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Form Taps For Different Materials

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  • Form Taps For Different Materials

    I have posted this on the PM site but got no responses. So I am posting it here in the hope that someone has an answer or at least some thoughts.

    I am doing some experiments with taps and am thinking about getting some form taps (chip-free taps) to play with. I have been testing with both aluminum and a mild steel.

    Looking on the McMaster web site I see that they have these taps for both steel/stainless and for aluminum. Their illustrations and text do not show any difference. In fact, it looks like they used the same illustration for both. The only difference I can see there is they say the steel taps are coated while the aluminum ones are described with "The threads are specially shaped to produce smooth, accurate threads in soft materials." Both types are described as to be driven with drill press or CNC.

    So my question is do I have to buy both styles if I want to experiment with both steel and aluminum? Is anyone familiar enough with this style of tap to know what the real differences are, beyond coated vs. not coated? Or is that it? I would prefer answers from people who have actually used both types although not necessarily ones from McMaster. And does anyone know what brand of form taps that McMaster is presently selling. Perhaps the manufacturer's web site would provide more information.
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.

  • #2
    Some form taps have a groove up the side for hydraulic pressure relief of cutting fluids. For years Balax taps with the groove were all my local tooling house carried in stock. I found in soft materials the grooves tend to fill with swarf eliminating the pressure relief and causing the taps to break.

    Taps with a lobed design worked better in soft material. By lobed, I mean the tap has an almost squarish cross section with the forming done by the four lobes while still allowing pressure relief via the area between the lobes.

    Generally, uncoated for aluminum and coated for harder materials. The coating resists tap wear.

    Form taps "dull" differently than cutting taps. Since there are no sharp edges it's difficult to determine dulling. What happens is the taps begin wearing down on the leading threads so the tap has a taper on the end. Your tapped hole then has incomplete formed threads at the bottom of the hole which aren't noticeable until you try to screw a bolt in.

    Using correct technique and fluids form taps can last "forever". My all time record for a production run in an automatic lathe was over 4,000 10-32 holes tapped 7/8" deep in 12L14 with one tap before it broke.