Subject: Re: A challenge for the BEST of you.
From: Robert Bastow <Teenut@hotmail.com>
Date: Tue, 09 Feb 1999 17:07:51 GMT
Spehro Pefhany wrote:
>How much would you expect a 1/16" hole to drift going through 12" of
>steel? Would it depend on the quality of the steel (something like P20
>having less inclusions and being better?)
As in all things Spehro...It all depends!!
First of all, 1/16" dia x 12" is an awfully deep hole and not the sort of hole I
would expect to see in an oilhole drill. Most of these are not much longer than
standard drills I have one that is 1.093" dia x 8" long and the oil holes are
about 3/16" dia.
Having said that I am sure that it can be done. I have watched .177" dia holes
being gundrilled through a 22" rifle barrel with no problem.
Getting to the "it all depends" bit...Obviously there are a number of factors.
Gun drills have to be set up DEAD true and have to be started DEAD tru into the
cut. This is achieved by either pre-boring a starting hole to dead size and 2-5
x hole diameter, or, more usually by using a hardened and ground starting
The question also arises as to whether the part or the drill is rotated...or in
some cases BOTH. Obviously if the part is rotated the setup is inherently far
more accurate as the drill head is constantly self centering.
Given all that the forces that tend to push the drill of center include using a
too high feed rate, insuficient oilpressure sothat chips jam and blunt or
chipped drill head.
By FAR the most common cause is variations..Hard or soft sponts in the
material. Remember my oft used statement, to the effect that NOTHING is
perfect, we are in a situation in gundrilling where we are "pushing the
envelope" and everything had better be as close to perfect as possible.
To this end, manufacturers of custom gun barrels go to inordinate lengths to
ensure consistancy in thier raw materials which will be specially rolled and
normalised. On receipt the bars will be checked, straightened and re-heat
Given that, runouts and straightness are expected to be within pretty tight
limits. Even so there is a selection process for "Premium" barrels.
Gun drilling is a facinating and versatile process..did you know that
overlapping "8" shaped holes can be done quite easily. In this respect the
gundrill is similar to a woodworkers "Forstner" bit.
Subject: Re: A challenge for the BEST of you.
From: Robert Bastow <Teenut@hotmail.com>
Date: Wed, 10 Feb 1999 18:55:47 GMT
The ones I saw being made were forged to a straight flute section before being
They were then flash butt welded to a shank before being heated by an induction
coil and twisted.
The assembly was then machined, heat treated and finish ground.
Don't forget that , in Industry, processes that we might find incredibly
difficult to imagine, are carried out daily, with casual aplomb.
It would also be possible to extrude the required twist drill section, holes,
flutes, twist, and all. If anything, extruding a STRAIGHT section is more
Danny Hopping wrote:
> So, let me get this straight. They drill the two small holes, then
> twist the blank, then machine out the groves that form the drill twist?
> This LOOKS like how this bit must have been formed as the small holes
> are perfectly round. I am amazed that a 1/16" hole can be drilled 14"
> long. Do they heat up the blank before drilling then drill it hot?
> Wouldn't the twisting affect the OD of the blank?
> Is it turned down after twisting?
> Thanks for the answer, now I can sleep again :-)
Subject: Re: Boring a gun barrel (long)
From: Robert Bastow <"teenut"@ hotmail.com>
Date: Sat, 18 Mar 2000 00:50:10 GMT
Jacob, you covered the subject beautifully..especially the bit about extreme
attention to detail. Above all else in deep hole drilling..There ain't much
room for mistakes.
teenut..who has made most of 'em!!
Jacob Henteleff wrote:
> There are a number of "secrets to truly concentric boring. The drill bit is
> the first critical item. It should be of the gun drill (also called deep
> hole drill) variety which has two flutes like a conventional twist drill but
> they intersect at point off center and therefore cut a sort of "W" shaped
> surface if you look at in cross section.
> There are no spiral flutes, just a long trough along the length of the bit.
> Oil under pressure is fed through a pipe to the back end of the gun drill.
> It emerges at one of the relief faces of the bit and lubricates the area
> behind the cut. It then finds its way to the trough and carries the chips
> back out of the bore.
> When the barrel is readied for boring it is set up in a chuck that will
> support the blank at both ends and with both ends running as truly as
> possible. Since most rifle blanks come with a rough exterior you generally
> turn a smooth area at each end to run a dial test indicator on.
