Recommended tap for Helicoil/STI thread?

[robert123]

Can anyone recommend a good tap for rigid tapping for 1/4"-28 Helicoils in blind holes in 6061-T6? I ordered bottoming taps from McMaster and they are Helicoil brand and push chips into the blind hole. We have broken two of them. I have almost 2000 more holes to tap.

 

[Finegrain]

Form tap. No chips.

 

[robert123]

We use form taps most small diameter regular threads, but I don't know if form tapping is allowed for thread inserts. I think the spec is MS21029. Anyway, don't most bottoming cut taps push the chips upward out of the hole?

 

[Limy Sami]

https://www.stanleyengineeredfastening.com/tools-and-systems/accessories/heli-coil-taps-and-gages

 

[djm77]

I don’t think you can get form taps for helicoil threads due to the form required to hold the helicoil in place. We use Walter taps and we produce thousands of holes in 6082-T6 aluminium all day long with no problems.


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[Mike1974]

You want a spiral flute tap that pulls chips out. Also, you don't want a true bottoming tap, unless you are going back in by hand...? Try looking for a modified bottoming with a 2-3 thread lead.

 

[Shop Supply Guru]

Emuge sells a large variety of STI taps.

 

[gregormarwick]

YG1 make decent quality STI machine taps. Not in the same league as Emuge et. al., but plenty good enough for 6061.

 

[JP Machining]

Balax STI form taps, I think MSC sells them not sure of others.
1/4-28 BH2 STI THREDFLOER | Balax | Forming Taps, Cutting Taps, Thread Gages

 

[Larry Dickman]

I would recommend an OSG VA-3 bottom tap.

 

[Jrill]

I use OSG list S108 for tapping 6061 STI holes. Tapped thousands of 2-56/4-40/6-32/10-24s and it works fine. Also, you can get much closer to the bottom of the hole than the suggestions in the helicoil STI book.

GENERAL PURPOSE Taps - OSG

 

[MachEng]

Definitely stay away from form taps with heli-coils. They may gage perfectly but the crest will have a groove in it. You will see it under magnification. It cause problems installing the heli-coils. I would thread mill them if you don't like using spiral flute taps.

 

[Bigbore050]

Form taps will work just fine if you know how to use a form tap. If you don't just buy a good spiral bottoming tap.

There's a reason all the big tap manufactures make roll form sti taps. We tap 1000s of holes in 6061 using Balax and Osg form taps. We also install all the coils after plating. We use tanged and tangles from 4-40 to 5/16-18 with no problems. The taps only break due to operator error. I would almost bet I got a 4-40 tap that's been running since 2017.

 

[robert123]

I use OSG list S108 for tapping 6061 STI holes. Tapped thousands of 2-56/4-40/6-32/10-24s and it works fine. Also, you can get much closer to the bottom of the hole than the suggestions in the helicoil STI book.

GENERAL PURPOSE Taps - OSG

That's good to know. The S108 they show as "good" for aluminum. I ordered list 13039 which they show as "best" for aluminum (and good for only aluminum).

 

[robert123]

Definitely stay away from form taps with heli-coils. They may gage perfectly but the crest will have a groove in it. You will see it under magnification. It cause problems installing the heli-coils. I would thread mill them if you don't like using spiral flute taps.

I have read that form taps are not good for helical inserts before, but I also see major tap manufacturers making them. This makes me curious, but I simply don't have time to investigate right now. I need to cut parts. They may function fine but still not meet the old mil specs. As these are reasonably critical aircraft parts, I'm not willing to experiment.