broken tap!!!!!!!!!

True, I just had to jump in somewhere. It seems the acid would need several weeks to work in this case. OP is looking for a homemade edm (one off disposable.) He should watch tose hole sizes more closely, too.

wws

I second that! I've used this technique to remove broken drill bits and taps from aluminum and copper. It does take time. My experience has been with small parts that I could through in a beaker an a stirring hotplate. I love to see that stream of bubbles coming from the hole as the steel is disolved.

Alum???? is that the same stuff that they cook with, a taste less product or is that short for aluminum somthing or other??????? not being funny, but i would like to know... and will it work with a broken tap in a car engine???? (aluminum motor parts)....????? thanks for a reply...

I use the same technique when I brake drill bits inside precious metal. Works like a charm every time. Of course my parts are very small and I can boil them in the alum solution.

Ya, it's the same stuff. My part was small so I could put the aluminum in a saucepan of almost boiling water. I mixed about 4 or 5 containers of the Alum in the hot water, dropped in the part, set the heat to "simmer," came back once in a while to stir the mixture (the Alum tended to precipitate out of solution) and in about an hour and a half, the broken 2-56 tap had completet dissolved. There was a VERY slight pitting of the aluminum after the process but I managed to polish it out.

During the process, I could see bubbles coming from the hole containing the tap. It was as though the tap was just "fizzing" away!!

The stuff I use is Aluminum Ammonium Sulfate, from our Chemistry Stockroom. There several chemicals that use the generic name Alum.

Aluminum sulphate is available from garden centers to acidify soil for things like blueberries. 5$ 5lb bag It is also used for setting dyes when marbelizing fabrics Darma Trading Co sells it. Find em online if the spelling is off look under tie dye supplies.

thanks for the suggestions. Someone suggested alum, funnily enough my dad was here and he said "I think alum dissolves steel but not aluminium" strange what surgeons know.. he then appeared with a couple of boxes of it..

I haven't tried the alum, yet. I etched into steel recently caustic soda[NaOH] and some volts. I cut ~.125" depth in no time in some steel. Remebering that I knocked up a sort of clamp on tube, sealed to the surface with an O ring, it will hold 50 CC of acid and it also holds an electctrode. It's on the casting and merrily fizzing away at the moment, I used battery acid as electrolyte. The fact it's a roll tap obviously makes it slower to remove, though it does help that I sprung for the tap with lubrication grooves, sureely your likely to hydraulic in blind holes without them, after all the 'cam height' of the lobed ones isn't eater than thread depth, is it? After an hour with 12volts and 2 amps, it's eaten enough to pick out one fractured bit that I couldn't get out before. not that it looks any smaller;-)

I couldn't feasibly weld to it, it would take a lot to unscrew it. I tried a carbide mill, well the remains uf a broken one and I ground it like a D bit and ran it backwards, well it got hot but didn't do a lot. stiff like a Bridgeport, no stiff like an Ajax.

a small tap busting edm seems the best solution, but it wouldn't get a lot of action, I take the piss when I see people break taps. I haven't done it for so long.

The hole size for the tap is 8.7mm and I am drilling with an 8.6 and getting 8.6>

You do know that caustic soda is used to etch aluminum? Creates hydrogen gas?

IIRC you are tapping 3/8-16. Balax says .344 diameter for a 75% thread, and you are drilling to .338/.340? Do you need to have an 85 or so % thread? You might try using OSG AL type cutting taps. They have a short length of thread, helical style, with relief above. They pull the chip out of the hole and work very well. I use them regularly.

What to do for tight threads. I guess you could keep drilling the holes small and take your chances. Get some Alumatap, A-9 is pretty good, but I think Alumatap is a little better. Maybe kerosine, but I don't recall ever using it for form tapping, cut taps, or other 'cutting' operations it works great. And stinks, and a PITA to wash off.

michael

Perfect! Sulfuric acid, with the workpiece positive will anodize the aluminum, but etch the iron tap. After a few minutes the aluminum will become insulated and no current will flow from the exposed aluminum..only the tap.

There is a commercial acid based product for this called Tap Out. It has the acid and neutrializer for steel taps in alum parts.

A fondness for hydrocarbon aromas is a prerequisite to metalworking crafts.

True. One of my most memorable and favorite aromas was going to the gas station early on a weekend morning with the old man to fill 'er up. Well before unleaded gas, that was. Kerosine smell is not bad, I find it really sucks to get it washed off.

mj

The part of the tapped hole that SHOULD be making the tap a tight fit is the pitch diameter, not the minor diameter. Decreasing the tap drill size when using form taps has a dramatic effect on the minor diameter and the torque required to tap the hole. In a 3/8 -16 form tapped hole .3431 gives 75 percent thread, while a .3474 gives 65 percent and .3516 gets you 55 percent. Usually, form tapped holes are tapped to 65 percent. You can see that it is possible, with a form tap, to have the tap fitting so tight that the metal has no where to go but ahead of the tap. This reduces the hole diameter ahead of the tap more and more until the torque required is greater than the breaking point of the tap. Looking at the MSC catalog shows that form taps start out at the H5 designation which theoretically puts the tapped hole at .0025 above basic pitch diameter. The fit will usually be tighter. That's why form taps start out at H5 but cutting taps usually start at H2. You can order taps ground to smaller H numbers from MSC. They are not that much more expensive than regular taps. Reduce the tap drill size accordingly and you will get the tight fit you want along with increased tap life. You should get more way than

264 holes out of 1 tap. When first starting to work in a machine shop I tapped over 11,000 holes with one 10-32 roll tap. These holes were about 3/8 long and were through holes. The holes were drilled to give 70 percent thread. However, the castings were die castings of aluminum and were good castings with no hard spots etc.. Also, I just tapped 'em. The setup was done by the boss and the holes drilled by someone with experience. I do know the holes produced 70 percent theoretical threads though. We had class after work and since I was doing all those holes I learned all about the hole size , cutting oil, speed etc. It was my job to inspect the hole size with pins every so often to double check the inspections done by the guy drilling the holes. ERS