Knurled surfaces are commonly seen on cylindrical parts such as handles, knobs and rollers. Knurling is a process that creates a pleasing, easy-to-grip geometric pattern on a part's surface.

When knurling, machinists apply a tool with two freely rotating rollers that deform, or push, the material rather than cut it. Creating a crisp, clean knurl on a lathe is easy if you follow these suggestions.

Some machinists get into trouble when they try to start a knurl over an arbitrary diameter and find that the knurl doesn't track properly by repeating itself in the previously laid grooves.

A proper diameter to knurl is any diameter that is a multiple of the spacing, or distance, between the teeth of a knurling tool divided by π (3.14159). The relationship is the same whether the knurl is diamond or a straight knurl. However, you must measure the spacing of a diamond knurl's teeth along the axis of the part or roller for the relationship to hold true.

Suppose you want to impress a diamond knurl on a 1"-dia. shaft and the distance between each tooth of the knurling tool measured, with calipers, along the roller's axis is 0.060". The measurement is not extremely critical in that ultimately you will determine the final diameter by trial and error. However, the measurement will give you a decent starting point.

Following the formula stated previously, 0.060 divided by 3.14159 is 0.019. Therefore, any multiple of 0.019 should provide a diameter for a perfect knurl. For example, 0.019 times an arbitrary number such as 40 equals 0.760. In theory, then, if you turned a shaft to 0.760", you should be able to create a perfect knurl.

However, because you want to knurl a 1"-dia. shaft, you have to find a multiple of 0.019 that gets you close to 1", such as 52 (0.019 × 52 = 0.988). Based on experience, though, the chances of producing a perfect knurl on the 0.988"-dia. shaft are not great. An error occurs as a result of an imperfect measurement between the teeth of the knurling tool, which isn't a big deal because ultimately you are going to sneak up on a usable diameter. Therefore, begin by machining the part about 0.010" larger than the calculated diameter.

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Once you've machined a diameter that is a few thousandths of an inch larger than the calculated diameter, you can begin testing.

Mount the knurling tool approximately perpendicular and on center to the workpiece. It is not critical that the tool be exactly perpendicular or exactly on center.

Start a test knurl by lightly pressing the rollers of the knurling tool against the workpiece. Start knurling close to the headstock because that is where the workpiece is most rigid. Rotate the lathe spindle slowly by hand as you watch the pattern being impressed on the workpiece.

After you complete one turn of the spindle, the pattern will start to repeat itself on the workpiece. If the grooves impressed on the part near the beginning of the second rotation don't line up with the grooves impressed on the workpiece from the first rotation, that's an indication you'll have to adjust the workpiece's diameter.

Machine 0.002" off the shaft's diameter and perform another test. A little over one turn is sufficient to see if you have a winner. Once you find a diameter where the grooves fall on top of each other, record the diameter and proceed.

Put some pressure on the knurling tool and start feeding the tool slowly along the length of the shaft. Feeding slowly on the first pass helps maintain proper tracking of the rollers in the previously laid grooves.

Start with slow to moderate settings and then increase them as you see fit. A lot depends on part rigidity and setup. In areas where the knurl comes in shallow, you can dwell and concentrate the tool in that area to help balance groove depth.

Because knurling is a material-displacement operation, it's best to use a lubricant that aids in slipping-not cutting. You want the knurling tool's teeth to slip back into the previously laid grooves. Additionally, select an oil that doesn't retain the flakes of material that inevitably are created during knurling.

Blowing a hard stream of air over the rollers and workpiece during knurling helps prevent those flakes from being pressed into the knurled surface.

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A sharp knurl can be uncomfortable to handle. File or sand the knurl to adjust for the desired feel and diameter. Finish the knurl using a wire brush to remove burrs and flakes. ∴

This ProShop Machinist Article is adapted from information in the book "Machine Shop Trade Secrets: A Guide to Manufacturing Machine Shop Practices" published by ProShop Publishing. A special thanks to 'Cutting Tool Engineering Magazine' for teaming up with James A. Harvey to produce this article.