Why Wavy Threads in a Tapped Hole?

[PW8697]

Seeing something in a part today that I don't understand why.

Below is a 0-80 tapped hole. Why are the threads wavy? Goes the whole depth of the hole.

Using a Haas TM-1P, drilling with a .056" drill at 3000rpm and 2.5" plunge feedrate. Then rigid tapping with a Balax 0-80 form tap. 400 rpm, peck tapping .050 each time.

This is in .090" thick 360 Brass. Have done thousands of holes in the past which were fine. May have adjusted the drilling or tapping parameters slightly since the last batch a few months ago, but as conservative as they are, I don't have a clue with the science why this hole has wavy threads.

 

[Larry Dickman]

why are you peck tapping with a form tap?
Do you have the parameter set for orienting the spindle when peck tapping?

 

[DDoug]

What did the blank hole look like ?

 

[PW8697]

Peck tapping, like I said, being conservative. Thru tapping has not resulted in a very good success rate on tap survival. If it was 4-40 or more, then sure, nail it home all day long.

Good question about the blank hole, I don't know. Assumed it was fine, being that the tap didn't break and the holes are spot on location and the drill bit still looks like all the brand new ones, no chipped edges, no wobbling, etc.

 

[Mike1974]

We form tap 0-80 all the time in brass (not sure if it is 360 however), 2-3x diam, 1200rpm 15ipm. We use a .055" drill.

 

[PW8697]

Ok, we got that everyone runs a form tap thru or 2-3x diameter, but what is the theory on the waviness of the hole?

 

[Ember]

Ok, we got that everyone runs a form tap thru or 2-3x diameter, but what is the theory on the waviness of the hole?

OK, Never seen that before but here's a theory: If a drill is slightly blunt you can get that triangular look to the hole, you say the drill looked fine and wasn't wobbling so it wasn't that, if the tap wasn't perfectly centered in the chuck or was slightly bent maybe it could oscillate in the same way, giving you a slightly larger than average not quite round thread? It the tap was fine then perhaps the same thing could be caused by uneven hardness in the metal stock caused by a defect?

 

[PW8697]

Could be the material.... I've got extra left over and will run a test. The brass machined perfectly, even using .039 and .0625 end mills, but drilling and tapping could be another story. When things slow down I will do a test. All in all, the parts aren't scrap, so that is a good thing.

Thanks for staying on topic.

 

[TDegenhart]

Cut one of the scrap pieces in half and look at the thread.

Tom

 

[rcoope]

+1 on wondering about the lobularity of the hole. But also yes saw through one and take pictures.

 

[Ember]

Could be the material.... I've got extra left over and will run a test. The brass machined perfectly, even using .039 and .0625 end mills, but drilling and tapping could be another story. When things slow down I will do a test. All in all, the parts aren't scrap, so that is a good thing.

Thanks for staying on topic.

If you don't wan't to cut the material try measuring the ID of the hole and comparing it to the ID of a straight taped hole of the same thread, if the tap was oscillating as it was cutting the ID will be slightly bigger

 

[Matt_Maguire]

The OP said they’ve done this before successfully so I’d agree with checking the drilled hole carefully on the setup before forming the thread.

That said, DON’T FORGET the material choice… 360 free cutting brass (screw machine stock) is a piss poor candidate for cold forming. It commonly is knurled but that's different than this. You don’t have to go nuts with copper content to get a better candidate either.

Attaching some info… If the problem persists and I really wanted to do 360 by cold working I’d get a coolant mixture recommendation from the brass people.

Good luck,
Matt

 

[CountryBoy19]

I'm going to bet it has to do with the peck tapping. When the tap hits metal it loads/flexes a bit. A peck will let it unload and pecking changes the dynamics of the tap especially right when it gets back to the point where you left off and it loads and flexes. Do that multiple times in a material that doesn't form well and I wouldn't be surprised to see a result like you're seeing.

 

[hanermo]

My idea ..
Kinda like countryboy in post 13 above.

The current hole might not be true as in cylindrical, straight, uniform size, and uniform tir.
This might be due to hard spots in the material, heating effects, etc.

Past material might have been different.

If you can check holes in current material vs old material you might see a difference.

Likewise, perhaps single-point boring a start-hole pre-drilling might make the hole straighter.
3-5d, typically.
Or drill-ream, even a lot undersize, then finish to size with drill at 10x less pressure.

Even pre-drill then final-drill might fix the problem.
The better you document your efforts (phone pics)+note sheet the better your processes will get.

 

[DDoug]

. All in all, the parts aren't scrap, so that is a good thing.

Thanks for staying on topic.

They aren't ?

 

[steve45]

I think the hole is small enough that it is approaching the wavelength of the light. This is causing refraction that is giving you the appearance of waves.