Tapping/Threading aluminum

I am not really the best person (not even pretty good) to get involved in this conversation.I have been a maintenance man in steel mills for years. And part time boss where having to think on my feet was a primary function. With several hundred tons of hot and molten steel in play, there isn't time to think about the problem. Just cobble it together it and keep the mill running. Certainly no spills...... messy!

A few points on your threading:

For external threads, tapering the shaft such as mentioned above is applicable for all metals and all grades of dies. I often turn a starter shaft to the smaller diameter of the thread to give the die a place to start true. This in addition to the taper. It helps on the lathe. Mandatory if the threading is going to take place later in a vise or whatever.

For internal threads, starting with metric is easier to describe. Imperial works the same, just harder to figure.

Start with a 10mm tap. The usual thread is 1.5mm pitch. Take the 10mm bolt minus the 1.5mm pitch and there is the size to be drilled. 8.5mm. This may not deliver the optimum 86% thread tooth engagement, but it is close enough to hold an automotive engine assembly together. And get you up and running without worrying about a chart or wires.The diameter minus the thread pitch.....

For imperial threads, think 3/8-16. It would be 3/8 (0.375) minus 1/16 (0.0625) for drill size. 5/16" as I recall.... And 5/16 is a good starter size for a 10mm tap as well. A letter drill a few thou larger would be better, of course.

I happen to be able to juggle numbers in my head pretty good so it's easy for me to juggle imperial and metric. Comes from model building. Some folks not so easy..

Hope that helps in your endeavers,
Bill Hudson​
 
Alright, decided to give another M8 die a chance in case it was a die issue. I switched to the fine thread, and got it to finally go. Though something seems off here, wayyy off. After threading, the size of the shank is now almost a whole mm smaller. That doesn't sound right at all?

Used more than one caliper just in case my measuring was off... I'll post photos tomorrow maybe.
 
The major diameter is the nominal diameter of the thread. i.e. an M10-1.5mm would have an unthreaded rod diameter of 10mm. For tapping internal threads with about 75% thread, the tap drill diameter is the major diameter less the pitch. i.eo, the M10 1.5mm tap drill would be 10-1.5 =8.5mm.

This also works for SAE threads if you convert tpi to pitch. A 1/4 -20 thread has a major diameter of .250" the pitch = 1/tpi =1/20 = .050" so a .200" tap drill is used.

When threading rod with a die, I find it useful to turn a short lead to the tap drill diameter. There is a slight interference with the die which helps to get the die to bite in and the lead helps to getting the die to thread straight.

As Brino said, there are two sides to the die. Most dies are labeled"start this side". If the die isn't labeled. look at it closely. On side has more of a chamfer than the other and it is the start side.
 
I tried both sides but still things are just fail. To be honest, I've heard the cheap Pittsburg Dies at HF are crap, so that could be part of the reason. Though naturally, I don't want to start blaming tools when I am the newbie at play here.

In any case, it seems I'm having the issue of the teeth just digging in, and then stripping. So far it's still a fail, with standard and fine dies for M8 no different. What I will do is some more research on this during March, and just maybe I'll eventually get myself up to a master-thread cutter. LOL.

I will also experiment with some hot-rolled steel, may that will be easer.
IMG_7619.jpg
 
Chuck it in a drillpress and give it some feed pressure and clamp the die and turn the pulley by hand. It may help to cut a groove witha dremel or jewlers file.
 
Your die is slipping and stripping the threads. I have had this problem with large diameter taps. You may be starting with too large of a shaft. Forward pressure on the die holder will help as AGFRVF stated above. Others may have some good tips.
Robert
 
Those 'entry' level taps and dies are part of the problem. Have you inspected the die closely to see if you can see an obvious problem with it? If you ever need to cut a full quality thread then spring for a reputable brand tap or die of the size you need.
But for now to make those HF dies work you have to make it easy for them, by cutting start tapers and other things as mentioned above. If that does not work then pitch the die.

BTW This chart has two different thread engagement choices that is my favourite for quick reference.
https://littlemachineshop.com/reference/tapdrill.php
 
Good tips. I'm just going to put this here though. You owe it to yourself to try one known good tap and die. I am in a similar place with HF stuff I already had. I was trying to run a 3/8 tap and just couldn't get it to cut past the starting threads. I ordered this and it came in today.

https://www.amazon.com/gp/aw/d/B00F8RFEZ6

OMG. This thing cut 304 stainless like butter. The HF tap couldn't cut 12L14. I know part of it is that this is a brand new tap, while I did use the other on mild steel before. But I was shocked at the difference. I struggled with the other one for about an hour trying to get it to work. I was done with the new one in about a minute.

Sure, the one tap cost about what the HF set did. But it's worth it. I do this stuff for myself, so I can just standardize on a few thread types. And I'm planning on doing more threading on the lathe than not going forward.
 
Aluminum will be easier than steel. What size shaft are you starting with for the 8mm die? Do you have a method to exert forward pressure like a lathe or a drill press?
Robert
 
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