/documents/Engineering/Timing%20Belt%20Pulley%20PD%20and%20OD/(3mm%20HTD)%20Timing%20Belt%20Pulley%20PD%20and%20OD.pdf

Sure enough, when you look for the "approved" dimensions for the HTD3 tooth profile, you find:

To summarise - approx 1.9mm across the throat and 1.1mm across the top of the teeth. 

And what do we find?

Let's call that 0.6 - 0.7mm across the teeth. So I'm seeing around 0.5mm backlash on each pulley, equivalent to ~1mm total backlash. You can hear the wind howling through the gap between the belt and the pulley. Jesus shit and piss.

With a circumference of around 72mm for the 24t pulley, 1mm of backlash is equivalent to ~70um. That's in the sort of ballpark I'm expecting - ballpark being the operative word here.

So that makes a mockery of my ground ballscrews and 2500 steps per rev servo motor. 

The choice seems to be to randomly buy a load of HTD3 pulleys from a wide range of suppliers and hope some are actually better than this - or perhaps try my hand at making my own. Never one to avoid an unnecessary complication, I may be tempted to go for the latter. Dumb fuck or what? Watch this space...