Churlish Knurls
July 22nd, 2009DVDs Deceive
Man, I am irate. I have two sets of lathe DVDs, and neither one has the right information about knurling.
As I noted earlier, I did not understand how the machinists in the videos got the knurls to track in the same cuts, over and over as they turned the workpieces. When I do it, on successive turns of the workpiece, the knurls make new marks overlaying the marks made on the earlier turns. You don’t get a sharp diamond pattern this way. You get a mess. The guys in the videos say nothing about this. They just knurl away, and everything comes out perfect.
I just found an article which explains it. Apparently, you have to measure the distance between the ridges on the knurls and then turn your work to a circumference that is a multiple of that distance. Why don’t they mention this in the videos?
The more I read about this, the more I see that I don’t know anything. I’ll quit until tomorrow. But I’m pretty sure I should get a scissor-type tool.
July 22nd, 2009 at 9:08 PM
The knurl/circumference thing is kind of tribal knowledge. And it’s not completely a science either, in roll knurling- cut knurling is different.
Most people will do a test knurl, and if it’s a little off, take a thin pass over the area to be knurled. Remember you only have to reduce the circumference by a fraction of the distance between two points on the knurl. Sometimes a bit of sandpaper will be all it takes to do it. And if you’re close, and the material is soft, you can just push in a bit harder and the knurl will find it’s home. Then just crank the feed until you’ve got what you want.
July 22nd, 2009 at 9:53 PM
“But I’m pretty sure I should get a scissor-type tool.”
Run with it!
Cheers
July 23rd, 2009 at 6:42 AM
It’s like making a pair of gears of a different diameter match up…the distance between the teeth on each has to match and thus you can’t have less than some number of “full teeth” on your workpiece thus if the knurl tool has a 0.05″ pitch the workpiece diameter must be 0.50″, 0.60″ (not 5/8″), 0.70″ (not 3/4″), 0.80 (not 7/8″), or 1″ else you get cross tracking.
Obviously the smaller the workpiece diameter the tighter the knurl pattern, and on the courser knurls you can run a file or some coarse sandpaper across the top when finished knurling to smooth things over a little and improve the feel in the hand.
July 23rd, 2009 at 8:26 AM
It’s a long time since I did any knurling, once I’ve made a tool I’ll be able to try some experiments. This article has an interesting comment
“Longitudinal feed must be according to spiral?knurling pitch to avoid overlapping.”
http://www.gtz.de/en/dokumente/en-metalwork-knurling-course-vocational-training.pdf
Here’s another useful link
“Once the diamond pattern is visible the power feed is engaged at a slow rate. .003-.005/revolution”
http://www.practicalmachinist.com/vb/sitemap/index.php/index.php/does-knurling-work-t110664.html
July 23rd, 2009 at 9:03 AM
Knurling: nobody explained it because it seems obvious to them.
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Actually, what you need to know is the diameter of the knurling tool and the number of cutters on it. Then you adjust the diameter of the work in the same increments. Doing it that way lets you avoid having to deal with pi.
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Example: the knurling tool is 1″ in diameter and has 24 cutters. The diameter of the work then must be in units of 1/24th of an inch.
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You can (in fact must) adjust that, because the diameter of the knurl cut at the bottom is less than the diameter of the remaining metal and the diameter of the tool at the tips of the cutters is more than the diameter of the tool at the base of the cutters. If you want a deep knurl, you measure to the bottom of the cutters. If you want a shallow one you measure nearer the cutting point(s). If you measure at the cutting points, you get a “kiss” that doesn’t cut the work.
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Once you “get it”, the concept of “pitch diameter” for gears will be clear to you, which is one of the reasons they teach knurling in the first place.
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Regards,
Ric
July 23rd, 2009 at 9:05 AM
“Knurling: nobody explained it because it seems obvious to them.”
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So they made instructional videos about it? Okay.
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“Once the diamond pattern is visible the power feed is engaged at a slow rate. .003-.005/revolution”
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Far as I can tell, there is no way to determine the speed of the power feed on my lathe. The threading feed has measurements listed on it, so I suppose that is what I would have to use.