Parting 316 Stainless Steel on the Lathe

[marcoskaiser]

Dear All,

Any experienced maker could enlighten me on the secrets of parting 316 Stainless Steel?

I usually hate parting big pieces on the lathe, but so far Brass and Steel could be handled. But this 44mm diameter 316 SS piece is challenging me.

Tool has been shaped to 8 degrees of back rake and end relief, has side clearance and is aligned to the centre. Best speed calculated at 115-135 feet/minute equalling 330 RPM.

But the parting tool looses every time. 

All I manage to make are extremely dangerous micro stainless steel needles, and dented tools. The chattering is horrible.

Is it possible that I unintentionally hardened the blank while trying to cut, and now it's harder? Should I use a lot of coolant? 

 

Best regards,

 

Marcos

 

[nickelsilver]

What lathe are you using? 316 (and 316L, most common for case parts) is usually ok as far as stainless goes, but is going to be more difficult than a typical steel for sure.

 

When I part a piece that size in my lathe (Schaublin 102), I do everything possible to maximize rigidity. Cross slide is positioned so the tool is over a dovetail, not hanging out. Tool retracted into holder as far as possible. I use two parting tools, one quite stubby, maybe 15mm stickout, and another longer to take over from there. I'll even snug up the gibs for a little more rigidity.

 

Use a good cutting oil, preferably one for stainless or difficult metals.

 

330rpm would be too fast for me. I kick mine into backgear and inch up the speed until I get chatter, then back off. With a parting tool in 316 you may or may not get a real chip, depending on the foundry that made it and even the batch. If you do get a chip it often doesn't want to break and you have nice long moving razors around the workpiece- resist all temptation to grab it and pull it out of the way! Use pliers. And interrupt the cut to break it.

 

If you're getting chatter you either need to increase the feed or reduce the speed. On a Monarch 10EE or Schaublin 150 or Weiler yeah go ahead and increase the feed. On a smaller machine you can only reduce speed.

 

316 will work harden in 1 revolution. Your tool has to be SHARP. Stop and touch it up as often as you have to.

 

Sometimes once chatter has started it won't go away no matter how you mess with speed and feed, in really crap chatter situations I might take a few strokes with a hacksaw in the groove to knock off the chatter ridges on the part. This can remove a work hardened skin too.

 

Are you using carbide or HSS?

 

 

 

 

[jdm]

45 minutes ago, marcoskaiser said:

Any experienced maker could enlighten me on the secrets of parting 316 Stainless Steel?

Together with the good advice by master nickelsiver, I repeat the fundamental question, what lathe are you using? What you're attempting to do requires a pretty sizable lathe,  a very minimum of 1Kw motor perhaps. And from your writing I gather you're using HSS, but for hard metals you really need indexable tools.

[marcoskaiser]

7 minutes ago, nickelsilver said:

What lathe are you using? 316 (and 316L, most common for case parts) is usually ok as far as stainless goes, but is going to be more difficult than a typical steel for sure.

 

When I part a piece that size in my lathe (Schaublin 102), I do everything possible to maximize rigidity. Cross slide is positioned so the tool is over a dovetail, not hanging out. Tool retracted into holder as far as possible. I use two parting tools, one quite stubby, maybe 15mm stickout, and another longer to take over from there. I'll even snug up the gibs for a little more rigidity.

 

Use a good cutting oil, preferably one for stainless or difficult metals.

 

330rpm would be too fast for me. I kick mine into backgear and inch up the speed until I get chatter, then back off. With a parting tool in 316 you may or may not get a real chip, depending on the foundry that made it and even the batch. If you do get a chip it often doesn't want to break and you have nice long moving razors around the workpiece- resist all temptation to grab it and pull it out of the way! Use pliers. And interrupt the cut to break it.

 

If you're getting chatter you either need to increase the feed or reduce the speed. On a Monarch 10EE or Schaublin 150 or Weiler yeah go ahead and increase the feed. On a smaller machine you can only reduce speed.

 

316 will work harden in 1 revolution. Your tool has to be SHARP. Stop and touch it up as often as you have to.

 

Sometimes once chatter has started it won't go away no matter how you mess with speed and feed, in really crap chatter situations I might take a few strokes with a hacksaw in the groove to knock off the chatter ridges on the part. This can remove a work hardened skin too.

 

Are you using carbide or HSS?

 

 

 

 

Thank you Nickelsilver.

 

I am using a BV-20 , 1/2CV, six-speed lathe.

Tools used: HSS 12% and 50% cobalt.

I will try to get rid of the ridges and cut again, with the slower speeds. 

[jdm]

35 minutes ago, marcoskaiser said:

I am using a BV-20 , 1/2CV, six-speed lathe.

With a 370W motor, it should be a geared headstock type? However parting 44mm is hard work even for lathes much bigger than your.

Quote

Tools used: HSS 12% and 50% cobalt.

Definitely an hard proposition.

Quote

I will try to get rid of the ridges and cut again, with the slower speeds. 

Check what the carriage does. Even when fully locked it may still move a little. When it does, all bets are off, chatter comes, the cutting point wanders, machinist swears and the suppliers get a chance to sell.

Below, good machinist receives 316 work just to be parted off

 

 

Edited July 30, 2020 by jdm

[nickelsilver]

Ok, that's not a baby lathe, but not one known for its rigidity. HSS is ok but you will need to sharpen often.

