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Russell Eberhardt11/01/2019 09:51:50
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Posted by Andrew Johnston on 10/01/2019 20:16:33:

Ah, I'd place a small bet it's an RDG cutter?

Most clockmakers stick with Thorntons cutters, expensive but good. I buy mine from Meadows and Passmore and never had a problem. Most clock designs for amateur construction specify cycloidal cutters. You might have difficulty sourcing involute gear cutters in the appropriate MOD.

Russell

David Noble11/01/2019 11:06:41
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Thank you all for this info' I am getting there an allied question is 'what is the recommended way to work out the tooth depth'? I have had a look on-line and found various answers. Some give a fixed depth based on my cutter of MOD 0.75 and some vary the answer by using number of teeth.

Andrew Johnston11/01/2019 12:00:23
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Posted by David Noble on 11/01/2019 11:06:41:

Some give a fixed depth based on my cutter of MOD 0.75 and some vary the answer by using number of teeth.

Which just goes to show you can't always trust the internet. The real skill is in determining who is an expert, and who is an ex-spurt. smile

I'll give the cutting depth details for gears defined by diametral pitch P, as I've got the numbers in my head. Shouldn't be difficult to translate to MOD defined gears. For involute gears the depth of the cut is determined solely by the diametral pitch; it is not dependent upon the number of teeth. Logical if you think about; otherwise a large gear and small pinion may not mesh properly because the tooth depths are different.

The depth of cut is the sum of two parts, one above the pitch circle and one below. Above the pitch circle is the addendum, defined as 1/P. Below the pitch circle is the dedendum, also defined as 1/P. However, also below the pitch circle, some clearance is needed at the bottom of the space. The clearance is arbitrary, but two common values are 0.157/P or 0.25/P. The total cut depth is the sum of the addendum, dedendum and clearance, so either 2.157/P or 2.25/P. A value of 2.157/P is the more common. The total depth of cut should be marked on a quality cutter. If not then assume the lower value and either measure the tooth width at the pitch circle with a gear vernier, or see how they mesh compared to the calculated centre to centre distance. As an aside my experience of gear verniers is that they're rather difficult to use, especially as the teeth get smaller so I no longer use mine.

Andrew

JasonB11/01/2019 12:08:04
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I tend to look at HPC who as well as selling gears have a handy table of how to work out various things gear related.

So for your MOD 0.75

Addendum is 0.75mm

Dedendum is 1.4 x 0.75 = 1.05mm

Total depth is the two added together so 0.75 + 1.05 = 1.8mm

Or you can add the depth constants together first to get 2.4 and use total depth = 2.4 x MOD

NOTE they do give different depths depending on DP or MOD with smaller teeth not being cut deeper

 

Edited By JasonB on 11/01/2019 12:15:11

Michael Gilligan11/01/2019 12:31:08
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Despite the [hopefully] interesting assertion that I to linked yesterday: One of the advantages of cycloidal gearing for clocks is that its tooth depth is non-critical ... The wheel centre distances are optimised when the train is planted.

MichaelG.

Andrew Johnston11/01/2019 12:44:58
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The ISO standard for tooth depth on MOD gears is 2.25 times the module. So for 0.75Mod that gives 1.6875mm.

Call it 1.69mm. smile

Andrew

JasonB11/01/2019 13:09:52
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That ties in with what HPC give for the larger gear sizes Andrew, is any mention made for smaller ones?

Andrew Johnston11/01/2019 13:49:13
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Small Mod value gears, or small numbers of teeth?

Andrew

David Noble11/01/2019 13:49:24
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Brilliant. Thank you so much.

JasonB11/01/2019 14:13:32
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Small MOD values, HPC give 2.4 times MOD for 0.25 - 1 MOD and 2.25 x MOD for 1.25 - 8 MOD

Not corrected PCD or OD for small numbers of teeth

Andrew Johnston11/01/2019 14:32:03
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Posted by JasonB on 11/01/2019 14:13:32:

Small MOD values, HPC give 2.4 times MOD for 0.25 - 1 MOD and 2.25 x MOD for 1.25 - 8 MOD

In that case no - I don't see why the total tooth depth equation should vary with MOD value? Surely different MOD values are simply scaled versions of one another.

Andrew

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