I regard commercial “bottom taps” and “plug taps” as one and the same. If I want to tap right to the bottom of a hole, I either (reluctantly) grind the tip off a commercial tap or make one from silver steel.

I also grind the end “point” but not the lead

But rather than buy three taps just get a spiral flute which has a similar lead to a plug tap. It’s all I use these days.

YG-1 go down to M2 though the smallest of theirs I have is M2.5 from Cutwel. My M2 came from Ali and is branded UCAN as the small YG are not cheap.

I have quite a lot of plug taps that have all the lead ground off in the ME 32 and 40 rates they are brilliant when making nuts for pipe fittings

Just to state the obvious. When attempting to use a tap to thread to full depth in a blind hole do remember to clear all the swarf out of the bottom before the final cut.

On 14 February 2025 at 16:32 Georgineer Said:

I haven’t time to check at present, but memory says that the tap we call a “second” is what the Americans call a “plug”.  If that’s the case (correct me somebody if I’m wrong) it could explain any confusion.

George

 

A quick google of a .com site says you are correct. US Plugs are UK 2nd taps

Just to clarify, in Canada (and the USA) taps are listed as Taper, 8-10 thread chamfer, Plug, 3-5 thread chamfer, and Bottoming, 1-11/2 thread chamfer. Smaller ones will have a pointed end and larger ones will be flat with a centre. Like others I regularly grind off the point on the small bottoming taps to be able to tap closer to the bottom of a blind hole where it is shallow.

John

Another thing which may help when tapping with small taps at least on flat surfaces is a home made tap guide. Just a little block of any shape with a hole drilled that the tap will pass through to reduce the wobbling which is a danger when handtapping. As already mentioned, if a tap feels wrong, it may be getting blunt and be sure to clear swarf several times as you go. I also grind off any tip cones on plug/bottoming taps, but that means even more caution when finishing a blind hole.

I have made several lathe tools with CCMT06 and DCMT 07 inserts out of Densimet (sintered tungsten alloy), and chose a new tap intended for hard steel and it did not feel right. The Densimet probably would have been better to tap if I had bought a tap intended for aluminium. I found an old tap in amongst the hundreds that we have at the museum which finished the holes much better, you don’t know what may work until you try it.

A few minutes with a decent CAD program is a great help when visualising exactly whats going on as a tap reaches the bottom of a blind hole. Whether a standard drilled hole or an (almost) true flat bottom created with a slot drill.

Fairly obviously its impossible to tap into the conical end of a drilled hole so some of the theoretical lost depth of thread is accounted for.

Its also essentially impossible to create a true thread with a tap having effectively zero taper, so the starting cut is full depth, without pretty sophisticated guidance to ensure it enters squarely.

The typical 2 or 3 thread taper on the common variety of spiral flute taps is probably the minimum taper for effective starting when hand tapping where guidance can never be as good as an accurately set up machine. It’s important that the screw or bolt is not so long as to enter the tapered portion. Given the impossibility of completely cleaning all the tapping lubricant out of such small holes over-long screws can easily be driven so hard into the short taper that removal in one piece is a fraught, or even impossible, task. I prefer to run a tap ground off dead square down the hole after tapping to remove any taper and reduce the risk of jamming. Its far easier to feel if a screw comes up to the end of the thread.

Naturally we wouldn’t make the mistake of using an over long screw in our work. “Ahem”! But I see a considerable amount of repair and fixing work and need to minimise the chances of getting  job back with an even more intractable problem. Extracting sheared 2.5 mm screws is not a favourite pastime. Bridgeports aren’t designed for the weeny centre cutting end mills needed. (Will teh guy with a quill master attachment please stop sniggering!)

Effective lubrication during tapping is problem. I tend to poke some Trefolex down the hole with a thin wire before starting work. Seems to me that anointing the tap is ineffective as its the business end that needs lubrication but any compound on the tip will surely be wiped off after a turn or three.

I’ve seen it suggested that short, slender, sticks could be cast from candlewax or similar and dropped down the hole. As the tap advances it crushes the stick not only producing a continuous supply of lubricant at the tip but also helping to push the swarf up and out. Fully cleaning any residue would seem impossible tho’. It is of course arguable that some lubrication of a teeny thread is desirable debit increasing teh risk of overtigntening and jamming it the screw entered the tapered thread left at the bottom end.

Clive