Is this a bottoming tap?

[rpbidgood]

Hi,
I am making a Cracker locomotive and have to put a thread in the oscillating cylinder - the trouble is I only have a few mm of thickness to play with before I break into the cylinder bore. I bought a 'suitable' tap from Ebay, a dormer - the box says 'bottom', but it doesn't look like my idea of a bottom tap. I am new to this game, so I would like some advice before I complain. I did expect the nose to be flat.
Keith.

 

[tel]

Well yes, it looks like a bottom tap - you could carefully grind that point off it to get it in a little deeper.

 

[arnoldb]

Keith, that is a bottoming tap - I don't know why they make them like that though...

I've had a similar problem with buying bottoming taps - if you have a bench grinder, just grind the point flat; don't let it heat up too much while grinding though.
I think the taps are made that way assuming one would tap to the bottom of a drilled hole where the drill bit left a cone for the point to go into.

Regards, Arnold

 

[hitandmissman]

I would have to say that is not a bottom tap. It appears in the photo that the firs thread is tappered to start the tap therefore not a bottom tap. If ya grind the tip off and grind that first thread off, ya than you will have a bottom tap. Any bottom tap I have seen is flat at the bottom and will not start to tap a hole that was not already tapped first with another tap. Most high end taps come three in a set, a starter, second tap, and bottom tap.

 

[shred]

I'm guessing you've already soldered the cylinder to the valve plate or you could drill & tap right through. I think I ended up soldering mine in anyway as it would come loose, then carefully turning down the solder fillet on the outside.

 

[doc1955]

Set Type Hand Tap Sets (Taper, Plug, Bottoming)

 

[FIXIT]

Hi
Just to put my bit of experience in , I've found that the 'budjet' tap sets from eBay @ £3 -4 set are normally as you have (the ones i tend to buy and replace regularly ) have this point and the 'real' quality ones as a toolmaker would buy @ £20-30 set. have a flat bottom.

So as everyone has advised just carefully grind off the bottom

Steve

 

[Bernd]

Nice pics Doc.

I thought I'd add a one more.

From left to right: taper, plug, bottom.

Bernd

 

[Noitoen]

Out here if I ask for a bottoming tap, I get a spiral one. When you cut, the shavings come out like a drill's.

 

[radfordc]

When I break the tip off a normal tap I grind the end flat and call it a bottom tap.

 

[Stan]

This from Machinery's Handbook 22nd Edition Later editions may vary
Taper Taps are chamfered 7 to 10 threads
Plug Taps are chamfered 3 to 5 threads
Bottom Taps are chamfered 1 to 2 threads

 

[peatoluser]

yes it's a bottom tap, and FIXIT is not quite right when he suggests that only cheap sets have a point on the end. my dormer HSS ground thread tap sets (M3 - M6) aren't cheap. they're made up to a standard not down to a price, all have points. I always thought this was so that you COULDN'T run the tap right to the bottom of the hole. The swarf has to go somewhere and the point will leave a little a little 'pocket' at the bottom of the hole for it. You shouldn't be jamming the tap on top of the swarf. But iv'e also been told that it's just part of the manufacturing process so can't be realy sure. In general engineering, if a blind hole is to be tapped you generaly drill about a couple of threads deeper than required, it's only in our scale of engineering where hole depths can be shalllow that we need every thread we can sqeeze into it, so yes, just grind the point of it as everybody suggests, but remember to empty the swarf out before trying to put that last thread in. it's a real bummer when the little buggers break.
Noitoen's spiral taps aren't, strictly speaking, hand taps but are for use in tapping machines where the tapping is done in one go. so they need a spiral to get the swarf out. no reason not to use them by hand, and they're invariably good quality. HSS ground thread.
incidently, if you come across any taps that are straight fluted but seem to have a lot of negative rake at the end, these are also for use in machinery but for through holes. the slope at the front is to make sure the swarf is pushed out of the hole. I dont find them that easy to use by hand even as a bottom tap and stick to hand taps, but the spiral point are fine.

