wareagle said:
John, as we all know gear cutting is a science all into itself.
Yes it is a science but it's a mis represented one because by the time it's filtered down to us muppets at the bottom all the work has been done.
Forget Addendum, Deddendum, working clearance, plane of forces, ear of bat, tongue of newt, etc. is of no consequence to us because WE CAN'T DO SOD ALL ABOUT IT.
All we are worried about are 7 pieces of information and we need to be given at least 2
Lets take it in it's most simplest form using a gear cutter.
We need a gear cutter, right that bit is obvious but what?
Well we normally have to cut so many teeth on a wheel of a certain diameter called a gear. This is the first hurdle trying to work this out as this is where the Herbert's try to confuse you as a gear has umpteen diameters all set to confuse you but we ignore all but two diameters, forget base circle clearance circle they all follow on from the right calculation, WCDSAAI [ WE CAN'T DO SOD ALL ABOUT IT ]
So the two we are interested in are Pitch Circle Diameter [ PCD ] and Outside Diameter [ OD ]
OD is easy it's the outside of the blank before cutting the teeth.
PCD is the theoretical diameter at which the gears make contact, image two rubber rollers running together.
Gears are often listed by PCD as it plays the most important part of gear calculations.
Say you have two gears on 4" PCD and one 2" PCD then the distance between them is PCD1 plus PCD 2 divided by 2 so in this case bung two holes in 3" apart and they will mesh - easy init ?
But that doesn't help up make the blank as you can't see PCD or easily measure it so we work on OD.
Now we will use the same 2" gear as an example, it's 2" on the PCD but ?? on the OD, the only other information we need is how many teeth. So say we want to cut 40 teeth on this gear we next need to work out what size tooth makes up the gear, in imperial they are called Diametrical Pitch [ DP ] in metric they are called Module but we won't go there .
Working DP out from the PCD is easy as its a straight division of PCD into number of teeth so 2 into 40 means we are looking at a tooth form of 20 DP.
Next we need the OD and the magic number here is 2, not 2" just 2 and it's always 2.
So add 2 to the number of teeth, 42, divide that by the DP and we get 2.100"
So now to recap we had PCD at 2", number of teeth at 40 and we now know the cutter size needed, 20 DP and the OD
That's 4 out of the 7.
Next we choose a cutter, Because the teeth on a gear change with every change of tooth number in their shape we need 1000's of cutters but as this isn't possible the Herbert's have simplified them into 8 bands.
No# 1 cuts from 135 to a rack
No# 2 cuts from 55 to 54
No# 3 cuts from 35 to 54
No# 4 cuts from 26 to 34
No# 5 cuts from 21 to 25
No# 6 cuts from 17 to 20
No# 7 cuts from 14 to 16
No# 8 cuts from 12 to 13
So from this table we need a No# 3 20 DP cutter, that's the 5th bit of information.
Next is pressure angle which is that technical even gear cutters don't understand the technicalities of it so we muppets will simplify it or forget it .
There are two main ones 14.5 which dates back years and the more modern 20 degrees, they can't run together correctly. Now if we are making all the gears in a train it doesn't matter which we use as long as they are all the same.
If we are having to mesh with existing gears it's quite hard to find out usually age is a good pointer, most lathe change wheels are 14.5 because of having to mate with older models. If you have to choose a new design make them 20 degrees as they are stronger, so this is our 6th piece of information and we now have enought to make the blank and choose a cutter.
Cutting the teeth is a straight forward milling operation where you feed the cutter in, pass thru the work, index round one tooth and repeat.
The missing link is the depth that the cutter has to be fed into the work and is specified as D+f which means depth plus clearance and is expressed as 2.157 divided by the DP, in this case 0.108"
An easer way to work this out is to look on the side of the cutter as it usually says something like -
20DP No# 3 357T to 54T D+f 0.1082 engraved on it.
That's the last part of the 7 steps, get cutting
EDIT: - [Cutter number edited to # 6 because I'm a dummy and can't count]
