Is there a minimum number/type of gear cutters needed to do basic gear
Ivan Law's book is very good, but leaves me high and dry knowing what
John to the rescue?
Norfolk UK (not to be confuzed with Norfolk VA)
First off gears are worked out in size of tooth called Diametrical Pitch or
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
Everything was going hunky dory until that bl**dy last sentence of
Always a fly or wasp in the jam pot isn't there.
Piggy bank just will not take the strain, it will really squeal.
I suppose eBay could be the answer?
Yes or No guys.
Some on ebay at the moment from RDG but it's hard to get a set up in bits
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
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
the generating circle is one half the radius of the pinion's pitch circle)
What a fount of info you are !
You just got out the way in time mate ! ;-)
Who was this?
Never considered using a hob.
Much cheaper solution too.
CNCing a shaper would be the answer albeit a little expensive.
What an interesting project this would be.
Winter approaches so more time on the CAD.
I was coming to this conclusion, and especially after your first
paragraph about the number of cutters needed.
Can see myself trawling the various me mags for details and reading
the gear cutting books, and try to get the info into brain.
Thanks for all the info.
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.
Thanks for the pointers to the magazines.
Piggy bank has a stay of execution.
Only temporary mind you.
Money still _has_ to be spent.
On tools, not food.
Thank to all for responding.
It's turned into a very interesting thread.
next week is still a lifetime of decisions away