Gear cutters- DP,Mod and Pressure angle

Im told that a cutter with a DP of 32 is the same or as near as dammit to one Mod 0.8 however the Mod cutters available are of 20 degree PA versus that required of 32DP where the PA is 14.5....

How compatible are the cutters likely to be?

DP 32 cuters appear in very short supply...like hens teeth almost....

Hi John

Unfortunately the pressure angle (pa) of the cutters mean that they are not compatible. I must admit that I have not checked the size properly, but I think the PCD of 32dp is probably about 10pc bigger than .8 mod which sounds too much difference for any speed.

Practically if the speed is slow and you are not bothered about wear or noise (and you calculate the new centre distance correctly) then yes they will probably run together, but if you want a nice smooth running pair, then no.

Regards - Will

The 32DP cutters RDG do seem to work OK for me.

Edited By JasonB on 12/09/2015 07:40:03

0.8 MOD is 31.75 DP, close enough to be interchangeable for most practical purposes. The difference in pressure angle may be more of an issue but 32 DP gears are pretty small and there are some approximations in using milling cutters to cut gears anyway and a thou or two difference could get lost in the manufacturing process. So while a homemade gear may not be as "accurate" as a professionaly made hobbed gear but they seem to work ok. A 14.5 degree PA tooth will have a slightly thicker root than a 20 degree so the centre distance may need to be increased slightly to get the gears to mesh freely. I think it depends on the application as to whether such a mismatch would be satisfactory.

Cheers,

Rod.

Not a big selection though - only 1, 6 or 8 available

Rod

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Think its just a case of out of stock, they had them all listed when I last bought some.

J

Edited By JasonB on 12/09/2015 07:58:24

Gents,

Thanks for the replies.

Jason, perhaps I wasnt specific enough.....I need to cut gears with 24 and 48 teeth, the ctters required are not available...I have spokent to,the guys att RDG, they are awaiting new stock and have no idea on their arrival.

Will/Roderick, thanks for detailing the differences. I was after a simple answer without going into all the theory.

Im just going to have to wait....

I do have a copy of Ivan

Laws book, but trying to digest the information is not easy.......,well not for me...

Are they just timing gears for an engine? You may be able to get away with using 0.8MOD and slightly altering the depth of cut as they won't be taking any great load, would only need to be cut 0.002" deeper.

PCD of 32DP is 0.563" and if using 0.8MOD then 0.567"

Whats the engine?

Jason,

The gears are for the Webster......

Then a bit of playing with the depth of cut will be fine as its slow running or just move the pivot for the larger gear towards the cylinder slightly.

If you're marking both gears then the PA will make no difference. I'd just go for it with the .8 MOD. The cutters will be cheaper anyway, I got mine from CTC.

Rod

There are some much cheaper hobs about now. It might be worth looking around. ARC do a few but not this one.

Most of the cheap new ones are MOD. I managed to buy a Russian one very cheaply recently but most are from China. I asked them via ebay if they did any for timing belt pulleys - no was the answer but they may be able to obtain DP. It might be worth asking.

John

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Curious - has anybody got something against hobs ? One cutter does all.

Actually there is some on in the Ukraine on ebay selling a 0.8M hob for around £32. I bought another size off him some months ago. No problems but I haven't used it yet. It's brand new razor sharp and came in a very neat plastic case. Russian. Tungsten loaded HSS too - and covered with bear fat. That's what they seem to use for protecting microscopes - doubt if it is but it's hard to get off.

I bought another one of ARC for if I need to cut a change wheel for my lathe - they are now on clearance which suggests no one is buying them - I hope - rather than being rubbish.

John

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It's the mechanism required to gear the job to the blank that puts me off hobbing. Free hobbing seems too prone to producing n-1 teeth.

Rod

I might regret buying them then Rod.

I can't see why that should happen though. I did realise that straight indexed cuts with them will produce gear teeth with an incorrect tooth shape but I doubt if I would ever buy a complete set of the ordinary form cutters so would finish up with incorrect teeth anyway even with the usual part set. In both cases I suspect that a bit of lapping would clean things up if needed.

One thing for sure if diameters and cutting depths were out the wrong way a hob will cause problems.

John

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If you try to cut teeth by indexing with a dividing using a helical hob, you will end up with no teeth at all.

Best regards,

Steve.

That's pretty obvious Steve - that's why they are straight cut and not helical which might be used for worm wheels.

People who make worm wheels at home usually gash at the worms lead angle to serve as a guide and then use a tap to finish it off - or just gash to the rack form. Some just use the tap but the tooth count doesn't always come out correctly

John

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I was under the impression that most (if not all) commercial hobs were helical. They have to be like this to keep synchronised with the blank as it rotates.

Best regards,

Steve.

Egg on my face. The ones I have are helical - over 1 degree less than 2. Mislead as I have seen info on making straight rack form hobs so assumed that they would be like that, also gear shaping and at first sight they look to be straight cut.

I'm going to have to try using a slitting saw slot to guide them with a freely rotating blank. This does work when taps are used for making worm wheels but the lead angles are usually much greater so I'd guess it wont work with these at all. When worm wheels are made like this people claim they turn out correct without the gashes but I would be inclined to put that down to luck - if it happened at all.

Dammmmmmmmmmmmmmmmmmmm!

I did realise that without rotation they wont produce perfect teeth but think that they would be usable.

It's not difficult to make hobs that are just a rack form eg

**LINK**

I think that there was some one around in the UK once that had a vacuum furnace making then in HSS mainly for clock making. I doubt if I have the link any more.

John

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