Tapping holes in aluminum with the actual screw to be used

[RCMFMaxxMan]

Originally Posted by BigSki

tap-tite being a brand name, is a 'tri-oval' self tapping screw designed to tap holes in plastic and thin sheet metal. Like mentioned above, get the proper taps and cutting fluid and MAKE your own holes. Rule of thumb for a tap hole diameter is 70% of minor diameter.

Your rule of thumb is off. First, you must have meant major diameter, not minor, and in that case, 80% is more accurate. The standards for tapping are always between about 80%-85% of the major diameter. For example: a hole for an M10x1.5 tap is 8.5mm in diameter.

Back on the self tapping screws that Mercedes uses, I talked to a friend of mine who used to be a mechanic (and we are both BSME students in our final year) and he said Mercedes definitely does not use such screws in anything but plastic or sheet metal. Cast aluminum parts would be tapped, otherwise you are introducing stress concentrations. And the reason self tapping screws are used in production applications in general is $$$, no other reason.

[rockhugger]

Originally Posted by JoeThePro

Thanks for the input, everyone. Metric tap set is in the mail. This is for (at this point) 1/8" and 1/16" aluminum. I'm going to be using nuts on the 1/16" anyway so that wont really matter.

I would recommend getting a set of numbered drill bits as well. I keep a set with all the small RC related taps I've collected since getting back into RC. 3mm is just the beginning. A set of numbered bits will cover pretty much all the taps in the set you ordered.

I use WD-40 for my "cutting oil", and rinse with water or blow off the the tap between each use.

If you break a tap, you did something wrong, period dot end of sentence.

[maslin]

Originally Posted by RCMFMaxxMan

Back on the self tapping screws that Mercedes uses, I talked to a friend of mine who used to be a mechanic (and we are both BSME students in our final year) and he said Mercedes definitely does not use such screws in anything but plastic or sheet metal. Cast aluminum parts would be tapped, otherwise you are introducing stress concentrations. And the reason self tapping screws are used in production applications in general is $$$, no other reason.

He's wrong. Engine blocks, valve covers, heads, ABS pumps, transmission housings... they all come with blank drilled holes. all cast aluminum parts. Fairly deep holes too, 10-20mm regularly.


I fix the shit that engineers do wrong every day. Last week I spent 47 hours replacing the wiring harness for an entire car because the engineers didn't remember water runs down hill.

From the all-to-familiar SBC fiasco... "• Using hand tools, tap threads into SBC hydraulic unit bores by running self tapping screw (B, Figure 17) into lower bore"

[BigSki]

Originally Posted by RCMFMaxxMan

Your rule of thumb is off. First, you must have meant major diameter, not minor, and in that case, 80% is more accurate. The standards for tapping are always between about 80%-85% of the major diameter. For example: a hole for an M10x1.5 tap is 8.5mm in diameter.

Yup, I got my minor and majors backwards as I was typing. The 80% is rule of thumb is ok for a class 2 fit. The 70% rule is what we used in aerospace. I've always used it with the small metric and machine screw pitches, it helps to hold the screws in a bit better.

[58_Willys]

I built my 1:6 scale CJ frame from 5/16" T6 aluminum, and every joint is drilled with the drill press and tapped using oil. Backing up 1 turn for every 1/4 cutting turn, really saved my taps. I only broke 2 taps in the beginning. One, because I got a little over zealous, and thought I could just punch through the 5/16 T6, and the other, because I was rushing the last hole.

This is a job requiring patience, so when you're ready to do it, set your mind to, going slow. You can feel the binding as you cut the threads, and if it feels like you have to wrench down on the handle to continue, that's when it 's time to relax. When my 2nd tap broke, it was on the last hole of 4 for a cross brace. The only way to remedy it was to start over and make a new one. So take your time.

[JoeThePro]

Originally Posted by 58_Willys

I built my 1:6 scale CJ frame from 5/16" T6 aluminum, and every joint is drilled with the drill press and tapped using oil. Backing up 1 turn for every 1/4 cutting turn, really saved my taps. I only broke 2 taps in the beginning. One, because I got a little over zealous, and thought I could just punch through the 5/16 T6, and the other, because I was rushing the last hole.

This is a job requiring patience, so when you're ready to do it, set your mind to, going slow. You can feel the binding as you cut the threads, and if it feels like you have to wrench down on the handle to continue, that's when it 's time to relax. When my 2nd tap broke, it was on the last hole of 4 for a cross brace. The only way to remedy it was to start over and make a new one. So take your time.

You had to start over and make a new chassis? Was it because the bit got stuck in the hole? Could you not drill it out?

[Correll]

Can I play too fellas? OP, just use your tap and have fun. On another note, I work on MB everyday too, at an MB dealer. There's lots of times where I would rather strap C4 to an MB product and blow it to the moon than repair another dead Sprinter.

But all flaming and back and forth aside in this thread. Those German Kraut gobbling engineers do send the Techs transmissions and engines covered in blank holes. And we just run the bolt in. As far as re-using these self tapped holes. Well, your talking about cars that break Frequently. So yes, these self tapped holes receive a workout even under warranty. And the bolt can be re-torqued and not strip out.

[58_Willys]

Originally Posted by JoeThePro

You had to start over and make a new chassis? Was it because the bit got stuck in the hole? Could you not drill it out?

Sorry for the confusion...I only remade the brace I was working on. Taps are made from very hard, high carbon steel. Drilling it out would have been difficult at best and it would have made the hole to big to re-tap for 3mm screws. It was easier to reshape and drill a new brace. I went slow and steady on all the rest of the holes.