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Cutting Clock Gears


Registered User
NAWCC Member
Apr 8, 2016
I've been diligently using Robert Porter's excellent "The Clock and Watch Makers Guide to Gear making" to both educate myself on the subject, and also to construct the clock that he has included the design for in the book. Overall, it's a great book and a fine introduction to this dark art, however I'me stumbling over a detail and hope someone can help me out.

To cut a gear you do, of course, first have to acquire the correct gear cutter, and he explains how to do fabricate them from scratch, and I've encountered few problems with this. He then goes on to include the necessary info regarding the actual cutting of the gears. My problem is that in some cases, he calculates the cutter profile length as being shorter than the cutter working depth. Specifically, in page 95, for the 5 minute pinion he specifies the cutter profile length as 0.052" and the cutter working depth as 0.068".

I'm sure it's me being dimwitted about this but I can't understand how you can make a cutter of a given profile length then expect it to cut deeper than that when actually making the gear - am I missing something?

Any help will be most appreciated as I'm at a total standstill with this at the moment.

Best Regards

Roger Adams
Edmonton, Alberta, Canada

John MacArthur

Registered User
NAWCC Member
Feb 13, 2007
This does seem a little excessive, but in most cases the cutter profile length is somewhat less than the working depth. This ensures that the teeth will have a sharp tip, as opposed to an uncut flat on the outer end. It also ensures that the tips and working faces will be absolutely concentric with the center hole. In other words, if there were a very slight eccentricity of the edge of the disc, it will be completely removed during cutting. Best of luck, and keep us posted.


Staff member
NAWCC Member
Oct 19, 2005
North Carolina
Let's see if we can attract @Jerry Kieffer over here to add his expertise ;)

Jerry Kieffer

Registered User
NAWCC Member
May 31, 2005
Let's see if we can attract @Jerry Kieffer over here to add his expertise ;)
Per Shutterbugs request.

While I are not familiar Robert Porters book, every wheel and pinion cutters profile needs to be longer than the tooth cavity that it is cutting. The reason for this is that the cutting depth is not based on the cutter length, but observing when the depth is such that the tooth is fully formed. Once one tooth is fully formed, that depth is used to cut the rest of the teeth.

An example of this can be seen in the attached photo where I machined a cutter to machine ten replaced teeth to match the rest of the existing teeth. The cutter is actually longer than the tooth but required to properly form the tooth.

Jerry Kieffer


Joseph Malpeli

Registered User
NAWCC Member
Mar 23, 2015
Roger's difficulty can be resolved by examining Porter's figure on page 14, which I have attempted to attach (let me know if it doesn't show up). In this figure, the "cutter working depth" is the distance between the major diameter of the wheel (the circle that touches the tips of the teeth) and the space between teeth at the root diameter. The "cutter profile length", on the other hand, has nothing directly to do with the tooth dimensions, but is related to the overall width of the cutter. You can see that for any given tooth configuration, the cutter profile length will increase as the overall width of the cutter increases, and as long as the cutter spans the distance between two adjacent tooth tips, its width is irrelevant to the configuration of the teeth being cut. The reason he gives this figure, however, is that it's a critical consideration when the form tool is used to make the cutter, because it's used to determine how far to advance the form tool forward before moving the lathe carriage laterally toward the headstock to machine the cutter. For these reasons, sometimes Porter's working depth is larger than the profile depth, and sometime it is smaller, which probably adds to the confusion.

In Jerry's picture, you can appreciate that if he widened the portion of the cutter that forms the addendum radius, it would not affect the tooth dimensions, although it would increase the cutter profile length.

Porter's book is very carefully written, and I have yet to identify any errors in it. I've made form tools and cutters using his methods, and find his explanations are very complete. However, you may want to also look at Malcolm Wild's book "Wheel and Pinion Cutting in Horology", which goes into great depth on the theory and practice of gear cutting. There are two major standards for cycloidal gears used in clocks, Swiss and British. The British standards are probably more relevant (especially if you don't read French), and with some minor exceptions, this are the standard followed for Thornton cutter. The relevant British standard is BS 978 Part 2, which you can find on the internet, or in Wild's book, along with additional explanations. Wild also shows how to make four-point cutters with relief.

Joe Malpeli

Cutter configuration.jpg
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