# machining stainless



## dalem9 (Nov 13, 2011)

I am trying to cut threads on some stainless steel on the lathe , it is coming out more like a rasp then a thread .What am I doing wrong ? I beleive it is #316 .Thanks Dale


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## SBWHART (Nov 13, 2011)

Some grades of stainless can be dificult to machine Dale, 303, 304, 305 are the best for maching, never tried the 316. But try a good sharp HSS cutter, if your using TC tip it may not be the correct grade or geometry, also try a bit of tapping fluid on the job (Rocol) , I've always found that improves the finish, also drop the speed down a bit.

Hope this helps

Stew


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## dalem9 (Nov 13, 2011)

Thanks Stew I will go out and try this .Thanks Dale


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## Tin Falcon (Nov 13, 2011)

Dale I posted a link to this in the getting started in machining thread as it is a often adressed topic. 
It should answer most of your questions it is actually a Design Handbook (9 page pamphlet) on Stainless Steel for Machining) enjoy. 
http://www.ssina.com/download_a_file/machining.pdf
Hope this helps. 
and there is always
USAF aeorospace metals a bit longer read. 
http://www.robins.af.mil/shared/media/document/AFD-091006-039.pdf
Tin


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## steamer (Nov 13, 2011)

YUP 316 is not pleasant stuff to work  303 is much better.

Dave


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## dalem9 (Nov 13, 2011)

Thanks Everyone I will work with some of these ideas. Dale


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## /// (Nov 14, 2011)

I must have tapped thousands of holes in 316/316L with a HSS tap, Rocol and cordless drill... never had an issue.
Maybe try a good quality die?
Ignore me if the thread is huge/tiny and the die would cost big $$$


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## steamer (Nov 14, 2011)

Leaded 316 ....316L cuts easier.....

It's been my experience that 304 and 316 are miserable...

Dave


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## /// (Nov 14, 2011)

I've never realy noticed any difference, but then it's only 1/4" or thinner and vast majority has been 316, only occasionally tap 316L and on the rare occasion CF8M.


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## max corrigan (Nov 14, 2011)

In my past life as a sheetmetalworker to the food industry the only acceptable stainless steel was 316 which was classed as food quality stainless, 304 was not allowed, as for working it was fairly easy to drill and tap if the tools were really sharp and using slow speeds plenty of coolant and a steady no messing cut, if the stainless got hot through using a blunt drill or tap it would turn blue, get as hard as glass and no way would you get through it
The turners at the firm would say the same thing, sharp tools plenty of coolant and a constant cut, as already mentioned it could be a pr..ck of a metal to work although very tough material, it would scratch very easily, which was classed as a bug trap and have to be polished out, loosening or tightning a nut and bolt would for no apparent reason (that i could see) seize up solid, and would need to be cut of with and angle grinder! (no joke when you up a ladder)
Anyhow good luck with it Dale
Regards Max........


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## purpleknif (Nov 14, 2011)

Biggest reason it sux is that it work hardens. If you keep that in mind things might go easier.


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## /// (Nov 15, 2011)

max corrigan  said:
			
		

> ....
> loosening or tightning a nut and bolt would for no apparent reason (that i could see) seize up solid, and would need to be cut of with and angle grinder! (no joke when you up a ladder)
> ....



Yep, known as Galling.
For offshore oil and gas, 316 is a minimum. Nickel alloys like Monel, Nimonic, Inconel etc are preferred.
Fortunately, I have had only one 316 bolt/nut seize.. my own damn fault for forgetting to put Rocol Anti-Seize on it, you only make that mistake once!


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## MachineTom (Nov 15, 2011)

If the cuts look like a rasp, it is first a dull tool, second low cutter on center. Preground carbide is the best unless you are a really good tool grinder, keep a stone around to touch up the tool, also check for chip welding on tool tip, that will kill a perfect thread in one pass. I use the dark sulfur oil, smells a bit but works well.


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## Entropy455 (Nov 15, 2011)

Stainless work-hardens when cut. Making matters worse, stainless is a very tough material that requires a lot of energy to plastically deform it.

The trick with stainless is to take larger cuts, as small cuts will cause your bit to remove material from an area thats been previously work-hardened by the prior cut. The phenomenon is very noticeable with 316  not as bad with 304  mainly because of the additional strain energy required to shear 316 under the cutter head. You can take small cuts if you have a beefy machine, and your work piece is rigid enough to prevent deflection  as pushing the bit against the work piece without actually removing material is the quickest way to obtain a very hard surface. . . .

The high sulfur content of 303 reduces the work hardening effect, making it easier to machine. However the high sulfur makes it very prone to cracking when welded, which is why most manufacturers consider 303 unsuitable for welding. 303 is commonly used for mass-producing parts that require a lot of machining operations, with zero welding.


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## Ken I (Nov 15, 2011)

The work hardening property is a SOB.

It's a case of he who hesitates is lost.

A moments hesitation leads to rubbing - game over.

As Entropy 45 pointed out, heavy cuts work best.

Feed aggressively (that doesn't mean go at it like a bull in a china shop) similarly terminate cuts rapidly - particularly when peck drilling deep holes - back away rapidly at the end of each "peck" resume "at feed" rate - don't "search" for the material.

