I will leave others to give their centering wisdom

The thread itself you should do 95% on the lathe and use the tap only for finishing off

I do this even for 10mm 12mm holes in steel

It's 3mm pitch therefore just under 8tpi.

So you can screwcut the majority and then finish off with a tap in case your tool is not quite ground correctly.

Yes you can angle your plunger type indicator as you are only looking for needle movement not an actual measurement

If youf bar wad truly 60mm in diameter and you put a hole in the centre, remounting in the lathe can be aligned by clocking on the outside diameter.

As above, removing a small amount of bur on the start of the attempted tapping will make little difference to the end product

Cutting any fresh hole, with a 24mm tap in steel (if this is the the actual material – you don’t say) will require a great deal of manual effort, even with a taper tap.. I hope your drill vise is secure!

Ady1’s method is the obvious way to go – especially as you gave a lathe!

Cutting an M24 thread by hand ?? Should be a good trick.

Anything over 12mm, I cut by machine and dress up by hand, if required.

First, moving the job to a drill-press is a legitimate technique. It allows more force to be applied, but is likely to create a new problem – how to hold it!

Exactly as Ady and others have said, I use the lathe to remove most of the metal in threads of above 10mm or so, and then use a tap or die to finish off. The lathe produces an accurate straight pitch and does all the hard work without busting a gut. Then the tap (or die) runs straight down the partial thread and cleans and forms it to the correct profile. The combination plays to the strength of both tools and avoids their weakesses. As found, tapping an M24x3 thread is hard work, with a risk of breaking the tap, skewing off, and chewing the thread.

Another common trick is reduce the amount of work the tap or die has to do by widening the hole or slimming the rod. Boring out to 21mm for M24 means the tap has to remove enough metal for a 75% engagement. 70% engagement is more usual in soft materials, and 50% in steel. About 65% on average. Home-workshops can go even lower. Opening the bore to 21.5mm would roughly halve the torque needed to tap an M24 thread.

Apart from Brass and Cast-Iron, it's essential to lubricate when threading. Proprietary cutting fluids are best, I've always used CT90 because my local emporium sells it! Paraffin is OK for Aluminium and engine oil helps everything else.

Tapping horizontally in a lathe, or vertically in a blind hole, tends to choke the tap with swarf, so withdraw the tap and clear the hole every so often. Also, manually driven taps and dies should be reversed a half turn after every other full turn. This breaks any swarf ribbons that are forming – the break can be felt giving with a sort of click – and helps swarf drop clear of the cutting edges. (Mincing swarf is hard work and blunts the cutter!)

Dave

Good morning all.

I second the machine cut most of the thread & use the tap for the final pass method.

Also opening up the hole a little won't affect the strength by much if you're going to tap by hand.

I had a job some years ago where I had to make a dozen threaded pieces with an M24x3 thread. The customer supplied the steel & it was, I suspect something like en19. I did the first two by hand with a 2 foot tap wrench. It was jolly hard work, & it was difficult to stop the part turning in the vice. So I gave up & bought a spiral point tap & did the rest them on the lathe.

Cheers.

Stewart.

John McCulla

Where abouts are you?. If in sensible reach of West Glos (nearly S Wales) you're welcome to borrow a finger style indicator, or I can bore the hole true for you assuming the material isn't too exotic.

If some help is of interest to you send me a PM and we can sort something out. I'm sure you realise you're trying to do something quite difficult/strenuous!

HTH Simon

PS I've got a big tap wrench too. I try and keep it in a secret drawer with the shovel. It needs a big packet of chocolate biscuits before it'll show itself.

Taper taps do help to get the thread straight and a few visual checks as the first few turns are cut will also be worth doing.