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Newsgroups: rec.crafts.metalworking
Subject: Re: Gunbarrel Drilling again...
From: (murr rhame)
Date: 31 May 1995 01:26:24 -0500

My news server would not let me post during the thick of this thread so
I'm doing to throw in my two cents worth now. 

A typical gun drill bit is a single point tool with a carbide tip braised
to a hollow steel shaft of slightly smaller diameter.  These are coolant
drills with cutting oil being forced though the shaft and out a coolant
port in the tip under high presure.  This system can clear chips to a
great depth without pecking the drill (over 150 diameters deep for some
rifle bores). 

The gun drilling machine spins the work piece not the drill.  Any drill
will tend wander less when the bit it fixed and the workpiece spins (all
else being equal).  Most rifle barrels are reamed after drilling.  The
outsides of modern barrels are turned to size and shape using the bore as
a center before rifling. 

There are three ways I know of to cut (or form) the rifling in a barrel.
Most modern barrels are rifled by the button broach method.  The barrel is
filled with a special grease.  A form tool is pressed though the barrel. 
All the grooves are formed is one pass and no stock is removed.  Grooves
are formed into the bore by displacing the medal.  Technicly this is not a
broaching operation despite the name. 

A second contemporary method is to hammer forge the barrel around a
mandrel.  Again no metal is removed.  The grooves are formed by squeezing
the metal onto a carbide form which has a negative image of the grooves
cut into it.  The pressing is done by rollers that spin around the barrel
somewhat like a pipe cutter.  The rollers move from one end of the barrel
to the other.  The rifle form mandrel is drawn though the barrel and
twisted as the rollers progress.  The barrels come out of this process
considerably longer and thiner that they were before.  It's an impressive
and very noisy machine to watch.  This process also has a misleading name. 
It's called hammer forging but it's done on a cold workpiece using nothing
that resembles a hammer. 

The third method is the oldest and is still in use for some muzzle loaders
and custom guns.  A single tooth broach is used to cut each groove.  This
is drawn though the barrel with the twist being controlled by what amounts
to a very large screw with a thread pitch the same a the rate of twist
desired.  Approximately .001" of stock is cut on each pass.  The depth of
the broach is hand adjusted for each pass.  Approximately 15 passes per
groove.  The broach is indexed to cut each grove separately....  A heck of
a lot of work done by hand. 

Some military guns and a few custom guns have their barrels lapped as the
final bore finishing process.  This improves the surface finish and
removes any slight end to end diameter variations.  Some barrels may also
have their bores trued buy hand in special presses before final assembly. 

On modern guns the chambers are formed by a series of twist drills, form
drills and reamers.  For a complex bottle neck rifle cartridge there may
be as many as three rough and semi-finish tools used before the final form
reamer.  Each tool gets the chamber closer to the finished shape.  The last
reamer controls both the final dimensions and the surface finish. 

         murr rhame       /\/\ |_| |~ |~
         Show-Fire entertainment pyrotechnics mailing list.

From: Robert Bastow <>
Newsgroups: rec.crafts.metalworking
Subject: Re: A challenge for the BEST of you.
Date: Tue, 09 Feb 1999 05:54:52 GMT

The type of drill being discussed is NOT a "Gundrill" but is referred to as an
"oil hole" or "oil feed" drill...A very different animal!  And yes, they REALLY
ARE produced by pre-drilling and twisting!!

"Gun" drills don't have holes drilled down them!!

A "traditional" gun drill is like a "D" bit and is brazed into the end of a "D"
section tube.  Oil is pumped under high pressure though the tube, it exits via a
SHORT hole in the cutting bit and exits back up the outside of the tube carrying
the chips with it.

In actual fact, the tube and bit describe somewhat more than the half circle of
a true "D" bit the actual section being nearer to 225 degrees and the tubes are
given this section by roller swaging from round tubing.

Commercially, this type of gundrill is available in sizes down to 0.050" and

These drills are usually run on very precise, specialised Gundrilling machines
although it is not difficult to adapt a standard center lathe to do the job.
Speeds are high and feeds of the order of 0.0005" to 0.003" are the norm.

Multiple spindle setups are common, indeed the operatio I observed drilling the
twin holes in oil hole drill blanks, was done on a specially adapted ACME or NEW
BRITAIN type of multispindle automatic lathe Each of the multiple stations had
twin spindle gundrills such that there may be ten or twelve gun drills all doing
their thing at the same time!!

