Cycloidal gear hobs

Does anybdy even use these any more? I have a bunch of cycloidal gear tooth hobs of 8mm and 10mm bore but after doing a search I don't see any used ones for sale which means they are either undesirable or unobtainable, I suspect that it's a bit of both. I also have a few ratchet cutting hobs.

Having no interest in horology or instrumentation I don't suppose I will ever use them. They don't take up much space so keeping hold of them is not much of an issue but on the other hand I'm conscious of the fact that with the advent of additive machining in metals these hobs might soon become completely obsolete that is if in fact it has not already happened.

If these things are still in common use somewhere I should like to find out where. Alternatively, if there's a common range of gears used I wouldn't be averse to producing cycloidal gear form bar stock for people to part off their own gears from but again I wouldn't know where to start looking.

It just seems such a waste to have them sitting in a box for the next twenty years (hopefully, at least) for them to then end up in a skip.

Looking for ideas, suggestions etc.

" advent of additive machining in metals "...wot dat?

Very intriguing, Pete

What module sizes are they ?

MichaelG.

Ah, now there's a question. Most of them have only numbers so I am laboriously measuring each one and labelling them. I have a nice little measuring microscope which I have fitted with a mount for the hobs and tenth-reading dials so that I can accurately take pitch and form readings. Some are module but most of the numbered ones so far conform to DP sizes. Some are marked with the module or pitch. I will be working through them today so I'll have a good list. I'm using paper tags at the moment whilst I gather the bits to make an etching pen.

Edited By Pete Rimmer on 23/05/2021 10:34:11

That’s even more intriguing, Pete

Cycloidal usually implies Horological, and Horological usually implies Module

Mmm

MichaelG.

I'm (slowly) working through them. I have cycloidal hobs that conform to sensible numbers in DP, module and metric circular pitch so far. For instance the latest one I've measured only makes sense for it to be called a 2.25mm circular pitch. The module would be 0.7163.

I've knocked up a spreadsheet so I can input the normal pitch and it spits out all the conversion numbers. I was using the chart from Davall which is very good but I have loads that just don't appear on it.

Using the microscope shows me that I have not so many cycloidals as I thought, some are all kinds of weird shapes. For instance I have one which is triangular shaped, straight sides 55deg rounded root and crest just like a Whitworth thread (except a straight sided hob will produce involute teeth) and a pitch of 0.0394" or 1mm CP. Somebody somewhere must be chuckling at me.

I would dearly love to get some insight from someone who has hobbed cycloidal gears in the past. Some of my cutters have tooth counts on them too.

I believe that cycloidal hobs can only cut a limited range of tooth sizes, unlike involute, for which a rack form can cut any pitch provided the tooth doesn't undercut.

That would make sense John because the shape wuld change on smaller tooth counts.

As I said I have no clue about cutting these types of gear. I do recall being most upset when I found out that my 33DP gear cutter (a very rare involute size) turned out a lovely cycloidal gear

I have catalogued the first twenty cutters and they are a mic of DP, CP and module. Some are actually marked as module but an insensible size (like 0.505 mod) until you realise that it's exactly 1/16"CP. Seems to be no rhyme nor reason and I guess conventions have changed over the years my collections span.

I will continue to catalogue them and see what I end up with. I have about 20-25 ratchet/escapement hobs to measure also. Some are new in the box.