Taping bronze? (2023 Update)

Use a forming tap, rather than a cutting tap. Bronzes of various kinds can be a stone bitch to tap, much like the parent copper.

Be sure to check for the proper (larger) size hole needed for a forming tap.

Gunner

"You cannot invade the mainland United States. There would be a rifle behind each blade of grass." --Japanese Admiral Isoroku Yamamoto

One thing you can do is change your lubricant. Depending on which tapping fluid you used, it's probably chemically active in steel (or aluminum). Very likely it's useless in bronze.

As for engine oil, it's not much of a tapping lubricant. Try using some of the black tapping oil used for cutting threads in plumbing pipe, available at any plumbing supply. It's a heavy mineral oil with surfur and chlorine additives. The sulfur should give you some of the extreme-pressure lubrication you need.

Ed Huntress

I used "White Lightning" from Monroe. It says "good for all metal except for aluminum". I tried engine oil because the problem seemed to be friction in the hole. It is a very smooth friction just like a press fit. I will get some pipe taping fluid as you suggest.

It seems to me that you could grind a tap specifically for bronze. It would have all the threads ground a few tenths undersize except for the threads near the cutting edge. Does such a thing exist?

Why are brass and bronze grabby? Is it because the tooling cuts a hole a little bit smaller than the tool? Is steel not grabby because the tool cuts a hole a little bit larger than the tool?

Pete.

Ed sez:

"... It's a heavy mineral oil with surfur and chlorine

Thanks Ed. I always wondered what the sulphur did. Can you elaborate a bit more on what happens with the sulphur?

Bob Swinney

Indeed, lube oil like that will make the tap bind worse. Almost anything will make the job easier. If he cannot get some of that sulfurized cutting oil from home depot, even some pork chop grease would work better than motor oil.

Jim

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That's screw machine blood, ED ;)

Just beware of one thing: it probably will stain the bronze. You can make a dam around the hole with putty or clay to confine it to the hole itself.

'Don't know. I'd call a big tap company and ask what they recommend, and why.

The biggest problem with copper alloys is that their chips tend to be stringy, and, in the case of tough bronzes like manganese bronze and aluminum bronze, they're tough. A lot of zinc in the alloy (yellow brass, and some "bronzes") seems to make chips break easily. But there is still a lot of friction in cutting it.

Somebody has the answers written down somewhere, I'm sure. When I machine tough bronzes, which I used to do quite a lot because I made a lot of bits and pieces for ocean-going boats back in the '80s, I use the best cutting lubricants and the sharpest tools I have. And I watch out for the rake angle. Positive rake can really make cutting tools dig in and grab with brass and some bronzes.

The properties of various bronze alloys cover a wide range. Some are easy to machine, others are very difficult. I dread cutting 8% aluminum bronze, for example, because the chips are really stringy and tough. I don't know how much you can generalize about operations like tapping or reaming in bronze.

I wish I could, Bob, but I'm a chemistry ignoramus and the sources tend to be contradictory. If I had to write an article about it, I'd call a couple of experts and quote them extensively.

The only thing I can tell you is that sulfur, by itself, is considered to be an extreme-pressure lubricant. But the sources talk about the *chemical* aspects of it, in which it interacts with the metal being machined. They lose me there. And then they talk about interaction of the sulfur and the chlorine, and I'm lost in the dust.

It works. There's a reason. Somebody else knows the reason.

Ed Huntress

I certainly don't have your experience, but it does not seem to be a chip problem for me. Yes, my aluminum bronze does form long stringy chips but they are not a problem for me. I am making just a few pieces by hand and am not concerned with the time per part. When I drill I can clear the chips often and with no problem.

With steel you can drill a hole and afterwards drop the same drill in the hole without it binding. Thus, the hole is a little larger than the drill. With bronze the drill will not fit in the hole without binding. The drilled hole seems to be a little bit smaller than the drill used to make it. This is after everything has cooled to room temperature.

The same thing happens with taping a hole. The tap will not go into the thread that it just tapped the way it will with steel. I can see where an extreme pressure lubricant would help.

Is seems that there is some basic property of materials issue here.

Pete.

watch out for crappy Chineese taps now being marketed as delux bronze chaser taps @ $10 per

I had the same problem tapping a 3/8" x 10TPI x 1" long thread in phosphor bronze. The first broken tap I put down to heavy handedness - the second broke when being extra careful and I think it was impossible with the lead-in on the tap. So many teeth cutting created more torque than the tap could withstand however carefully done. The tap flutes are deeper at the front causing further weakness. I salvaged a stub tap from the remains of the broken one which utilized the strongest part of the tap and also had a much higher specific tooth load as fewer teeth were cutting. Going carefully after that did the job but, as there is little lead-in on the tap, the lead-in has to be created by tapering the hole which works fine. Nigel