Hi all, Is there a minimum number/type of gear cutters needed to do basic gear cutting. Ivan Law's book is very good, but leaves me high and dry knowing what is needed. John to the rescue? Regards Geoff Halstead Norfolk UK (not to be confuzed with Norfolk VA)
First off gears are worked out in size of tooth called Diametrical Pitch or DP. Any gear of the same DP will mesh with any other. An example of size is the Myford change wheels which are 20 DP. Further subdivided in each DP size are the number of cutters to cover the whole range of teeth from 12 up to a rack and this is 8 in number. Ivan's excellent book shows this on page 65 where the following is published :- Cutter 8 will cut gears of 12 - 13 teeth Cutter 7 will cut gears of 14 - 16 teeth Cutter 6 will cut gears of 17 - 20 teeth Cutter 5 will cut gears of 21 - 25 teeth Cutter 4 will cut gears of 26 - 34 teeth Cutter 3 will cut gears of 35 - 54 teeth Cutter 2 will cut gears of 55 - 134 teeth Cutter 1 will cut gears of 135 - to a rack.
The reason you need 8 to do a set is that the involute shape changes as the size gets bigger or smaller.
To do basic gear cutting you first need to find out what DP your gear is. Easiest way is to measure the OD and the number of teeth, add 2 to the number of teeth and divide by the OD, so again using the Myford example a gear with an OD of 2.100" and 40 teeth will work out at 42 over 2.1 = 20 DP
Next work out what range of teeth you need to cut and then obtain the cutters relevant to that range from the table above.
Bad news is there are 8 cutters per set for each size DP of gear so you need 8 for 20 DP, 8 for 16 DP, 8 for 14 DP ad nauseum
Some on ebay at the moment from RDG but it's hard to get a set up in bits and bobs. Not wanting to upset you even more but there are two common pressure angles of 14 1/2 and 20 degrees so in reality there are 16 per set [ ducking and running ]
Three ways out:- Hobbing, covered by our friend whos original posts I don't see due to Agents impressive kill file [ pity it don't work in real life ] Where one hob per size and pressure angle will cut the whole range. Disadvantage is the hobbing machine or a decent setup to do this on a horizontal and also obtaining the hobs.
Shaping on a shaping machine, disadvantage is it's slow but the advantages of cheap home ground cutters costing pence can out weigh this.
Last is to make your own form cutters. Quite a few articles have been written about this and Ivan Laws book covers the 20 degree cutters.
It's an interesting subject when you get into it and very worthwhile after you have cut your first pair of wheels.
The other area where working tolerances change what you do is in the shape of the teeth! Whereas the books go into great detail about the derivation of involute, cycloidal, epicycloidal and hypocycloidal curves, when it comes to making your own cutters, you end up doing a circular approximation to the curves (except for hypocycloidal where you use a straight line because the generating circle is one half the radius of the pinion's pitch circle)
Unless you are going into production of gears of all sizes it is very unlikely that you will need all the sets of cutters to cover the full range. Having made two traction engines in 1 1/2" and 2" scales I found that I only needed 6 cutters to cover the range of teeth needing to be cut. So over 18 years (as long as it took to make both engines!) it was no great outlay. All my cutters came from Tracy Tools which I found to be the cheapest.
That's a good point. If your need is actually for a very few gears, then have a look at pp54&55 of MEW, March/April 2001; this shows using a hand shaper to cut wheels using no more than a standard lathe tool ground as a single bit.