Taps and Dies

[milotrain]

I'll start by saying that I don't tap that often. However in the limited exposure I've had to tapping I've discovered a few important things.

1. It's all about the material you are tapping into and the diameter of the hole. You can use a steel screw to tap a 6-32 thread into aluminum if you have too. You need a nice tap (or epic patience) to tap a 3/8-16 thread into SS.

2. Don't forget that different materials want different pilot holes. If you find you are having trouble tapping look at the chart and open up the hole to the next size that's acceptable for a different material. For example, that 3/8-16 want's a 5/16" pilot in aluminum, brass and plastic but it was a fight to tap into SS with a 5/16" pilot. Should have used a Q size bit. Either way though 3/8-16 in plastic want's a 5/16" (.3125") hole while 3/8-32 wants a T (.3580") hole. That's a pretty big range, if you are making a big hole use a boring tool to open up the hole in steps within the holes tap range until the tap runs smoothly.

3. Cutting oil and verticality cannot be overstated. Use whatever methods you must to keep that tap straight, and run engine oil if you have nothing else (or Olive oil). Don't tap dry.

4. Patience. I made that 3/8-16 thread in a SS block with a crappy Vermont American tap (I don't like them as much as even the Irwin taps you can get at Home Depot) and an undersized hole of 5/16" but I only advanced about 10 degrees before backing off and making sure the new threads were pretty clean.

 

[JohnT]

FWIW, I have bought NOS taps in the small sizes I need on EBAY like 3-48, 4-40, 5-40, 6-32, 8-32 in small batches. The taps are Greenfield, Winter, Rieff&Nestor. I recently bought 30 taps total from one seller consisting of the various sizes I just mentions for $90 shipped. That's about three bucks a tap. Not too bad, I'd say. Other than doing it that way, if I buy them new I will only buy American taps like Greenfield, or Japanese taps like OSG . No others. Not worth it. Same with dies.

 

[Tonnetto]

Tapping large coarse threads in tough materials can be quite difficult, if it's possible try to make the hole extra deep or even go through with it to give room for the chips to go somewhere. In my fulltime job we put threads into parts using many different methods depending on the size of thread and what the material is. (Of course working on a $750,000.00 CNC doesn't hurt). Generally speaking, for thread sizes 3/8" and smaller in aluminum we'll use rollform taps, they require a complete different hole size chart, the advantage is that they don't make chips and you can tap to the bottom of holes without stopping. For harder materials and larger sizes in aluminum we try to threadmill whenever possible (CNC only method). Although it would be possible to threadmill on a manual mill using a rotary table that's somehow geared into the Z axis. Another option for tough materials would be a high production nut tap it has a very long lead to it that gradually cuts the thread to final size, the downside is that the hole has to be a thru-hole or very deep.