Straight Form Knurls Breaking

Hi Everyone,

Machine - Tsugami B325 Swiss with oil
Material - Annealed 41L40 (see pic)
Tool - Accutrak 64XDP cobalt straight form knurl wheel

I am setting up an old job on a new machine and wanted to review my knurling process because on the old machine I had some trouble with double tracking. We always make sure to infeed relatively fast to keep the wheels from slipping and also control the turned diameter to make sure the knurl pitch divides into it evenly.

So I called accutrak and they said for this application I should use a single bump wheel because using a double knurl can introduce double tracking as a risk. So I said fine. I got a cobalt 64 pitch straight form knurl, .250" wide. I turn the 41L40 to .438" I go into X.414 at F.010 with a G4U.5 (S1000). Everything looks good but the knurl is underformed...must be material deflection, so I end at X.398 with a pretty good form, but not "full." After 100 pcs the knurl cracks (see pic.) I slow the speed down to S300 to make sure it is tracking properly and cracks again at 100 pcs (approximately.) Accutrak suggested I slow the feed rate down to F.003. Again, it cracks around 100 pcs. I tried dropping to F.002 but double tracked, so I go to X.428F.004 and then X.398F.002. Everything looks good....until it cracks again around 100 pcs.

They said I should try some HSS wheels as they are tougher so they are on order, but does anyone have any ideas as to what's going on? I really don't think I am feeding too far because the knurl is not fully formed (see pic...I'm afraid to form it more since they are breaking!) and when I measure parts that start coming out without knurl they measure right at .438 diameter so it's not like the turning insert is wearing. I'm flooding it with oil and it is just a straight in and out form with a dwell...this shouldn't be that hard haha!

Thank you!

I've learned to not use cobalt knurls because they've almost always cracked on me. I just stick with the HSS ones now.

Your dwell is a bit long. 1/2 second is more than 8 revolutions of the spindle at 1000 rpm. I try to limit myself to 3 revolutions to prevent work hardening of the part.

If there's anything you can do to limit deflection of the part (i.e. support part with sub, use extended GB, knurl closer to the collet), I would do that. Knurls work best when the part is held rigidly.

wmpy said:

I've learned to not use cobalt knurls because they've almost always cracked on me. I just stick with the HSS ones now.

Your dwell is a bit long. 1/2 second is more than 8 revolutions of the spindle at 1000 rpm. I try to limit myself to 3 revolutions to prevent work hardening of the part.

If there's anything you can do to limit deflection of the part (i.e. support part with sub, use extended GB, knurl closer to the collet), I would do that. Knurls work best when the part is held rigidly.

Click to expand...

Well I'm really excited to hear that you have, in fact, run into problems with cobalt...hopefully that's about all there is to it! Thank you! The Accutrak guide says to dwell for 5-25 revs, so that' where I got the dwell from. I will start with just changing to HSS and then play with dwell time. Thank you, again!

We finally said to heck with form knurling and went with cut knurls. Never looked back.

Gobo said:

We finally said to heck with form knurling and went with cut knurls. Never looked back.

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Even on a straight knurl? That's what I wanted to do since our diamond pattern cut knurls never give us trouble but the accutrak guy talked me out of it. Maybe I should have trusted my gut...

Honestly, we have never had to cut straight knurls. They work so well with diamond pattern, I just assumed they would work as well on straight. If he tells you why form are better for straight, post his reasoning. I would like to know.

We use cut knurls for straight with no problem.
Tens of thousands of parts out the door without a second thought.
I think ours are from Zeus, if I remember right.

FWIW,
Brian

Time to move on from here:
Machinistboard.com

ALSANTAYOS said:

Time to move on from here:
Machinistboard.com

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Banned people's posts stay up, anyone remember when Gordo was able to erase the memory of a few members?

Nerdlinger said:

Well I'm really excited to hear that you have, in fact, run into problems with cobalt...hopefully that's about all there is to it! Thank you! The Accutrak guide says to dwell for 5-25 revs, so that' where I got the dwell from. I will start with just changing to HSS and then play with dwell time. Thank you, again!

Click to expand...

The last time I tried cobalt knurls, I was trying to get better tool life than the HSS knurls I was using. I was doing a diamond pattern on 1.5" diameter 303 with a two-die straddle holder. After not too many parts, the knurls exploded and damaged the holder. I went back to the HSS rolls and finished the job with the lesson learned.

