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Harrison m250

Rack gear

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Sean Stimpson20/04/2022 07:23:19
27 forum posts
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Hi there the gear that runs along the rack for the carriage is badly warn on my m250 I can't seem to workout the mod to cut a new one mine measures 23mm and it's 12 teeth unless someone has messed around with this is anyone sure many thanks

JasonB20/04/2022 07:37:31
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Some smaller gears have an increased OD which may explain why you can't find it. Measure the length over 10 teeth of the rack and then divide by 31.42 which should give you the MOD of the rack.

DC31k20/04/2022 07:51:22
686 forum posts
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Posted by JasonB on 20/04/2022 07:37:31:

....give you the MOD of the rack.

... and be cognisant of the possiblity that it might be a circular pitch gear (i.e the rack teeth spacing are an 'easy' number of units apart).

The numbers you give are hovering around 0.200" or 5mm circular pitch, but it is not possible to draw conclusions as the low number of teeth on the gear mean it may have been profile shifted as Jason suggests.

A very good, accurate measurement of the rack is your best way forward in this case. If it does turn out to be a CP gear, it would be wise to measure more than ten teeth as that makes it easier to distinguish whether it is imperail or metric (somewhere around 8" or 200mm would do).

Nigel McBurney 120/04/2022 08:51:18
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The 3 Colcester squarehead lathes I have owned (early 1970 models) had metric pitch racks ,irespective of them being metric or imperial machines, I dont know when Colchester and Harrison came under one one ownership so there could be a chance that both makers used a common metric rack pitch. The metric rack allows a dial to be fitted to the handwheel to indicate the amount of saddle travel.

Sean Stimpson20/04/2022 20:33:48
27 forum posts
14 photos

The rack is definitely a 5mm pitch 50mm 10 teeth 100mm 20 15mm 30 teeth etc etc

The mod for that works out at 1.59 is this a special cutter or is it a mod 1.5 little suspicious thanks

JasonB20/04/2022 20:37:23
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That would be 5mm metric circular pitch, though you may get away with 16DP which is very close.

Andrew Johnston20/04/2022 20:55:30
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Posted by Sean Stimpson on 20/04/2022 20:33:48:

...mod for that works out at 1.59 is this a special cutter...

It's a special cutter. If the circular pitch is rational then by definition the module will be irrational as it involves a factor of pi.

Andrew

Pete Rimmer20/04/2022 22:45:51
1233 forum posts
65 photos
Posted by JasonB on 20/04/2022 20:37:23:

That would be 5mm metric circular pitch, though you may get away with 16DP which is very close.

Not just very close - VERY close - half a thou per pitch which is the same as nothing at all for the application. Cut a 12T 16DP gear.

In theory, your 16DP 12T gear should have an OD of 22.23mm plus the long addendum adjustment of 1.98mm to give 24.21mm diameter but they might not have added so much addendum due to it being engaged with a rack I dunno. If you are confident that your OD measurement is correct and hasn't been affected by wear, cut a pinion of that diameter and cut 12 teeth of 16DP into it.

JasonB21/04/2022 06:57:07
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Just watch the pressure angle which is likely to be 20pa as some of the cheaper DP cutters are 14.5pa.

For future use this is the table that I use when comparing DP, MOD and circular pitches.

DC31k21/04/2022 07:47:13
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There's a manual here: http://vintagemachinery.org/pubs/detail.aspx?id=18429 and on page 35 (904/2) it shows the pinion as integral with its shaft (so it is a kind of blind gear). That might have influenced the measurement they chose for the OD.

If you can measure the centre distance somehow (pitch line of the rack is halfway up the tooth), that might give some more information to ponder upon. An easy measurement is to go from the shaft centre to the rack tip, just to give you an idea of the absolute maximum root diameter that your gear can be.

Given that you are now faced with the challenge of stopping the new gear rotating on its shaft (key and keyway?) and securing it laterally (space on its inboard side for the projecting head of a fastener), the further from centre you can make the tooth spaces start, the more options you have.

I would also gang up three blanks and cut three gears, then you have spares for the future or in case something gets spoiled.

Pete Rimmer21/04/2022 07:57:09
1233 forum posts
65 photos
Posted by JasonB on 21/04/2022 06:57:07:

Just watch the pressure angle which is likely to be 20pa as some of the cheaper DP cutters are 14.5pa.

For future use this is the table that I use when comparing DP, MOD and circular pitches.

I hav e the same chart on my wall laminated in A3, it's one of the handiest charts I know and so well laid out.

Howard Lewis21/04/2022 16:05:11
6104 forum posts
14 photos

If this is any help.

When I damaged the rather soft rack gear on my lathe, rather than replace with the "Genuine article". I cut my own 13T 1.5 Module gear, with a short integral shaft. This was Loctited into, and pinned to, the 16 mm Silver Steel shaft. The gear / shaft assembly was located relative to the apron by a brass spacer bush.

Howard

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