Drilling & Tapping

I have to say I hadn't spotted that J&L sometimes call 2nd taps plug taps even in the UK catalogue. Very confusing for the novice. On page 114 they call the 2nd tap a plug tap in the Hertel section but in the value tools section they call the general purpose tap a "second lead" tap. On page 115 they then use the terms taper, second and bottoming but if you look at the metric fine listing they call the general purpose tap a bottoming tap which clearly it isn't. Someone there needs a good kick up the arse.

To the OP again. In the UK we refer to taps as follows.

1st (or taper) tap - Has a long taper lead-in for starting holes, especially useful in tough materials and small tap sizes.

2nd tap - general purpose tap with some lead-in. Can be used for both starting and finishing holes most of the time and is what you'll get in a tap and die set with only one tap per size.

Bottoming (or plug) tap - No lead-in and with full diameter threads right to the end for tapping to the bottom of blind holes but can't be used for starting holes. It's pretty rare you need a blind hole with a thread right to the bottom and you can't just make the hole a bit deeper so that a 2nd tap will do.

In America they call a 2nd tap a plug tap for reasons unknown.

As a general point there shouldn't be a difference on tolerance on the full size threads on a tap so for a through hole you can use either a 1st or a

2nd equally well as long as you run it all the way through the hole.

Good point! But also a reason for not requiring 100% thread engagement.

I recently used some hardened and ground 4mm dowel pins to locate some parts.

As an experiment I drilled some with a 1.5mm pilot hole and them drilled 4mm and the second lot I took to 3.8mm and then 4mm. The first resulted in a easy fit that I push by finger where as the second required the pin to be tapped in with a hammer (which is what i wanted).

I would imagine that the effects get more pronounced as the size increases.

Tim South Africa

That agrees with what I've found over the years and which I'm hoping I'm not going to have to question myself too hard on based on other comment. Double drilling with the 2nd drill close to the 1st in size ought to lead to a very tight tolerance on the drill size because it rubs as much as it cuts. There's an element of plastic deformation in the workpiece which springs back after the cutting edge has passed. This is aided by a slow speed on the

2nd drilling which helps preserve alignment by cutting down on chatter and wander of the drill bit and also leads to a coarser cut with more high spots left.

With very rigid equipment and constant machine feeds I suppose you might do almost as well with single drilling at a slow speed compared to double drilling but I've not found it to be the case drilling by hand on a Colchester Student or a Bridgeport clone. Also taking all the stock out at a slower than ideal speed is not good for the tool life. There are usually good reasons for why we do what we do but in my case they're so lost in the mists of time I'm more tempted to question them again than perhaps I ought to be because I've forgotten the hundreds of tests I've already done to establish why I ended up with this viewpoint in the first place.

I always rather assumed that a normal drill would want to 'unwind' a bit when taking a heavier cut and that would be part of the reason for the hole to be over nominal size.

A machinist of my acquaintance once suggested rounding off the end of the drill a bit - like a ball end-mill but only slightly. This also seems to help them cut to size.

Pete Harrison

You'll probably find that the holes in the second case have ended up slightly triangular. That's what's giving you the interference fit.

Mark Rand RTFM

I have always believed there were two main causes:

(1) If the drill lips are slightly unequal in length, the drill will rotate slightly off centre, and thus give an oversized hole.

(2) Pushing on too far without clearing the flutes will result in their packing with swarf (as someone mentioned earlier). This is almost certain to occur more on one side than the other, which would push the drill slightly off centre. This would result in (a) a slightly oversized hole, and (b) a slightly off-centre hole.

In really critical cases the first can be corrected by drilling undersize, then using a small boring tool down to at least 1 diameter deep, then using the correct sized drill or reamer. (If the hole is too small for your smallest boring tool - and they come pretty tiny - you can in a pinch use a slot drill, as this will cut straight down without being noticeably deflected by the off-centre walls.)

The cure for the second is fairly obvious.

I think GHT had a good write up about long hole drilling in one of his books.

Interesting. Anyone else heard this, or have any idea why it might work?

David

The above are indeed factors but the single biggest cause of oversize holes when drilling or reaming is chatter from excessive speed. The tool literally bounces from side to side in the hole removing material with the lands as well as the cutting edges. You can do an easy experiment with a small reamer, say about 8mm, in the lathe. Drill a hole to 7.8mm and then run an

8mm reamer through it at drilling speed, 800 to 1000 rpm and see what size hole you end up with. Then try again at 250 rpm.

