DIY Large Thread Tap

[Phillip AllenParticipant @phillipallen34597]

I have a need for a single use tap to cut a 1-1/2 inch BSP thread in brass. The brass tube is embedded in the wall of a concrete tank and the thread only goes half way and I need it to go all the way through.

Do you think that non heat-treated steel will be sufficient for this one-off job? There is about 50mm of thread left to cut.

Any and all opinions and ideas appreciated.

Thank you.

Phillip

[John HaineParticipant @johnhaine32865]

That would be a big tap! I assume you would screwcut say silver steel then form flutes? I guess you wouldn’t be able to form relieve so there would be no top rake on the teeth. So lots of flutes to reduce friction. Inevitably though only the leading tooth on one flute would do most of the cutting. Danger is that this gets blunt before you are through and the tap jams. Even worse the increased friction breaks the bond between pipe and concrete. Could you find a way to work around the existing thread?

[John HaineParticipant @johnhaine32865]

Another thought. Since there is an existing thread part way through could you screw cut a matching bar then mill a slot across the end to accommodate a single point threading tool made in HSS?

[FulmenParticipant @fulmen]

I seriously doubt mild steel will last long enough, but any heat treatable steel should do so I’m sure you can find a way without too much work.

[JasonBModerator @jasonb]

Why bother making one when you can buy a HSS tap for £22.00 which is about 1/4 the price of a suitable length of silver steel to make one from. Postage would be about the same for both

[SillyOldDufferModerator @sillyoldduffer]

I agree with John – risky job.  Brass likes a sharp cutter, and non-hardenable steels don’t hold an edge.  And soft metal lilke mild-steel is likely to deform and jamb.  Maybe make several taps and swap for a new one as soon as they don’t cut easily?

Rapid Metals charge £18 for 100mm of 40mm dia D2 cut to length.  I’d buy some and harden it.  I’ve had considerable success with home made taps for threading Aluminium and mild-steel.  All below 20mm diameter, and never mission critical. Nor have I tried them on Brass.   Main problem the way I make ’em, is getting a sharp edge and suitable relief.   Not as good as bought items.

But, how much will it cost if a DIY tap jambs in the pipe, mangles the thread, or the pipe comes adrift?   If more than the cost of a professionally made tap, then buy a tap.  Dormer over £400, Presto £350.   After the job is safely done, recoup some of the cost by selling the tap second-hand…

Be interested to hear what others say.  My take is this is more about managing what happens if it goes wrong than making a cheap tap!

Dave

 

[MacolmParticipant @macolm]

Safest to buy a new carbon steel or HSS tap, and best it is a full taper (first). I would not risk anything else when failure such as jamming could result in a serious problem. Avoid buying a used tap even if best quality, since sharpness is vital.

[cedric 1Participant @cedric]

No. Buy a tap.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, you’ll be a long while trying to make a 1 1/2″BSP tap from a piece of 40mm bar with any grade of steel, seeing how the O/D of a 1/1/2″ BSP is 1.882″ A commercial tap would most likely have five flutes, if you intend making one. Mild steel would not be suitable as others have said.

Regards Nick.

[noel shelleyParticipant @noelshelley55608]

Not worth messing about for all the reasons given above. Just give the boys at Tracy Tools a ring and get a carbon steel one. Noel.

PS it’s scaffold tube size.

[Martin ConnellyParticipant @martinconnelly55370]

I am just wondering what needs to go all the way through. Is it something you can cut the threads off in a lathe leaving just the required threads on it to screw into what you already have? You have also not stated if it is a parallel or taper thread. It is easy to assume parallel from what you have said but also easy to be wrong.

Martin C

[Clive FosterParticipant @clivefoster55965]

As it’s a one time job how about making a threaded light alloy carrier long enough to go right through for a single 11 tpi Whitworth chaser to match the existing BSP thread. With a thread going near enough right to the end it ought to provide plenty of support for the inevitably single sided cutting load. Slow job with the usual 1/2 turn cut, back off 2 turns then go again process. Chip space will be an issue so probably looking at pulling right out to clear things every three turns at least.

