Turning 3/16" Mild Steel

[RVJimD]

Hi guys,

I am turning down some cold rolled steel and having trouble with it wanting to crawl up on the tool. I am starting with 3/16" rod and only need to take off about .025". If I try to take it all in one pass it crawls up on the tool and bends it at the chuck. The parts are 1" long and I chuck it as close to that as possible. If I take two or three passes I can get some of them finished. I was thinking I would be able to do them in a single pass... I need them turned down just enough to fit inside some brass tube. Not sure there is any better method of I am just stuck making several light cuts?

I am stuck with reducing the size vs using a stock size because of the parts they mate up with, otherwise I would use tube and shaft that matched.



Jim

 

[Rex Walters]

Fair warning: I'm a complete novice and really don't know what I'm talking about, but I thought I'd share my experience anyway.

Are you taking 0.025" off the diameter or off the radius? Hand ground HSS tooling or inserts? By "crawling up on the tool" do you mean the part is deflecting away from you (from tool pressure) or actually deflecting upward (which I suspect might mean a dull tool that isn't cutting well).

I only use hand-ground HSS tools. For thin rods, I usually have more success by taking larger bites than small ones. Taking a cut of 0.010" to 0.020" (radially) ensures more on-axis load from tool pressure than radial load. Trying to take off just a thou or two is much harder since the part will just deflect. That is, with more depth-of-cut it feels that you're pushing more on the end than you are deflecting the part side-to-side.

The success factors for me are: collets, freshly sharpened and honed HSS tools, and an optivisor to see whether I'm really on center and cutting the way I want.

One inch really isn't that short for a 3/16" mild steel rod, but I did recently learn a neat trick for reducing much longer thin rods. Say you're making a long thin bolt or pin where the head is, say, 1/4" and the shaft is 2" long but only 1/16" in diameter. Start with the stock in a collet with only a short length extended. Turn down the first half inch or so to size, then pull the stock further out of the collet to turn down the next half inch or so. Doing it incrementally this way ensures the thin section is always supported by the remainder of the stock.

Dunno if that technique might help or not, but it's worth a try.

Good luck.
--
Rex

 

[tertiaryjim]

Can you support it with a live center in the tailstock ?

 

[RVJimD]

Good points/questions Rex. I am using HSS and I put it on the stone after I started having trouble and got it nice and sharp but didn't help. I didn't want to do it between centers just to avoid the extra steps. It doesn't need a hole in the end of it... I am pretty sure I could shorten it up like you suggest and make that work.

I think I can get them made, but I was just checking to see if I was forgetting some obvious solution.

Jim

 

[brino]

Rex touches on many good questions, tool type, sharpness, and tool height.....

one more question is what is your tool shape?

I like to shape the cutting tool so that most of the cutting pressure is on the side, you push the tool (and workpiece) more towards the headstock than towards the "back" of the lathe.

-brino

 

[Silverbullet]

If you were using a turret lathe a box tool is your answer. The have a set up that will not let the rod deflect. If you had one you could use it in your tail stock or your compound with some modifications . Or just set up the follow rest if you have one.

 

[higgite]

Jim,

Some good points so far. I'm curious, what rpm are you running?

Tom

 

[RVJimD]

It is good for me to ask a noob question occasionally , since I am most certainly STILL A NOOB.

All the what and how are you doing this or that helps me remember stuff I would know if I had more experience. I DO appreciate the help!

I am running about 500rpm.

I think one of Rex's initial point about shorting up the stock is my biggest trouble. I am now pulling out about 3/8 at a time and using a cutoff blade and it is working great in one DOC pass. Then when I get an inch turned down I can simply part it off and start the next one.

Here is the setup that is working fine. I have about 15 of these to make and this isn't too bad now.
Thanks for the tips!

 

[higgite]

It is good for me to ask a noob question occasionally , since I am most certainly STILL A NOOB.

All the what and how are you doing this or that helps me remember stuff I would know if I had more experience. I DO appreciate the help!

I am running about 500rpm.

Hi Jim,

I'm pretty much a noob, myself, but the rule of thumb I keep reading is to run mild CRS at 100-150 sfpm. For 3/16" rod, that's in the neighborhood of 2000+ rpm. I don't always follow the rule, but usually find that best results are with at least half of calculated rpm. Have you tried turning it at higher rpm?

You also might try taking lighter cuts on such thin stock, say .015" and .010" or 15/5/5. Hope this helps.

Tom

 

[brino]

Here is the setup that is working fine. I have about 15 of these to make and this isn't too bad now.
Thanks for the tips!

Hi Jim, I'm glad you got something that works!

I found some 3/16 steel rod and tried a few myself. I started with about 2" hanging out of the collet (for worst case) and around 350 rpm(for no particular reason than that's where I left it!). I was never able to cut more than about 5 thou. radius (or 10 thou. off diameter) without the stock rolling up onto the tool bit. Even when it did cut, the stock ended up tapered; bigger away from the headstock because the rod was pushed away from the tool.

One inch stick out from collet was reasonable though. Changing to around 930 rpm did seem to help a little.

the rule of thumb I keep reading is to run mild CRS at 100-150 sfpm. For 3/16" rod, that's in the neighborhood of 2000+ rpm.

Tom, the references I usually go by say 60-80 sfpm for cutting steel with HSS tool bit, so for 3/16" stock around 1200-1600 rpm.

-brino