I found this out the hard way when I was trying to make some 8mm bronze valve guides for the first time about 15 years ago. I turned and drilled a bunch of blanks without any problem and then reamed them all without stopping to check one first. Every one came out not just a bit oversize but several thou over and completely useless for their intended purpose. At least bronze wasn't so bloody expensive to scrap in those days. I was well miffed because I'd followed the advice in the Dormer twist drill and reamer handbook which said use half the drilling speed for reaming and for 8mm in bronze this was in the range 1000 to 2000 rpm for drilling and 500 to 1000 for reaming. On rigid machines you might get away with this but on an old Student you clearly couldn't. I tried again at 250 rpm and achieved the perfect size and finish.

Same scenario with drilling which is why I started double drilling with the

2nd pass at a slow speed. It cuts down on chatter because of both the speed and the minimal stock removal which also helps the drill cut straight instead of veering off to one side.

Anything that makes the drill rub instead of cut will help but it's the wrong way to go about it. Also rounding off a drill by hand is going to lead to some eccentricity which might promote wander instead of cutting in a straight line. The correct speed is the key and no special measures are needed to get a good hole size tolerance other than this.

yes,

and (b) a slightly off-centre hole.

but how will it be off-center?

-- Peter Fairbrother

I'm sorry but that's really nonsense. Drills have a cutting tolerance in correct usage which is always +0.00 to -something. They are intended to make holes that are never bigger than the nominal size. The Dormer tolerance on a

4mm drill is -0.018mm or about 7/10ths of a thou which is a light tap fit on a nominal sized dowel. Running too fast or not double drilling will use this up, and perhaps more, leading to a nominal sized hole or an oversized hole.

Reamers by contrast have a tighter tolerance but which is slightly oversize. They are intended to make holes which are not smaller than the nominal size so that the mating part, say a gudgeon pin or shaft, is a running fit. The tolerance on a 4mm reamer is +0.004mm to +0.012mm or about bugger all to

+5/10ths. Again running too fast will also lead to an oversized hole.

If you want a press fit on a job then you're better off using a drill at a slow speed than a reamer of the same nominal size. Tim's results were exactly what you'd predict from correct usage of a decent quality 4mm drill. No reaon at all to suppose that triangularity played any part in it.

It's commonly thought that reamers are intended to cut to exact size and that drills cut oversize but this is invariably caused by incorrect use rather than design. In fact the whole raison d'etre behind how things worked out this way is you have a choice between a press fit hole or a sliding fit hole using readily available nominal sized tooling by either just drilling or drilling then reaming. If both drills and reamers were designed to cut oversize you'd have to have special tooling made to get press fit holes.

That also explains why it's drills that cut undersize and reamers oversize. If you want a press fit you're not so worried about the quality of the finish. If you want a running fit, say for a bearing, you are. If reamers were designed to cut undersize and drills oversize this would end up the wrong way round.

Now that explains a lot. Now I see why instructions for making holes for bearings say drill then ream. I could never work out how a bearing was supposed to work if the hole was the exact size of the shaft. The common belief that drills always cut oversize simply implied that the reamer just gave a tighter tolerance. Thank you.

We were always taught to loose mount reamers to ensure accuracy. I am talking about 60 years ago.

I don't think they were triangular - clamped and done on a drill press with new Somta drill bits.

regards Tim South Africa

Somta may not be a familiar name to you guys in the UK - from the Somta site -

"Somta Tools was founded in 1954 by Samual Osborn Ltd of Sheffield, England, through its subsidiaries Osborn Steels and Osborn Mushet Tools. The name Somta was chosen as the acronym for these companies operating in South Africa."

It is commonly used in industry here.

Tim

It is a feature of twin flute twist drills that if you enlarge a hole with them or drill through thin stock, they will tend to drill a three lobed hole rather than a round one. Similarly, three flute drills will tend to cut a four lobed hole in similar circumstances etc. By pre-drilling the hole, you remove the ability for the conical end of the drill to stabilize the drill. This then allows the flutes of the drill to try to move in the shape of a Reuleaux triangle.

Unless you hand drill through thin sheet with a large bit, you are unlikely to see it by eye, but a Talyrond or similar equipment will show the out-of-roundness of the hole.

Note that perfectly symmetrical cutting edges are not necessarily required for accurate, to-size, holes. Gun drill bits specifically make use of the asymmetric end to press the bit to one side, which supports the bit and avoids errors.

You don't have to believe this explanation, but don't dismiss it without measuring roundness of the hole!

regards Mark Rand RTFM

Cutting fluid also plays a part in the size of the finished hole. Drill and ream a hole using soluble oil then do the same on another part using neat cutting oil. The hole done with oil will be a tighter fit.

It's also possible to play with reamer sizes over about a 2 tenth's range to dial in a hole. If the reamer is cutting oversize and all else is correct take a hardened dowel pin and stroke if down the top lands of the reamer, once per tooth.

If the reamer is too tight then again use the dowel pin but stroke it on the face of the lands and this will take the reamer back up to size. It's a good tip to remember if you have to ream aluminium bronze at any time as this stuff is a total bitch at blunting cutting tools.

John S.