Only need to cut a slot to locate the chaser at the correct angle and position with some sort of loving device such as a grub screw or three to keep it in place. I’d probably use a hand chaser as there is nice long handle part to get hold of, heck I might even have one, although a coventry die head version might be sharper. I suppose something could he done with a Whitworth tap.

Whatever you do make up a couple of dummy pipes to practice on. Far safer if rethink is needed.

Clive

[John HaineParticipant @johnhaine32865]

https://www.tracytools.com/product/11-2-x-11-tpi-bsp/

£45.

[JasonBModerator @jasonb]

Traceys will be £54 by the time VAT is added, as I said they can be found in the 20s for HSS  not the Carbon of Traceys

[MacolmParticipant @macolm]

Even using a sharp new tap, the torque necessary to cut this size of thread may surprise most people who have never done it before. Do be sure the fitment being threaded is well enough fixed!

[John HaineParticipant @johnhaine32865]

Carbon steel is reputedly sharper.

[Macolm]

On Macolm Said:

Even using a sharp new tap, the torque necessary to cut this size of thread may surprise most people who have never done it before. Do be sure the fitment being threaded is well enough fixed!

That is a key point.  The tube may appear to be firmly fixed but the join may be tested to destruction trying to cut the thread.  Really I think that if a way can be found to avoid having to lengthen the threaded bit it would be better (and cheaper!).

[old martParticipant @oldmart]

I assume the existing thread is not tapered as the taps mentioned are parallel. You might be able to turn the tap with a pair of 18″ Stillsons because the square end may be 25mm. As already mentioned the torque needed could cause damage if the brass turns while being tapped.

[DC31kParticipant @dc31k]

A possible way to ‘lengthen’ the thread (at the expense of a short length of reduced bore) could be a 1 1/2″ to 1 1/4″ bush, a 1 1/4″ long nipple, another 1 1/4″ to 1 1/2″ bush and a 1 1/2″ coupler. That gets you a 1 1/2″ BSP female thread on the other side.

You would need to check the ID of the existing pipe (1 1/2″ BSP has a minor diameter of 45mm 1 1/4″ black iron pipe is noted as 42mm OD).

As the large amount of torque required to drive the tap has been noted, a possible option could be to make the tube ID larger (lower percentage thread engagement).

Depending on the length of the tube, the OP’s enthusiasm to machine something might better be channelled into a tap extension.

[old mart]

On old mart Said:

I assume the existing thread is not tapered as the taps mentioned are parallel.

A through-thread would have to be parallel as a taper thread has a gauge length. If it needs ‘converting’, it is going in the correct direction.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, if there is a 50mm length of blank, I would be using a tapered tap to start with, as has been said there will be a fair amount of toque at that size, and depending on how it’s embedded in the concrete, it might be too weak to take the force.

Regards Nick.

[Nigel Graham 2Participant @nigelgraham2]

How long is the present, useable portion of thread?

Can you acheive the same aims with a fitting that has sufficient thread length for that, plus a spigot of core diameter to go the rest of the way?

[cedric 1Participant @cedric]

Yes, given the force required to turn a 1.5″ diameter tap, and the possible fragility of the brass tube, combined with the difficulty of replacing it if damaged, you might be best to look at alternatives as suggested. Some kind of spigot using the existing thread etc might be safer. Depends on what exactly you are wanting to achieve.

[not done it yetParticipant @notdoneityet]

Personally I cannot envisage a requirement for a 1 1/2 BSP thread deeper than about 50mm.  Extending it further seems pointless.   Why would this thread need to be ~100mm is the question?

[noel shelleyParticipant @noelshelley55608]

This is BSP so the 1.5 ” refers to nominal bore – the tap will be about 48.3mm OD = even bigger than some thought and will require a very considerable turning force – and it’s brass. Not an easy job, even with the right tools. A proper BSPP tap is the only way to go.   Noel.

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