owdy. long time no see.

i been racking my brain lately over how to grind involutes. one of those ideas that lurks for ages then rears its ugly head.

no particular reason, im not about to cut gears or anything. just using grey matter.

the idea of grinding the correct involute for a given gear size has always interested me. how to generate an involute mechanically. done parabolas and spheres with XY links before. the principle behind an involute has always seemingly defied my mental imagery until now!

Attachment 233330

may the animation make it clear what has to be achieved.

so, im thinking of making a jig some time.

basic theory of a gear, sorry to rehash old info for the gurus, but is basically, a series of involute curves in both directions, generated from the base circle diameter, spaced evenly along the reference circle diameter, which automatically gives the curve required for a given gear size. i wont get into addendums and thrust angles yet...

hobbing generates the required curves on any size blank with one cutter, but has small "facets", and a hobbing machine is a big, single purpose machine... sure, i can do it in the mill but i require a faster, preferably servo driven 4th axis, with good spindle synch. isnt going to happen on an x2... yet ive done it before but the process wasnt pleasant!

use pre fab cutters. they cost money. grrrrr. and i destroy things that cost money! the longest lasting endmills are always the cheapest and we all know they only do the tooth counts theyre designated for. and to cut the smaller ranges of gears requires more cutters than the large end... anyway. needs versus cost. whatever. a hss blank in a boring bar is far nicer than an arbor and at least one cutter for a one off gear!

back to the jig.

Attachment 233330

and animation. consider the flat plane as the grinding wheel, faced and dressed to a small radius. (tiny. either radius or a sharp point... im thinking a dremel disc or a chainsaw sharpener disc)

the central "bearing" of the jig, what the tool being ground rotates on, is the circle.

we would move the tool, not the grinder, ideally... im just throwing out an idea is all.

so the tool would have to START at 90 degrees to the grinding wheel.

in the animation, think of it as pointing down from 12 oclock. the involute is generated in the tool....in reverse.

running parallel to the grinding wheel is a flat face, linear bearing type affair.

the tool is bolted to a platen, that rotates with the bearing. this bearing supports an interchangeable drum, that is turned to the reference diameter being dealt with. the whole assembly is mounted rigidly to the linear slide.

a cord, some type of un-stretchable belt, is connected to the drum, the other end being anchored securely somewhere convenient i guess. pointless left loose. would also need to run parallel to the linear slide.

the drums circumference is bought in line with the side of the grinding wheel. tool adjusted to correct position on the platen in relation to wheel.

as one rotates tool past grinder, the drum winds up the cord, and pulls the drum along, generating the involute. the rotation is also only quite small. 360 deg / no. of teeth / 2

could do it with a gear/rack but the idea is to make the cutter for making the gears that would be required for grinding the cutter for making the gear that is required for....

fiddly bits would be grinding the tool evenly both sides, and the correct distance apart for clearance etc. otherwise, one could basically and i reckon, with some good jig design, quite quickly grind the perfect fly cutter for any size gear... even funky custom ones, without being overly complex or tricky. all you have to do is machine a drum the appropriate diameter and adjust the linear slide to suit, insert tool, adjust and bang...

if i wasnt half asleep i might do i quick sketch but nah. my sketching is lousy...

hope my description gets someones creative juices flowing... i like seeing my ideas being made even if i dont ever get to benefit myself