> The end at which the hole is started is turned flat across the end with a
> little of the concentric turning marks vivsible. This will help retain a
> tight oil pressure seal while the oil pressure is applied to the bit.
> The bit itself is passed through a closely fitted bushing. Normal drilling
> practice on the lathe or drill press is to chuck the twist drill by its
> shank and just shove it at the work piece. We all know that the small chisel
> tip isn't very happy about this even if you started with a pilot hole.
> Because the average twist drill bit is upwards of 10 times as long as it is
> in diameter and given that the bore of the tails stock and the head stock
> aren't allways precisely in line and given that even the nicest factory
> drill bits may not have precisely identical flute lengths on each side, the
> result is the bit wobbles a bit until it's in deep enough that the hole
> controls it.
> That's where the bushing comes in on barrel drilling. It is mounted in a
> housing that has been set up to hold the bushing so that it aligns perfectly
> with the axis of rotation of the barrel. The face of the bushing is greased
> lightly and brought into close contact with the face of the barrel blank.
> The turning grooves act as a seal.
> The gun drill which is like a groud rod with a trough along the side is a
> perfect sliding fit in the bushing. The barrel blank is set to turning. Then
> the gun drill is fed through the bushing until it touches the face of the
> barrel blank. A little oil can be fed but not much. Too much flow or
> pressure might affect something. Because the bushing is exactly in alignment
> with the center of rotation and because it holds the bit so snugly the bit
> starts a perfect hole down the barrel.
> Once the drill is started and has gotten deep enough that the barrel hole
> now acts like a bushing for the drill bit the oil pressure can be brought up
> so that it flushes the chips out well and reliably. Getting a pile up of
> chips is guaranteed disaster. A 6mm bore might need over 400 psi to flush
> well and perhaps higher. Remember that the drill bit is going to be 20 plus
> inches down the hole and that's a big stack of chips to move. Also the oil
> pipe is what is holding the bit down there so it's doubly fragile.
> You can see that as the bit passes farther into the bore and no longer has
> the support of the bushing and because the bore it makes is less than
> perfectly smooth, there is bound to be a slight tendency to wander, however,
> slight. Really close attention to the details of bit grind, oil pressure,
> rate of feed, and rotational speed of the blank can all contribute to
> success or failure.
> In an amateur rifle making environment you will not want to bore right
> through the blank. If you pop through the end with 400+ psi of oil on tap
> you'll rust-proof everything in sight before you can say holy crow.
> Stop an inch or so short and just cut the "cap" off to expose the hole.
> Now that you have a hole as concentric as care and attention to details can
> provide, say less than 20 thou TIR over 24 inches you can set up the barrel
> for reaming. Reamers with oil pressure feed are drawn through the bored
> barrel to clean up the surface. Ususally this is done in two steps, each
> taking out a few thousands of an inch only.
> The bore is then ready for rifling and if you are cut rifling for lapping.
> most manufacturers of rifle barrels use a carbide button to swage the
> rifling into the drilled bore which is much faster than cutting individual
> grooves a few ten thousandths at a pass. It also produces a smoother bore
> finish and probably work hardens the bore surface a bit too. A button takes
> only one pass to produce full rifling.
> There you have it. There is no secret to drilling rifle barrels. All you
> need is a full understanding of the process, infinite patience in
> preparation or tools and set-up and then loads of experience (in other words
> you screwed up a lot of other people's barrels first). What could be
> Want more info? Buy a copy of Guy Lautard's video on amateur rifle barrel
> making. It will tell the whole story and is well worth the price. You can
> find ads for his books and videos in Home Shop Machinist and other magazines
> and I believe he also has a Web Site now. Sorry I can't reproduce it here.
> Whether you make your own barrel or not you will know a lot more about the
> subject than I just gave you.
> Paul Koning wrote in message <38D14E87.DFD49BE5@lucent.com>...
> >Lou Boyd wrote:
> >> "Orrin B. Iseminger" wrote:
> >> >...
> >> > Seriously, I wonder what the secrets of truly concentric boring
> >> > are. Does anyone know them? If they're in the book, the
> >> > information will easily be worth the price of it.
> >> The way to end up with a concentric bore is to turn the outside after
> >> the hole is bored.
> >Not from what I understand reading the articles.
> >For one thing, if the drilling process itself couldn't be trusted
> >to go concentric, it couldn't be trusted to go straight either.
> >And turning the outside last is no help at all if the bore has
> >a curve in it!
> > paul