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Thread Forming Taps

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by EmilioG, Jan 2, 2017.

  1. EmilioG

    EmilioG United States Active User H-M Supporter-Premium

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    I need to tap a very short hole, about 4mm deep. I was thinking of using a thread forming tap.
    Tap size is m5 x .80 (4.6mm drill for 70% DOT) I read that thread forming taps must be used on a DP or mill. Can they not be
    hand/wrench tapped? Most taps don't have enough starting threads.,( like tapping sheet metal)
    the hole is on a 1/2" OD x 8mm ID, so not a lot of tapping room. Not a blind hole.

    If not a thread forming tap, what type of tap is best for such a short hole?
     
  2. Tony Wells

    Tony Wells United States Former Vice President Staff Member Administrator

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    Why do you believe a form tap to be suitable for this? If it is a shallow thread, the usual method is to use either a taper or plug tap, followed by a bottoming tap. There have been instances where I have had to modify even a bottoming tap to get a little deeper. It can get a little dicey once you start grinding even what little lead in that is on a bottoming tap, but it can be done.

    Some parts I have had to run I said were tapped two turns past the bottom of the hole. At least it seemed like it.
     
  3. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    Form taps produce no chips which helps with blind holes, this is a big plus in production work as cleaning chips from thousands of small tapped holes is time consuming.

    Hole size is important, you are forming not cutting. I see no reason why hand tapping will not work if all parameters are met.

    Forming a full thread all the way to the bottom of a shallow hole will require a non standard tool as Mr. Wells mentioned.

    Good Luck
     
  4. 2volts

    2volts Australia H-M Supporter - Premium Content H-M Supporter-Premium

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    I'm trying to work out where the 4mm depth comes from.

    Are you trying to thread a hole cross drilled through a tube 1/2" OD and 8mm ID?
    so the wall thickness is only (12.7-8)/2 = 4.7/2 = 2.35mm
    which isn't even 3 full threads at 0.8mm pitch

    IMG_1960-M.jpg

    anyway this is a 5mm x 0,8 taper tap so you would get close to full thread and be able to finish with a bottom tap

    pete
     
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  5. EmilioG

    EmilioG United States Active User H-M Supporter-Premium

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    Thank you. Yes, you are correct. Wall thickness left after reaming will be 2.35mm. This is why I was thinking to use a roll form tap, but if I can use
    a bottoming tap to finish, great. I will also need to put a flat on the part that slides into the 8mm ID so that I can use a set screw and clear the surface.
    I don't want the set screw to stick out., unless I grind the set screw. The shortest set screw in m5 is 3mm L.
     
  6. Tony Wells

    Tony Wells United States Former Vice President Staff Member Administrator

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    Well, a very common cheat in this scenario is to drill into the far side of the hole to gain some length to work with. I have made extensions for air motors and taps and a few other things that needed a set screw to bear against the accessory being extended. It's not always, but never hurts, even to drill all the way thru the far side and just tap it all the way. That way you even have a backup in case the original tapped hole wears out. With such a short engagement, you should be careful of how much torque you put on the set screw. Of course, it's ideal not to have any set screw sticking out of the OD, but as long as it's not much, generally there isn't a problem with it, unless you are working in a very tight spot, or are in a bore that you can't risk putting some pretty heavy scratches and dings in.
    Overall, I don't favor using these types of extension in high speed situations, but it can be done, with care.
     
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  7. EmilioG

    EmilioG United States Active User H-M Supporter-Premium

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    The sleeve will be used for an Indicol mod. No high speed or high torque., so it will be fine.
    I thought about drilling into the opposite side of the wall a bit to gain more threading room, but may take your suggestion and drill
    all the way through for a secondary tap hole as a back up. The part will slip into an r8 collet so the set screw cannot stick out.,
    hence the small weldon flat on the 8mm ID part. To get the squared off weldon flat, I need to use a square end/ end mill correct?
     
  8. Tony Wells

    Tony Wells United States Former Vice President Staff Member Administrator

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    That's probably the easiest way, and although genuine Weldon flats have beveled lead-outs, you shouldn't need them for what you are doing. You can just run an end mill across it at the desired location. Bear in mind that the length of set screw and depth of that flat work together.
     
  9. Jimsehr

    Jimsehr United States Active User Active Member

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    Remember form taps need a bigger id size then cut taps.
    jimsehr
     
  10. EmilioG

    EmilioG United States Active User H-M Supporter-Premium

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    Yes they do. I have Guhrings' chart for roll form taps/drill size. Going with standard tap sets anyway.
    Yes, Weldon flats do have bevel ends, but for my parts they are not necessary. Just a little dressing to remove sharp corners.
     
  11. Tony Wells

    Tony Wells United States Former Vice President Staff Member Administrator

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    I've only tried form tapping by hand a couple of times as an experiment. Both times were failures. Tap broke. The great success they are capable seems to me to be with a rigid power tap cycle in a CNC machine, and all done in one shot (Fanuc G84). I've seen them break many times with a regular tapping head. I ran some on a SpeedyCut tapping machine that is gear driven with the proper lead to match the tap and even then, if you are hand holding the part, the slightest move of the part can result in a broken tap. I'm sure that was exacerbated by the size of the tap I was using, but it sure changed my personal opinion on where they belong.

    If anyone has used them by hand, without breaking (and they are a pain to get out) then more power to them. They do make a stronger thread, and are great when the application allows.
     
  12. EmilioG

    EmilioG United States Active User H-M Supporter-Premium

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    Glad I asked first before buying them. I will pass on roll form for now. I don't need precision for this part.
    I'm going with standard tap sets. I also found short set screws that I'll take a closer look at. They're from McMasters and
    come in 1/8" lengths. Hollow lock set screws. They may work well enough. They have have brass tip set screws, but I'd
    rather go with Mikeys' suggestion and use Delrin rod as a buffer to prevent marring.
     

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