Machinability of 304 Stainless

Patrickknives

Patrickknives

Joined
Dec 13, 2006
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I have a bar of 304 stainless to use for guards. It was really difficult to drill 3/32 holes last night, fast, slow medium with cutting fluid. Buck, squeel and broke two bits. Is this stuff that bad??
 
It can be done, maybe cobalt bits?

This from azom.com

Machinability
Stainless steel 304 has good machinability. Machining can be enhanced by using the following rules:

· Cutting edges must be kept sharp. Dull edges cause excess work hardening.

· Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material.

· Chip breakers should be employed to assist in ensuring swarf remains clear of the work

· Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities.
 
It may be work hardening, I used new drills and they started fine but started to make a squeel noise and then really did not cut, a new bit in the same hole did not cut either. I have some 420, this bar may just set in the back of the shop a while.
 
I don't know if this will help you or not but I have ran a great deal of this stuff at another shop I worked at. One of the tricks that we used to make the drills last longer....We would "dub" them. By this I mean to put a VERY small flat on the cutting edges (perpendicular). This sounds counterproductive until you understand the reasoning. The small flat basically gets rid of the fracture point on the drill. (sharp edges fracture under load). Yes, it will squeal a bit but it should last longer.
Keep decent pressure as not to cause undue heat and plenty of lube.


HTH


Bill
 
Work hardening is caused usually by poorly ground bit or insufficient feed.
 
Afaik the reason the 300 series is hard to machine is due to the high chrome and nickel content. At least, that's what I hear from machine shops whenever they whine when I design a part from these materials (usually 310(s)/314). Nathan should be able to give more details.

Cheers Rody
 
I use 304/316 on a regular basis, just a couple of simple rules.

Use sharp cobalt drills.

Use lubricants.

Once you start to feed keep it up with even pressure until it begins to chatter then back out and check the drill bit for damage, resharpen if necessary.

George
 
I quit using 304 a long time ago. It's just too soft. Caused me a lot of headaches and broken bits and taps because of being so soft.
Switched to 303 and all of those problem disappeared.
It'll take the teeth right off of a store bought band saw blade and requires a bi-metal blade to cut it, but it drills, taps and otherwise freely machines.
303 also gets a much better polish and is far more difficult to scratch.
 
Another tip for drilling this stuff that just occurred to me. We would also use a lubricant called "Moly D". I have used carbide on 304SS with limited success. Honestly, to me 304 was on an order worse than machining titanium but given the proper techniques could be done with good results. This again is another material that we would purposely drill oversize on taps for the minor diameter for tapping.

Ahhh...I miss machining this stuff...Seemed like I ran 304 for about a year straight on differing parts. I would machine this over Kovar or Invar (beautiful stuff when you can get a finish).
 
I use 304 for balisong handles.i can tell you its a bugger to work compared to 303.304 is very gummy and tends to break bits easily.its machinability sucks as it smears instead of chips,even with carbide.just drill/ream slow with lots of cutting fluid and you should be alright.it will finish out well and be pretty darn tough!
 
If you have a choce for bolsters etc, 304 is the worst as it is 'gummy', free machining grade 303 is better but 416 is the best stainless .416 is the free machining grade of 410.
 
As already stated, stainless steel is bad about work hardening. This tends to be caused by feeding too slow, or a sloppy setup that allows chatter or hammering. A rigid setup and steady in feed will fix the problem.

I tend to disagree with the suggestion to use a carbide drill bit. I only ever use carbide drills in production drilling of plastic, aluminum and bronze because these are abrasive applications and tend to be lower strength materials. 304 doesn't dictate carbide, and the processing window would be narrower.

An ordinary HSS drill bit (not carbide, or even necessarily cobalt HSS) should be able to drill hundreds, perhaps thousands, of holes in 304 without a problem. Coolant helps because the heat conductivity of stainless is poor, power feed helps because the chip thickness window is more narrow than most materials and it prevents overfeeding during exit, a rigid setup helps reduce shock to the cutting edge and work hardening, and moderate surface feet per minute speed is important.

One of the most useful things folks on this forums could add to their drill press, band saw or mill is flood coolant with a recovery tank and pump. Then flood coolant (which is just water with some rust preventatives, frequently with cutting aids added) could be used to prevent the need to retract from the cut.

I wasn't aware that 303 was more scratch resistant and less gummy than 304.... I just thought it was just more free machining. I know 303 costs a good bit more. 304 is fairly soft but has very good corrosion resistance, particularly red rust resistance. 304 also welds nice. I have never had any problems machining it, even on older lighter machines.



Mete,
I think, for me, the single biggest pill in machining to date has to have been that tang you had me modify. I have filled up entire gaylords with chips from a days machining, but that thing took most of a day to make a small handful of tiny chips. That was some kind of stainless wasn't it? heh.
 
Nathan , I'm glad you came through on the challenge .That's now my special knife with all the desirable features, a neat project . Thanks to you and Butch !
 
Thanks for the input, I have some other stainless that I will use until I feel the need for more pain. I did put the work in a medium drill vice but it was not bolted to the table. The drill press is a 1hp free standing. and I had a drip of light weight machine oil on the hole. But... it would make a long cut on SS then start to squeel, chatter and shout and then would stop about 1/4 through some more pressure and the bit would break. Thanks everyone,
 
We drill, tap, and countersink 304 and 316 everyday at my work. We get by with sloppy old machines and crappy import HSS bits. When they dull, we just re-sharpen them.

I use a wash-bottle (little squirt bottle) filled with soluble oil as lubricant/coolant.

Countersinking is a challenge. The import countersinks will do about 5 holes.
 
I know I'm replying to an ancient thread, but I did not see anyone here mention this solution to drilling materials with work-hardening properties. Many years ago I obtained a number of discarded ice cream machine blades from the milk processing facility I worked at. They were 1" to 1 1/2" wide, 3/32" thick stainless, maybe three feet long, and possibly hardened 420 series stainless. I used it for handles on the Barry Wood style handle swing folders I make. I could barely saw it, and drilling holes was very tough till I discovered the simple technique of pausing the rather hard fed drill immediately when the drill point broke through. I would back off for a second to let the heat dissipate, then feed the bit in hard the rest of the way, no coolant. It would still eat up a few bits, but worked. The technique should work great if the material is not heat treated, and I am fixin to order some annealed 304 and try a new batch of folders. What got me here was researching it's machinability.
 
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