STI Taps

M.Brown · Dec 1, 2021

Is there a GL class for a 1/4-28 Tap out there in the world? Or, is this a custom grind from the manufacturer? GL meaning below PD by .0005 vs H1 being .0005 above PD. I would very much like to get to the bottom of this for my boss.

dandrummerman21 · Dec 1, 2021

What is GL?

1'/'4"-28 Straight-Flute STI Taps - Grainger Industrial Supply

Edit: I see your edit. I was going to paste this link, but see below after reviewing it. I doubt an undersize is available but i didn't look: 1'/'4"-28 Screw Thread Insert '('STI')' Taps - Grainger Industrial Supply

I would threadmill it if it needed to be under.

MaxPrairie · Dec 1, 2021

+1 for threadmilling.

Micmac1 · Dec 1, 2021

Emuge special order or thread mill as long as you can measure id pitch dia or have a go no go

M.Brown · Dec 1, 2021

Yes, we thread milled the part. However, we are looking at a rework issue that requires us to chase the threads and I don't want to blow the holes out. (which one guy has already done to one part).

Thanks for replying.

MaxPrairie · Dec 1, 2021

Are you looking for an undersize tap or an undersize helicoil tap? Title says STI tap.

Larry Dickman · Dec 1, 2021

if the holes are already threaded, how are you going to chase them and make them smaller?
Can you plate (anodize, electroless nickel) the holes?
Try a place like North American Tool to get a u/s tap made

M.Brown · Dec 1, 2021

It's not about making them smaller. It's about not making them blown out. Meaning taking the NOGO gauge.

M.Brown · Dec 1, 2021

STI tap. Just trying to keep from blowing the hole out while chasing them that's all.

MaxPrairie · Dec 1, 2021

STI = Standard Threaded Insert. Made for inserting a Helicoil in the tapped hole after threading, they also use a larger drill size. If you are chasing a standard tapped hole with an STI tap your pitch diameter is wayyyyy to big. Does the tap have a drill size recommendation on it? Also, what brand is the tap?

dandrummerman21 · Dec 1, 2021

Okay, what's the application? I mean, why are you reworking the holes?

Did the plating come in too thick? Were parts welded in the area and it isn't round anymore? Were they HT and the gage just barely doesn't go because they sandblasted it too hard?

Depending on the reason you need to chase the threads, I was wondering if you could use a form tap instead.

Larry Dickman · Dec 1, 2021

MaxPrairie said:
Does the tap have a drill size recommendation on it? Also, what brand is the tap?

if the tap has a drill size on it, then it is without question a piece of shit. (Hobbyist Harry time)
So you're really saying you can't chase a hole without fucking it up? Time to maybe look for a new line of work.

MaxPrairie · Dec 1, 2021

Larry Dickman said:
if the tap has a drill size on it, then it is without question a piece of shit. (Hobbyist Harry time)
So you're really saying you can't chase a hole without fucking it up? Time to maybe look for a new line of work.

I think they are trying to tap a standard hole with a Helicoil tap...we will see what he replies.

TeachMePlease · Dec 1, 2021

You guys are working really hard at misunderstanding what he wants.

It's pretty clear, he has an STI threaded hole. He wants an undersized STI tap to chase the threads (presumably due to a burr, or other schmutz in the threads, NOT due to the threads being oversized), because his operator is unable to use a normal sized tap for chasing without wiping out enough of the minor to allow the no-go gage to go.

That said.... You gotta be pretty bad at chasing a thread to do that... Just IMHO.

in2glamisgirl · Dec 1, 2021

Balax makes a 1/4-28 STI roll tap in GH2. A form tap would be less likely to oversize chasing your thread.

I am surprised the guy chasing the thread didn't ball the first thread to make it gauge ok.

TeachMePlease · Dec 1, 2021

in2glamisgirl said:
Balax makes a 1/4-28 STI roll tap in GH2. A form tap would be less likely to oversize chasing your thread.

I am surprised the guy chasing the thread didn't ball the first thread to make it gauge ok.

If the guy can't chase a thread... He probably doesn't know all the tricks yet

GENERALDISARRAY · Dec 1, 2021

Ward, I think you are being a little hard on the Beaver.

I have been in that boat before, Class 3 thread in aluminum. I could not put the tap that tapped the hole, back in it and not oversize it. Like if it was too shallow.

friesen · Dec 1, 2021

Beware of STI taps, sort of a guilty until proven innocent approach.

Personal experiences with 10-32 and M4.0X0.7 sizes:

Balax form taps are oblong, basically don't work in aluminum, seem to cut oversize if I recall correctly.

Helicoil taps from McMaster cut the threads about 0.010 oversize.

I ended up using a threadmill, otherwise the end result was too loose.

SeymourDumore · Dec 1, 2021

Balax STI Threadfloers do work quite well in AL, though I am not sure it will be effective in making a slightly undersized thread to come out in-size.
It will however help if the threads are nasty with chips and stuff.

Also, the No-Go gage CAN! enter the hole all the way, but it must have a noticeable drag!

That said, a nice sharp cut-tap should work in the right hands.

dandrummerman21 · Dec 2, 2021

SeymourDumore said:
Also, the No-Go gage CAN! enter the hole all the way, but it must have a noticeable drag!

Do you have a source for that? I googled it and do see that an article published by "Quality Magazine" says that about metric threads, and 3 turns for standard threads.

In the case of the No-Go thread gage, the most common misconception is that it should not enter or travel over the thread feature at all. In other words, it should “Not Go” at all. This is one of the largest areas of debate between machinists and quality inspectors.

According to the national thread standards, the no go can be allowed up to but not exceeding three full threads for standard gages or two full threads for metric thread gages prior to feeling a definite drag. In theory, you could travel the length of the thread with a metric No-Go thread gage providing you feel a drag after the second full turn.

I am sure there are more places that agree but I didn't see an authoritative source for the "all the way" bit. I did see that Machinery's handbook says 3 turns for standard threads. hmm, maybe I'll pull out the book and have a look in the metric section...

Naturally, good luck convincing most inspectors that a part that the nogo fits all the way into is actually good.

STI Taps

Plastic

Stainless

Hot Rolled

Cast Iron

Plastic

Hot Rolled

Titanium

Plastic

Plastic

Hot Rolled

Stainless

Titanium

Hot Rolled

Diamond

Cast Iron

Diamond

Hot Rolled

Cast Iron

Diamond

Stainless

Similar threads