Gear Cutting

Hi Guys,,,, This is my first general post. I’m struggling to find an 18DP No2 20PA involute gear cutter at a reasonable price. It’s easy to get a 141/2 Pressure Angle but not 20PA. I was wondering if a MOD 1.25 with 20PA is close enough. I’ve seen somewhere that MOD 1.5 20PA would produce a bit too much backlash. If MOD 1.25 was just a bit tight I could possibly generate some on the mill. I wouldn’t have a centre to centre gear problem because the gear to be made is on a swing plate,,,, Anyone got any ideas Regards Ian

What will the tooth load be on the gear? If it is low enough like in the geartrain of a lathe where you want the ratio to be correct but the load low a 3D printed gear works pretty well. For power transmission with a higher tooth load then metal would be needed.

If you had to choose between meshing a 14.5pa gear with a 20pa gear of the same DP, and meshing a 18dp gear with a 20.319dp gear (equal to 1.25MOD) then you'd go for the PA mis-match every time.

If I needed a 20pa I would buy it ready made from HPC or similar

I’ve been in machine shops/toolroom most of my working life (now retired) but gear cutting is not my forte. I’ve built/modified gearboxes but understanding gear cutting??,,,, I need help! Anyway back to the situation, I accidentally bought some Boxford back gears some years ago (18DP 141/2PA) and meshed them with my Kerry lathe gears 18DP 20PA on the bench. The rough running was quite evident and,, to me,, not good at all, so I took them back. I bought new 18DP 20PA gears from HPC gears (for metric thread cutting) The 127 gear from them was not a quick delivery so I purchased it from a machinist, I think in Yorkshire, unfortunately I cannot find his name/address. I have the material (tufnol) for the gear ready to gear cut (diameter/width/bore done) and would be a shame not to use it. The correct MOD equivalent to 18DP being technically 1.411 I think,,,, but it’s not made. That’s why I was thinking I could cut a slightly smaller MOD gear at 20PA to get the contact and therefore better smoothness than 141/2 PA. I was hoping I could tweak a little clearance to help the situation. Any more suggestions?? just get back to me Regards Ian

If you want to cut your own gears, Tracy Tools list from 2 DP to 120 at £20 each. But advise phoning to check stock

I have bought Module cutters from RDG (£22 )so it may be worth checking to see what they have.

I found folk like HPC and Davall to be expensive, but maybe that's just me being tight!

And with a cutter in stock, you can cut a limited range of mothers, should ,you wish.

Howard

I don't think you meant back gears in your post above but change wheels. In case anyone reads this in the future
Boxford Back Gears are 16DP and the changewheels are 18DP both 14.5 pressure angle.

If your main target is the 127 then look at the screwcutting tables for the threads you are actually going to cut rather than all of them then select all the gears that will actually be meshing with the 127. Make the 127 and the gears it mates with all in 20degree PA. You can also make them 20DP to make them smaller or go for 1 Module and buy some of the smaller gears which are quite common as they are used for lots of small lathes.

It's no wonder you had rough meshing on your back gear because the spindle and back gear shaft are at fixed centres. If you're meshing gears on the screwcutting banjo you should be able to adjust the centres for smoother meshing. It's not ideal but it's a lot more ideal than trying to mesh gears of two different pitches and very much do-able.

If it needs to be said, each Involute gear cutter only covers a range of tooth numbers, so that to cover from 12 to a rack, you would need eight cutters. With Brown and Sharpe type DP cutters a No.1 will cut from 135T to a rack, while a No.8 will cut 12T and 13T. So if you only want a 127T, you need a No.2.

Each cutter should be marked with the depth of cut, as "D+f", as well as the number and tooth range.

Workshop Practice Series No 17, "Gears and Gear Cutting" by Ivan Law will provide a lot of information on gears and cutting them in the home workshop.

My practice, following Ivan Law's advice, is to cut each tooth space in one pass, with a very light feed. My view is that multiple passes could introduce error from cut to cut.

I am NOT expert in gear cutting!

Howard

Hello Guys,,,, I did mean change wheel gears NOT back gears, sorry for getting my twickers in a nist on the subject. I do have a full set of metric/imperial change wheel gears (including 127T) for my Kerry lathe. I bought some Tufnol to make a127T gear wheel myself but found a machinist who would cut it in cast iron. I would still like to finish it off. If I was still at work I could have made a hss fly cutting tool for it using a shadowgraph for tool accuracy (copies from my good 127T gear. By the way I’m just getting up to speed with the terminology used on Boxford lathes etc. I’ve spent best part of my working life spent on “all singing and dancing” Colchester lathes just having to move a lever for any thread you need,,,, have I been spoilt??!! Probably!! I bought the Kerry 51/2 for home use and to save its life from an unknown future. I’m so glad I did, I’ve fitted a nearly new metric TRAV-A-DIAL to it to bring it into today’s world. Regards Ian