Gear spec for threading dial

Hello, I need to make a threading dial for my metric Harrison M250. The leadscrew is 25mm diameter 6mm pitch.

I plan to buy the necessary gears, can anyone advise me on the right gear sizes? Not the number of teeth, but the type and size of gears.

Thanks

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But what profile ?

Without that detail, it will [I believe] be impossible for anyone to give a 'correct' answer.

The loads are very light, so arguably "anything that meshes without jamming" would suffice.

MichaelG.

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Let the argument commence ...

Presumably it's an Acme thread...?

I'd prefer to find the best possible match to the original without having to buy more gears than necessary.

Hi Michael Cross,

On my lathe (a bit maller than yours) with a 3mm pitch leadscrew I made a 28T module 1 gear from a piece of Delrin for the threading dial, I assume that on your lathe it should be possible to use a module 2 gear. I know the tooth spacing might be a bit on the large side.

Thor

Thanks Thor that's a good start. Looking at it now - do you think mod 1.5 would fit your lathe or does mod 1 look about right? All else being equal I guess less slop is better.

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Presumably not

If it's metric, and that general shape, it should be 'Trapezoidal' which has a different flank angle [despite what most ebay sellers will tell you].

MichaelG.

Edited By Michael Gilligan on 23/07/2019 10:13:15

On my lathe with a 3mm pitch leadscrew (Trapez form and not Acme I assumed) there is 3mm between each "groove" so I needed a gear with about the same tooth spacing. A very large module 1 gear (i.e. rack) has a tooth spacing of about 3.14mm. I used an ordinary (home made) module 1 gear cutter, and as Michael Gilligan says: "anything that meshes without jamming would suffice". So you could buy one cheap 30T gear in module 2 and try.

There was a discussion on Practical Machinist here, about thread dial indicators on metric lathes. Scroll down a bit and read Clive603 post 07:07AM From this it seems at least some Harrison lathes uses a 30T gear on the thread indicator.

Thor.

Edited By Thor on 23/07/2019 10:22:05

Thanks Thor.

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[only kidding] MichaelG.

I'm still interested to hear any advance on Thor's advice to use metric mod 2 - I bet someone knows of an even better fit from something off the shelf. I know that anything that meshes will work but since I'm going to the trouble of doing it I might as well do the best I can.

I wondered about using a metric worm wheel but he original Harrison gears look more like a spur gear.

30 teeth at mod 2 would be 60mm diameter which would make the thing a bit bulky so I'll use smaller gears and switch them like they do on the real one.

Have you ruled out making a quick hob and freehobbing them in plastic or brass?

I'd prefer to save myself the bother.....

Read your PMs.

Acme thread angle is 29 degrees and trapezoidal is 30 degrees., I doubt if a threading dial gear would notice the difference.

Your two statements contradict each other. Either you want to do it right, in which case you need gears with 6mm circular pitch and very likely 30 degree pressure angle, or you want something close, in which case 2 MOD (6.28mm CP), 20 degree pressure angle will do adequately.

Looking at the gear on an imperial Chipmaster TDI, it is closer to parallel-sided stub teeth with a reasonable chamfer on the tips than a true involute form.

I hope you understand that one single gear will not do all metric pitches. Please glance at post number 6 here:

http://bbs.homeshopmachinist.net/threads/45876-Why-doesn-t-a-thread-dial-work-with-metric

as it contains an image that is worth saving.

I've written an article on the subject for ME but it's not yet published.

you can use a module 2 mm. 30 teeth for pitches submultiple of 10mm (5, 2.5, 1.25, 0.5mm); 42 teeth for pitches submultiple of 7 (3.5, 1.75).

JM

Thanks James,

Is there any advantage to using 30&42 over using 16, 18 & 14 teeth with 8 divisions on the dial?

Will the smaller gears bind or anything?

All else being equal I'd prefer to keep it compact.

sorry Jacques - I meant 'Thanks Jacques......'

For future reference for anyone who might need it, I received this in a helpful PM (many thanks):

Being metric the leadscrew will be trapizoidal, meaning that the included angle is 30 degrees, not 29. SInce the pitch is 6mm in theory that means you need Mod 1.91 gears. I doubt you'll be able to buy those. However, Mod 2 will probably work. Again in theory the gears should be helical with the teeth at the helix angle of the thread. In practice using a very thin gear means that a straight spur gear can be used. A pressure angle of 14.5 degrees should be fine.

Implicit in your original post is that a thread dial indicator on a metric lathe has more than one gear, the appropriate one being selected according to the pitch required. In the manual for my Harrison M300 the parts list shows 5 gears for the metric version. The metric leadscrew is 6mm pitch, but I don't know if the gearbox is the same, so the tooth counts might be different.

Oh well ... There wasn't any interesting argument after all.

MichaelG.