Form tapping sand cast aluminum?

Makes some parts, tap them with a couple different processes and clearances, then put a bolt in them and twist with a torque wrench to see when the threads strip. Use the best compromise between machinability and real world strength requirements.

If thread strength and durability in aluminum is really an important issue, you might want to ask your customer to modify the design to use helicoils.

I think that's a great idea. I'll ask the customer today for some of the pieces cut from the castings and test them. Thanks, Eric

Many of the castings I machine do get thread inserts. In fact, that's part of the reason I got some of the jobs. I convinced the customer that I could install the hundreds of heli-coils faster and cheaper than they were doing them. I use a tapping head to install them. But even the parts with thread inserts would benefit from stronger threads. So I'll be using Bob's idea and measuring the pull out strength of cut versus formed threads for each particular alloy that I'm threading. Eric

Because you wanted someone with experience tapping 356, I didn't respond earlier, but the other issue you mentioned is that there are some different alloys involved.

I haven't personally tapped aluminum casting alloys but I know you're going to find some big differences in their ductility and, thus, their response to form-tapping.

356 is moderately ductile for a casting alloy; A-356 is much better. Neither of them will be as good in that department as wrought 6061. The high-silicon casting alloys will be problematic.

In other words, if you're going to run some tests, and you're going to be dealing with a variety of alloys, they could respond very differently to form-tapping.

Possibly you should look at the likes of Keenserts or Timeserts as I believe those provide a stronger installed thread than a heli-coil. It may have been one of those that a company I worked for used in a magnesium alloy casting for a racing gearbox and as it would be dismantled regularly a thread insert was a must, IIRC the inserts used required a more complicated machining than heli-coil and they were intended to be replaceable if they became damaged, also due to the wall thickness the thread in the alloy had a greater diameter and so was stronger.

*** I use a tapping head to install them. ***

Now that is something to remember. I could make an insert tool to install them out of drill stock pretty darn quick too.

I use a Procunier head because it does not self feed. The install depth of the inserts varies less than .005" Eric

The customer requires helical type screw thread inserts. Eric

Greetings Ed, I spoke with the customer yesterday about test pieces. I will be getting test pieces made from the various alloys I will be tapping. Some bronze alloys along with the aluminum. I'm pretty sure that the

356 alloy will be better off cut rather than formed but I've been surprised before. Bob's good idea of actual pull out torque tests will be put to the test. Eris

That kind of practical testing is always a good idea, and there are enough variables involved that predicting results would be tough in any case.

It sounds like you have the right plan. BTW, the engineers at Emuge have been helpful to me in the past (10 years ago) in writing articles, and they know their stuff. Maybe you'd want to call them to get some perspective: The testing you're planning sounds like the best idea, however.

I just have a cheap no name tapping head, and the self feed does cause me headaches sometimes. I just can't afford a Procunier head. Still, if I take the time to do a setup properly and set my drill stop with some experimentation I can eliminate most of my troubles. Not convenient for one or two holes, but worth it when I have to do 20-30. I read your comment and thought... a reason to buy another tapping head and another drill press. LOL.

and like them. They do make good taps. Even though I will be relying on my real world tests I'm sure that getting info from the engineers at Emuge will help. Eric

If you try it you'll like it. I have bought all my Procuniers on ebay. Except the first one I bought. I got that one at a flea market. It was in pieces in a coffee can. The cork lining for the clutch had come off of the aluminum hub. I epoxied it back on and it has worked fine for the last 25 years or so. And the tapping head itself was made before WW2. I never paid more than 60 bucks for one by the way. Eric

You know, my dad has an electronic variable speed reversible drill press he bought from Sears atleast 35 years ago. I've often thought if I had that I could make a simple spring centering lockup tapping head without all those fancy internal gears. I always thought it was "cheap" and light duty, but when I asked he sais the only thing he ever had to do to it was replace the speed control pot when I broke off the shaft as a kid.