Carbides are fine if you have the horsepower - and you use appropriate grades - galling buildup on the tool can be a big problem with some tips / stainless grades.

Otherwise use sharp heavilly raked HSS (hook rake) and HP lubricants (the dark smelly stuff Machine Tom suggests).

When screw cutting use the half thread angle approach so as you are only cutting on one flank - plunge cutting on stainless is just looking for trouble.

Ken


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## steamer (Nov 15, 2011)

Ken I  said:
			
		

> The work hardening property is a SOB.
> 
> It's a case of he who hesitates is lost.
> 
> ...



Good advice Ken!

Dave


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## Swede (Dec 6, 2011)

Ken is dead-on correct. We in the home world can work on these nasty stainless steels, and on even nastier metals like inconel, if we keep a couple of things in mind...

First, we don't have the HP or rigidity to use zero or negative rake carbide tools. These cutters need to take big, continuous bites to avoid the work hardening issues. So the way to go is with HSS tools that are especially ground for the job. Make the tool VERY aggressive, with a small tip radius and a large positive rake, and this tool will do two things, with some drawbacks:

1) It'll BITE (cut), rather than rubbing (and work-hardening) with a 0.004" infeed, it'll remove metal.
2) It'll do so with much less HP, and rigidity will not be as critical. It won't spring the work away from the turning axis.

Drawbacks:

1) Cuts cannot be too aggressive, or the high pos. rake will cause the tool to dig in. The small radius can break if you get too aggressive. So more passes are needed.

2) Small tip radius can mean poorer finish unless the longitudinal feed can be set to pretty slow.

Here's the sort of tool I'd grind for a nasty turning job on 316. It looks more like an Al turning tool. Some older lathe heads might cringe at the extreme rake for steel, but it works if the limitations are kept in mind.


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## mu38&Bg# (Dec 6, 2011)

I used inserts for aluminum turning to cut 304SS. It worked fine , but the life is not going to be great. I could take off less than .001", but it began to get inconsistent.


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## Jeremy_BP (Jan 4, 2012)

I had to machine some slots with radiused ends in some 304 today. What a pain.
As I had no rotary table, the ends had to be drilled out with a 11/16 drill -- no fun!






I started the hole with a 1/8" pilot.






And enlarged it a bit.






And finally brought it out to size with a 11/16 bit. Oh, my poor mill was really suffering, but it came out well. I had to be aggressive, but not too much so. It's a balance.






And then I was able to break out a nice carbide endmill and plow on with the slot. I kept it cool with a bit of air and it just ripped through the stainless. What a joy.

304 is way up there on the list of materials I avoid as often as possible.


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## Mosey (Feb 11, 2012)

I am having trouble milling some though slots in .060" 304 SS sheet. HSS mill (1/8" 2 flute) burned up the tip right away. So, next I got a 5/32" solid carbide straight flute mill, and promptly broke it off attempting aggressive feed. This is a manual mill. 
What would be best way to go? A could use flood coolant, but don't like the mess of liquid all over the table, potential rust of machine. 
What spindle speed for this size mill? Solid carbide, spiral flutes?
I am making a 2" diameter multi-blade fan.


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## MachineTom (Feb 11, 2012)

A two flute is not for SS, use a 3-4 flute, rpm should be 1500 with HSS and 3K + for carbide. What speeds were you using? I remember the first time using carbide drills at 5K rpm, I was thinking it was nuts to do so, but man does it work.


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## Mosey (Feb 11, 2012)

600 rpm! Embarrassing isn't it.


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## MachineTom (Feb 11, 2012)

Not at all. But if you do it again,


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## Mosey (Feb 11, 2012)

MachineTom
Here's the strategy going forward.
5/32" 4 spiral-flute HSS mill. Lot's of brushed-on cutting fluid at the cutter tip.
Grind off the burnt end of the mill, as I don't need to center cut.
1500 rpm
Feed gently into the work, burying the tip into the aluminum pallet about 1/16"
Work is bonded to aluminum pallet with hot-melt glue. 
Sound OK?
Wish me luck.


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## Swede (Feb 13, 2012)

Mosey, I am concerned about



> Grind off the burnt end of the mill, as I don't need to center cut.



The ends of the cutter are for more than center cutting. If you are contacting ONLY on the flank of the cutter, then you can grind the end off all day long, but if you are expecting to machine a surface, it doesn't sound good to me.

Look at the chips when you cut. They will probably come off tan to blue. If chips are too small, then the steel is work hardening, and the cutting edges will be ruined in short order.

I personally like carbide when using small cutters on stainless. Although they are brittle, they retain sharpness so much better than HSS, and each tooth is more likely to bite and cut rather than rub. Once HSS begins to rub, it'll simultaneously work harden the material, AND dull the cutter fatally.

Good luck!


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## MachineTom (Feb 14, 2012)

Mosey, That will create another issue, an end mill with a flat bottom, will not work if the bottom rubs on the material being cut. If you are only milling with the flank no problem, but if you were cutting 1/4 material 1/8 deep, not good, if you are cutting 1/4 material and 1/4 deep so the end of the mill cuts air its fine. And under those conditions you can leave the end burnt anyway, since it does nothing.


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