hammerscale

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Archive for June, 2013

A quick primer on knurling

Posted by hammerscale on June 10, 2013

Knurling is often a vexing issue as well as a lathe operation prone to contentious debate.

In no particular order, please take note of the following tips/advice:

We’re talking embossing a pattern on steel, brass, whatever; NOT cutting a knurl pattern. In a typical setup, knurl wheels are either pressed into the piece at 90 degrees to the work piece axis (bump knurling), or better, a scissors type knurl holder is cranked down to emboss a pattern with the knurl wheels 180 opposed from one another.

Diameter is irrelevant. I say again irrelevant. It would make sense to sync the pitch of the knurl pattern with the diameter of the work piece if the knurl wheels where travelling at a constant, predictable speed. However, knurl wheels are free spinning; they can speed up, slow down and even reverse direction depending on what forces they encounter. So it’s pretty easy for a knurl wheel to slip into an existing pattern as it is formed while the wheel travels the perimeter of the piece on the lathe.

Make RPMs slow. Even to the point of using backgear to slow down the spindle. Trying to impart a pattern on a work piece spinning too fast is akin to jumping on a treadmill that’s going 10 mph. Bad things will happen.

Make longitudinal travel fast. Fast travel equals less flaking and less of a tendency for the knurl pattern to wander, especially noticeable on straight knurls.

Use sharp knurl wheels. Sharp as in new,- obviously you can’t sharpen knurl wheels. When they reach a certain level of dullness after prolonged use they must be discarded. I prefer cobalt knurl tooling, as import HSS is probably not that high a quality material.

A “cheat” technique is to tilt the knurl wheels slightly into the work piece, maybe 3 to 5 degrees off the axis. This allows a deeper, surer emboss without excessive pressure. Not applicable with a scissors knurl tool.

Make sure both work and knurl wheels are clean, any chips from previous ops, flaking, etc. can affect the final outcome.

Lubrication. Take care here, often you’ll find advice saying “use alot.” That’s machinists’ speak for “I’m bluffing my way thru this interview.” Use just enough to insure smooth spinning of the knurl tooling. Too much lube tends to accumulate and hold gunk (swarf) which can disrupt a clean knurl from forming.

The biggest problem when knurling is double cutting. That is, not forming the primary knurl pattern, but the pattern skipping and tracking over itself creating a light, fuzzy texture. Solutions: more pressure, slower rpms, or replacing dull knurl tooling. Also consider using a scissors type tool or just taking a break and trying again later !

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