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Knurling

Knurling is one of those operations that give a part or assembly the final touch of functionality and elegance.  I've thought of many mods and projects for the Mini since I got it that would include knurls.  They are not limited to handles and grips but also work well for press fits and reducing components in assemblies.  Back in my automotive days I remember having valve guides knurled to give them a longer life, better lubrication and heat dissipation, so the uses are pretty vast.  You can see them on everything from flashlights to watch buttons...even most socket head cap screws have straight knurls on the heads.  With the advent of CNC machines they are everywhere now.

However, what I didn't realize is the skill and wherewithal it takes to create good knurls.  This has been quite a steep learning and skill curve to get to the point where I am generally satisfied with the results, to say the least!  After the last month or so of trials and tribulations I now have to take my hat off to the many Machinists I have known, for what it takes in practice, calculation and tooling to get a good precise knurl.

As an engineer I specified a fair number of knurls on drawings and even inspected some that needed critical dimensions.  I didn't spend much time on it other than correctly specifying the dimensions and type on the drawing and occasionally referring to the
"Old Standard", Machinery's Handbook for detail.  So, with a Mini-lathe and knurling tools at hand, I went back to the Machinery's Handbook to start my journey which had many twist and turns to get here.

What I hope to accomplish on these knurling pages is give the novice like myself, a place to tie it all together by simplifying and explaining this process.  Like Richard Bach once said, "We teach best what we most need to learn."  Please keep in mind this is written from my perspective as a relative novice...which may still be wrong. ;-)


What is a Good Knurl?

A good knurl is one that has crisp and defined roots and peaks with no deformations.  Deformations can be caused by many factors like; run-out, tool rigidity (post and holder), material rigidity (type and the way its held), and wheel quality, to name a few.  However, the key factor is the ratio of Knurl Pitch* to material diameter, to prevent mistracking on each successive revolution of the material.  Think of it like a gear set.  If the teeth on the larger gear are not the same pitch as the smaller gear, they will jam up.  In knurling the first revolution sets the tone, but if the number of teeth/Pitch on the knurl wheel are not an integer ratio (1:1, 1:2, etc.) with the material diameter, the next revolution will overwrite the previous lines, resulting in a muddled knurl.  This is because knurling is a deformation process.  It basically squeezes excess material out into the teeth of the knurl as pressure is applied.

*Knurl Pitch is the Diametral pitch of the knurl, basically the No. teeth/Nominal Dia. of knurl.  More on this later.

Knurling Tools

There are several types of holders for lathe work.  The two primary styles are the scissor type and bump type.  Mine is the scissor type on the left from Little Machine shop and comes with course, medium, and fine pitch angular pattern wheel pairs.  The Bump type on the right, comes in several styles; rigid, self centering, multiple wheel sets (like shown below), etc.


They both contain changeable Knurling wheels and come in a variety of pitches, sizes, patterns and shapes.  Most companies that manufacture the wheels provide them in two pitches: DP- Diametral Pitch and CP- Circular Pitch.  DP knurls are manufactured to tighter tolerances, based on the ANSI Standard pitches.  CP knurls are based on the circumference of the wheel and the number of teeth on it.  They are typically specified in TPI or LPI (teeth/in. or lines/in.).

Sizes are specified in diameter, width, and hole size.  The typical sizes are to mate with various holders and vary from about 5/16" to 1.25" diameter, widths from 5/32"-1/2" and hole size from 1/8"-1/2".  So knowing your tool holder is important when ordering additional or replacement wheels.

Here are the most common patterns on the market.  The straight pattern will produce a spline pattern and can be used for grip or press fits.  Notice the angular pattern wheels comes in Left & Right Handed versions.  Using one of each will produce a diamond knurl pattern in a scissor or bump holder.  The Diamond Pattern knurl wheel is typically used in a single wheel bump holder.


Shapes are typically cylindrical but also may include a 45° chamfer on both sides
to the bottom of the tooth.  They are also available in beveled, convex, male/female and other shapes.  Holders and wheels also come in inside and outside versions.

The Knurling Process

Both styles of holders are mounted in the lathe tool post.  Pressure is applied to the material through the knurl wheels to deform the material and create the pattern. The scissor type is typically applied at the top and bottom of the material (6 & 12 o'clock) by tightening the pinch knob successively until the desired depth and pattern are created.  The bump type is applied through pressure from the cross slide at a right angle to the material (9 o'clock).  The two close proximity wheels create the pattern based on centerline of the material.  My research found that the scissor type was best for the Mini because the bump type requires excessive cross slide pressure and over stresses it and the spindle bearings.  Larger lathes can handle bump type because of their robustness.

Knurling is typically a material deformation (Forming) process.  However, Cutting knurls are available, generally for CNC operations.  The triangular shape of the tooth bites into the surface of the material and with each successive revolution pushes deeper into the material, squeezing the excess material into the root of the tooth, thereby increasing the diameter of the material.  The forming process will increase the diameter of the material about the depth of the knurl tooth once completed.

First thing to do in the process is figure out is what type of knurl you want: Straight or Diamond pattern, and the pitch - course, medium, or fine.  The big challenge is determining the appropriate stock diameter to create it and what tooling you actually have to create it with.  It's difficult, primarily because there are two or three completely different schools of thought on how to do it well. 

The three schools I found are: the ANSI Standard based on Diametral Pitch (DP), the Integer Rounding method based on Circular Pitch (CP), and the Hunt and Peck method.  They are all a bit confusing, laborious and convoluted.  To my way of thinking, there should be a simple algorithm for creating good knurls...every time.  The key to all three is: what wheels you actually have.

The following pages take you through each type in some detail, and then describe what I came up with as a hybrid of the DP & CP methods with the knurls I have. 

I highly recommend going through the pages sequentially as the information in them is presented in a linear manner that builds toward understanding how I arrived at My Method.