Ther are several othe deep hole systems..on of the better lnow being the Sandvik
"Ejector" system.  Here the tubes are circular and the head has three brazed
carbide bearing pads, one of which also acts as the cutter.
In this system the Cutter Head, is screwed by a multi start thread into a
thickwalled outer tube.  Inside this tube is a smaller, thinner walled tube.

Oil at pressures from 300 to 1000psi and in great volume is pumped through the
annular gap between the two tubes and exits through ports in the cutter head.
Thence it returns via large throats in the cutter heads, flushing chips with it
down the center of the inner tube.

The name "Ejector" stems from the fact that, just before reaching the cutter
head, a small proportion of the oil is bypassed straight into the inner tube via
specially shaped, backward facing ports.  This creats a venturi effect and
greatly enhances the ability to "Eject" chips.

Speeds and feeds are much faster..0.005 to 0.015" per revolution. The horsepower
requirements, both for the workhead and the fluid pump is high.  One such
machine that I custom built to drill out, from solid steel forgings, the bodies
of large jackhammers, had three independently controlled DC drive motors.  The
spindle had 50 HP the feed had 25HP and coolant pump had 75 HP!!!

The 3 3/8 diameter x 21" deep hole, in preheat treated 48RC alloy steel was
drilled in four minutes...floor to floor!!  Lotsa chips..had a
1 1/2hp DC motor on the chip conveyor!!

Robert Bastow wrote:

> In article <>,
>   Robert Bastow <> wrote:
> > Your question was answered by several people Danny,
> >
> > The holes were drilled and then the "blank" was twisted, before HT and
> > finishing!!
> Right, but now how did they gun drill the holes in the drill
> that was used to drill the holes in the larger blank??
> And please don't say they made two short blanks with short holes
> and welded 'em end to end to make the long one!

From: Robert Bastow <>
Newsgroups: rec.crafts.metalworking
Subject: Re: Drilling small straight hole
Date: Tue, 28 Sep 1999 14:24:54 GMT

Yup!  Dead on Dan!

I have drilled muzzle loading, match target rifle barrels, (.452 cal x 36" long)
with the same slow but sure method.  The only luxury touch was to braze the "D"
bit onto a tubular shank so I could get lube right down to where it was needed.
I was also able to use a high speed steel cutting tip.

Eventually I made a tip with brazed on carbide cutting tip and rubbing pads.
300 sfpm cutting speed and about half a thou per rev feed.  The holes would come
out within a couple of thou TIR and would be ready for rifling and a finish reaming required.

The only real difference between that and a REAL gun drill, is that the pro.
tool has a grooved, tubular shank, down which oil is pumped at VERY high
pressure and it, reurning up the groove in the shank, washed the chips out
continuously.  Thus there is no need to "Peck Drill"..the tool goes end to end
in one push.

The one VITAL consideration is that the drill be give a DEAD TRUE has
already been mentioned..a short starter hole is first bored true to dead size.
(or a hardened starter bush is used in industrial practice)


Dan Bollinger wrote:
> Allan,  Here's how I've drilled holes straight that where about 20-50 times
> as long as the diameter.  It requires making a simple, "D" drill.  Get a
> length of 7/32 drill rod.  On one end grind a flat almost half way through
> so the end-view looks like the letter D.  The flat ought to be about 3/4"
> long.  Now grind the end with a little cutting and relief angle.  The goal
> here is to make a drill which only cuts on its end, and not its sides.
> Using a torch, heat and quench to harden.  Polish.  Heat to "Straw" colored.
> Start a 7/32" hole with a twist drill after making a centering hole.  Use
> lots of coolant/lubricant.  Drill for about 1/16" and extract the bit to
> clean the chips being stored on the "flat."  I usually loosen the tailstock,
> slide the drill out, swipe off the chips and return the bit into the hole.
> With a little luck, you can drill a 6" hole that will require a caliper to
> measure how off center it is.  :-)
> --
> Dan Bollinger
> Clay Critters
> Allan Hudson <> wrote in message
> > How do I drill a long straight round hole approx. 7/32 dia. Obviously a
> > regular twist drill is not the answer. Runout soon becomes obvious as I
> > proceed. Professionals who do rifle barrels must conquer this problem.
> > Any ideas ?  thanks Allan the Aussie
> >

From: Robert Bastow <>
Newsgroups: rec.crafts.metalworking
Subject: Re: Drilling small straight hole
Date: Tue, 28 Sep 1999 15:13:17 GMT

Sure, it can drill the hole.

