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Gear form advice needed

I picked up a nearly new Mikron gear hob at a car boot sale last week (amazing what turns up at these places sometimes). No markings, but It looked to be a useful model engineering size, so worth risking a couple of pounds I thought.

However, the hob form baffles me. The circular pitch, as well as I can measure it, is about 98 thou, which seems to stack up with 32 DP. Fine. However the tooth depth is only 40 thou.

From my Machinery's Handbook a 32 DP, 20 degree PA, standard involute tooth should be 62.5 Working Depth, or 67.4 thou Whole Depth.

Is it perhaps a stub form? Again from my Machinery's Handbook, 32 DP,

20 degree PA, standard stub form Working Depth would be 50 thou. Still not right.

Can someone with better knowledge of gears than me throw some light on this (before the hob ends up being thrown)

Mike

Reply to
mikecb1
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Oh come on, you know perfectly well that if you throw it, you'll find a use for it next week!

Reply to
newshound

From what I see available there are more special hobs than standard ones. It seems that manufacturers often design their gears for a desired center spacing and number of teeth. Most hobs seem to have some data etched on the end, but it may not be useful. It is definitlely not true that all gears are standard DP, CP, or Module sizes, even if metric dimensions are used. There are also a variety of pressure angles and tooth depths used in addition to the common standards. About the only thing standard seems to be that a whole number of teeth are always used.

I was told of a newly hired machinist who claimed gear cutting ability and was asked by his boss how his first gear was coming along. He replied, "It is nearly finished, I just need to know if you want me to finish it up with one big tooth or two small ones." ;>)

Don Young (USA)

Reply to
Don Young

Thanks Don, I was perhaps being rather naive expecting it to be something standard.

One for the White Elephant drawer then!

Mike

Reply to
mikecb1

I have a lot of Mikron hobs and all have some marking on then but often it is very faint. They give the DP, mod or CP and the d&f. In this case it seems to be very shallow, are you certain that the teeth are properly shaped ie involute or similar, and thart you have the pitch at 32 DP it could be a ratchet or some other special . Peter

Reply to
petercolman45

Thanks for the input Peter

I had another careful look today, and apart from the maker's name and location, the only information on the side of the hob is the number

99608N. I must admit that I don't know if the teeth are truly involute form, but they are trapezoidal and perfectly symetric. A very careful circular pitch measurement with a calibrated eyepiece come out at 2.50 mm, pretty close to my first measurement of 98 thou.

Mike

Reply to
mikecb1

That's how it should be - the straight sided teeth of a rack will generate the correct curve for any number of teeth of an involute gear wheel.

Reply to
Alun

Could it be intended for an involute spline rather than a gear? --Glenn Lyford

Reply to
Glenn Lyford

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