ACME Threading Problem

[YotaBota]

I'm practicing acme threading and am having an issue with cutting (or pushing) the material. I've done the usual searching of youtube and other sites but am coming up empty. The picture shows the problem where the metal seems to be pushing up rather than being cut cleanly. There is a lot of metal being cut as well but the ridges don't seem to go away and it starts being noticeable at about .025 deep and I'm only using a .002 DOC. The pushed material is proud of the rod so it can be trimmed to work but I don't think I should have to do that. I've reground the bit a few times to get the geometry to what I can find to be correct but something is still not right. The lathe isn't labouring or chattering so I don't think that's a problem. I do have a thread gage, not a Starrett but it seems to give the right shape. The steel is inexpensive CR if that would make a difference or not.
I need to get this right before ruining the 1144 rod that is coming.
There are lots of youtube how to thread videos out there but I don't find many "how to fix" threading problems videos.
All help is appreciated.

 

[DavidR8]

I saw somewhere a video about needing the grind the bit such that the helix angle was taken into account.
Basically, skewing the front edge of the tool to match the helix angle. I'll see if I can find the video.

 

[YotaBota]

ToT did a video about that and that was one of my regrinds, didn't help.

 

[DavidR8]

Right, that was the one.
Are you threading toward or away from the chuck?

 

[YotaBota]

LH 10tpi going away from the chuck.

 

[YYCHM]

How does that rod behave when just turning it down? I've had some real gummy stuff that just doesn't turn worth a darn and probably wouldn't thread worth a darn either. It's like it displaces rather than shears.

 

[YotaBota]

It turns down okay, not a spectacular finish but okay for what I do. I'm wondering if this is normal for the cheap CR steel and acme threads. I'll have a few extra inches of the 1144 so I can afford to try a short piece and see how it cuts. I've cut standard 60 degree threads in regular rod with no problems so this is a new experience.

 

[DavidR8]

I’d give it a try.
There’s a lot more material being removed with the Acme profile so perhaps any crappy results due to material is being exacerbated?

Sent from my iPhone using Tapatalk

 

[YotaBota]

lol, I feel like Wylie beating my head on the wall and getting beat by a machine.

 

[RobinHood]

Are you plunging in at 90*? Some people prefer that. It looks like you have the compound set to 14.5* (nothing wrong with that) - as only the leading flank has a burr. The trailing flank looks quite good.

Maybe try and hone the top of the tool so that you have a mirror surface. Also, the leading flank cutting edge should be razor sharp (the edge on the RH side of the tool - as viewed from the top). I presume you have checked for sufficient clearance with the tool in the cut? With the compound at 14.5*, the critical side is the RH edge of the tool.

 

[YotaBota]

The bit is set via the gauge and I'm feeding using the compound at 14.5*.
I'm finding that with each pass there is a small burr at the tip of the tool that I hone off with the stone and repeat.
I'll check the clearances again, hone it sharp (again) and give another go tomorrow (again). I bought a ten foot length of rod so I still have a bunch more to chip up.

 

[Dabbler]

I was taught to remove as much material as possible witha 60 degree tool, and then use an acme threading tool to do the final material removal. This would only reduce the pressure and amount of material being removed, but it is what Bert taught me (for good or bad)...

 

[YotaBota]

remove as much material as possible with a 60 degree tool,

I've read that somewhere as well, haven't seen any videos of that method but I'll try anything once.
Do you keep an eye on the width of the top of the thread to know when you are close?

 

[Dabbler]

-- You look up in Machinery's Handbook what the crest width is for your pitch, and then you cut so you leave about .010 on each side to take of with the Acme threading tool. With less pressure, you usually get a better finish...

This is of course advice for smaller lathes. If you have a 5000lb behemoth, that this advice doesn't buy you anything...

 

[YotaBota]

No behemoths here, just me and my little 1120.

 

[DavidR8]

And its 500 lb stand

 

[YYCHM]

How's the threading coming along?

 

[YotaBota]

Non existent the last few days, it's spring and hasn't been raining the last few days so yard work has been the priority. I've been getting beat up by bramble bushes and had a 90' fir tree come down in the back yard so had to get that chopped up and cleaned up. It's raining today so I should get a chance to get back at it.

 

[YotaBota]

The problem turned out to be the CR steel that I bought to practice threading with. I tried a normal 60* thread it was worse than the acme, it literally peeled/tore out pieces of metal. Even with a .001 DOC it wouldn't cut nicely, the stuff is sh_t.
I tried the acme thread on about 4" of the C1144 rod and it came out beautifully, no muss no fuss, a light touch with the file and the nut spun freely with no play. WAHOO,,,,,, success.

I used the 90* plunge method that RobinHood mentioned, used fine cuts and it came out perfect. Next time I'll try using the 60* Dabbler method just for fun.
Thanks to all for the comments and help, I'll do a quick write up and pics in the projects file.

 

[DavidR8]

Wow, that looks fantastic! Well done mate.