Gear, belt, chain offsets?

Hi all, was wondering what values you use for extra offsets when it comes to these calculations. Let’s say that a positive value means “farther apart”

Typically I design with these numbers:
32DP Gears: .003"
20DP Gears: .003"
25 Chain: .018"
35 Chain: .012"
3mm GT2 Belt:0-0.002"
5mm HTD Belt:0-0.002"

If the part is being made on a machine that can hold tighter than .0005" tolerances generally I omit the belt C-C add.

For 20dp gears, I’ve always heard (and used) +0.003". With 32dp gears, I’ve heard either exact distance or the same +0.003"

Chain gets a bit more complicated, as different manufacturers and even batches can have different tolerances. It may be worth it to measure for yourself what the proper add-on is for your specific chain. If you don’t want to do that, I’ve heard/used +0.012" for #25 chain, +0.016" for #35 chain.

For belts, I’ve always used exact distance. I have heard of some teams adding one or two thous if they want the belts extra tight, or taking away a few (even a lot of) thous if they want better efficiency and don’t mind a bit of backlash.