Specific question on tapping aluminum what pitch tap to use?

[The_Tuner]

Ok I am going to hand tap some 1/4" diameter holes into 6061 aluminum they will also be about 1/4" to at most 5/16" deep .. I will be using 1/4" steel set screws in them to hold some plastic parts.


Now my question is what thread pitch is the strongest for this ... coarse, medium or fine?

Oh yeah and if you could explain why, it may make it easier for me to decide in the future for other projects when tapping aluminum, so I won't have to bug people lol.


Thanks for any replies they will be really appreciated

[MrWild]

There is no medium. You get two flavors. Coarse and fine. Course threads 1/4-20 are strongest in aluminum and for aluminum, you want at least 3/8" of full thread engagement. Why is course better than fine? I don't know the physics/engineering answer, it just is.

[Eurisko]

A course pitch thread is a bad choice in this application. Use a fine pitch for tapping thin material. You'll have more thread engagement.

Use a 1/4-28 uncoated spiral flute tap, and some cutting oil.

btw, fine pitch threads actually have greater holding force than the same diameter course thread.

Fine Threads vs Course Threads

[Geof]

1/4" to 5/16" length of thread for 1/4" does not qualify as thin material, at least not the way I look at 'thin'.

I have been making my own product line of aluminum parts for over twenty years and have fooled around extensively with different threads. My experience is that coarse is definitely the best in all sizes up to 1/2". You get a higher strip-out strength and a longer life under frequent loosen and re-tighten with coarse.

You can get a higher holding force, at the same torque with fine compared to coarse, but it is my experience that you can take coarse threads to a higher torque and a higher holding force, before failure.

[The_Tuner]

Thanks for the replies, see I looked for this over the internet and as you can see above this is what I found, people really have differing opinions on this ... its hard for me to find a fine thread in exactly what I want so i'll try the coarse first and if thats not working I will try the other, thanks for the replies they are really interesting ... too me anyway ... i'll report back in case anyone else searches something like this.

[gahydtoolman]

another way to get stronger threads, if that is important, is to use a heli-coil. you tap the hole larger (with the helicoil kit), and install the insert. now you have the strength of around a 3/8 dia thread and still use the 1/4-20 bolt. also, if you can't helicoil, then form threading is stronger than cut threading, with a form tap you are displacing the metal in the hole which makes it much harder to pull the threads out with the bolt. great for aluminum, it takes a larger tap drill for a form tap than a cut tap, be sure to get the correct size.

[juergenwt]

What ever tap you use follow up with a bottom going (flat bottom) tap to get the most depth. You can make one yourselves by grinding an old tap flat and backing off the first thread. You can also ask any toolmaker to do it for you. That way your set screws will go all the way to the bottom.