Drilling and Tapping 304SS

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Rickoo

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Feb 23, 2017
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I'm planning a jig and want to drill and tap approximately 25 1/4 inch holes in a piece of 1/4 thick 304SS I picked up. From what I'm reading it's not easy to tap 304. Haven't tried it yet, but if I'm in for a nightmare, I might just abandon the idea. Anyone have any advice? Wondering if I should look for a different steel?
 
Get a quality spiral flute tap and some good cutting fluid, and you shouldn't have a huge problem. I like moly-dee. 1/4x20 holes in 1/4" material should be fine.

I'd power tap with a drill, just don't stop half way. ;)
 
I work with 304 quite often. Karl makes a good suggestion. You don't "need" 55% thread engagement. Going a little bigger than the chart recommends will make tapping easier.

For drilling, go slow RPM. 400-500. Keep a chip coming at all times, don't allow the drill to just dwell and rub in the hole. Use plenty of lubrication. It's not that bad.

Boelube is phenomenal for tapping 304. Works fine for drilling too, but heat is the biggest thing to avoid during drilling, so coolant would be my preference.
 
I'll give it a try and see what happens. If it's too much trouble, what would be a better choice of steel for easily drilling and tapping? Is there a better form of stainless or maybe a carbon steel?
 
If you need it to be stainless, 303 is easier to machine and still relatively cheap. 416 is great to machine but relatively expensive. I don't know what you're building or why it needs to be stainless. Most of my knifmaking jigs are mild steel or aluminum.
 
Tried a few test holes today. Wasn't as bad as I expected. I drilled the holes a few thousandths over at about 400 rpm with plain old 3 in 1 oil for lube. Then tried a cheap harbor freight tap. Firm pressure and went right through. Just have to stop the drill press pretty quick once through. Not sure how many holes I'll get with this tap though.

Think a better lube would really make a difference?

Didn't really need to be stainless. I bought the stuff cheap and thought I'd give it a try. Hindsight I would have used something else.
 
304 is simply not a machinable grade of stainless. It's not designed to be drilled, tapped, threaded, etc. It lacks the sulfur that is added to grades like 303 and 416. That's what makes them easily machined.
 
Any steel can be machined, just some take an exponentially larger amount of (or much more expensive) tooling to do so... ;)

I agree that 304 should never be the first choice (or 2nd, 3rd, 4th, 5th, etc....) for drilling and tapping stainless, but it can certainly be done with success given proper tools and techniques. In some cases, you might want to weigh the cost of taps, cutting oil, drill bits, headaches, etc... vs. just getting the amount of 303 that you need and using what you have.

For only 1/4" stock, I'd wager that it was POSSIBLE to tap 25 holes, given a quality tap and some good cutting lube, especially if you use a size larger on your drill bit. I highly doubt you're HF tap will thread more than 6 or 8 holes before your piece work hardens and you break your tap, but that's just my experience with import taps. YMMV.
 
Like Karl said sulfur increases machinability. Oils like 3 in 1 or motor oil are very low sulfur oils. You don't want your piston rings machining your cylinder walls right?

Oils like black pipe threading oil or tap magic are formulated to lubricate with without trying to prevent cutting action and most have some sulfur in them to accomplish this. The benefit you're getting from 3 in 1 is more likely a tiny bit of lubricity and a great bit of heat carrying through evaporation.

The danger in tapping 304 is galling of the chip to the cutting edge. Without sulfur, it's a matter of when and not if you break a tap, especially power tapping.

Buy a bottle of pipe threading oil from Ace at least. It will last you a long time and pay for itself in saved drills and taps.
 
I was a bit surprised how easily the few test holes I did were. Even with 3 in 1 and the cheap tap I tried (and expected to fail.) I'm sure the over-sized holes helped. I tightened a few bolts and they fit snug and tighten fine. Threads appear fairly clean as well.

I have no problem picking up better cutting oil and will look for some. As well as a spiral tap. Any opinions on one of these?

http://www.ebay.com/itm/151062635516?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT
 
Spiral taps like that are for a specific purpose, namely blind holes because they draw the chip up rather than pushing it down. They also have reduced shanks and are physically weaker than a typical taper tap.

If you're willing to spend $8 per tap, if I were you I would simply order a tapered 2 flute tap from McMaster Carr. It will generally be a good brand like Hertel.
 
Thanks. Will check out McMaster. Maybe I'll pick up a decent set. For occasional use, are HSS worth the extra money?
 
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