UKC

I have a blind hole in aluminium that is approx 9mm deep and is already threaded to approx 5mm. Thread is 10-24 UNC. I want to extend the threading a bit further into the hole, 3mm further would be ideal. So that is only about 3 new threads. I really don't want to mess it up so I am looking for some advice and tips from someone who knows what they are doing.

I would be doing it with a manual hand tap. 

As the hole is part threaded could I use just a bottoming tap or should I use an intermediate taper tap first to partially cut the threads?

Would wd-40 or a light machining oil be ok as lubricant?

thanks

I work in a technical sales role for a lubricant blender specialising in metal working. For Aluminium you need lots of lubricity, specially formulated AL tapping fluids are usually kerosene based with ester lubricity improvers blended in. As you are only going to need a miniscule amount you need something easily available, I'd recommend mixing some high quality kerosene with a drop of baby oil or glycerine.  

I would have thought a bottom tap should be fine. It has probably already been cut to the max extent of an intermediate/second tap. A bottom tap still has taper will only partially cut the first 1-2 threads anyway. Go slow and regularly clear the swarf. 

(Watch out for tap terminology. In the UK a 'plug' tap is a bottom tap. In the US (i.e unc) a 'plug' tap is an intermediate/second tap. )

As Steve says the plug tap will have a taper so if you want to tap 8mm of a 9mm hole you won't do it with a standard plug, it's risky but you might be able to grind the end of the tap a bit flatter, use it as deep as you can to start then flatten it a bit to get the last thread. 

If you're buying taps you probably don't save much by not buying a set, if you've only got the bottom tap you'll probably get away with it. I'd personally want to run the intermediate tap in right to the bottom first, if the alloy is hard you'll appreciate the help from the taper, if it's soft you reduce the risk of stripping the few threads you have getting it going. Any old oil will do including WD40, it's not a tough job, just go slow cut for half a turn, back it out half a turn to break the chips, cut for half a turn, repeat. Wash it out between taps or the chips will stop the plug going full depth. Start the tap with your fingers rather than in the wrench to make sure it's straight. Also double check the pitch against a screw before you wind it in!

edit: As Dax said if you need the last little bit of depth grind the first couple of threads off the bottom tap then use it again, it works fine with care, I've used a couple of small taps like that in stainless with cutting oil.

jk (tinkerer not machinist)

A bit like making love to a beautiful woman?

Swiss

Thanks for the replies. 

There is a slight depression at the bottom of the hole. Measured to the mid point it is approx 10mm deep but measured down the sidewall is approx 9mm so I may have a little space. It currently has 5 threads. If I could extend it to 7 that would be ok, 8 would be a bonus.

I have a reasonable tool shop locally that usually stocks good stuff and the owner is knowledgeable so I will try him tomorrow.   If I have no luck there and need to order taps which brands should I be looking out for? I am guessing that a good quality tap will make success more likely.  Also are there any specifications I should look for? Steel type etc?

Dormer are nice, and I've had good use from a metric set by Goliath but for most jobs anything with the right thread will do.

jk

If there's a way to describe tapping holes that doesn't end up sounding like a weird clipping from a jazz mag I'm damned if I can figure it out 

jk

Taps come in 3 types and are priced accordingly. 

Type 1 is made of cheese and very cheap. EBay and the like

Type 2 is made of Carbon steel more expensive (last I got was 1/2 bps and it cost about £6) but work perfectly but not up to machine room repetition.

Type 3 is high speed steel, more expensive designed for repetitive use and way over the top for what you want.

I buy mine from http://www.uni-thread.com/index.php  

This is definitely one of the best threads on the forum.

Any advice on where I can learn about this sort of thing, specifically sliding head lathes? Books/Websites ect.