Gear cutting advice

[deeferdog]

Help! Cutting gears is not my long suit (not my short suit either), in fact I've never done it. This project I'm looking at requires making four gears:
1 x 33T diameter 49.7mm
2 x 50T " 72.5mm
1 x 115T " 160mm
All the gears are in mesh and a tooth profile is included with the drawings (see attached). I understand a some of the theory and have read bits of Ivan Law's book "Gears and Gear cutting" but I really need a shove in the right direction. If I grind a tool to the profile, cut the blanks to the drawn size and then index around each at the appropriate angle, is this the right way to go about it? I have a dividing head. If I wanted to buy the cutters, is there an easy way to work out which ones I need to cut these gears? Really grateful for any advice, Regards, Peter

 

[Cogsy]

I have made a few gears but I'm not an authority on them. I do not own any cutters though, I've made all the 'hobs' I've needed. Here's the LINK to all the info you'll need. It's really simple, costs nothing (made from scrap) and you'll have made your hob (only need one for each module/DP) and your gears before a set of expensive cutters would have arrived in the mail. Enjoy!

 

[deeferdog]

You're a gem Cogsy, for that I'll hope the Eagles win on Sunday! Cheers, Peter

 

[kvom]

If you take the radii of two meshing gears along with the distance between the axes, you'll see how much the two circles overlap. Subtract half of the overlap from each to get the radius of the pitch circle. Once you have the pitch circle, number of teeth, and pressure angle, the formulas in Law's book will enable you to calculate the needed cutter, should you choose to buy rather than make.

There are various online gear calculators that allow you to enter parameters to design the gear train and verify the fit.

 

[Fluffy]

G'Day Peter,
Cogsy is right with the info on using a hob like cutter. They are easy to make & give a reasonable tooth profile. I make a simple jig to fit the gears in question & run then together with lapping paste until I'm happy with the running surface of the teeth. These cutters have been made from 0.75% plain carbon steel & heat treated. The important points are to make sure the HSS tool bit used to cut the grooves in the hob blank is twice the pressure angle (generally 40&#730 & the pitch of the grooves is equal to the gear module x pi, with the depth groove depth being 2.25 x module plus 0.05 mm. Make the hobs with five teeth & set the middle tooth to the centre height of the dividing head when cutting the gear.
Send a PM if you need more info.
Regards,
Don.

 

[deeferdog]

I wish I could say that I have read it all and now understand it fully. I could say that but I'd be lying. I get the concept of the hobbing cutter described in the link from Cogsy, some of the math is still a bit hazy. Fluffy has inspired me to give it a go so all I can say is "watch this space". It might be a while before I report back, there is a little matter of a new kitchen cooktop and oven installation that I'm getting nagged to death over so I think that the smart thing would be to get that out of the way, then on to making chips! Thanks for all your help fellers, Cogsy, kvom and Fluffy, much appreciated. Cheers, Peter

 

[coulsea]

the original post requires specific size gears, I presume to fit in a particular size space which can be difficult because you need to understand the system that the designer was using.
I have been experimenting making gears just to see if I could(for future IC projects) and have found that if you can adjust your plans slightly gears can be simple.
Method - buy a couple of involute gear cutters (a few dollars each from ebay) each cutter will do a range of teeth sizes but if you want a 2:1 ratio you will need 2 cutters eg one for 13 teeth and another for 26 teeth
using metric (module) cutters the only thing that you need to know is the size of blank and how deep to cut. so using module 1 the blank will be number of teeth plus 2 and you cut 2.25mm deep.

works well with brass gears but I don't know how well a $5 cutter will do harder metals

 

[trizman]

I with the rack hob method and I admire the hollow cutter and mandrel approach. Here in the UK, involute gear cutters very expensive if you only going to make a few gears. There is a good explanation here http://www.jeffree.co.uk/pages/multi-tooth-gear-cutter.htm as how to make the tool.

Peter

 

[bluejets]

I use module 1 cutters which I believe originated in India. Relatively cheap for a set of 8 cutters at around Aus$120. Compare this to a single " bought " gear in some instances of at least $100 each. Of course you will need a hob but most cheapies are around $30 and quite good. Slow feeds and cutting depth with plenty of lubrication seems to work well and cutters so far have lasted more than 5 years. Of course it goes without saying to many who have cut their own, do it when no one is around chattering in one ear while you try to remember number of turns plus number of holes on the dividing head. It is a must to write it down and refer back each move but still the former isolation applies or you will end up with a portion of a geartooth somewhere.