I don't think the guide on Accu-Trak's website means that you should dwell for 5-25 revs. It means that from the time the knurl contacts the part to when it is fully rolled, the spindle should have made 5-20 revolutions. Form Roll claims a similar 10-20 revolutions. Once it's rolled, you should get the tool out of there because you're just wearing out the rolls and work hardening the part.

I hope the HSS knurls fix your problem. I'm guessing they will.

Plug - the accutrak guy emailed me saying he saw the order come through for the HSS wheels and is sending them to me for no charge. Despite the initial challenges with the wheels blowing up they have had good service thus far helping me try to figure this out!

Nerdlinger said:

Plug - the accutrak guy emailed me saying he saw the order come through for the HSS wheels and is sending them to me for no charge. Despite the initial challenges with the wheels blowing up they have had good service thus far helping me try to figure this out!

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The guys at Accu-trak are top notch.

I've never cut a straight knurl, but we form literally thousands of diamond knurls every day using cobalt Accu-trak knurls, tool life in the thousands of parts, no issues.

Form knurls are for soft materials. Also, I have always used HSS knurls and never had a problem...even in 4140. Do not dwell if you do not have to. In some materials dwelling work hardens the material, especially with form knurls.

Cut straight knurls - yes! The cut straight knurl tool is a one wheel tool. The wheel is mounted at an angle and on center. I have one and do like it very much. Like all kurling, it seems, I have found it still to be a challenge on some material but it does a nice job.

And +1 for Accu-trak. They seem like an itty bitty company, not sure, hope they don't go away in my lifetime. Great to deal with for knurling.

Looking at your part- What I did most recently with my cut knurler was similar. I roughed the part out and left the knurled diameter a little long. Then poked the knurl tool into the extra material and knurled it then roughed and finished the extra length off. Trying to get the nicest knurls I figured maybe the entry wouldn't look as good. Not sure if this is possible with a swiss since maybe you'd need to pull the knurled section back into the bushing to do so.

Well sure as shit the change to HSS wheel has made all the difference in the world. The cobalt wheels were breaking between 80 and 100 pcs and we are up over 500 pcs (and still going) with the HSS wheel without changing any other factors (i.e. dwell time, feed rate, etc.)

I really did not think the hardness/toughness between those two materials would have made that much of a difference!

Thank you, again, for the help!

P.S. - Gobo - the reason they recommenced a form knurl was because it's typically faster to go in and out in X (with a form) than going in in X and then feeding over in Z (with a cut knurl.)

Nerdlinger said:

Well sure as shit the change to HSS wheel has made all the difference in the world. The cobalt wheels were breaking between 80 and 100 pcs and we are up over 500 pcs (and still going) with the HSS wheel without changing any other factors (i.e. dwell time, feed rate, etc.)

I really did not think the hardness/toughness between those two materials would have made that much of a difference!

Thank you, again, for the help!

P.S. - Gobo - the reason they recommenced a form knurl was because it's typically faster to go in and out in X (with a form) than going in in X and then feeding over in Z (with a cut knurl.)

Click to expand...

You do understand that form knurls require a lot more pressure, right?

ALSANTAYOS said:

Time to move on from here:
Machinistboard.com

Click to expand...

Yeah. Those snazzy icons really give me a hard on. But, I will not join until you get some neon, flashy dashy things and cartoon characters to keep my attention.

CORONA VIRUS said:

You do understand that form knurls require a lot more pressure, right?

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Yup! I guess if deflection or overloading the axis was a problem I would go straight to cut knurling but if the HSS wheel solve the cracking problem I will probably just call this done for now. The only reason I may consider cutting in the future is for a better (fuller) tooth form.

CORONA VIRUS said:

Yeah. Those snazzy icons really give me a hard on. But, I will not join until you get some neon, flashy dashy things and cartoon characters to keep my attention.

Click to expand...

Interesting viewpoint lol

Final update - the cobalt knurls were lasting around 80-100 pcs before fracturing. After replacing them with a HSS wheel we made over 1,100 parts with no problem (no change to the program.) The job is now done so we'll see how long it lasts into the next one but I consider this case closed. Thank you for the help, everyone!