I'm not sure I can tell you how to get so much wander though!!  ;^)

Are you SURE you want it so far out?  I usually find that .003 to .005 is more
than sufficient!

Kidding apart!  If you do as suggested you should have no problems.


Brian Evans wrote:
> Can a D-tip drill made from drill rod, heat-treated and tempered at
> home, drill a 1/4" hole 16" deep in un-hardened drill rod, with say
> .030" - .050" wander?

From: Robert Bastow <>
Newsgroups: rec.crafts.metalworking
Subject: Re: Drilling small straight hole
Date: Tue, 28 Sep 1999 15:48:58 GMT

I would add just one thing!

The "Shank" of your drill MUST be a few thou (0.005" - 0.015") less in diameter
than the hole and the D bit producing it.  In other words..don't make a D drill
from 1/4" drill rod and expect it to drill a 1/4" hole of that depth.  You must
relieve the shank.

Otherwise it WILL sieze up solid in short order!

For a shorter hole..say up to 3-5 times diameter deep you might get away with
it..But even regular jobbers drills have a back taper on them.  (or SHOULD


Robert Bastow wrote:

> Sure, it can drill the hole.
> I'm not sure I can tell you how to get so much wander though!!  ;^)
> Are you SURE you want it so far out?  I usually find that .003 to .005 is more
> than sufficient!
> Kidding apart!  If you do as suggested you should have no problems.
> teenut
> Brian Evans wrote:
> >
> > Can a D-tip drill made from drill rod, heat-treated and tempered at home, drill a
> > 1/4" hole 16" deep in un-hardened drill rod, with say .030" - .050" wander?
> >

From: Robert Bastow <>
Newsgroups: rec.crafts.metalworking
Subject: Re: headpicking
Date: Tue, 28 Sep 1999 18:00:06 GMT

I surely can Hoyt!

For REAL gundrilling, call the REAL gundrilling experts..Eldorado Tool.

The have a Web page at < >

I am not trying to duck the question, but there are too many variables to even
guess at speeds and feeds..but these people are the cat's meow in this field.


Hoyt McKagen wrote:

> Obviously this means gun drilling. I hope Robert Bastow will be kind
> enough to make specific suggestions about those holes in feeds/speeds,
> coolant supply, starting holes, and everything else germaine.

From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Deep hole drilling
Date: Thu, 18 Nov 1999 21:22:24 GMT

Hmm!  Yes a parabolic drill will clear chips faster and easier than a regular
"Jobber" type.  However, a regular drill should not be loading up to that extent
after only 1/2 a diameter of peck.  Check that the drill is not excessively worn
on the lands..a taper here will NOT help!

Also check your grind and the chips it the drill loading up with
compacted, fine chips, such that it is difficult to tell discrete chips apart?
You may need to grind a "flatter point" (to direct chips in a more axial
direction) and give it a bit more back relief..this with a thinned point will
give easier penetration..Thus a stronger, more free-flowing chip and with less
feed pressure (which will tend to force the point off track)

Feeding too "gingerly" can also create broken "scrapings" rather than chips!
When you have all the conditions right..speed, grind, alignment, coolant
etc..then FEED it don't fudge it nervously along..the chip, produced as the
drill designer intended, will scoot right up the flute, rather than collapsing
on itself and creating a traffic jam at the tip.  You should be feeding at .003"
to .005" per rev.**

Of course..all the above points apply equally to a parabollic fluted drill
too..perhaps even more so.

With that depth of hole, I would be using two or three different drills of
graduated length..taking each one to within an inch of the end of the flutes
before changing to the next longer drill.  "peck" drilling should only be
resorted to when the depth is such that the chips are not self
long as they are clearing..Press on!  So long as the work is rotating and the
drill is sharp and "fed" not "pushed", the drill will tend to remain self
centered..unless you hit a hard spot or let the drill get loaded.

Good luck


** "Gun" drills on the other hand feed at about .0005" per rev but rely on high
pressure oil, returning from the tip, to push the chips out.  The Sandvic
"Ejector" type drills I used to use would demand a feed rate from .015 to .025"
per rev!!  Not for the faint hearted!!

