A Knurl Pitch Chart

[jpaul]

Recently at my club’s meeting, a question was raised about Knurl Pitches and getting knurl patterns to track repeatedly through part revolutions. It was conceded that for knurling soft materials such as 6061 Aluminum, one could “mash” the knurl die into the part until the die tracked. But what about a steel part, or Straight Knurls?

What follows is the method I use in my shop. It is not the right way or necessarily the preferred way. It is what works for me. First, knurl pitch and its relationship to part diameter. Then my method

Knurls are described by pitch, teeth angle, roll diameter and hole diameter. Knurl Pitch is simply the number of knurls (teeth) per unit length of measure. Some knurl (rolls) are labeled with the pitch or you may have to make the calculation yourself. My knurls were not labeled so I had to count the number of teeth on the roll and divide that number by the circumference of the roll. In my case, I had 3 sets of knurls with pitch sizes of 14 tpi, 21 tpi, and 33 tpi (teeth per inch). In my shop these knurls are categorize as Course, Medium and Fine, respectively.

Knowing the pitch I can calculate the number of teeth (marks) for any part diameter by applying the pitch to the circumference of the part

N = (pi ) (dia) (tpi)

If N is a whole number I am done, but what if it isn't? In that case I will have to increase or decrease the diameter of my part an amount to care for the fractional tooth. To do this I apply a plus or minus diameter factor to add to or subtract the fractional tooth. Here is the formula:

d(diameter change) = n (partial) / (pi)(tpi)

That's the math but here is what I usually do. I use my custom chart. Typically I know my part diameter and which knurl pattern , course or fine, I want. I make the first calculation. Then I decide if I want to add or subtract a partial/fractional tooth. I mentality mentally interpolate the required change in diameter to account for the fractional tooth. See the Example on my chart. Note that these are linear relationships between teeth and diameter so interpolation can be applied anywhere on the chart (see diameter 1.0 vs .diameter .010) and .375" dia = 39 knurls (FINE) same as .00375 dia = .39 knurls (FINE)

My Knurling Chart

My scissors type knurling tools.

More information

http://www.proshoppublishing.com/articles_knurling.html

 

[John S]

So your diameter is 0.308 when you start with the tips of the knurl just touching, What is the diameter at the root when the knurl is finished.

After all the tips are now at a lesser diameter than 0.308"

Can you explain this then?

A steel shaft turned down with steps of random diameter then knurled from end to end.

Why isn't the pitch ragged on various diameters ?

John S.

 

[mklotz]

Paul,

No interpolation in your table is required. Use the math that I use in the program on my page...

N = pi * D * tpi

If N is non-integerial, define 'n' as the largest integer less than 'N'. [In math, we call this the 'floor' function.]

Then, the new diameter, 'd' is given by:

d = n / (pi * tpi)

Using your example:

N = 32.4 so n = 32

d = 32 / (pi * 33) = 0.30866... ~= 0.309

John,

Knurling, as you know, is a metal displacement process. Once the knurl is tracking, small changes in the diameter, as in your rod, don't matter because the track carries over from the previous section, pushing the knurl peaks a bit higher or lower to account for the different diameter.

This is why crunching the knurling tool into the work until it tracks, your preferred method, does indeed work.

 

[kcmillin]

How do I know what the pitch of my knurler is? Or are they all common, 14, 21, and 33 (course, medium, and fine)

I have a wobbler knurler with 3 sets of cutters on it. There is no markings on it.
Kel

 

[jpaul]

KC,
knurls come in a range of pitches. http://www.mcmaster.com/#knurls/=9d26cs

My classification of Fine, Medium and Course was arbitrary based on the dies in my collection, and like you, my knurls were not labeled. To determine the pitch I counted the serrations on the die and divided that count by the circumference of the die.
pitch= number/ 3.142(Diameter)

 

[kcmillin]

Thanks Paul, I thought something like that, but wasn't sure.

Thanks Again

Kel