Knurling Question

[prasad]

Guys

I attempted my first knurling operation. I measured the knurl pitch accurately by running on a paper and measuring the marks with caliper, etc. Made all related calculations for the desired diameter I must use before trying to plunge the knurl. And finally plunged the knurl in after setting the roller to be parallel to the piece mounted in the chuck and speed was set to the lowest available (130 rpm). The result was no good. Please see the picture below.
Where did I go wrong?

Lathe used: G4000 by Grizzly, set at 130 rpm
Tool used: Straight single roller knurling tool,type H5936 from Grizzly. See below.


Thanks
Prasad

 

[higgite]

Did you flush the work with oil to remove the cuttings during the knurling operation?

Tom

 

[dlhoulton]

To my eye that looks like a good "Straight Line" knurling. Were you trying for something else?

 

[prasad]

Did you flush the work with oil to remove the cuttings during the knurling operation?

Tom

Yes, I did. I used WD40.

 

[prasad]

To my eye that looks like a good "Straight Line" knurling. Were you trying for something else?

Tom,

Thanks and I am glad if you really say so. Yes I was trying straight line finish. I am not happy with the appearance of the grooves. They look kind of rough to me. I thought something went wrong and I could not detect it. I have seen similar knobs and they look much finer in finish.

I guess I will try again soon and see if I can change some process.

Thanks again,
Prasad

 

[Bill C.]

Be sure to use a lot of cutting fluid. When I was cutting threads I used a wire brush to clean the grooves might give that a try.

 

[T Bredehoft]

Try going deeper, and feeding back and forth, to let the tool cut in as it moves sideways.

 

[Wreck™Wreck]

Knurling in a lathe is not a science in my experience, a great deal of measuring, calculating pitch diameters and adjusting part diameters does not seem to help. If you require 1 knurled part make 3 or more and one is likely to turn out nicely. In a production environment one may scrap dozens of parts before getting it right.

From past experience I may offer but one bit of advice, when using a bump tool as pictured, run the tool into the part as fast as you can, and by this I mean Scary Fast, this gives the knurl less time to double track which is the problem that you are having. If you do not have a physical stop on the cross slide for such purposes then make one, you are a machinist after all.

I did these parts several months ago, ran a test part on the same piece of stock and it worked fine, ran the first actual part and it double tracked which is unacceptable to the customer, the second part was perfect and the 5th was awful so I kept bumping up the infeed rate until 1 in 10 were right. This is on a CNC lathe so changing infeed rates is a matter of changing numbers with a key pad.

 

[mikey]

Bump knurlers usually require a good deal of rigidity in both the lathe and the set up due to the way the tool is used - you're pushing from one side only and deflection is going to be a problem. The normal solution for this is to always use a live center for support, and if you were not using it in this case then try it again with a live center in place. Then you can increase pressure so the edges of the knurl don't look all chewed up.

I would also recommend you use a good cutting oil, even on Aluminum. Knurling is not cutting; it is displacement and generates higher temperatures than you might think. Even your hardware store sulfur-based tapping fluid will work for this.

Bump knurlers normally require higher speeds than you're using to track well. I normally knurl at 50% normal turning speed for the material. I haven't used a bump knurler in a long time but I know I was turning at higher speeds than what you're using here. Maybe give it a try?

I prefer a scissors knurler. It allows for much higher knurling pressure, does not stress the spindle bearings, does not deflect the work piece, can be used at higher speeds and produces a really nice knurl whether you calculate diameters or not (in most materials). Below is a knurl done with a shop-made scissors knurler on the end of a 3/8" OD piece of aluminum without a tailstock support:



I'm not sure what this would have looked like with a bump knurler, though!

 

[Doubleeboy]

Just an aside, bump knurling is hard on your lathe, If you worked in a shop on a high precision lathe like a HLVH or similar and the shop owner saw you bump knurling you would be gone in a heartbeat. I have a couple bump knurlers, neither have been used in decades, scissors are much kinder to your machine.

michael