Gilles Jalbert wrote:

> The procedure I'm using is almost exactly what Robert describes but, I sure
> don't get 1 or 2 diameters of the drill in depth. Will a parabolic drill
> make this much difference? I currently don't get much more than 1/8" before
> I hear the drill loading up.
> Gil

From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Deep hole drilling
Date: Fri, 19 Nov 1999 20:07:16 GMT

I am not sure what is the point you are trying to make Brian..But I will make
the following observations:

brian whatcott wrote:

> In article <>,
> says...
> >> a thick wall tube could accomodate  a brazed
> >>bit AND a suds passage. This would really help clear the swarf ...
> >It's not over kill for a 50 to 1 hole but it will be a problem at a
> >quarter inch due to stiffness and clearance. It would work great
> >for a 1 inch hole 4 foot deep.
> >
> >gordon
> >Gordon Couger
> This worry about stiffness and clearance is a very natural concern,
> even for an expert machinist.
>    But an engineer is required to keep his prejudices to himself, and
>  work out the awkward questions like these:
> 1)How torsionally stiff is a twist drill?

Not very!  Which is one of the limiting factors in deep hole drilling with twist
drills....torsional "Chatter" becomes a major problem.

> 2) How much torsional stiffness does a thick wall tube lose?

More to the point...How much torsional stiffness does a 3/16" tube, (as
suggested) have when compared to the 1/4" solid shank it replaces?

> 3) How narrow can a thick wall tube get, and still be stiffer
> than a twist drill?

How thick a wall can you retain on a 3/16 tube and still get enough volume and
pressure though it to fluss chips up a 1/4" o/d x 3/16" I/d annular gap?

> 4) What is a really stiff engineering material.
> (solid carbide is very, very stiff...)

Carbon fibre?

But neither carbon fibre or solid carbide would survive the high pitched chatter
set up in deep hole drilling.  One of the "Arts"(if you like) is the setting and
tensioning of steadies ON THE DRILL SHANK to eliminate the high pitched chatter
that will destroy a deep hole drill in seconds.

Deep hole or GUN drills of these small sizes do not have circular tubular
shanks.  The shank is formed by rolling a VEE into the tubular material.  A
SINGLE lip cutting head is brazed to this preformed tube.  This cutter must be
VERY rigidly and accurately supported, usually in a hardened starter bushing and
on long drills a series of up to four or five travelling steadies are used to
support and dampen the drill shank.

Cutting oil is pumped down the tube at pressures up to 3000 psi, this exits the
cutting head via a port, lubricates the cut and then as it exits up the vee
shaped groove it forces the PAPER thin chips ahead of it.   Gun drills are
manufactured down to 1/16" diameter.  Above about 9/16" diameter the "Ejector"
type systems (Either single tube or double tube) start to take over the field.

> The answers can be surprising....

Oh go ahead then Brian..I know you are dying to surprise us.

BTW..I used to own and operate a Contract Deep Hole Drilling Company!!



From: Robert Bastow <"teenut"@>
Newsgroups: rec.crafts.metalworking
Subject: Re: Deep hole drilling
Date: Sat, 20 Nov 1999 02:28:56 GMT

brian whatcott wrote:

> I got a sense that you were thinking of using the central port for
>  exitting the swarf. That *is* unusual

Not at all unusual..The Sandvik "Ejector" system uses two concentric tubes.  Oil
under high pressure is pumped down through the annular space between the inner
and outer tubes to ports in the replaceable (At enormous expense!!) drill head.
It then returns, flushing chips with it via the central tube.

Just befor reaching the drill head, some of the oil is allowed to escape into
the inner tube via rearward facing slots..this creates a partial venturi effect,
accelerating the main rearward flow of oil and chips..hence the "Ejector" title.

There is another, similar system that only uses a single drill tube.  Hear the
oil is pumped down the gap between the tube and the wall of the hole..returning
with the chips, via the inside of the tube.  It is a cheaper system to tool
up..but relys on having to establish and maintain a high pressure seal between
the drill driving head and the work piece.

To give you some idea of the power involved...My largest Ejector drill had IIRC
a 50 HP V/S DC drive on the headstock, 20 HP V/S DC drive on the Feed shafts and
50 HP AC on the hydraulic coolant pump!!

It would drill a 30" deep X 3" diameter hole in solid 4140 @ Rc 35/37 material
in about 3 1/4 minutes!

Tolerance on the hole would be held within about .002"


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