Ask MAKE: Are My Gears Metric or Imperial?

CNC & Machining Workshop
Ask MAKE: Are My Gears Metric or Imperial?
Can you tell if this gear is metric or Imperial?
Can you tell if this gear is metric or Imperial?

MAKE reader Eric Chia of Singapore writes:

I have come across quite a few old HP Deskjet printers, and have taken them apart to salvage the gears inside. But I have no reliable way to identify whether they are metric or Imperial gears (diametrical pitch or modulus).

The compound gear in question is shown here. It has 60 teeth on the outside and 15 teeth in the center.

The OD measured by me is around 1.681″. My guess that since it came out of a HP printer, it could be using diametrical pitch (Imperial). I read up some books, and my rough guess was that the DP is 32.

Can any experts at MAKE help me on this issue? Thanks.

MAKE technical editor David Scheltema replies:

A friend suggested you could look at the mounting holes on the gear. The assumption is that you’d not find Imperial screw mounts on a metric-toothed gear. However, this doesn’t answer the question directly enough for me.

Here’s another approximation route which uses a few features of your gear and helps you guess. But all of these approximations don’t strike me as real solutions.

What do you say, makers? Can you tell a metric from an imperial gear? Share your tips and tricks in the comments below.

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