How sharp should the corners of Gear Tooth Calipers be?

[Azoth]

Shop I'm at has an old Brown and Sharpe #580.

I noticed the Depth of the teeth on a gear rack on a part I'm milling was undersize, so I checked the caliper with an optical comparator and found a 20°x.0025" chamfer and 20°x.0033" chamfer.

They aren't designed with a chamfer are they? I think it's wear from overtightening when checking so many 20° pressure angle gear teeth over the decades.

 

[guythatbrews]

Not sure about the chamfer.

Can you measure over a pin and the back of the rack and compare that derived tooth thickness to the caliper tooth thickness?

 

[Azoth]

That's what I did actually. Verified the error by checking the part directly on the optical comparator at the tooth addendum the same way the caliper measures. Also used a mic and pin and double verified.

The guy training me doesn't know anything about gears, so he got mad when I told him the calipers are damaged and we need to account for the error because he's always done it that way.

I'm 99.99% sure I'm right, but I thought there's a chance there's a reason for the little chamfer like the calipers are designed and calibrated for a specific pressure angle.

 

[sfriedberg]

I have never seen a set of gear calipers listed (or restricted) to measuring a specific pressure angle. So you are probably correct!

 

[Mud]

Related question - How free moving are the jaws of a gear tooth caliper when new? I have one in very nice condition, that has a lot of drag when adjusting it. I have to turn the thumbwheels to move the individual parts, and they are rather stiff, so there's no feel at all. I bought this one used and it's the only one I've had in my hands, so I have no frame of reference to compare it to.

 

[guythatbrews]

Related question - How free moving are the jaws of a gear tooth caliper when new? I have one in very nice condition, that has a lot of drag when adjusting it. I have to turn the thumbwheels to move the individual parts, and they are rather stiff, so there's no feel at all. I bought this one used and it's the only one I've had in my hands, so I have no frame of reference to compare it to.

The caliper should not be stiff enough it's hard to move. Maybe gunked up?

If the vernier reads zero with the jaws closed but there is a gap at the jaw tips then it's not right. Can you set the tongue at zero then grind the jaws and the tongue together until the tips are sharp again?

 

[Dan from Oakland]

If you know the amount of wear (and it appears you do), simply adjust the corrected addendum setting on the calipers accordingly. A .001 error in the addendum setting is not going to affect your tooth thickness reading to a great extent anyway. Plus reading the vernier to .001 is always a bit if a challenge so......... Measure 'em over pins and you will be a lot more accurate than tooth calipers.