View Full Version : Half-round drills.... uses for?

J Tiers
12-30-2005, 10:10 AM
Sorting out some "stuff", I found a packet of 0.091" dia half round drills.

What special use are they for? Similar to gundrills?

Searches on various forums seem to suggest that they act similar to gundrills, but most gundrills are 3/4 round, instead. That would prevent them wandering better than only a half-round solid part.

These are only about 4 thou over half diameter in the solid part. They are 0.049 solid, 0.091 total. Maybe enough, maybe not.

Curious, mostly, no application in mind.

12-30-2005, 10:48 AM
Half-round holes, of course!

That should be obvious.

12-30-2005, 10:56 AM
Years ago they used half round drills for drilling in glass and hard tile. Before the current carbide drills became affordable. You put a wall of clay or putty around the spot and mixed up some light oil and grit to form a paste used a drill press to put the hole in only did it once that it worked well. I guess that it was a quired touch. Perfer the modern diamond glass drills or the carbide grit drills of today LOL.

Have a number of Delta jig saw blade packets with blades for cutting bone, leather, cardboard or paper. About 6-8 teeth to the inch. weird looking blades.

Been there, probally broke it doing that

[This message has been edited by PTSideshow (edited 12-30-2005).]

12-30-2005, 11:38 AM
They are great for drilling thin stock and brass. I also use them for reaming holes in brass and aluminum.

I have a fractional and numbered set.


12-30-2005, 01:22 PM
I think you're referring to what the Brits call D-bits. Good for finishing a hole to-size in lieu of a reamer, relatively easy to make from drill rod. Early issues of Model Engineer magazine talk about them a lot. Not particularly good for drilling a "complete" hole...drill undersize first, with a regular drill, then finish off.

Norman Atkinson
12-30-2005, 01:49 PM
D Bits? We were here just a short while ago.
Tim Leech, if I am not mistaken.

So let's get back to Malbenbut. Oh, yes!
He did an article about bagpipes- and
D- Bits are the correct tool to drill the long holes.

So let's get back to our Mark of the Welsh Marches. So let's get back to Alistair.
Both were onto D-bits.

Anyone would think that memory training went out with the collapse of Pelmanism.

Write it down before you forget it!

Now, what was my name? Nor****? nor lose the Comm** t- tut, tut. It was Rudy- hard to remember.

Right- Norman.

PS- what a load of pillocks!

J Tiers
12-30-2005, 02:08 PM
<font face="Verdana, Arial" size="2">Originally posted by NORMAN ATKINSON:
D Bits? We were here just a short while ago.
Tim Leech, if I am not mistaken.


I searched AGAIN (including D bits), and your post from which the above was taken was the only result!

Perhaps I have not the mouth correctly held.
Ach so.

The search is apparently best used when you know what you are searching for already, and know what other words should be included (I could suggest a few, but I digress).

Thanks for not calling me an addlepated git, anyhow. That shows either diplomacy on your part, or perhaps merely poor skills at observation.....

So I take it these thinguses are more for a sort of reaming operation?

The gundrill type will drill from the solid, and seem to be best at that. They do need a starter hole on location and correct size.

The half-round look too fragile for much.

Norman Atkinson
12-30-2005, 02:29 PM

Would I call you - damn, I've forgotten.

Seriously, Tim was prattling on- sorry, matey- about drilling fancy holes.
Me computer has had a Malbenbut- sorry, Malware and I gawn an lorst me records.
At present, I am limping on small hard drive.
When you get to my age, everything goes soft.

I also prattled on about Ian Bradley- and the Drilling Machine and also the Grinding Machine. Both by Tee. They have D Bits and one goes on to doing bits for engraving machines.
Now, JT, might I quote the Gospel of St George Thomas, the Divine?
"For this chapter, a D bit is a tool for producing a parallel hole as nearly as possible true to size, therefore it must not be capable of cutting on its sides"

Obviously, GHT in true tradition expands his advice to- lesser mortals like me.

Oh, and a Happy New Year.


phil burman
12-30-2005, 03:42 PM
<font face="Verdana, Arial" size="2">Originally posted by J Tiers:

I searched AGAIN (including D bits), and your post from which the above was taken was the only result!

Perhaps I have not the mouth correctly held.
Ach so.

The search is apparently best used when you know what you are searching for already, and know what other words should be included

Try searching for this thread "D bit Geometry Question"
start by me on the 08.16.2005


Norman Atkinson
12-30-2005, 04:11 PM
Well, thank you Phil!
It finished on the 21st August 2005.

All protagonists- sorry, contributors- mentioned appeared in the plot.

Did I say all that? Yuk!

Nah then, J! You take a chunk of say African Blackwood of 13 inches long and have to drill a 3.5 mm hole all the way through it. Yer might have to do a bit shorter lengths but get a parallel hole of only 3mm. The easy bit is drilling the tone holes in the chanter.

Interesting stuff, I promise.



J Tiers
12-30-2005, 08:04 PM

I searched using "D bits" AND half-round drills, etc.

Must have searched for the combination, instead of "any of them". But I checked the right box.........

Just goes to show that if you don't have the right words, the search won't find the same subject unless the words you use are in there.

So if you use the wrong words to describe the thing, its no worky.

It's the only way a search CAN work, but it surely must account for many "WHY THE DICKENS DIDN"T YOU TRY SEARCHING FIRST?" replies......

12-30-2005, 08:50 PM
Half round drills have been in use for a long time to drills holes in brass run on automatic screw machines. I have used them a lot and it takes a lot of practice to regrind them to cut to size. I ran a job in 1/4"dia. brass that had to have a 0.159 hole 2 in. deep, did it in 1 poke at around 0.007 feed at 7200 rpm. Held less than 0.005 TIR. and + or - 0.002 on the dia. of the hole.

Al Messer
12-30-2005, 10:23 PM
They're also useful for drilling in "sticky" metals like Copper, such as drilling holes for rivets in miniature Copper boilers.

12-30-2005, 11:17 PM
I was under the impression D-bits were used to "square up"the end of a drilled hole.Idid know they are also used as a poor-man's reamer.Are they that much stronger than a spiral drill bit?Guess i'm never too old to learn http://bbs.homeshopmachinist.net//biggrin.gif


Norman Atkinson
12-31-2005, 12:54 AM

Without trying to teach you how to suck eggs, I went into the search mode.
Then I typed in the full title as suggested by Phil and gave the bloke his full moniker- Phil Burman and as it was possibly a half year ago, I gave the search facility a year to do its thing.
As the Germans say " Le Voila" or something like that. First time that I have tried it- honest Injun. As another honest admission, I was swanning about in the General stuff around the 70 odd mark- without success.
You can have a swipe at looking for the 21st August that way as an alternative.

I was having yet another thought.
The amount ground off a D-Bit should be .005" LESS than half the diameter. Yours, and I assume your accuracy, are not like that.

Most peculiar.


Al Messer
12-31-2005, 08:45 AM
The "Half round" bits such as sold by Traver's Tools, Page 22 of their 2005 catalogue, are not "D" Bits as "D" Bits are flat on the end and used for finishing previously drilled holes, especially when one has to have a flat bottom, while half round bits are pointed.

Norman Atkinson
01-03-2006, 03:07 PM

We are going through a find on Model Engineer archive index.
Might I suggest that you have a look at the Novices Corner from 30 th November 1950 for more on D Bits.

My memory is better than I thought!

Kind whatsits from