Interesting Tap Experience

[Merkava_4]

I picked up a Morse high speed steel cutting tap at the industrial supply today. The size is M8-1.25.
For some strange reason, when I spin an M8-1.25 nut on the tap, the nut will stop at the last 3 threads.

I tried this with two identical GM flange nuts and two identical Rockford flange nuts from NAPA and they all stop at the last 3 threads. Meanwhile, I experimented with a Vermont American hardware store tap in the size 3/8-24 and the nut spins all the way past the last 3 threads no problem.

 

[oilboy123]

Sounds weird, must be defective then......

 

[Johnny2Bad]

May be slightly oversize to allow for starting the bolt; the bottom 3 threads on the tap would correspond to the first three threads in a tapped hole if you ran the tap all the way through until it bottomed.

 

[JHZR2]

+1. It seems to me that sometimes threads are tapped such that the first few threads are a bit larger to allow easier alignment. But maybe it's wear too, I'm no expert. But I have observed this in some cases.

 

[Trav]

I think it might have something to do creating a slight bind to throw the machine into reverse when power tapping.

 

[tom slick]

What type of tap is it? Taper, plug, spiral point, etc? I've never seen taps with a different thread profile at the end and don't think such a tap would meet the standards for thread quality.

 

[VetteElite]

Different H number on the tap you bought than the one that cut the threads in the nut?

Also, when machine tapping, you'll notice that almost all wear on the tap is on the first few cutting edges. We typically don't bottom out taps when machine tapping (which the nut was certainly produced with

 

[Merkava_4]

You might be onto something Johnny. I was going to take a picture, but this Amazon link with the rollover zoom probably shows it better than I can. The last 3 threads of the tap look look like they widen ever so slightly.

 

[RF Overlord]

I have a situation where an M8x1.25 bolt will thread in about a quarter of the way and then binds up. Because the piece it's going into is aluminium and difficult to remove, I don't want to damage the threads. I've been told to use a "thread-chaser". Would running a tap down the hole be an acceptable alternative or are those two different tools??

 

[Mitch]

make sure it is not a 5/16 thread. The difference between 5/16 and 8mm is .3125 vs .314.
hard to visually see a .0015 differnce

 

[Merkava_4]

Originally Posted By: RF Overlord
I have a situation where an M8x1.25 bolt will thread in about a quarter of the way and then binds up. Because the piece it's going into is aluminium and difficult to remove, I don't want to damage the threads. I've been told to use a "thread-chaser". Would running a tap down the hole be an acceptable alternative or are those two different tools??


There's differing opinions on that. I've always used cutting taps to chase threads and have never done any damage to a threaded hole. When I went into the industrial supply store to buy that tap, I asked if I could first look at the thread chasing tap they were talking about. I was expecting them to bring out something that looked like a Kastar thread chasing tap, but instead, they brought out a Hanson carbon steel tap. I said, "that looks like a regular tap." They said, "it is a regular tap, but the steel is not suitable for cutting new threads into steel." I've always used carbon taps for cutting threads into mild steel and softer materials with no problem, but according to them, a carbon tap is a "thread chasing tap."

 

[RF Overlord]

Originally Posted By: Mitch
make sure it is not a 5/16 thread. The difference between 5/16 and 8mm is .3125 vs .314.
hard to visually see a .0015 differnce
I know it's the correct bolt because it came out of that hole in the first place and the same bolt works fine in the other hole. I just think that running a steel bolt in and out of an aluminium hole I probably galled the (hole's) threads a little and need to clean them up. At least that's my hope...

@ Merkava: so using a tap to clean threads in a softer material like aluminium is fine?