# Thread: Which gear cutter do I need?

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## Which gear cutter do I need?

I posted a photo of a worn gear in another thread. I'm going to use it as it is for now, but I was thinking that it might be an interesting project to make a replacement when time permits. I have a universal mill and a universal dividing head with the bits to join them together.

The diameter of the gear is 1" 11/32 and it has 17 teeth. From my limited understanding, that means I need a #3 cutter (or is it #6?). What else do I need to know to get the right cutter for this gear?

Last edited by pinstripe; 07-17-2017 at 12:59 PM.

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basic od formula is DP=(no. teeth +2)/OD.....so its a 14DP gear. you need the 17-20 size and the probability is a 20 degree PA. You will need the universal set up.....it'll be a set up you don't see often anymore so make sure post some pics

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Originally Posted by Mcgyver
basic od formula is DP=(no. teeth +2)/OD.....so its a 14DP gear. you need the 17-20 size and the probability is a 20 degree PA. You will need the universal set up.....it'll be a set up you don't see often anymore so make sure post some pics
Doesn't this require both a universal table and a dividing head geared to the table?

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Originally Posted by reggie_obe
Doesn't this require both a universal table and a dividing head geared to the table?
yes, that's what I meant by 'set up'....says he has the bits and pieces.

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Thanks Mcgyver. I'll post photos when I cut it, but it's a low priority for now.

reggie_obe, the table on the mill swings 90 degrees. It also came with the gearing for the dividing head. I don't leave the gearing on normally, but it doesn't take much to bolt it back on. The 100 gear is where the hand wheel goes.

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Well the thing is with helical gears they don't take the same cutter per tooth count as spur gears. Because of the angle of of the helix your would use the same diametral pitch cutter (26 32,48) but for a different tooth count. Years ago I downloaded a Navy machinist manual which gave the proper approximation of cutter depending on the helix angle.
gbritnell

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So based on the pics the OP posted how does one determine the lead needed to match the helical? I understand (I think) the leads, but how do you take an existing gear especially a worn one and determine what to cut for the new? I don't mean to highjack your thread.

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Better make BOTH gears. If the pinion is chewed up the gear is also. After all, they did wear out together.

An added advantage to make both (or re-cutting the gear) is you have control over the lead by calculating from tooth counts and center distances.

Accurate determination of the helix angle or lead by physical measurement of worn gearing is a very chancy business. Making a new gear that properly meshes with good tooth bearing on a worn gear is nearly impossible.

It would make sense to re-cut the worn gear to clean up (and reduce the OD by a like amount) and make a pinion with an enlarged addendum to suit.
Last edited by Forrest Addy; 07-17-2017 at 03:05 PM.

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Originally Posted by gbritnell
Well the thing is with helical gears they don't take the same cutter per tooth count as spur gears. Because of the angle of of the helix your would use the same diametral pitch cutter (26 32,48) but for a different tooth count. Years ago I downloaded a Navy machinist manual which gave the proper approximation of cutter depending on the helix angle.
gbritnell
George is 100% right....sorry for the earlier sloppy advice. Nevertheless....my redemption may come through.... depending on the helix angle. No. of teeth cutter to use is = No of teeth of gear/ cube of cosine of the helix angle.....so if helix is under 20 degrees its still in the 17-20 range

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In helical gearing the tooth form is measured in two ways: normal (at right angles to the tooth) and transverse parallel to a radial plane. The difference is the lead or helix angle. The normal tooth form in a helical gear is the same as for spur gears.": is a spur gear the lead is infinite and the helix angle is zero so the normal and transverse pitches are identical.

The pitch diameter enlarges as a reciprocal function of the cosine of the helix angle measured at the pitch line. The normal tooth profile is identical with a spur gear of the same pitch. The transverse pitch will be greater by the reciprocal of the cosine of the helix angle. The pressure angle widens as well and by how much is academic but an interesting solid trig problem.

In other words, you'd use the same hob or wheel cutter to cut a helical gear as your would cutting a spur gear. Petty quibble: Fussy shop would use a RH hob to cut a LH helical gear and vise versa: it's a question of cutting forces acting on the hobber. I've hobbed helical gears using the "wrong" hand hob dozens of time (sometimes the proper hand isn't available) and the finished gears tested just fine.

It's been many years since I cut gears for a living so maybe I'm rusty. Maybe TenFingers will chime is. He's still in the business.
Last edited by Forrest Addy; 07-17-2017 at 04:25 PM.

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