chasing tapped hole with roll form tap

welder2022 · Feb 8, 2018

I have some 3-56 i.d. threaded parts, mild steel. The male threaded mating parts are oversized from heat treat and won't fit now. Can I use a H3 roll form tap to chase out (enlarge) the threads so the mating parts will fit? Should I drill out the holes a bit first?

Gobo · Feb 8, 2018

How hard are the mating parts? Roll forming has its limits concerning material hardness.

MrWhoopee · Feb 9, 2018

Remember that roll forming doesn't cut, it just pushes the material. With the thread already fully formed, there may be nowhere for the material to go without enlarging the minor diameter. Other than strength of the tap, is there a reason not to use a cutting tap?

welder2022 · Feb 9, 2018

MrWhoopee said:
Remember that roll forming doesn't cut, it just pushes the material. With the thread already fully formed, there may be nowhere for the material to go without enlarging the minor diameter. Other than strength of the tap, is there a reason not to use a cutting tap?

I haven't found a cutting tap more than H2 oversize. I am thinking I need H4-5.
Btw this will be done by hand...

welder2022 · Feb 9, 2018

Gobo said:
How hard are the mating parts? Roll forming has its limits concerning material hardness.

The hole is soft. The male threaded part is hard, like 50-60 Rc.

FredC · Feb 9, 2018

If I was doing a bunch of these I would buy more than one tap in case one breaks, if this fits, try one form tap and have back up cut taps in case it does not work. A lot of commercial nuts have a fairly low percentage of threads so they will have room for a little displaced material from forming. If the nuts are plated a form tap may leave some of the plating in the threads.

Let us know how it goes and we will all be smarter when you finish.

Gobo · Feb 9, 2018

welder2022 said:
The hole is soft. The male threaded part is hard, like 50-60 Rc.

What percentage of thread does your female part have? If it is anything less than 70, your deformation will be so slight that a form tap will probably work fine.

Bobw · Feb 9, 2018

MSC has Hertel spiral points that are an H4..

I was thinking taps for plating, but a quick look didn't find anything
under a #10.

How many of these do you have to fix/modify???

How far over are the male parts? Is it just scale? Did they
damage the treads somehow? Or did they just grow??

A 2.5mm X .45 is a standard tap, and is almost identical to a 3-56, except
for a tiny pitch error of about .0015".. I wonder if the pitch error would
open things up enough for your mating parts to fit.

Here is a bad idea.. Put a regular old #3-56 in an electric drill, and then run
it in and then run it out, take it out, put it back in, rinse repeat... I guarantee
you will open those threads up after a few passes. DO NOT ask how or why I know this.

If you are making the parts from scratch and are rigid tapping, tap once and then do
it again, but start a few thou difference in height.

TDegenhart · Feb 9, 2018

Actually, unless you order differently, I have found that H3 is the normal tap size. If you snoop long and hard enough, you should be able to get 4,5 and possibly 6.

Tom

Gobo · Feb 9, 2018

Bobw said:
MSC has Hertel spiral points that are an H4..

I was thinking taps for plating, but a quick look didn't find anything
under a #10.

How many of these do you have to fix/modify???

How far over are the male parts? Is it just scale? Did they
damage the treads somehow? Or did they just grow??

A 2.5mm X .45 is a standard tap, and is almost identical to a 3-56, except
for a tiny pitch error of about .0015".. I wonder if the pitch error would
open things up enough for your mating parts to fit.

Here is a bad idea.. Put a regular old #3-56 in an electric drill, and then run
it in and then run it out, take it out, put it back in, rinse repeat... I guarantee
you will open those threads up after a few passes. DO NOT ask how or why I know this.

If you are making the parts from scratch and are rigid tapping, tap once and then do
it again, but start a few thou difference in height.

How do you know?

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