tapping? – on the lathe

[pgrbffParticipant @pgrbff]

I’m completely new to metal having spent the last 30 years tinkering with wood. I need to tap the inside of a 16mm OD piece of 2011 aluminium bar with a 6mm deep M10x0.75 thread and had intended doing this on a friends lathe, he will be there to help me. What are the principles of tapping on the lathe? Can someone point me to a link for beginners?

I assume it is more difficult to cut a wide fine pitch thread than a coarse one?

[pgrbffParticipant @pgrbff]

[JasonBModerator @jasonb]

Your last sentance seems to suggest you want to screwcut the thread eg use the lathes gearbox to advance the tool 0.75mm/rev. Yet Tapping tends to be used when using a Tap to cut the thread.

 

If you can say which then we can point you in the right direction.

 

Also is it a blind jole or does the thread go all the way through.

 

J

[Lawrie Alush-JaggsParticipant @lawriealush-jaggs50843]

Hi pgrbff

 

For threading that size, I would just use a tap. Trying to make a boring bar for an 8.8mm hole is to difficult to think about – for me at least.

 

The process is simple and made easier if you friend has a handle attached to the rar of the spindle of the lathe, but it is not essential.

There are two ways you can proceed for aluminium with that sized thread.

One is to hold the tap in a drill chuck in the tail stock and move the tail stock up to the work. Don’t bolt the tail stock down, use you right hand to keep pressure on the tap. If you don’t have a crank for the spindle, just turn the chuck by hand. the reason for not bolting the tail stock down and using the handle to wind the tap in is that it is rather unlikely that the thread in the tail stock has a 1.75mm pitch. Being inexperienced it is very likely that in winding in the tap, you will bugger up the thread.

You want the tap to pull itself into the work and that is most easliy accomplished but just pressing the tail stock into the work.

 

Number two. A 10mm tap should have a centre hole drilled at the square end. You use a live or dead centre in the tailstock to press the tap against the work. You can use the topslide to prevent the tap wrench from spinning as you rotate the chuck by hand.

 

Things to watch out for.

Lightly countersink the hole you are tapping prior to tapping, it will stop deformation of the top of the work.

You may want to start the cut with an intermediate tap before using a plug tap to finish. A taper tap won’t really do anything in a hole that shallow. Being Aluminium, you can get away with just using a plug (bottoming) tap.

A hole that deep will only yield about 4mm -4.5mm of thread or just over two full threads which is not very much. You don’t mention if the full depth of the hole is 6mm or the threaded portion is 6mm. To get 6mm of thread, you will need a hole about 8.5mm deep. If you need more threads and the hole can only be 6mm deep, then I would go for a 10mm fine thread – 10 x 1.25mm. That may be impractiale for you though because you will then need a 10mm fine bolt.

Not a bad idea to use a couple of drops of Rapid Tap or if not available, kerosene (parrafin) is good for aluminium.

Don’t cut the thread under power, use hand power.

 

 

For the depth of hole you are tapping, you shouldn’t need to back out but you may want to do so just because it is good practice. The idea is that for every full turn you make, you reverse the cut for half or a third of a turn to break up the swarf. Howver the stuff you are cutting should not cause you a problem and the size of the thread is such that you won’t generate a huge amount of metal to clog the flutes.

 

Don’t be nervous. If you can’t wreck the piece of metal you have, just try a few practice threads on some scrap so that you get a feel for it. You develop a feel for it pretty quickly.

I had a mate over a fortnight ago who wants to play metal. So I had him turn some thing rubbish on the lathe and then put a 3mm thread down the middle. I was reminded how sensitive one becomes to tapping, I haven’t broken a tap in thirty years, he broke his first one on his first attempt. So have a practice.

 

Lawrie

 

[pgrbffParticipant @pgrbff]

Thank you both for your replies.

I expected to use a tap, but I’m not sure if there is any difference between a hand tap and one you would use in a lathe, I guess not from the extremely useful and detailed reply. A picture of wat I hope to do below.

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