This page contains the P.P. Thornton chart for gear and cutter calculations. It provides every
formula needed for gear cutting. This chart is invaluable for determining sizes of both gears
and cutters. You can print this page from your web browser or download the PDF to print and
keep. If you do not have Adobe® Acrobat installed on your
computer, please download it at http://www.adobe.com/products/acrobat/readstep.html.

 

 

 

Table of Addendum allowances

Number
of leaves

Ogive

Ratio
Tooth/Space

Add to number
of teeth or leaves

6

Full

1/2

1.71

7

Full

1/2

1.71

8

Full

1/2

1.71

10

1/3

2/3

1.61

12

1/3

2/3

1.61

16

1/3

2/3

1.61

Wheel

—

1/1

2.76
For Cutting Pinions for old work with thicker leaves, use cutter 0.05 module smaller than calculated. The addendum allowance is unchanged.
Recommended cutting speeds
For cutting carbon steel pinions with high speed steel cutters:

Cutter diameter 14mm 430-500 rpm
Cutter diameter 20mm 300-350 rpm
Cutter diameter 24mm 250-290 rpm
Cutter diameter 26mm 230-270 rpm

A copious stream of cutting oil should be used to keep the cutter cool and remove the chips. Mobilmet 745 or similar.

Pinions should never be cut dry.

For cutting brass a speed of up to 3-4000 rpm can be used, but a much lower speed will preserve the tooth cutting edges.

 

 

 

 

   Recommended brass and steel
BRASS for wheels should be hard or half-hard leaded engraving brass such as CZ 118, CZ 119, or CZ 120, with a hardness of 120-130 VNP Do not use soft brass which will clog the cutter and spoil the work.
STEEL Clock pinions may be cut from silver steel which HAS BEEN FULLY ANNEALED (it is supplied in normalized condition) but steels with a lower carbon content such as EN8 or EN9 are to be preferred.
  The profiles are made in accordance with BS 978 Part 2, NHS 56702/3 and have square bottoms, also NHS 56704 with round bottoms. These cutters can be sharpened without loss of form. For suitable grinding wheels see right.

The cutters are relieved using profile ground solid carbide tools; the finish on the form of the cutter teeth is therefore of a high quality.

All cutters are made from 8% Cobalt M42 (HSS-E) high speed steel hardened and treble tempered under vacuum, there is therefore no change in the surface chemistry of the tool. This steel is of first quality, and is known for its ability to keep a good cutting edge to give long duration between sharpenings. Hardness circa 68 ROCKWELL C.

 

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Profile Dimensions For Clocks & Watch Wheel & Pinion Cutters

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