Tapping Holes in Extrusion

For the mods to my Glensound mixer frame I had to cut down some existing extrusion. This meant I needed to tap the holes in the cut ends so I could fix the extrusions into the frame.  I have done many taps in sheet metal and with help from Frank Rollen and the right lubricant I can now do this easily in most thin panels in a single pass. I naively assumed a simple single pass tap could be used to cut a half inch long thread in an existing hole in the extrusion but I soon found out this doe not work. So what is the correct way to do this? Will I need first, second and final taps or can I get away with just a pair?

Cheers

Ian

What material are you trying to tap? For aluminum you want a two flute tap. For steel you want a four flute tap. Are you using a tap wrench? A tap with a rounded end is used to start the thread. With a closed end hole you want use a tap with a flat bottom to cut the threads. I use tap wax for lubrication. I think you bought some of that stuff when you got your mill.

With a tap wrench I cut threads until I feel it bind. Then I back the tap out. Clean off the swarf from the tap and continue until done.I've never used a tap chuck or anything like that. A friend says he taps aluminum with a hand drill but I know I would snap a lot of taps  if i did that.

Gold said:

What material are you trying to tap? For aluminum you want a two flute tap. For steel you want a four flute tap.

Click to expand...

It is aluminium extrusion.

Are you using a tap wrench? A tap with a rounded end is used to start the thread. With a closed end hole you want use a tap with a flat bottom to cut the threads. I use tap wax for lubrication. I think you bought some of that stuff when you got your mill.

Click to expand...

Yes I am using a tap wrench. You can tap sheet metal with an electric drill but I am not so sure with extrusion. I should have mentioned the tap need to be about half an inch long. The holes run right through the extrusion so I don't have bottoming issues. Yes I have plenty of lubricants!!

I have ordered a set of three (first, second and third) to try.

Cheers

Ian

ruffrecords said:

I have ordered a set of three (first, second and third) to try.

Click to expand...

I've never heard of using three types of tap for a hole. I'm certainly no expert though. McMaster-Carr sells three basic types of taps. One type with a round end is for starting a thread. The other type with a round end is for through holes. I know I've bought both types but I can never tell them apart. The third type has a flat bottom for a closed end hole.

I've been doing a lot of cutting and tapping the past week. I set up my drill press again. It was a floor stand model but is now a desktop model. I have a cross slide that I tricked out to make working on face plates easy. There are a lot of threaded mounting holes. While one type of tap may be best for a particular task I just use what I have.  The different tap types all cut the right sized threads and work well enough no matter if it's a through hole or closed hole.

With a half inch of  M6 thread in aluminum I’d expect to back the tap out three or four times.

Gold said:

With a half inch of  M6 thread in aluminum I’d expect to back the tap out three or four times.

Click to expand...

It is an M5 tap but just the same it is definitely not a one pass operation.

Cheers

Ian

My summer job back in the 60's one year involve tapping holes in steel pipes (used for taking core samples of ocean bottom after bolting a bunch together).

I used a gun tap (two flute)  that is designed to curl the chip and not clog up when used with a power driver, so you can tap holes without backing up to break the chip. (I did break a lot of taps that summer  : )

For aluminum I would be inclined to use the standard 4 flute tap, and reverse to break the chip as needed. Also keep it well lubricated, but aluminum is soft, so should be easy to cut, just take care to not clog up the tapped hole with the aluminum chip.

I suspect either a 2 flute or 4 flute tap will work OK, just take your time.

JR

PS: If the extrusion already has a hole in it, it may be designed to accept a thread cutting screw, without tapping.

I think what you might need is called a "bottoming tap".  Just my $0.02.
Best,
Bruno2000

bruno2000 said:

I think what you might need is called a "bottoming tap".  Just my $0.02.
Best,
Bruno2000

Click to expand...

A bottoming tap is for blind (closed) holes, a bore in an aluminum extrusion would be open.

But a bottoming tap would probably work too... aluminum is soft, so main problem is the hole clogging up with chip curl.

JR

If it’s standard soft 6061 aluminum I’ve had much better luck with two flute taps. If it’s been hardened a four flute tap might work better.

Gold said:

If it’s standard soft 6061 aluminum I’ve had much better luck with two flute taps. If it’s been hardened a four flute tap might work better.

Click to expand...

OK. I will check what comes in the three tap set when it arrives.

Cheers

Ian

When  I've bought a  three tap set it has always been the three types I described above.  It's not for a three step process. The difference between the thread starting and through hole taps seems minor to me. Either one works about as well. So that's a one tap process. Even wth a bottoming tap I don't usually use a thread starting tap. I push it until it grips and away I go.

JohnRoberts said:

A bottoming tap is for blind (closed) holes, a bore in an aluminum extrusion would be open.

But a bottoming tap would probably work too... aluminum is soft, so main problem is the hole clogging up with chip curl.

