Advice For Tapping Thread In Aluminium Tube

[The Coal Man]

Hi,

Would anyone please be able to advise me of my best options to tap some threads in round aluminium tube? It is decades since I last tapped a thread at school so any tips most appreciated.

I have some round aluminium tubes (19.05mm OD x 6.35mm wall) with aprox. 6.35mm hole and wish to tap both ends of the tubes with an M8 thread. The hole tapping size from a chart lists the hole size for an M8 x 1.25 thread as 6.8mm

My first question is, do I need to drill out the hole to 6.8mm with a tapping drill or can I tap the thread using the hole as it is at 6.35mm?

I would like to get the threads cut as straight as possible in the hole to ensure a more aligned fixing to the aluminium tubes and have a pillar drill. Is it practical to place the tap in the pillar drill and turn the drill chuck by hand in an effort to get a straight cut into the aluminium tube? Or should I just go for a plain hand tapping procedure with a tap wrench.

Any advice would be welcome

Thx

 

[knighty]

how deep are you going to tap into it, and how far will the bolt/whatever screw into it ?

and what's it for/how strong does it need to be ?

the deeper you go the better (stronger) it will be

 

[The Coal Man]

Helicoil it, threads strip easily in aluminium and helicoil improves the strength massively

I'm lost, what is a helicoil with respect to tapping a thread?

Thx

 

[gordon stephenson]

As aove, Is it something that will only be bolted up once or will it be frequently

 

[The Coal Man]

I would like to tap approximately 20mm of thread into each hole. It does not have to be very strong, the load bearing capacity placed on the threads will be quite small, its more to locate the aluminium tubes at fixed points rather than carry load.


Rgds

 

[The Coal Man]

As aove, Is it something that will only be bolted up once or will it be frequently

Bolted up once and left for months, possibly dismantled and bolted together again and left for months or longer.


Rgds

 

[123hotchef]

that will tap easy then you may be ok tapping into the hole you already have if you have decent taps

 

[slim_boy_fat]

that will tap easy then you may be ok tapping into the hole you already have if you have decent taps

And use plenty of lube.

 

[Seadog]

WD40 is perfect as tapping lube for ali

 

[gordon stephenson]

I recon the hole did wants opening up a bit, And tap as deep as you can (with-in reason) and threaded bolt to suit the length,

 

[8ob]

As the hole is close to size anyway you would get away with one of the drill/ tap combos and use a cordless drill. As above with a squirt of WD, but thats just me because I am lazy

Bob

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[gordon stephenson]

If you are so lazy Bo8, You could just use those taps as a bolt with the hex for the head,

 

[Rocketman]

Just drill and then tap it and forget all this worry for 5 min job. No need to mess around with helicoil.

Generally speaking 1.5x dia bolt thread location is fine in 7075 t6. If it's a cheaper alloy go to 2x dia thread location.

 

[bricol]

Stainless steel threaded coil insert, you drill a hole with the supplied bit, tap for the coil and insert the coil breaking off the tang when done, bolt screws into helicoil, which spreads the load across the whole insert and is far more resistant to pull out than a thread into aluminium

I don't believe that's quite true - it's still a tread type cross section into the ali - just larger than the original bolt which might lead to that conclusion - I don't think Stanley still claim it either. Certainly in our tests, a high tensile cap head screw inserted to a sensible length up to M16 will snap before stripping a properly formed tapped hole in a decent grade of aluminium - so I don't use them in component design that is very often removed. Somewhere on desk somewhere at work is a block of ali with a selection of snapped screws in it ;-)

Now if we are talking wear resistance for parts that are often disturbed - then I would use them.

Amusingly, some years ago, we were making aluminium components, 300mm dia, 300 long, that fitted onto 100dia shafts, with 12 locations for slides on them - each hole had a helicoil in as the slides could be removed quite a lot . . . then sent them for anodising . . .who removed the inserts to avoid contamination . . . which we then had to put back in . . . we got quite discount on helicoils for a while until someone planned it a bit better ;-)

 

[MrFluffy]

I don't believe that's quite true - it's still a tread type cross section into the ali - just larger than the original bolt which might lead to that conclusion - I don't think Stanley still claim it either. Certainly in our tests, a high tensile cap head screw inserted to a sensible length up to M16 will snap before stripping a properly formed tapped hole in a decent grade of aluminium - so I don't use them in component design that is very often removed. Somewhere on desk somewhere at work is a block of ali with a selection of snapped screws in it ;-)

Now if we are talking wear resistance for parts that are often disturbed - then I would use them.