 

[Deanofid]

Well, here's another one to add to the mix; That is not a bottoming tap. The whole idea of a
bottoming tap is to get threads almost to the very bottom of the hole. If you have to grind on
a new tap to make it thread down to the bottom of a hole, then it seems plain that it wasn't a
bottoming tap in the first place.

The pictures that Doc and Bernd put up show the difference clearly.
Of course, no one here knows where you are from. Maybe things are different for you.

 

[joeby]

The subject of taps may very well be one of those that has no definite end. I think much of it has to do with manufacturing consideration. The cone-pointed taps, as the OP has, are very common. It seems logical to do this, especially on the smaller sizes, so the manufacturer can use a cup or reversed center on the grinder instead of a center hole. Besides, a drilled hole doesn't typically have a flat bottom, so there will be some room for a cone point.

As the tap sizes get larger, the flat end with the center drilled seems to become more common. I'm thinking 3/8", or thereabout, seems to be the turning point.

As a general rule, for a bottom-tapped hole, I drill them full depth on the diameter. Simply put, for a hole 1/2" deep, drill 1/2" + the drill point. Quick and dirty math is .3 x drill diameter = point length. This, of course, doesn't work if the depth is a concern because of break-through or other design limitations.

As for type of tap, the categories are pretty well laid out, as Stan mentioned. However, as Dean stated, if it doesn't thread to the bottom how can you call it a bottoming tap. The problem here is that the "Standard" calls out the threads chamfered and not the length from the end to the first full thread, not that I can remember anyway.

Kevin

 

[tel]

You've pretty much hit the reason for the cone Kevin. Tap nomenclature seems to vary a bit, but we've always referred to them as bottom taps rather than bottoming because, as we have seen, the tap will not always reach to the bottom of a hole, even if they are pretty much full thread right to the end of the main diameter. Flat ended bottom taps are often called 'plug taps', unlike the 'plug' shown in Bernd's pic, which is referred to as an intermediate tap.

Dunno 'bout you blokes, but I think I've confused meself now! :shrug:

 

[Maryak]

A tap will not flow with its' bottom plugged. The difference for each side of the pacific pond is purely related to the angle of the dangle from the equator and a desire by our forefathers to confuse the hell out of younger folk.

So endeth today's gobbledegook taken from the book of Job and translated by a lusty wrench.

Best Regards
Bob

 

[rpbidgood]

Thanks for all your help and suggestions. I am trying to tap a thread in the rear of the cylinder to take the trunnion pin, and even though I have made the cylinder chunkier than the plan, I still have very little material to play with. To be honest I made a right dog's dinner of the 3mm thread - I'm going to enlarge the hole and go for a 4mm thread using my 4mm 'bottoming' tap with the nose ground off as suggested. If that fails I might try putting a small shoulder on the pin and (soft?) solder it to cylinder. Of course I could shave a few mm off the rear face of the cylinder, tap a through hole on on a piece of brass plate of a suitable thickness and solder it to cylinder.
By the way, I am from the UK - a few miles down the road from a very wet Ryder Cup.

 

[Tin Falcon]

in general I like spiral flute taps for blid holes these also come in various lead IE plug taper and bottoming but they push the chip up and out eliminating the need to break the chip every turn or two. Also reduces tap breakage.
Tin

 

[Stan]

Aside from the discussion on bottom taps, we now know what the tapped hole is for (trunnion). As a general rule, you only need a depth of thread equal to the diameter for a thread under tension. Your requirement is entirely different in that you are mounting a pin subject to wiggle (technical term). Your concern is not holding the pin in the cylinder but stopping it from moving back and forth with each oscillation.

My suggestion for this would be to have the tapped hole in a flat surface on the cylinder larger than the diameter of the pin and then machine the pin with a shoulder to seat on the flat surface. The larger you can make the shoulder, the more support you get. For final assembly, put a drop of Loctite on the threads.

 

[rpbidgood]

Thanks for all the help and suggestions. As advised, I ground the point off the tap and with the modified tool managed to put a decent thread in the hole (the picture does not show the thread too well). I also put a a shoulder on the pin (belt and braces). The threaded part of the pin is a cut-off 4mm screw (the brass part of the pin is threaded to take this screw). The whole lot will be locked in position with loctite.
Thanks again for all the help.