Stickout from the chuck needs to be as small as possible. For something like this I use my 4 jaw chuck; for one the gripping power is greater than a 3 jaw, and on my machine it sits about a centimeter closer to the headstock- that cm makes a world of difference.

You can think of overhang (stickout) as decreasing rigidity as a square; if you have 15mm and that gives you x rigidity, then go to 30mm it not half as rigid, it's way way less rigid. So if your raw stock is 50mm and you need a 15mm slice, stick 16mm in your chuck, and part off right next to the jaws. Then face it off to thickness.

[rodabod]

I’m sure this may be considered a faux pas to some, but for a rough cut on larger items, I sometimes mark a groove with my graver and then cut with with a piercing saw. 

[nickelsilver]

I’m sure this may be considered a faux pas to some, but for a rough cut on larger items, I sometimes mark a groove with my graver and then cut with with a piercing saw. 

That's 100% approved A-OK especially when needing to part something that's challenging the rigidity of a machine. If on a larger lathe best to put a block of wood under the cut or the saw can surprise you when it gets through and nick the bed (I do the saw trick with the lathe running). Just remember to reverse the spindle direction id using a jeweler's saw where the cut direction is opposite.

[rodabod]

Oh, I certainly meant with the lathe running! Works a treat for a quick job. Make sure to move the saw blade to prevent heat build-up too. 

[jdm]

2 hours ago, nickelsilver said:

Just remember to reverse the spindle direction id using a jeweler's saw where the cut direction is opposite.

Trivia bit, japanese woodsaws are opposite too. The reason given is that their trade was done mosty done outside the shop, so work was held without a vise.

[marcoskaiser]

On 7/31/2020 at 1:31 PM, rodabod said:

I’m sure this may be considered a faux pas to some, but for a rough cut on larger items, I sometimes mark a groove with my graver and then cut with with a piercing saw. 

That’s very tempting, hahaha. I feel ashamed of letting people hear the squealing from the shop..

[marcoskaiser]

On 7/31/2020 at 3:42 PM, nickelsilver said:

On 7/31/2020 at 1:31 PM, rodabod said:

I’m sure this may be considered a faux pas to some, but for a rough cut on larger items, I sometimes mark a groove with my graver and then cut with with a piercing saw. 

That's 100% approved A-OK especially when needing to part something that's challenging the rigidity of a machine. If on a larger lathe best to put a block of wood under the cut or the saw can surprise you when it gets through and nick the bed (I do the saw trick with the lathe running). Just remember to reverse the spindle direction id using a jeweler's saw where the cut direction is opposite.

I actually cut and welded my toolholder so I can cut using reverse rotation. That’s why I could not post anything in answer of your kind tips!

I may seem like those people who take more pleasure in cleaning the desk than getting the work done

[marcoskaiser]

On 7/30/2020 at 7:30 AM, nickelsilver said:

Ok, that's not a baby lathe, but not one known for its rigidity. HSS is ok but you will need to sharpen often.

Stickout from the chuck needs to be as small as possible. For something like this I use my 4 jaw chuck; for one the gripping power is greater than a 3 jaw, and on my machine it sits about a centimeter closer to the headstock- that cm makes a world of difference.

You can think of overhang (stickout) as decreasing rigidity as a square; if you have 15mm and that gives you x rigidity, then go to 30mm it not half as rigid, it's way way less rigid. So if your raw stock is 50mm and you need a 15mm slice, stick 16mm in your chuck, and part off right next to the jaws. Then face it off to thickness.

I will bolt the lathe to a granite bed and then test this. Got enough of low standard jobs already..
thank you for the tip!!

Edited August 1, 2020 by marcoskaiser
Incomplete

[marcoskaiser]

Thank you Nickelsilver, Rodabod and Jdm, It was a success! First getting rid of the part already started and full of bumps with a hacksaw took only a few minutes. Not difficult, and the saw came back still good, not blunt... I will adopt this approach whenever possible. 
picture 1

Then overcoming the lack of rigidity by working close to the chuck, with minimum overhang worked fine. Good chips, and a nice sound of metal actually being cut.. finally.

Funny thing, and in accordance to what nickelsilver said, the recommendation to cut stainless at 115-135 feet per minute of surface speed does not apply to parting, and with a much lower speed the parting was easier. I used 140 RPM and it was fine.

picture 2. Nice chips

Then to the end. At some point the tool got stuck, and had to be resharpened. I was going straight, without any lateral clearance, encouraged by the smooth operation. Apart from this, a good experience. I finished the blank with a hacksaw, but only because the tool could be off-center after resharpening and was cutting less towards the end.

picture 3.

With these tips, I guess other people will find easier to part 316L. 
The lathe is still waiting for the granite bed. Eventually I will bolt it better. 
Thank you again for the help!

[nickelsilver]

I'm glad it worked out!

Couple of more parting observations. The tool should be on center, but if it's not, you want to be below rather than above. If above center, with the flex of the machine and slides it will go down, which effectively makes you cut depth double or close to it. This is where tools break, parts pull out of chucks, and in extreme cases with enough power the tool post rips out the top of the cross slide. If the tool is lower and flex decreases the cut.

And, when parting a piece that doesn't have a center hole, especially larger work like this (all things considered), it's very common to finish off the last bit with a saw. With a bit of care you won't even mark the part.