Rigid Tapping - Thread Question

[RJS100]

I started on my first project that requires tapping a total of thirty 1/4 x 20 holes in 6061 aluminum (see attached). I completed rigid tapping fifteen of them today and have one question. When you tap a hole, should the screw thread into the hole easily, or should you have to work it a bit. I found that once I got the screw started, I could easily turn it in with an allen wrench, but when turning it by hand, there was a bit of resistance.

Notes:
The tap I used has an H3 pitch diameter limit: https://www.mcmaster.com/catalog/130/2923/2529A18.
I tapped these threads at 700 rpm.

Is the fact that the screw fits tightly in the threaded hole because of the H3 pitch diameter rating of the tap, or do I need to tweak one of the rigid tapping parameters somewhat?

Best... Richard

[cncsnw]

I am not a machinist, so I am sure others here will have ideas better grounded in reality. But my take is:
1) In tapping (as opposed to thread milling or lathe threading) the fit is determined almost entirely by the size and shape of the tap.
2) You might get a slightly looser fit if you tap each hole twice (which you can do, with rigid tapping).

[RJS100]

Thanks. These holes .75" deep. I tapped them once at .375 and again at .75 and they feel better. I do think the H3 pitch diameter also makes for a tight thread.

Best... Richard

[tblough]

I'm sure you've checked, but have you tested in wax? If there are any inconsistencies in pitch from an incorrect encoder setup, you could be "pulling" the last couple of threads.

[RJS100]

Thanks Tom. I did indeed test in wax and all looks good. I have some other holes to tap in on my next project and the taps I will be using have a more "relaxed" fit. I think that is the culprit.

Have a great day... Richard

[suntravel]

I would also use another tap for a looser fit.
On soft metals, I prefer thread forming.

Uwe

[RJS100]

Hello Uwe,

I will be using another tap for my next project. I have been using the GWizard to get a starting point for the speeds & feeds.  With aluminum, I am beginning to get an idea as to what is in the ballpark.  But when I work with some new material that I have never done before, I have no idea.  For example, the attached is a set of jaws for talon grips I am making from 1018 steel.  I used GWizard to estimate the feeds and speeds... but some look a little aggressive to me.   We will find out in the next few days!

Jaws for talon grips: https://www.dropbox.com/scl/fi/6s8gmfjp ... wr56q&dl=0


Thanks... Richard

[cnckeith]

i moved this post into the machining strategies forum!

[RJS100]

Thanks Keith.