Various examples of knurls, knurl holders and the diamond and straight patterns produced by the knurling process. Knurling tends to slightly increase the diameter of the workpiece. In the old days it was used to recondition worn out pistons!
Knurling is the process of embossing a pattern onto a rotating cylinder using a lathe. It is a brute-force method that squeezes the surface of the workpiece to conform to the knurl wheel (or wheels). A knurl wheel is a free-rotating tool with sharp teeth in either a straight or a 30 angular pattern.
Lathe-made knurling patterns are self-generating. As the workpiece rotates (at a very low speed: 50 to 150 rpm) the knurl creates grooves. With each succeeding rotation, the knurl tends to index with the previous grooves. Whether you get a fine or coarse pattern depends on the knurl pitch and how deep you force the knurl into the workpiece (more on that later).
A fine, 33-lines-per-inch knurl (left) and a coarse, 14-lines-per-inch knurl (right).
Knurl holders come in a variety of configurations. For this article I used two types: an adjustable, floating-/scissor-arm knurler, and a combination-style knurler/tool holder. The combo holder is a two-in-one tool: combining a boring bar holder with a knurling tool. The combo knurler, with a set of knurls, sells for $30 to $80. The adjustable floating-/scissor-arm knurler is from LittleMachineShop.com and includes three sets of knurls. It costs around $50.
I’ve been told that production shops favor straight knurls in angle holders to make diamond knurling and angle knurls in angle holders to make straight knurling. These reportedly produce cleaner knurling at higher rates of production. I’ve never used an angled knurl holder, so I can’t confirm the claim. I can say that, on soft materials like aluminum and mild steel, the low-cost, home-shop-grade holders—like the ones I have—are easy to set up and produce perfectly good knurlings.
This top view illustration shows how to set the tool post and knurling tool to a 3 angle.
Every knurl has a limit to how deep it can be pressed into the workpiece. On soft materials like aluminum, you can force a knurl deeper into the workpiece than the depth of the knurl pattern. All that happens is material will squish out and flake off around the edges of the knurl. If flakes clog up the knurl, use a wire brush to clean the grooves. If the knurl gets too clogged, it will affect the quality of the pattern.
Both these knurlings were embossed by the same tool. The one on the left was to the full depth of the knurl, about 0.025-inch deep, and the one on the right was embossed to about 0.018-inch deep. Your results may vary!
All three of these straight knurlings were done using the same tool. The left and center knurlings were made using two knurls in the combo holder. The knurling on the right was made with the same combo holder, but positioned for one knurl to contact the work.
Knurling, like a lot of machining operations, can be as simple or complicated as you want to make it. You could, for example, adjust your workpiece to the nearest “ideal” diameter for any given knurl. The ideal diameter is a function of the starting diameter of your workpiece, multiplied by the pitch factor of the knurl (the number of lines-per-inch x π); round the result to the next lowest whole number, and then divide by the pitch factor for a “true” OD. The diameter adjustments are often very minor (as in less than 0.010 inch), so they’re usually not relevant, especially when knurling aluminum. In practical terms, whatever the diameter of your workpiece is, just go for it. If you’re knurling a tough material like 4130 alloy steel, then it’s worth making the calculation and adjusting the workpiece diameter accordingly—but I would first try knurling the workpiece, as is, and see what happens. If the knurling doesn’t work out, simply turn off the bad knurl and redo it to the calculated diameter.
Common Knurls and Their Pitch Factors:
| Knurl LPI | Pitch Factor (LPI x π) |
|---|---|
| 33 (fine) | 103.67 |
| 21 (medium) | 65.97 |
| 14 (coarse) | 43.98 |
Example: 7/8-inch OD using 21 LPI knurl = 0.875 x 65.97 = 57.7, round down to 57 and divide by 65.97 = 0.864 OD. As you can see, the target “ideal” OD is only 0.011 smaller than the original OD.
These numbers assume you’re knurling to the full depth of the knurl. It’s possible to get a pattern to “double up” by not pushing the knurl as deeply into the part. This is something that takes a little practice, but can be useful to vary the effect or diagnose how your patterns are coming out. How “deep” to knurl? A fine knurl is 0.010-0.015 inch, a medium knurl 0.015-0.025, and a coarse knurl 0.025-0.035. Whatever depth you are knurling to, the procedure is to go all in. In other words, you move the cross slide or adjust the squeeze (scissor-type knurler) to the full depth, right from the initial contact. Deep knurling takes a lot of pressure and horse-power to keep the spindle moving under the load. My -hp bench lathe bogged noticeably as the knurl fully engaged.
One final tip before diving into the step-by-step sequence: The tool post should be angled 2 to 3 into the direction of feed (see the illustration). This helps the knurls bite into the workpiece and allows the grooves to form progressively deeper as you traverse the tool across the workpiece.
I selected the scissor-arm knurler for the photo sequence because the position of the knurls on the holder better reveals the action. The traversing procedure is the same for either style holder, as well as setting the tool to the previously mentioned 3 angle.
Use a wire brush to scrub the knurls clean (left). Turn a relief groove to define the width of the knurling and chamfer the ends.
Center the scissor arms over the workpiece (left). Touch off the knurls to “zero” pressure. Note the position of the blue pointer on the thumbscrew (right).
Using the cross slide, retract the tool off the workpiece and then tighten the tensioner about 1/3 turn (equal to about 0.020 inch on this particular knurling tool). Note the position of the blue pointer (left). Add some cutting oil to the workpiece (right).
With the lathe running 50-150 rpm and the knurls lined-up to engage of the knurl width (left), advance the cross slide slowly into the workpiece until the knurls are centered and the pattern begins to form (right).
Slowly traverse the carriage toward the headstock (left). Continue traversing slowly (right).
When the knurls reach the inside edge (left), retract the cross slide to disengage the tool (right).
Use some degreaser and a wire brush to clean the swarf (fine chips produced by a machining operation) from the workpiece. That’s it! You’ve graduated from knurling 101!
