The thread dial drive gear

I'm moving up a bit from the beginners section ! Lathe is a Smart & Brown model M mk2 , my dials drive gear is missing sadly, although the rest is still there .. Probably would have a job to get a new one now and it'll be 50 quid which i haven't got . Would this gear originally have helical teeth , and if i just made bodged a thin spur gear by hand file would it work ? There are good pictures on the Lathe uk site ,of a restored black one and it looks like straight teeth...but it cant be can it , if it engages the lead screw ? If it was thinner though...?

Edited By Hacksaw on 11/01/2017 23:44:17

Straight teeth are normally used

Thread dial indicator

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... and the idicator assembly is canted to match the lelix angle of the leasdscrew

But here is a nice example of a 'worm-wheel' **LINK**

https://goo.gl/images/tDghfW

MichaelG.

a better bodge would be, screw cut a small section the same as the lead screw. grinded out to make a hob, harden.

put back in lathe and set a blank on the cross slide (on a pin at centre height) and you should be able to hob one out. you may need to lower and heighten to get some clearance. if the blank has an index to start with the correct number of teeth it will just require slowly feeding in the blank. if your bodge hob is sharp it should cut Ok.

only use soft materials brass ally .etc..,.

There is some small angle to the gear teeth. It's a bit late now so I will get more details later.

That looks familiar ! An end on photo would be good

The gear is 1.04" diameter. The lead screw is 1" diameter with 6tpi so by calculation the angle for the gear is 3 degrees.

Hope these are of use to you.

Martin

Thank you kindly , that is great information

Providing it's for the same lead screw pitch one from any lathe will work. There is a but - if the tooth count on the gear is different you'll need the dial to go with it. Mostly on smaller lathes with 8 tpi leadscrews they tend to be the same. Some lathes though do vary but I've only come across something really different on a DSG which uses a much larger pitch leadscrew anyway.

They are generally are cut with angled straight teeth to match the helix angle of the leadscrew thread. Some are cut more like a worm. Straight teeth can be used if the gear is angled. You should get away with a 1 mod gear. The circular pitch of that is 0.1237 - pretty close to 1/8"

John

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I'm 6 tpi i think . . I'll try and make up something like the photo .I have the casting and the cable , i just need the gear

I know nothing about dp or mod... I'm very hands on , I make do and mend.. If we were stuck in the desert with a busted motor , I'd get us home .

Unless it was a cambelt snapped..then we'd die !

Edited By Hacksaw on 12/01/2017 23:19:42

Buy in a plasticspur gear and go for it, if the load on the gear is light. If you wished for more thread engagement, run it while giving a blast from a hot air gun? Looks like a 20DP pinion to me.

The last thread dial I sold was then fitted with a plastic gear and worked A1.

HPC, or similar, are cheap enough.

Muffets Gears are just down the road, can you see anything on their website products pdf's? I don't know what i'm looking for ! 20 dp .Ok its diametrical pitch , but it means nowt to me ! Gawd ,this is embarrassing..

Well if it is 6 tpi - unusual that's close to 4 mod. Mod - metric gears. DP imperial. Oddly in terms of circular pitch mod often comes out the same a imperial thread pitches,

One thing for sure if you don't get the tooth count to what it should be and a pretty reasonable fit it wont be worth doing.

Plastic will probably be ok but I'd use metal if I could.

John

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Bollocks.

6 tpi can be expressed at 1" / 6 = 0.16666" between threads which is also know as the circular pitch

So if we look 0.1666 up as a CP it equates to 19 DP [ or 1.33 MOD ]

4 MOD is freeking massive, it's nearly 1/2" between teeth.

That's the problem with the internet and forums when some village somewhere has lost it's idiot.

Then the chart I have is a load of bollocks. Looking now it is. 0.167 cp is reckoned to be 18.812 dp and then 4 mod. Looks like the american that did it dropped a digit or his spreadsheet has a problem.

I'd better find another chart or next time I'm bored and not much to do make up my own.

John

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Unless you are doing a lot of long screw cutting,I found that leaving the drive engaged was far less stressful, although this depends upon how the lathe responds to quick stop and reversing.

I'd use metal if I could.

Same as JS - bollox!

Why metal?

Plastic gears are perfectly serviceable and much more easily 'reformed by heat and pressure' if needed. 'Metal' has to be a softer than one which might wear the lead screw eventually. Brass gears are expensive, particularly cut at an angle.

Delrin gears are often used in the gear train before the lead screw, so are perfectly able to withstand a low load situation, which this is - it is only an indicator, after all!

Sounds like an easy task for a 3D printer. Do you know anyone near you with one? Or perhaps a nearby hackspace/makerspace?

Russell.

Glad you included "if I could" in the quote.

John

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18 tooth 20dp has a pcd of 0.9". Get a brass or plastic one and a thin file to open and angle the gaps a bit to let it mesh. There is no need for a great fit, there is plenty of play in the correct part. All that is needed is no jumping of the gear in use. The dial has six divisions marked 1 to 6 which are 1/2" movement of the carriage relative to the lead screw so any mark will work for an even number tpi count, odd numbers only for an odd tpi count (easier to remember than even numbers for an odd tpi count).

Martin