Thomas Industrial Library

A Note About Knurling

As many of you know, knurled parts are used in wide variety of products. Knurled surfaces are commonly seen on cylindrical parts such as handles, knobs and rollers. Knurling is a process that creates a pleasing, functional, geometric pattern of some sort on the surface of a part.

Knurling is accomplished by using a tool that uses two freely rotating rollers that deform or push material rather than cut it. Creating a crisp, clean knurl in a lathe is easy if you employ a few of the following suggestions. (See Fig. 8-14)

F IGURE 8–14 A clean knurl is produced.

Understand the relationship between a knurling tool and the diameter of the part to be knurled.

This is where some people get into trouble. They try to start a knurl over an arbitrary diameter and find that the knurl doesn’t track in the previously laid grooves.

The relationship is as follows:

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 PI . (3.14159)

The relationship is the same whether the knurl is a diamond or a straight knurl.However, the spacing of the teeth of a diamond knurl must be measured along the axis of the part or roller for the relationship to hold true.

Let’s do an example:

Suppose you want to impress a diamond knurl on a one inch diameter shaft. Suppose also that the spacing or distance between each tooth of the knurling tool measured along the axis of the roller is approximately .060 9 . You can measure the spacing with calipers. The measurement is not extremely critical in that ultimately your final diameter will be determined by trial and error. However, the measurement will give you a decent starting point.

According to the above relationship, if we divide .060 by PI (3.14159) we get .019.

Accordingly, any multiple of .019 should give you a diameter that would give you a perfect knurl. For example: .019 times an arbitrary number such as 40 equals .760. In theory then, if you turned a shaft to .760 9 you would be able to create a perfect knurl.

However, since we want to knurl a shaft that is approximately one inch in diameter we have to find a multiple of .019 that gets us close to one inch. After a little trial and error and playing around on a calculator we find that .019 times 52 equals .988 which is close to one inch. .988 then would be a good theoretical starting point.

In practice though and from experience the chances of getting a perfect knurl on the .988 shaft diameter are not great. The error happens as a result of an imperfect measurement made between the teeth of the knurling tool which is no big deal anyhow because ultimately you are going to sneak up on a usable diameter. Begin by machining the part about .010 9 larger than the calculated diameter. Let’s proceed.

Lay a short test knurl by hand. (See Fig. 8-15)

F IGURE 8–15 A test knurl is laid by hand to see if the diameter produces a knurl that tracks in previously laid grooves.

Once you’ve machined a diameter that is a few thousandths larger than the calculated diameter you can begin testing.

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

Start a test knurl by lightly pressing the rollers of the knurling tool against the work. I like to start knurling close to the headstock since that is where the part is most rigid. Rotate the lathe spindle slowly by hand as you watch the pattern being impressed on the work. As one turn of the spindle is completed 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 part from the first rotation, that’s an indication that you’ll have to make an adjustment to the diameter of the work.

Machine a couple of thousandths off the diameter of the shaft and do 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. The rest is easy.

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.

Use feeds, speeds and pressures you feel comfortable with.

These parameters are not overly critical. Start with slow to moderate settings then increase them as you see fit. A lot depends on the rigidity of your part and setup. In areas where the knurl comes in shallow you can dwell and concentrate the tool in that area to help balance the groove depth.

Use air and lightweight lubricating oil for knurling.

Since knurling is not a cutting operation, it’s best to use a lubricant that aids in slipping not cutting. You want the teeth of the knurling tool to slip back into previously laid grooves.

I like to use lightweight oil such as WD -40® so the oil doesn’t retain the inevitable flakes of material that are created during the knurling process.

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

Run a smooth file over the top of the knurl to remove sharp points.

A sharp knurl can be uncomfortable to handle. File or sand the knurl to adjust for the feel and diameter you want.

Finish the knurl using a wire brush to remove burrs and flakes. (See Fig. 8-16)

F IGURE 8–16 A wire brush is used to remove flakes of material that have pressed into the knurl.