A Problem with Knurling

clevinski

Active User
Registered
Joined
Apr 26, 2012
Messages
294
Hello, All,


I made a couple of aluminum knobs yesterday, and wanted to put the coarsest, straight knurls I could on them. My problem is that, when done, instead of pretty, smooth knurls, the material surface itself looks pitted and chipped, as you can see in the photo below. I have made several nice aluminum knurls in the past, but used the finer wheels and those were cross-hatched, not straight.

I was knurling aluminum, using the coarsest, straight knurling wheels I have (14 lines per inch). I have a "pinch-type" knurler from LMS; see below. I cut the outer diameter of the part to 0.978 inches, which makes the circumference in inches evenly divisible by 14, so that the knurl lines should line up after a full revolution. I started with a very light pass and with the wheels about 50% engaged, and moved slowly across the part by hand. I used old-school, dark cutting oil (mineral oil) as the lubricant and oiled the crap out of the surface and the tool during the process. I wiped the part after each pass and used a chip brush, but it wasn’t too effective at removing all the aluminum particles. You could see that the oil was basically a mix of thousands of small aluminum particles and the dark oil. In fact, after each pass, the lubricant was more silver than dark, in spite of my adding oil as we went. After each pass, I would tighten the knurling wheel some more and reverse direction. I was using a Real Bull 7x14 mini-lathe from Big Dog Metals.

I suspect the issue is that there was too much aluminum powder from the knurling process in the lubricant, and it essentially ground the knurls, but I don't know what to do about that as I was already oiling it very heavily.

Any suggestions would be appreciated.


Knurls w Bad Finish.jpg
knurler.jpg

Knurls w Bad Finish.jpg knurler.jpg
 
Possibly try compressed air and coolant, another thing we used to do was tip the knurls slightly so they were cutting mostly on the leading edge as opposed to the full width--feeding along of course. I suspect the oil is acting like glue, I've never used oil when knurling aluminum.
 
Lots of pressure and one single pass over it works better for me than trying to be nice to it and adding pressure over multiple passes.
I don't use any lube on aluminum when knurling it either.
I have the same knurling tool. Much of the time I will put it over the part and tighten it down hand tight then retract the cross slide turn the tool another 1/4 to 1/2 turn tighter then advance the cross slide back to centered over the part and then run the lathe. Vary the look of long knurles by changing the carrige travel speed.
 
It could be that you just went to deep also. I have done the same thing on brass b4. The knurl would look great then id feed in a bit more and my finish would strart getting flackey.
 
The pitch of the knurl has to go into the circumference of the part evenly with coarse knurls. Otherwise the knurl will overlap and create a mess like you are seeing. It's not as critical with fine knurls because they tend to index without issue, but coarse ones can be a pain. For example: If you had a 1.512" diameter part, the pitch of your knurl (1/14=.071) would go into the circumference of the part 66.5 times (pi times 1.512 divided by 1/14 = 66.5), so you would need to adjust the diameter up or down one half the pitch to 1.500 or 1.524 to get a good knurl. It doesn't seem like much but it can make a huge difference in how your knurl will turn out.

Tom
 
Thanks, Guys!

My bad, I guess... I thought that you always used lubricant when knurling anything. Come to think of it, when I knurled the finer, diamond pattern parts, I used WD-40, which is very thin and light. This dark cutting oil is very thick, so maybe I should try again dry. I assume that you guys use a cutting fluid when knurling steel?

Eightball, I think you're right, and I went too deep at the end, but I would check the appearance of the knurl after each pass and the rough surface was always there. You know you went too deep when the knob doesn't want to tighten any more.

Thanks for all the input, guys! I'm going to use these for now, since they're parts for a lathe stop I'm making, but I will probably remake them in the future on a slow machining day...
 
The pitch of the knurl has to go into the circumference of the part evenly with coarse knurls. Otherwise the knurl will overlap and create a mess like you are seeing. It's not as critical with fine knurls because they tend to index without issue, but coarse ones can be a pain. For example: If you had a 1.512" diameter part, the pitch of your knurl (1/14=.071) would go into the circumference of the part 66.5 times (pi times 1.512 divided by 1/14 = 66.5), so you would need to adjust the diameter up or down one half the pitch to 1.500 or 1.524 to get a good knurl. It doesn't seem like much but it can make a huge difference in how your knurl will turn out.

Tom

Hi, Tom,

Thanks for your comments. Yes, I know how critical the diameter is from my research before I started knurling. I actually made an Excel spreadsheet for the exact diameter for knurling each common, nominal diameter material when knurling with the three different line pitch wheels that I own. That's why I made this part 0.978 inch diameter. (Well, 0.9777 would have been more accurate, but I figured accurate to .0003 would be close enough.) .9777 x pi = 3.0715. 3.0715 inch circumference X 14 lines/inch (same as dividing by 1/14) = 43.001 lines per revolution, an error of only 0.1%. I would think that was accurate enough, but this is the first time I've tried to do a coarse knurl, so I could be wrong. What do you think?
 
Not sure if its just me, but last week I knurled alluminum for the first time (foot pegs for a CX650) and even though I set the diameter right, it did a similar "powdering up". I tried again with coolant (water based milky stuff) and it made a huge difference, as i used a spray bottle (with a thin stream) to "blow" the crud out as it went.
 
I like to use my kool mist system when knurling. Even though i have coolant on my lathe, I can set my koolmist nozzle to blow directly at my knurl and still see what im doing.
 
Hi, Tom,

Thanks for your comments. Yes, I know how critical the diameter is from my research before I started knurling. I actually made an Excel spreadsheet for the exact diameter for knurling each common, nominal diameter material when knurling with the three different line pitch wheels that I own. That's why I made this part 0.978 inch diameter. (Well, 0.9777 would have been more accurate, but I figured accurate to .0003 would be close enough.) .9777 x pi = 3.0715. 3.0715 inch circumference X 14 lines/inch (same as dividing by 1/14) = 43.001 lines per revolution, an error of only 0.1%. I would think that was accurate enough, but this is the first time I've tried to do a coarse knurl, so I could be wrong. What do you think?

Perhaps I should have read the entire thread rather than just look at the pic. :p

Yes, the diameter you chose should work fine. What type of alloy are you using? Unless it's cast, it should knurl fine provided you use some lube (I like WD-40 on aluminum), keep the chips clear and don't go too deep.

Tom
 
Back
Top