Tapping a thread

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What is the component that you wish to tap?

Mick

Mick, personally I would have thought that a plug tap although not ideal should be ok

if you are careful.

Mark P.

If its a one off only why not screw cut it running in reverse so you are not worrying about hitting the bottom of the hole i.e. carriage moving left to right and starting at the bottom of the hole with an undercut.

Saves buying a tap for one piece ?

John

Edited By JohnF on 06/07/2019 17:32:05

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Bit awkward. Better to avoid that design if possible: much easier to thread the outside.

However, in special cases you're allowed to modify tools like taps. I've even used a plug tap to get started and then ground it flat to get the thread even closer to the bottom.

Taper taps are good for getting threads started, then following with a plug or second. Easier to do and the tools last longer. But if there isn't room for the end of the tap, I get the grinder out. You may prefer to do this to a less expensive tap than a top of the range item.

Dave

Why not try & grind the #2 ( intermediate ) tap of your ME set back a little further to get a start into your item then grind the plug tap back, then you will get to the bottom of it ( pardon the intended pun ). This is what I do with broken small size taps, yes we all break them eventually ! I now have various small taps ranging from 2 - 6mm with ground flat bottoms which are only approx 3 - 5 threads long, gives me that extra turn or two to get to the bottom of a blind hole.

George.

use the plug but make the piece longer with a tapered lead in that you can cut away later.

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I do all my 1/4 x 40 with just a plug tap and that has had the pointed end ground off square so I can get in as far as possible, no lead in on the part but do guide the tap wit the tailstock or mill/drill that you used to drill the hole. Works fine for me on steam, fuel and gas fittings as well as gland holes..

Edited By JasonB on 06/07/2019 20:06:52

Great idea John, wish I had thought of that myself. I will try to remember it in case I ever need it.

Years ago my firm had some apprentices and I had one practice tapping using small taps. Nothing like actual practice on scrap metal to learn. I got him started on 5/16" and gradually worked down in size. When I gave him a 6BA, he took a little longer, but ended up with a nice thread in 1/4" steel plate. When he gave me back the tap, I realised that I had mistakenly supplied him with a plug tap. He did well not to break it.

Hi, well I tapped 8 12mm threads with only a plug tap, in steel channel on site back in 2012 as my workmate and I had nothing else and the job had to be done that day. tapping the holes was not intended, but the scaffolding wasn't erected far enough as planed to reach the other side so as to use nuts and bolts.

That's a swivel brake adjuster being used as a tap wrench.

Regards Nick.

When Needs Must, Nick!

I put some levelling-screws in my Harrison lathes' cabinet base-plate, already drilled but not tapped. According to the Harrison manual, you don't bolt these lathes down but level them with the screws provide - in my edition of machine they weren't. "Level" is defined as sloping slightly to the chip-tray drain.

I can assure you completing hand-tapping five 1-1/8"BSF holes in 5/8" thick steel with a hard skin on the underside and the holes a bit under-size for the thread, entailing wielding an ordinary spanner as "tap wrench" while lying on the floor in very cramped and awkward positions, and expecting naively to tap the threads square to the surface....

..... is a labour of gritted determination over several days.

That's even with the help of a somewhere-near-diameter Metric tap to quarry away some of the steel first. ( I drew scaled-up thread profiles first to determine how many turns I could get away with before the pitch and angle errors became significant.)

What was wrong with a sliding-fit set screw with a nut either side of the frame? I think you made a lot of work for no real gain.

Just make sure that you back off the tap every quarter or third of a turn or so to break the swarf, so that the tap does not jam.

Howard

Take care with that kind of exercise, Nigel, it has been known to cause knee problems..........

DaveD

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