# Knurling, math, mystery



## gheumann (Jul 2, 2017)

I understand why a part to be knurled must be a diameter that meshes with the pitch of the knurling wheels. And I've done plenty of successful knurling. But I have found that the "right" diameter is often NOT the one the math recommends, and I've ended up finding it more by trial and error. I don't understand why.

1) I have measured my knurling wheels by rolling them on a piece of paper across, say, 10" - counting the impressions and dividing by 10. I admit that this procedure produces slightly different numbers each time I try it so i do it a few times and average. These never seem to be round numbers - I've always (perhaps mistakenly) assumed the wheels were some metric pitch. Is there a better way?

2) I use this formula in Excel: =ROUND(INT(A1*B1*PI())/B1/PI(),3) where A is the "goal" diameter  and B is the LPI of the wheels.

My most recent frustration arose trying to use an 11 LPI wheel set to knurl a piece of 1.250". The math says I should turn to 1.244 before knurling. Tried it, results were awful. 1.238 worked much better. 

What's the deal? Is my LPI measuring technique to blame? Is the formula wrong? Is there a better way?


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## WoodBee (Jul 2, 2017)

I have no practical experience with your technique, so I hope someone who does chimes in. From a theoretical point of view I have this observation:
Once knurled the wheel and workpiece are in essence two matched gears. With gears not the outside diameter is what counts, but the pitch circle diameter, which is a little smaller. I guess this might cause your experience? No idea how to calculate this for knurling though...
Peter


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## gheumann (Jul 2, 2017)

WoodBee said:


> I have no practical experience with your technique, so I hope someone who does chimes in. From a theoretical point of view I have this observation:
> Once knurled the wheel and workpiece are in essence two matched gears. With gears not the outside diameter is what counts, but the pitch circle diameter, which is a little smaller. I guess this might cause your experience? No idea how to calculate this for knurling though...
> Peter




This is EXACTLY what I was wondering about. It makes more sense, and explains why some people say they just tighten the scissors (or increase cross-slide pressure) until it works - as that would change the effective pitch diameter.  I would think this would be more pronounced with coarser wheels, like the 11lpi ones I just tried.


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## 4GSR (Jul 2, 2017)

You can't go off the OD of the knurl for figuring the OD to cut the part to.  Must determine the pitch diameter of the knurl and use this to calculate for the OD to cut your material to.  The pitch diameter will be some number smaller that the OD of the knurl.  Guessing I would say, OD of knurl minus the depth of the knurl cut would equal the pitch diameter.


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## gheumann (Jul 2, 2017)

Just found this  - gave me some good ideas. He shows some pretty good technique...

Thought I knew everything...... I was wrong, as usual .....


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## Holescreek (Jul 2, 2017)

I realize that you guys are right in calculating for knurling, but in over 40 years it's never even occurred to me to think about it.  My brain might blow up next time I set up the scissor knurler!


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## gheumann (Jul 2, 2017)

Holescreek said:


> I realize that you guys are right in calculating for knurling, but in over 40 years it's never even occurred to me to think about it.  My brain might blow up next time I set up the scissor knurler!


The video above is really good. Of all the on-line videos I learn the most from this guy. Through my own experience and what he says in that video I've come to the conclusion that it is more about technique than starting diameter - although some combinations of diameter and knurl pitch MAY not work - so test first before you ruin a perfectly good part!


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## mikey (Jul 2, 2017)

One point to keep in mind is that there are two general types of knurls - circular (Joe Pi's TPI) pitch and diametral pitch. The latter is intended for use with nominally sized work pieces and the former is not. You need to know which type of knurl you're using. 

I only use CP knurls and almost never calculate start diameters, the reason being that the part being knurled is usually not nominal size. I have both a straddle knurler and a scissors knurler and both work similarly - crank down to get a pattern and go. 

We've gone over this topic numerous times and there are guys who insist on calculating diameters and guys who say they don't. Neither camp is right or wrong; use what works for you. There are many techniques for knurling and over time you develop what works best for you and your machine.


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