Helical milling – imperial to metric ratio

[Adam HarrisParticipant @adamharris13683]

For anyone interested in the typical set of gears for differential and helical milling that come with a dividing head, it does vary between makes. Hoffman have (12) 24×2,28,32,40,44,48,56,64,72,86,100, Fritz Werner the same, Schaublin 13-900 model dividing head comes with  (14) 24×2,28,32,36,40,48,56,64,72,80,90,96×2, and an extra special addition was available of (7) 22,26,29,52,80,100,112

[Adam Harris]

On Adam Harris Said:

I will make a 127T gear … It will be an enormous 193.5mm diameter

You may have missed the subtleties of Nigel’s point above. Since the 127t gear only ever drives the 100t gear, and the 100t gear does not drive anything (only used as one of a compound pair), it is possible to  make both gears to a smaller Module and hence smaller physical size (just as Martin Cleeve did for his Myford).

You have to consider the centre-to-centre distance available on the banjo: either measure it and work out the maximum number of 1.5Mod teeth that will fit in the space or look in the manufacturer’s tables for the machine and see what is the maximum they use.

At the least, do a mock up with two plywood disks cut to the pitch diameters of the proposed gears.

In the Schaublin one above, the maximum ratio they employ is 112:24 = 136 teeth. 127 + 100 = 227 so considerably more space is needed.

In today’s world, if you need a special dividing plate, 3D printing is the way to go. The first job of the plastic plate would be to make a metal duplicate.

[Adam HarrisParticipant @adamharris13683]

Thanks DC, yes I did completely miss that point, great news! Well I already have a 127T and 100T in Mod 1.25 (change gears for my 102VM lathe) and all I need to do then is couple the 100T to a Mod 1.5 gear of a tooth count as required in the tables. If it all looks unmanageably large I will drop it down 24 DP giving me a 127T of 5.38″ and 100T of 4.25″ which would be tidier but with still quite robust teeth.

Yes John S did suggest I only use the plastic dividing plates a couple of times before I should devote some serious time to making steel copies

[Adam HarrisParticipant @adamharris13683]

In terms of metric usage of the imperial Mikron with 5 TPI leadscrew, someone told me that the imperial Bridgeports also have a 5TPI leadscrew and some have dual imperial metric gauges on the handwheels (with 127T/125T internal gearing). Are these handwheels still produced or come with any other machines?

[Adam HarrisParticipant @adamharris13683]

Hopper, I have just ordered Martin Cleeve’s book “Screw cutting in the lathe” on Amazon for delivery Tuesday. I look forward to perusing it – thanks for the pointer

[Adam Harris]

On Adam Harris Said:

In terms of metric usage of the imperial Mikron with 5 TPI leadscrew, someone told me that the imperial Bridgeports also have a 5TPI leadscrew and some have dual imperial metric gauges on the handwheels (with 127T/125T internal gearing). Are these handwheels still produced or come with any other machines?

Those are the ones that I mentioned earlier, Adam

https://www.model-engineer.co.uk/forums/topic/helical-milling-imperial-to-metric-ratio/#post-693690

… not sure if they are still produced [or affordable] but they are very elegant.

MichaelG.

[Adam HarrisParticipant @adamharris13683]

Thanks Michael. Wow what an amazing article that man David M MacMillan has written. An absolute perfection in method of explanation with pictures and very entertaining to read! I wonder why they are uncommon. I would have thought there would be great demand. Probably too expensive to make. Now my wonky memory is firing up on this cold and wet Monday, I think actually very many years ago I had a dual metric/inch dial on a Tom Senior ELT Universal mill. The mill was quite a big machine for me and it had fallen over on its pallet during transport in the pallet truck getting some damage so I ended up selling it cheaply quite soon after arrival.

[Adam HarrisParticipant @adamharris13683]

I quote from the Tom Senior brochure regarding the ELT Universal “Metric or Imperial Leadscrews, nuts and index collars are fitted; or alternatively Imperial to Metric / Metric to Imperial conversion dial units can be fitted at extra cost”. They did look very nice

[Howard LewisParticipant @howardlewis46836]

Purely as an aside, the Vertex HV series Rotary tables have 90:1 ratios, and probably so do the Vertex Dividing Heads.

I have seen Rotary Tables with 36:1 ratios.

Personally, prefer the 90;1 since it provides a greater resolution, 4 degrees per turn of the handle as opposed to the 9 degrees of a 40:1.

Howard

[Adam Harris]

On Adam Harris Said:

…a 5TPI leadscrew and some have dual imperial metric gauges on the handwheels (with 127T/125T internal gearing). Are these handwheels still produced or come with any other machines?

The one on the Circuitous Root website is a later version of the Gamet dial. It used internal gears for the 127t and 125t as it resulted in a more compact item. The orignal Patent is easily located on espacenet as there are not so many inventors named Gamet.

The earlier ones, fitted to Colchester Bantam and Chipmaster machines, use standard external gears, with the double width pinion on the outside. They were thus much bulkier, but would be considerably easier to make in a home workshop.

The secret is to cut the 125t gear and the 127t gear on a blank that is sized for a 126t gear. Then the double width pinion will mesh at the same centre distance with both gears.

You asked about other machines: Hardinge HLV was available with them, as was Elliott Omnispeed:

From: https://www.mig-welding.co.uk/forum/threads/elliott-omnispeed-lathe.125202/

Note the three definitions of the inch given above.

—-

Just as an aside, if you do multiply the leads in the table by 1.27, and use the standard gearing, you will be able to cut exact imperial leads in 1/4″ increments (e.g. 5 x 1.27 = 6.35).

[Adam HarrisParticipant @adamharris13683]

It seems these internally geared dials require the teeth to be cut (or slotted) on the inside of the ring which is not easy to do!

[Howard LewisParticipant @howardlewis46836]

Sorry, did not find my previous post so resubmitted .

The Vertex HV series of Rotary Tables use 90:1 ratio, (Giving a 4 degree increment for one full rotation of the handle, as opposed to the 9 degrres for a 40:1 ratio, so greater resolution)and I have seen Rotary Tables with 36:1 ratios, giving 10 degrees increment for a single handle rotation.

My HV6 is used as a Dividing Head when cutting gears.

Howard

[Adam Harris]

On Adam Harris Said:

It seems these internally geared dials require the teeth to be cut (or slotted) on the inside of the ring which is not easy to do!

The first (original) patent for the dials, GB1463732A, used external gears and this was the design used on the Bantam and Chipmaster.

The internal-gear design was a later improvement.

On a milling machine, the larger size of an external-geared version would not offer too many disadvantages.

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