Form tap trouble in 316?
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  1. #1
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    Default Form tap trouble in 316?

    I ran into a weird problem tapping 316 and I'm kinda scratching my head.

    The part in question is made in two operations: the top, where I hit five surfaces, and the bottom, where I surface the bottom (it's at an angle to the top). The top has 2 M3x0.5 blind holes. The part mates to three others, and it's a little fussy, so I made a prototype in 6061 to check fit -- works perfectly. The environment is wet and the part needs to last for decades so I made a final version out of 316. The part design is identical, I just reprogrammed it to account for the new material. The drill and tap are identical between the 6061 and 316 version. The tap is an OSG Exopro XPF form tap. The drill is a no-name HSS-TiN 7/64ths, which has cut exactly 4 holes in its entire little life.

    Before I broke the first setup, I checked the thread fit with a M3 screw, which was perfect. After the second operation, I went to assemble the part, and the M3 bolts won't thread. They turn in maybe two or three turns and then bind up.

    The thread looks good on visual inspection. I can chase the hole with the tap with zero resistance, the fit feels perfect. And this worked perfectly in the aluminum part -- same drill, same tap. But no bueno in stainless.

    What am I missing?

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    I assume you are using SS bolts since you mentioned environmental issues. SS will "gall" in SS. I have had 316 lock up on a known good thread and had to trash the part. Never did get them apart. Try some lube.

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    Stupid question but you're sure that you are using the M3 screw when you tried to assemble?

    I may have read incorrectly but does the screw thread into the part when you aren't assembling it to the other piece? If so, I'd guess a setup issue that caused your holes to be off in location.

    If your (new?) tap fits and your matching screw doesn't, I'm intrigued!

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    Are you running this on a Speedio/Brother? Did you program with G77 and I for the metric pitch (I.5). I find a lot of users misuse the G77 and try to convert the metric pitch into TPI and use J. To be fair the manual is not clear on this. The way I remembered it is J is Just inch (TPI) and I is not Inch (metric pitch). Had a guy call me from the Northwest the other day. He was trying to form tap chromoly steel with a M12 x 1.75 on a High Torque and was struggling. Had him switch code to I1.75 and worked beautifully.

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    Do you have a countersink at the entry just a little bigger than the major diameter? There may be better guidance on the countersink sizing.

    The first thread tends to be pulled up, it is pretty easy to push that raised thread back down and have the screw bind. Try re-chamfering the entrance to see if it fixes things.

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    I would change to a Cobalt drill with some sort of coating, coated form tap and check the %concentration of the coolant. Most of the problems with form tapping I have are when the coolant concentration is low. Maybe want to be at 7.5%min

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    Interested to hear about the cause. Any update?

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    if the threads in both holes are fine, but the screws bind when you use them to attach a mating part then one of your hole locations (tapped part or through drilled part) is off. The harder material may have caused the tap drill to wander enough to screw up your fit.

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    Quote Originally Posted by BROTHERFRANK View Post
    Are you running this on a Speedio/Brother? Did you program with G77 and I for the metric pitch (I.5). I find a lot of users misuse the G77 and try to convert the metric pitch into TPI and use J. To be fair the manual is not clear on this. The way I remembered it is J is Just inch (TPI) and I is not Inch (metric pitch).
    Yep, on a Speedio. Great callout, but the tap call sure looks correct to me: G77 X0 Y-11.975 Z-8.5 R3 I0.5

    (Note that the machine is in metric.)

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    Quote Originally Posted by triumph406 View Post
    I would change to a Cobalt drill with some sort of coating, coated form tap and check the %concentration of the coolant. Most of the problems with form tapping I have are when the coolant concentration is low. Maybe want to be at 7.5%min
    Coolant is Trim 690XT at 8.5%. The threads looked excellent and the form tap is pristine.

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    Quote Originally Posted by mattthemuppet View Post
    if the threads in both holes are fine, but the screws bind when you use them to attach a mating part then one of your hole locations (tapped part or through drilled part) is off. The harder material may have caused the tap drill to wander enough to screw up your fit.
    To be clear the M3 screws were binding up even without the mating part present....

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    I re-made the part with the addition of a 0.5mm chamfer at the M3 threaded hole prior to tapping. That seemed to do the trick -- the second try went together fine.

    So perhaps the screw deformed the thread in the first turn?

    Thanks everyone!

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    Quote Originally Posted by trochoidalpath View Post
    Yep, on a Speedio. Great callout, but the tap call sure looks correct to me: G77 X0 Y-11.975 Z-8.5 R3 I0.5

    (Note that the machine is in metric.)
    Your code is perfect. Awesome you program in metric with 1 micron resolution. Hopefully someone else will be helped from this thread too.

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    Quote Originally Posted by trochoidalpath View Post
    I re-made the part with the addition of a 0.5mm chamfer at the M3 threaded hole prior to tapping. That seemed to do the trick -- the second try went together fine.

    So perhaps the screw deformed the thread in the first turn?

    Thanks everyone!
    Good to hear this solved things. I had run into the same problem manually tapping with form taps. The tap would run great, then immediately go to thread a screw in and it doesn't go. Chamfering solved it.

    I have been meaning tap a part without a chamfer and check it out under a scope so I can see exactly what is happening. It certainly isn't intuitive that it works the way it does.


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