Amusingly, some years ago, we were making aluminium components, 300mm dia, 300 long, that fitted onto 100dia shafts, with 12 locations for slides on them - each hole had a helicoil in as the slides could be removed quite a lot . . . then sent them for anodising . . .who removed the inserts to avoid contamination . . . which we then had to put back in . . . we got quite discount on helicoils for a while until someone planned it a bit better ;-)

I thought the helicoils were supposed to distort when under pull out loads transfering some of the load to the next wire up distributing it, instead of focusing the stress on the most engaged thread root thus leading to a higher pull out strength?
Its something I'd like to see definites on done on lab test equipment in controlled conditions, I can only find this from the year dot.
http://crosstools.com.au/helicoil-type-inserts-technical-study-of-strength-and-pullout.html

I hate helicoils, they always strike me as nasty springy things, and for years much prefered timeserts, but a number of studies favouring helicoils for the above reason have sort of revised my opinion.
Sorry op, this has wandered off into insert discussion but its related.

 

[MrFluffy]

I was more thinking of some results from testing to iso 896 or similar by a accredited lab than someone on youtube squishing them in a hydraulic press.

 

[piman]

Hello The Coalman,

I would drill the tube to the correct size, attempting to tap an undersize hole will cause problems, soft though aluminium is, don't skimp on the correct procedure. Your idea about using your pillar drill as a guide is sound particularly as you say you have little tapping experience and is basically what I do when tapping in the lathe by using the tails stock to keep the tap true.

Alec

 

[The Coal Man]

To All,

Many thanks for all of your replies and advice, sorry for not getting back to this topic a little sooner.

The load bearing on the thread in the aluminium tube is not that great, it is a static load of only a few kilos, if that. On account of the light load I think I will try tapping the aluminium tube with the thread to be used rather than opt for a helicoil.

I need to get some tapping drills of the correct size, 6.8mm for the M8 x 1.25 thread and 8.5mm for the M10 x 1.5 thread. As I already have some Alco, Dormer and Presto taps I dont need to buy a tap and drill set. Has anyone a recommendation for a set of tapping drills or for individual tapping drills, is there anything I should look for, or avoid?

Thx

 

[Northumbrian]

The only thing I would add, sorry if I'm teaching egg sucking here.
Make sure you're dead on centre, mark with a centre punch, and start off with a centre drill.
In the absence of the pillar drill, I've used two wooden mitre blocks at 90 deg to each other. One for the tube, and other to guide the drill.
I've snapped a few taps in the past, because I didn't get it dead on square, and using a hand drill free hand you can't guarantee not to snap a tap.
WD40, works very well on Ali, even for milling & turning.

I also, if tapping by hand, start the tap off with a cordless drill, again in the setup I used to drill the hole. Then finish off with a tap wrench.
Since, I've been doing it like this I not snapped a tap in any material I've been tapping, including 17-4PH stainless steel.

 

[The Coal Man]

The only thing I would add, sorry if I'm teaching egg sucking here.
Make sure you're dead on centre, mark with a centre punch, and start off with a centre drill.
In the absence of the pillar drill, I've used two wooden mitre blocks at 90 deg to each other. One for the tube, and other to guide the drill.
I've snapped a few taps in the past, because I didn't get it dead on square, and using a hand drill free hand you can't guarantee not to snap a tap.
WD40, works very well on Ali, even for milling & turning.

I also, if tapping by hand, start the tap off with a cordless drill, again in the setup I used to drill the hole. Then finish off with a tap wrench.
Since, I've been doing it like this I not snapped a tap in any material I've been tapping, including 17-4PH stainless steel.

Northumbrian,

Thx for your reply.

As I am a total newbie to most things to do with metalwork and fabrication I am years away from becoming an egg sucker

I have a pillar drill and the aluminium round tube. With the tube I only need to increase the existing inner diameter to that required for tapping. I had planned on placing the tap in the chuck of the pillar drill and the tube in a machine vice with a V in the jaws to clamp the tube. Then I was going to turn the chuck of the pillar drill by hand to tap the thread.

Alternatively, to avoid one of my total bodge ups, if there was anyone local to me with the equipment to machine tap the aluminium tubes or insert helicoils centrally and vertically into the tube I would happily pay for the service. I realise its probably a routine job for an experienced fabricator but for a total bodge merchant like myself its a farce about to unravel.


Rgds