Machining 304 stainless

Pistolero

Well-Known Member
Keith and others with experience....

I am considering using 304 SS for a turned part. Need to thread one end, too. 5/8" stock. I
can buy the part, too, just wondering how much of a PITA I am biting off to make it?

I am aware of the extreme cold work hardening of this alloy, did a lot of analytical (FEA) work
on parts made of this, so very familiar with the characteristics but have not machined anything
very much of it on my own machines.

One real PITA was drilling a .020 304 firewall....I really screwed up and used a slightly dull
drill ..... spun, work hardened and that was the end of that drill and a new, sharp one, too.
Finally managed to worry it through with a carbide pin router in dremel....but what a great
example of what it can do with cold work! Boy does it harden! I have drilled a couple of holes
since in parts, with a sharp HSS drill and good pressure, no real problems.

Just figuring on lathe work on this particular part. Maybe just do it to learn. Any commentary
is welcomed.

Bill
 

Brad

Benevolent Overlord and site owner
Staff member
I will be watching this.
I have avoided stainless for these reasons. Made a single part from stainless and that was a simple rod for use in lapping forcing cones in revolvers.
 

KeithB

Resident Half Fast Machinist
Bill, I feel your pain buddy. Next week we are starting on a batch of tailwheel towbar bolts that start out with 7/8" diameter 304 SS rod and end up with a 3/8" body that is threaded on one end. It is a replacement for the standard tailwheel bolt/axle that has a special head and matching nut. They make tools that look like lawnmower handles that latch on to the head/nut and allow you to move a light plane around by hand. (http://www.flyboyaccessories.com/product-p/1111.htm)

Of everything we machine 304 is the LEAST favorite material. But hey, that's what my peeps are willing to pay for so we learned to do it. Stainless steel in general has some properties you have to deal with. It's strong and tough and gummy all at the same time. It expands more than carbon/alloy steel and yet doesn't seem to soak up heat, instead directing the heat into the tool. And the visual clues of temperature (silver/blue chips) are different than those of carbon/alloy steel. Here are my tips:

1. Sharp tools. Start with new tools, replace them at the first clue of dullness.
2. Moderate but always a positive feed. When you get to the end of a cut GET OFF OF IT and don't rub a surface, especially with a dull tool.
3. Take your time. Cut your RPMs down.

We use negative rake tools for turning but carbide positive rake inserts for threading. We drill and tap with HSS tooling, mostly use carbide end and face mills. Carbide is always nice but HSS is plenty hard enough for machining 304 SS as long as it is sharp and you take your time. We use soluble oil as a flood coolant but that just gains us a little more production speed and helps wash the chips away. A little cutting flooding applied with a brush can sometimes be helpful.

Give it a try, it isn't Satan in metal, it's just another material with its own set of properties that need to be respected.
 

VZerone

Active Member
Keith I heard the LW50 stainless that Lothar Walther uses (it's proprietary) is a bear to machine if whoever is machining it doesn't know how to work it. You ever worked on any of LW stainless barrels?
 

KeithB

Resident Half Fast Machinist
Nope. There are a number of proprietary grades of stainless that have been developed for specific uses. Quite likely that is what they use.
 

Eutectic

Active Member
I think most gun stuff is Martensitic stainless (400 series). 304 is Austenitic having more chromium and some nickel on board. What 400 series I fooled with seemed to machine better in general. The nasty one for all you restoring gunsmiths is Winchester's old "Nickel Steel" I'll usually have a good lipped tool (HS) for this cause it can 'tear' on you if you're not careful. Most reboring guys really shy from "Nickel Steel" rebores. Koozer did my 94 .38-55 when I was just a pup in the early 60's.... It was a originally a.25-35 Nickel steel! He did a great job cut rifling the barrel but his finish ream before rifling had one small spot that had a tear. On a land now of course about half way down...... Doesn't hurt as the old gal shoots! I'd rank old Nickel Steel worse than 304 for machining but I'm dated some!

Pete
 

RBHarter

West Central AR
My Dad was a plumber/pipe fitter later a pilot and A&P and probably a half a dozen things at more or less semi pro that I probably don't know about .

Any cutting tools we used on SS or AL stayed marked for it . As a result we kept 4 sets of number , letter and fraction drills , 4 sets of basic tap and dies , 3 sets of shears and 3 tubing cutters .
Files too . Not long ago I had a reminder of why we did that . I must have picked out one of the AL files , it's the only reason I can think of for a Nicholson flat mill to pack up with and gouge crs ........

Of course way out of my league here .
 

smokeywolf

Well-Known Member
Never cared much for 304. Actually had better experiences machining 316. Yes, 400 series stainless is a bit more of a chore to machine, but still nothing next to Nitralloy, Carpenter or Incoloy 20, Invar, Kovar or, my biggest nemesis, Hastelloy C.

Hard to add to Keith's recommendations.
I always favored high percentage cobalt (8%) high speed tools, Stellite, Tantalum Tungsten or Carbide for machining most stainless.

As Keith says, whatever you do, don't stop your feed. If you stop your feed anywhere during your cut, good luck getting it started again.
 

Brad

Benevolent Overlord and site owner
Staff member
How is 303? Never machined stainless but I have a few scraps of it.
 

Pistolero

Well-Known Member
Brad,
From an engineering standpoint, 301, 302, 303 and 304 are extremely similar alloys.
301 has slightly less chromium and nickel (the primary alloying elements in the 300 series)
than the others, and it seems that 302 and 303 are just a touch less. 304 has 18-20% chromium
and 8-10.5% nickel. 301 has a couple percentage points less of each. It isn't even clear to me
as an engineer with substantial materials experience why 303 even exists and why one might
specify it rather than the more common and nearly identical 304.

My guess would be that 303 is about identical to 304 in machining. The alloying is very similar,
although the sulfur content of 304 is spec'ed at <0.3 and for 303 sulfur is spec'ed at >0.15. Could
be identical, but possibly 303 has more sulfur, and that tends to help machinability. Probably doesn't
actually happen, since the amounts are so extremely tiny, and essentially overlapping, but differently
stated specs.

The only significantly different 300 series alloys I am familiar with are 316 and 321, which are very similar
to 304 except for the addition of 2% Mo in 316 and a fraction of a percent of Ti in 321. Both alloys are
modified to scavenge the small amount of carbon with Mo or Ti to avoid the formation of chromium
carbides when welding. Chromium carbides corrode readily in water electrolytes, and will result in a
"knife edge" corrosion separation around a weld, since the chromium carbides are created at an exact
and very narrow temp range. Welding 300 series stainless is asking for serious corrosion troubles unless
you use a few special versions.

The general class of SS are often called "18-8 SS" for 18 percent Cr and 8% Ni, although the allowable percentage
range is reasonably broad, at least a couple of % points. The balance is Fe.

Bill
 

Rick H

Well-Known Member
I worked in a small machine shop in the early 70's making jet engine parts. Most were small fittings and such turned on very slow moving old screw machines. We used cobalt tooling, slow speeds and high feeds. Most of the materials were 300 series stainless, particularly 321. We sharpened the drills to a flatter included angle and were careful to relieve the point to thin the center flute material. We used Cleveland Cobalt drill bits exclusively.

I didn't think it was bad to machine at all. When we started running titanium for the SST and Valkyrie is where we ran into problems. That stuff was almost impossible to put a deep drilled hole into without tearing things up.