Need help tapping 5/16-22 holes in 304

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DMS

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I have never used 304 before, so take all this with a grain of salt. My thoughts are in no particular order.

1) Could you single point thread them? It would solve your expense with taps, though single point threading an internal thread that small may be fiddly.
2) Maybe substitute 303 for the 304? 303 is more machinable. 304 will work harden readily and that equals bad times.
3) Maybe try a HSS tap. Most common taps I have seen are carbon steel. HSS may last a little longer.

The last thing is that 30x SS is not supposed to be hardenable through heat treating, so I doubt the did anything to it when you brazed it. In any case, the one you finished looks great. How many more?
 
Hi Jim,
I am thinking that the tubing itself is probably the problem.
Most tubing is drawn cold, and then annealed, IF specified. My guess is that what you have got is nice and shiny polished before you started? Usually if its been annealed it is more of a satiny finish.
I have done a fair bit of work with 316, and some 304, and although it is a tough metal it does machine fairly well. Whilst it is not heat hardenable, it does work harden readily.
I only ever used HSS, never needed cobalt or carbide tools for the job. Used a quality cutting fluid, "ROCOL Ultracut" undiluted. Its pretty hard on the back pocket tho.
You need to slow your speeds down with SS but keep the feed pressure high. Dont allow the tool to "rub" or it will work harden the job very quickly.
Taps do blunt comparitively quickly in SS.

Cheers Phil
 
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T304 is not the best selection for tapping holes. Proper tapping fluid helps. Also, is the OD is close to the ID, giving a thin wall, it will sort of follow the shape of the tap, including the relief between the flutes. The cutting pressures can exceed the material strength, so you get an egg shape that really doesn't get cut, but simply distorts and drags on the tap. A heavier wall part, or a solid, will act differently, since it will not give to the pressures of the tap.

If you need the final OD as small as that, leave it oversize until you tap it and then turn it to finish size. You might want to experiment with truncating a tap. That involves reducing the OD so as to reduce the cutting depth of the tap. Of course, you will need to follow this with a full size, unmodified tap. I have used this trick in hard materials where the torque required to cut was greater than the tap would take. It works.

If you could switch to 303, you would find it much easier to tap. But since you have 304, you should be able to make this work, just take your time.


Edit: Rereading your post, I see that you are using tubing. Worse material than bar in 304. You'd be better off turning this out of solid, tapping the hole on the lathe before turning the 3/8 OD.
 
I still think the possibility is strong that the wall is so thin it is conforming to the shape of the tap rather than cut. Are you using a 2, 3, or 4 flute tap? A taper tap will be better, as is exerts lest pressure on the part.

With the tap OD >5/16 a little, and the OD of the piece 3/8", you only have 1/32" wall. You might improve this if you are using a ful contact collet to hold the piece, if you aren't already doing so.

The TIG'd assembly may be better, depending on the filler used, and the care taken to clean the pieces, etc. Carbon inclusion is common if care is not taken, and that can lead to corrosion.
 
Then perhaps a new tap will be the remedy. I have used Moly-D as well as Tap Magic on 300 series stainless with success, although there are others. But let's not go into the bacon fat vs cocoa butter etc. ;)
 
Jim,

As has already been stated, 304 has very poor machinability, especially when it comes to tapping. Use a tapping fluid made for stainless like Tap Magic (the older variety worked better, but the new stuff should work as well) and be sure to hand tap, not machine tap. You'll need to clear the chips by backing off the tap every quarter turn or so. What's happening when you try to machine tap it is the chips are building up in the flutes of the tap and locking it up. Although 304 stainless steel cannot be heat treated, it will work harden if the surface feed of the tap is too high. If that happens, you may as well throw the part away and start over. I would be wary of the TIG weld as well. the back side of a TIG weld on stainless, where the oxidation occurs, can have hard spots and unfortunately that is right where your tap will be cutting. Be very cautious and don't try to force the tap if the going gets tough.

If I were making these parts, I would use 303 stainless and probably turn them out of a solid bar. By the time you fabricate them out of tubing and sheet, the time investment would be about the same.

Is this an MGA you are working on? I used to have a 1958 1500 and these parts look very familiar.

Good luck with the new tap.

Tom
 
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In case you haven't looked this up already:


Annealing: Heat to 1900 - 2050°F
(1038 - 1121°C), then cool rapidly. Thin
strip sections may be air cooled, but
heavy sections should be water
quenched to minimize exposure in the
carbide precipitation region.
Stress Relief Annealing: Cold worked
parts should be stress relieved at 750°F
(399°C) for 1/2 to 2 hours.
 
the best way you'll find to tap it will be to use some cutting oil called EZ-foamy, comes in a spray can, and its about the best i've seen on viscount/heat-treated h13, d2 and stainless..

buy 3 high quality high speed taps... take one tap and grind it down to create a roughing tap, then use the other two taps to finish tap.. also grind relief on the backside of the tap so it doesnt drag...... I agree with tony, I think when your tapping the tubing is collapsing around the tap which makes all the surface of the flute of the tap drag which is really creating a lot of drag, resulting in more pressure on the tap...

what i would do is when you order your taps, order some CNC taps, they are made specifically to reduce the amount of feed pressures on the tap...
this is the type of tap I'm speaking of http://www.osgtool.com/EXOTAPVA-3®-c300.htm you'll notice the short flutes of the tap, and also the overall length is shorter for more strength, the tap i linked is made for stainless specifically.. the thread size you need might be a problem though..

or purchase a threading bar and cut the threads with a carbide insert..


at work,we use order a lot of the EZ series stainless when possible, made life much easier.



heres a page a found when i did a general search for cnc taps. http://nextgentooling.com/profiles/blogs/tool-geometry-and-the
 
You could light-press the part into a piece of stock to keep the tubing from "following " the tap. I have used this method with steel tubing, not SS, but think it should work equally well. When you are done tapping, screw in a bolt and push it out of the stock.
 
I've used a small threading bar, and they do work, but they make me nervous. I've also sacrificed a tap and ground away everything but one complete profile tooth, and used it as a thread bar.

Seems I used a tool made by Kaiser the king of thinbits. Best thin grooving tools....maybe not, but it's been a while, and it was either them, or Scientific Cutting Tool. I know they make good thread hobs.
 
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