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PostPosted: Fri Sep 04, 2015 1:48 pm 
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Hi there, help needed!!

I want to cut an internal thread into a 12.7 dia x 3.2mm aluminium tube. Which tap size should I use? What thread will it cut? M6? This is for a school project and we do not have a centre lathe so threading is done by hand.

If the tube works out too expensive I thought of using a round bar and drill the holes on with the pillar drill. In the past this did not work very well. The drill bit would break, get clogged up with aluminium swarf and overheat. Can anyone recommend a drill bit that will work well with drilling holes to approx 20mm depth into solid aluminium. Hole size either for M5 or M6. What speed should I use with a standard pillar drill.
:-)
Misss DT


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PostPosted: Fri Sep 04, 2015 3:07 pm 
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Hello MisssDT.
Are you trying to drill into the END of the tube, or across/into the side of it?
If you are drilling across it(at 90 degrees) I would put a short length of bar (the same as the internal size of the tube) inside the tube to strengthen the hole(you can glue it in position with araldite or similar, as 3.2 mm won't give you much thread strength, and the tube might compress when you tighten anything down onto it. If you need a 6mm threaded hole then you need to drill a slightly smaller hole. There are lookup tables available for this sort of thing but I would go about 4.7-5.5mm.

http://www.littlemachineshop.com/refere ... lsizes.pdf

As for drills a standard HSS drill should be fine for Ally, and needs to rotate fairly fast. Make sure it's sharp though. Take it steady and don't pull on the pillar drill too hard. Aluminium is quite a good conductor of heat, so shouldn't get too hot unless the drill is blunt. Ally is "slippery" though so the drill needs to be sharp. I would centre punch the holes first, but put something inside(such as tube or dowel, even an offcut of electric cable) to support the walls of the tube.

U tube is your friend!!

https://www.youtube.com/watch?v=hdvs80rywi4

Drilling into the end is more tricky. I presume the 3.2mm is the wall thickness of the tube.) Is the 12mm inside or outside diameter? If it's outside diameter, you have nearly the right size hole as it is[12-2*(3.2) is about 5.6mm already]. You could put an M6 thread into that............or an M8.
If it's an inside diameter, and you need to make a plug to fit in the end, consider a smaller peice of tube cut to a longer length than you need and secured inside with a small screw through or pin through the tube.
A local engineering works might do it for you, or even an entheusiastic dad/uncle/grandad of one of your fellow students who is lucky enough to own a metal working lathe.


Then there is always the likes of http://www.emachineshop.com/, but that's cheating........................ :wink:

Photos or a drawing of what you need to make would be great.
Good luck with whatever it is you are making!

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Last edited by Timllfixit on Fri Sep 04, 2015 3:29 pm, edited 3 times in total.


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PostPosted: Fri Sep 04, 2015 3:22 pm 
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as above, when drilling metals or even tapping a thread use some lubricant, only reason you would break as drill is if you put to much pressure on it, or it gets to hot - lubrication will help prevent this



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PostPosted: Fri Sep 04, 2015 3:41 pm 
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Thank you for your answer. Very helpful.

We are making phone holders.
I am drilling into the ends to about 20mm depth.

If i use a ready made tube (12.7mm x 3.2mm) leaving an internal diameter of 6.3mm. Does that mean I can cut an M7 thread according to the table.

Best, Miss DT


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PostPosted: Fri Sep 04, 2015 3:59 pm 
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Hi,

If you are using tube with 12.7mm outside diameter and the tube has a wall thickness of 3.2mm then if I see this correctly a 5 mm tap will drop straight into the tube and a 6 mm tap won't have much metal at all to cut a thread into? A 7mm tap would do the job but is an odd size although 7mm taps are readily available even on eBay? A 7mm tap would possibly cut a thread into such tube without the need to drill?

http://www.ebay.co.uk/itm/like/160930883842?adgroupid=&hlpht=true&hlpv=2&rlsatarget=&adtype=pla&ff3=1&lpid=122&poi=&ul_noapp=true&limghlpsr=true&device=c&chn=ps&campaignid=&crdt=0&ff12=67&ff11=ICEP3.0.0-L&ff14=122&viphx=1&ops=true&ff13=80

In order to run the tap in straight I would grip the tube in a machine vice and use the drill press to hold the tap; using the chuck key whilst gently lowering the tap into contact with the work rotate the chuck by hand using the chuck key; if it starts to tighten up back off and clear swarf. Just my take on it. Good luck. :thumbleft:

http://www.metals4u.co.uk/aluminium/tube/12.7mm-x-3.2mm-(-12od-x-10swg-)/detail.asp?prd_id=1987

Kind regards, Col.

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Last edited by Retired on Fri Sep 04, 2015 4:07 pm, edited 1 time in total.

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PostPosted: Fri Sep 04, 2015 4:05 pm 
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Unless you actually want to do thread cutting why not cut a bit of m6 studding to the right length slide all the way through the tube. A dome nut either end would hold everything together



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PostPosted: Fri Sep 04, 2015 5:13 pm 
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elecengineer wrote:
Unless you actually want to do thread cutting why not cut a bit of m6 studding to the right length slide all the way through the tube. A dome nut either end would hold everything together



:withstupid:
This is the best option and very easily done, you just need a hacksaw.



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PostPosted: Fri Sep 04, 2015 5:43 pm 
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Thanks to all your messages.


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PostPosted: Sat Sep 05, 2015 8:21 pm 
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Id say for that size tube, use M6 studding, and dome nuts at each end, using the tube as a spacer only.

Tapping size for M6 bolt is usually 5mm, and M8 is usually 6.8mm. M7 isn't all that common, so taps and fasteners may be more expensive.

Obviously if you use the incorrect tapping size, you risk not being able to start the tap properly or it jamming and breaking... if the hole is to large, you wont get a correctly formed thread, and it will strip as soon as any torque is applied to the bolt.
This is especially true with aluminium.
Infact, if you want a good quality thread in aluminium, a 'roll tap' may be better anyway. That type of tap displaces the material to form the thread, rather than actually cutting the thread.

If you need to drill the holes in solid bar, as you have a pillar drill, I assume you have a suitable vice to go with it? if the bar is small enough, clamp a piece n the chuck lightly, then clamp the vice onto the bar and bolt down to the table. Remove the bar from the chuck, drop it through the hole in the table and put in a suitable drill bit.

Drill bit wise, any decent quality HSS bit will be fine. Use a little coolant/cutting oil and pull the drill out regularly to clear the swarf from the bit. Speed wise, i'd go somewhere around the 1500rpm mark.

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PostPosted: Sat Sep 05, 2015 10:06 pm 
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Now we have a picture, studding excellent idea.

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