JR

Click to expand...

Sorry, I thought it was a blind hole.
Best,
Bruno2000

Gold said:

When  I've bought a  three tap set it has always been the three types I described above.  It's not for a three step process. The difference between the thread starting and through hole taps seems minor to me. Either one works about as well. So that's a one tap process. Even wth a bottoming tap I don't usually use a thread starting tap. I push it until it grips and away I go.

Click to expand...

Except that extruded Al is a bear to tap (or for any type of milling) because it has a "fiber". Here in France, the 3 types of taps are specifically used for a two-tap process for open holes, 3-tap for blind holes.
Taps that are meant to be used with a tapping attachment (mechanical taps as opposed to hand taps) are different and the size of the hole must be exact, when for hand-tapping there is more leeway.

abbey road d enfer said:

Except that extruded Al is a bear to tap (or for any type of milling) because it has a "fiber".

Click to expand...

I tap T-Slot extrusions all the time. I haven't noticed them to be any more difficult. I thought most aluminum shapes (sheet, bar, round, U channel, etc) were extrusions?

Here in France, the 3 types of taps are specifically used for a two-tap process for open holes, 3-tap for blind holes.
Taps that are meant to be used with a tapping attachment (mechanical taps as opposed to hand taps) are different and the size of the hole must be exact, when for hand-tapping there is more leeway.

Click to expand...

I've only used hand taps. Tapping drill chucks and tapping hand drills are expensive. I don't do enough to justify buying.

Hi Ian,

as long as you stick to the rule of "one turn forward, half a turn back" - to crush the loose chaff, particularly on aluminium - I have always found small work like extrusions ok  with a taper tap and lots of 3 in 1 oil.  It does of course depend upon the hole depth.

I also use a proper tap wrench and not a hand vice as it allows you to balance the tap correctly to start with.  As other folks have said, a plug tap is only necesarry for a deep or blind hole or a very thick panel.

Regards

Mike

madswitcher said:

Hi Ian,

as long as you stick to the rule of "one turn forward, half a turn back" - to crush the loose chaff, particularly on aluminium - I have always found small work like extrusions ok  with a taper tap and lots of 3 in 1 oil.  It does of course depend upon the hole depth.

I also use a proper tap wrench and not a hand vice as it allows you to balance the tap correctly to start with.  As other folks have said, a plug tap is only necesarry for a deep or blind hole or a very thick panel.

Regards

Mike

Click to expand...

Thanks Mike. I am learning lots of new stuff. What is a 'taper tap'??

Cheers

Ian

Gold said:

I tap T-Slot extrusions all the time. I haven't noticed them to be any more difficult. I thought most aluminum shapes (sheet, bar, round, U channel, etc) were extrusions?

Click to expand...

yup... I still have a few hundred pounds sitting around from my first generation tuner package.  :'( At least I engineered in thread cutting screws.

I've only used hand taps. Tapping drill chucks and tapping hand drills are expensive. I don't do enough to justify buying.

Click to expand...

Last time i was tapping holes as my day job I had a power driver with an auto reverse,,, so power in, bottom (on the jig) and then reverse power out... I still broke lots of taps because I was a kid and hated the job, so was not very diligent about holding the tap square to the holes. 

Then I learned that working fiberglass was even worse. It sucks to be the bottom of the food chain in a machine shop.  :  (don't ask me about making chain bags out of lock washers only using vise grips ).

JR

Gold said:

McMaster-Carr sells three basic types of taps. One type with a round end is for starting a thread. The other type with a round end is for through holes. I know I've bought both types but I can never tell them apart. The third type has a flat bottom for a closed end hole.

Click to expand...

Four 1/4-20 taps below.

Top: A starting tap, these are very good for self-alignment when the materiel is thick and you are doing it by hand. They will self-center and square up on their own.

2nd: "Normal" tap, good for most average things that aren't too thick. Requires paying attention to going in nice and square. If you start off crooked, it'll stay crooked and bind up.

3rd: Bottoming tap for blind holes. Full threads almost all the way to the bottom of the hole. Do not start with this, it puts too much strain on not enough teeth. Bottom out a "normal" tap first, and then this tap will get a few more turns of full threads.

Bottom one: Two-flue gun tap. For thin materials. You just "shoot" it through, and back it out. Note the angle of the flues, it pushes the chips ahead and out the other side, not for blind holes. Great for sheet metal.

I have a cross slide that I tricked out to make working on face plates easy. There are a lot of threaded mounting holes.

Click to expand...

I'd like to hear more about this. I'm always looking for new ways to do something.

Gene

Gold said:

I tap T-Slot extrusions all the time. I haven't noticed them to be any more difficult. I thought most aluminum shapes (sheet, bar, round, U channel, etc) were extrusions?

Click to expand...

That was in comparison with flat sheet Al, that is? (are) a different alloy/process or even steel.