Gears on a 6 inch Atlas Lathe
A friend with a 6 inch Atlas bought a 24dp involute gear cutter to cut change gears. Neither the size of the gears nor the resulting tooth profile matches existing change gears. What is the correct pitch for this lathe?
Use the formula DP = (N+2)/OD
DP is diametral pitch
N is the number of teeth
OD is the outside diameter of one of the existing gears
Example using a 30 tooth gear
DP = (30+2)/2
DP = 32/2
DP = 16
If you come out with a DP, say, 16.090 round to 16 (or whatever the closest DP would be) The OD of the sample gear probably will be slightly smaller that the theoretical OD.
Also, be aware that involute cutters come in a series identified by number numbers 1-8 and he may need several cutters to cut a range of gears.
Number of Cutter Will cut Gears from Number of Cutter Will cut Gears from
1 135 teeth to a rack
2 55 to 134 teeth
3 35 to 54 teeth
4 26 to 34 teeth
5 21 to 25 teeth
6 17 to 20 teeth
7 14 to 16 teeth
8 12 to 13 teeth
Machinery' Handbook is your friend!
Last edited by Errol Groff; 09-05-2008 at 10:36 PM.
24 is the correct pitch, but they are 14 1/2 degree pressure angle. Could be that he got 20 degree cutters?
I also note that you say he got an involute cutter, is that to say just one. If that's the case, you can only cut gears with a certain range of teeth, it's marked on the cutter. The tooth profile will be off for other numbers of teeth.
Last edited by Optics Curmudgeon; 09-05-2008 at 10:28 PM.
DP gear cutting
Measure the pitch diameter of any gear (in inches) and divide it by the number of teeth - that will give you the diametral pitch (DP).
The DP is a circle that runs through (about) the centre height of the gear teeth.
Measuring the DP that way can be a PITA as only an approximation is needed. An alternative is to measure from the top of one tooth to the bottom of the "cut" directly (or nearly directly) opposite.
Be aware that there are a range of cutters for each DP. They will each (only) - or are intended to - cut between a high and a low number.
I'm also assuming you mean 6 inch swing, rather than center height (12 inch swing). 9, 10 and 12 inch swing Atlas lathes use 16 DP gears.
Several more questions. First do the "Zamzc" gears grow when they corrode/age? When I checked his cutter it was 14.5 degrees, 24 DP or at least it was so labeled. It also had the word "bevel" etched on the cutter.
So the issues are, the cutter makes teeth that are the wrong size. Using the numbers from above or from my copy of machinerys handbook the factory gears are larger than they should be for a 24 pitch gear.
The word "bevel" is instant trouble. The profile on a bevel gear cutter is not correct for a spur gear. Zamac doesn't "grow" enough to affect the pitch diameter without being obviously deteriorated.
The word bevel is the tip off, it will cut a bevel involute gear, not a plain involute gear, the profiles are different. You will need an involute gear cutter in the proper tooth count range for the gear you intend to cut.
What I learned from this is to check the tooling that my friends bring over to use on my machines. I was assured that it was a correct cutter and didn't see the "bevel" label until he was gone and I tore the machine down.
This is the first bevel gear cutter I have ever had my hands on.
As to the "growth", the gears in question are really in bad shape so I assume that is why they are of larger diameter.
Thanks for the help and I will pass on the information to my friend.