Centering a bore on the lathe

Hi all,

I have a cut section of 60mm round bar in which I want to tap an M24x3 thread. I drilled and bored it out to 21mm, and then attempted to tap it in the lathe, using the tailstock with a centre to keep the tap straight, however it was requiring too much force, I couldn't hold the chuck and turn the tap, and honestly it felt like I was going to break something. I was using the only tap I had to hand, a 2nd tap, but my new plan is to use a taper tap which I've ordered, and do the tapping on the drill press, using a MT2 centre held in the drill press to keep everything square. For this to work, I need to be able to clamp the piece on centre below the quill. My solution for this was to machine a bar of 21mm diameter, which is a snug fit in the work piece, with a centre hole drilled in the end. This can then be put into the workpiece, centred using the MT2 centre, the workpiece clamped up, and then the 21mm bar removed. My problem is that on the side where I started to cut the thread, the 21mm bar will no longer fit. I want to put it back in the lathe and bore it to 21mm again, but I'm struggling to know how to indicate it so it runs true to the bore on the lathe. I only have a dial indicator, not a dial test indicator. Can I just angle that into the bore as best I can and work from that? Or is there a better way?

Turn it round, poke your 21mm bar into the clean bore and clock off that, bore through. You can safely make it a tad bigger without losing significant strength.

I'd be tempted to screw cut it to remove most of the metal, just use the tap for final sizing. Do you actually need the thread full length of the bar? Counterboring so you only have say 25 mm thread length would make life easier.

Edited By duncan webster on 09/04/2023 23:23:03

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

Hi John, This is a coarse thread at 3 tpi and certainly outside the normal myford range 8tpi max, so possibly screw cutting would be difficult...you would definitely need the set of three taps though. if a good bored taper at the mouth of the bore would help tapping process, along with cutting paste... but wonder if the work would slip on the jaws and outside damage..?It might still be possible to rough screw cut it even so depending on change wheels available?Cutting some water pipe threads as a volunteer! And using a big wrench took some effort in the past...., it might be the same situation, good luck and look forward to hearing how you progress...

Vic

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 the tap went in straight the first time (even a little bit until you decided to stop), it will go in straight again. Screw it in as far as it will go and use its shank as a reference to re-centre the workpiece.

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!

I wouldn't contemplate tapping this thread without first screw-cutting in the lathe. However, if you really must do it that way, then 3 taps, used bit by bit, 1,2,3 - 1,2,3 etc etc might get through the job. The torque required will be very considerable. Holding the job will be a problem. You are going for a full depth thread. Is this necessary? Boring out to, say, 21.5 to 21.6mm would make the work easier, with little loss of thread strength.

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.

i think perhaps you have underestimated the power required to turn this tap check out the size of the correct tap wrench. i tapped a 1/2 BSP thread in steel last night. at 19mm bore quite a bit smaller than yours moved the lathe [smart and brown about 1/2 ton] swinging on it. as others have said screw cut it

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

Only if they are traditional parallel flute taps, if driving spiral point or spiral flute taps by hand backing off defeats the object of their design.

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.

I agree with the 21.5mm starting bore suggestion. If there is enough length of metal to allow an extra length bored to 23.5mm, it would take care of the squareness and could be cut off after tapping. Extra length would also allow flats at the other end to hold in a vise which could also be removed afterwards.

Edited By old mart on 11/04/2023 21:12:23

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.

Hi all,

Sorry for the slow reply. Life got a bit hectic and I guess I went AWOL

Thanks for all the great advice, and especially thanks to Simon for your very kind offer. Unfortunately I can't take you up on it as I'm over in Northern Ireland.

So just as a quick update, I bought a taper tap and a large (560mm I believe) tap wrench. I clamped the part in the vice, tapped it freehand, and it turned out beautifully. I guess I was overthinking it, maybe it's harder to tap crooked with a larger tap.

Many thanks for everyone's input!

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.

21.6mm is the hole size from a Mazac mill for suggested hole size for tapping M24X3 thread. So I would suggest indicating the outer of the Bar, bore to 21.6mm, then start screw cutting the 3mm pitch thread. You can make the threading with wither a dedicated 3mm pitch threading insert , or make one out of a triangular insert. Being a course thread, I would suggest putting the top slide on the angle of the thread flank, and cut essentially only on one side of the thread groove. Then finish with the Tap.

Depending on the steel , it will take strong arms to Tap that from a drilled hole.