View Full Version : Annealing 304 stainless...? Is that even possible?
04-21-2007, 04:09 AM
I have some SS rod that I'm pretty sure is 304. I need to tap it for 6-32, and I'm having absolutely no luck at all. I've already broken or jammed taps in three parts, and I'm getting quite sick of having to keep remaking the part.
I have fairly new, name-brand, ground taps (spiral-point, three-flute and 4-flute, one's TiNitrided) and I've used Tap Magic for stainless, plain motor oil, WD-40, and heavy, dark "sulfurized" cutting oil.
I also have to tap fairly deeply- about 3/4 inch. My chart says the nominal drill size is .106", I've gone as high as .118" which hasn't seemed to help much, and I worry about thread engagement if I go much larger.
So I've wondered- I know stainless is "gummy", but would localized annealing help it any? Heat it red with a torch and jam it in a can of kitty litter for an hour or two?
Or are there any other tricks I can try? I can't get a different alloy unless I special-order it, and they gets incredibly expensive. The local supplier carries this 304 rod and I can get just a few feet at a time (which is good in that I only use a few inches a year.)
04-21-2007, 04:30 AM
Can you get 6-32 in serial taps over there?
I know we can't over here but the number series Un was never popular here anyway, we used BA and then metric.
The serial taps have to be used in turn because as well as having a greater taper on the taper and second they also have increasing OD's so the taper tap only removes a sliver, the second removes a tad more and the plug goes up to full diameter.
They are a pain for everyday tapping having to use all three or possibly two but in tricky material they can be a boon.
04-21-2007, 05:42 AM
304 is miserable. If it is possible, change to 303. If not, is it possible to relieve the bore so the whole 3/4" is not tapped.
The other area to look at is taps that are specifically for stainless. Check the catalogs or suppliers, there are taps available that have a much shorter cutting area, and black oxide finish for stainless. You might be looking at taps in the $10-$20/each range.
My Greenfield tapping book lists .117" as an acceptable oversize drill for such a long length of thread engagement. 3/4" deep tapped hole with a 6-32 is not a good design.
I know 304 can be nasty to tap, but with such a large tap drill I can't imagine it'd be a problem. OSG EXO taps usually will do the job in nasty stainless.
What about your drills? Are they free cutting? You might be work hardening the hole surface too much.
Are you hand tapping or machine tapping?
Try 316, it seems a bit easier to work with.
As a last resort order some Carpenter Project 70 stainless. www.cartech.com It's expensive relative to regular stainless, but so is breaking taps and having to redo the parts. I order Project 70 whenever we have a really difficult part to machine, it's well worth the extra cost.
04-21-2007, 05:58 AM
Years ago I did a lot of hydraulic piping in stainless tubing with a 37° flare fitting on each end. cutting the tube with a pipe cutter was sufficient to work harden the SS and the tube would split when I attempted to flare it. I found that annealing solved the problem. A torch was OK but slow. I found that if I briefly but aggressively ground the end of the tube on the disk sander the tube heated to red for about the first 1/8". After cooling, flaring was no longer a problem. Could be that in addition to annealing, I was removing the hardened region. 'Course, I had to compensate in length for what I removed during sanding. Obviously, this method wouldn't work of your application, but demonstrates the value of annealing.
04-21-2007, 06:17 AM
Doc, I think you can anneal it, but IIRC, you do it by quench annealing, i.e. heat it red hot then dump it staight into cold water.
04-21-2007, 07:11 AM
Switch to 303 ss if you can.
304 round bar is all sold in the annealed condition but as others have stated you may be work hardening by drilling.
Yes you can anneal it. The tech info I've seen on 304 state : anneal temp is 1850-2050, then cool rapidly in the 1500-800f range to avoid carbide precipitation( which is the carbon in the alloy moving to the grain boundaries causing a point for intergranular corrosion).
04-21-2007, 07:19 AM
Doc,try aluminum never-seize for the lube,coat the tap and the inside of the hole.This will work for a few holes in a pinch,but if your planning on many get some Greenfield EM series taps.Spiral flute and made for stainless,they make the stuff thread like it's aluminum.I used to thread some 1/2-13 holes 2" deep with them,I have done 600 of those holes with one tap so it was worth the $28 I paid for the tap:D
04-21-2007, 08:14 PM
Your holes are way too small.
for stainless steel, you only need 50 % engagement.
6-32...Major diameter is .137....Minor diam is .097
The root diameter to major diameter ratio is terrible (of course 10-24 and 1/4-20 are worse !)
The .106 size is 78% engagement...ridiculas in SS
So stay with the .118 and you will never pull the threads..believe me
You are also going for 6x diameter engaagement.. that is way overkill, unless you want the range for adjustment purposes.
Our toughest jobs were handled nicely by OSG EXO taps as was suggested earlier..
Can't top that !
04-21-2007, 08:55 PM
I think DR found the key; I think my drill was just dull enough it was work-hardening the hole as it passed.
So with that in mind, I drilled it first to .090" or so (just grabbed a drill at random) then used a brand-new #32 (mic'd at .115" on the shank) to drill the final hole. Cut like butter.
That made a big improvement in the tapping- less "squeaking", less force needed, actually felt like it was cutting, not trying to shove metal aside. I also found what I think was a brand-new tap (I'm running out of 6-32s!) and that helped as well.
Between the two, that finally worked. Once I'm done here, I think I'm going to set the fresh drills and sharp taps aside, specifically for the next time I have to do stainless.
That and order some 303. :D
04-21-2007, 09:34 PM
Try Crisco instead of tapping fluid.
04-22-2007, 03:48 AM
I suspect that, if what I suspect about the work hardening (and possibly a slightly-dull tap) is true, then I doubt any miracle lube, homebrew or otherwise, will help much.
I will say, though, the dark/sulfurized oil did make a slight, but noticible improvement even when I was still trying the original setup.
Not enough to cure it, but it did seem to "squeak" less and turn a touch easier.
04-22-2007, 12:12 PM
I have a lot of 304 around to play with, and I've used it for all sorts of stuff. When I'm drilling and tapping 304, I spend a lot of effort making sure the drill is sharp and things don't get hot. I've heard about and experienced work hardening way too much. It sucks that, in an instant, you can get the drill red hot and dull, and the hole is then officially un-drillable. So it's worth the trouble to stay cool.
The "groaning gerbil" type of squeak is also a sign of a tap about to break. I try to walk away and regroup before damaging something.