Wish I were competent with CAD programs because I'd have some nice drawings to accompany this question. (Suggestions on CAD software welcome). This post will focus on the barrel's lower lug "radius". Maybe, you may think, it just doesn't matter that much. Perhaps not, but it is fun nonetheless, and my goal is to keep learning. No problem there.
When I refer to "bottom" and "top", "upper" and "lower", I mean those terms to be relative to the normal position of the pistol when fired. I am going to assume (have to start somewhere) that I have a mid-spec barrel, frame and slide so I can use the nominal dimensions thereof.
Begin with the barrel's link pin hole. The center of this hole will be the reference point (indicate CAD machinery here). Also given is the nominal barrel link with .278" spacing, c-c. Subtract 1/2 the diameter of the slide lock pin (link's lower hole) and you get .178". This number is the "nominal" radius for the lower curve of the bottom barrel lugs. And this is the distance from the link pin center to the outer edge of the slide stop pin. (Presumably where the s.s. pin touches the lug radius.)
Most discussions of the barrel's lower lug radius, as well as most drawings assume that an arc has been swung from the reference point (center of the link pin hole) across the lower left quadrant (quad 3) of a co-ordinate system. When a cartridge is fired, the slide stop's pin will travel from approx. 270 degrees (6 o'clock) to approx. 225 degrees (7:30 o'clock). That movement represents the barrel linkdown phase.
Intuitively, the above description makes sense because the slide stop pin sits in the lower hole of the link. Since the center of the top end of the link is our circular center of reference, the slide stop pin will slide smoothly along the circular arc (in contact) without any problems. Or will it?
In theory, I suppose this all could happen as above. But add the tremendous forces involved and the rather vigorous way that the barrel goes from in- battery to link-down, and we run into a few problems. So, the components will have to be "tweaked" slightly in order to avoid metal deformation and crashes.
First, we know we want the barrel to "stand" on the slide stop pin in the in- battery position. Furthermore, we also want things to stay that way while the slide begins its rearward travel (nominally, for .100" of rearward travel). That will mean that the lower part of our barrel lugs, just at the 270 degree point, will be flat (or nearly so). To make the barrel's lugs stand on the slide stop pin, we might have to put the link in tension, which is a fairly tricky thing to do without risking some type of metal deformation. One way to buy a little wiggle room at this point is to enlarge the bottom of the link pin's lower hole, so the link is completely free in the in-battery position.
But, considering full link-down, we would also like the barrel to be supported by the frame's VIS at the rear, and by the link in compression at the front. For the link to be in compression (via the slide stop), the surface of the barrel's lug radius will have to be shortened up at about the 225 degree point - that is the approximately where the link will be in full link- down position.
Already our added requirements for supporting the barrel have altered our original concept, which was based on a simple radius. Part of the original circular radius on the barrel lugs must be cut away (for linkdown). In addition, part of the radius is going to be be flattened out (for in-battery). And, finally, the lower link hole may have to be reshaped into an oval, to allow for complex movement between the slide stop, linkpin, and barrel.
Ed Brown has created a print (mechanical drawing) that makes allowances for these added barrel fitting requirements. His drawing makes sense to me, and I would like others to consider whether he has captured the essence of barrel fitting in the 1911. Ed does not indicate on the center of the barrel link pin hole. Instead, he places his radius CL .010" to the right of the link pin CL. That is, .010" horizontally (towards the rear of the gun) from the link pin center. Plus he uses a radius of .183", 5 thousandths longer than our original nominal radius.
If you plot the 2 curves out roughly, you can see the logic. The result of an arc swung from the "new" center gives us a little lug clearance (between slide stop pin and barrel lug) at linkdown (one requirement) while adding additional metal to the lower lug contact point for the barrel in-battery (the other requirement). Ed claims he cuts the barrel lugs exactly this way on a mill using a rotary table. Seems nice and neat, right? But remember he is also using his own slides, frames and barrels. So, I'll bet things come out pretty close.
Using Ed's concept for the lower lug fitting, we are going to have the barrel standing on the s.s. pin when its in-battery. We're going to have a small "flat" near 270 degrees to allow for short recoil. Then we are going to have the link in compression at full link-down. Obviously during link-up, the slide stop will ride the link, until the last part of lockup. Acceptable?
So, folks, I'd be interested in comments and corrections as well as insults and personal attacks if they are amusing. If any of you are wizards at CAD and can supply a couple of visual aids, that would be great. I'll be learning a bit of CAD for some drawings I want to do on trigger engagements for future posts.
If you e-mail the Ed Brown print of the barrel I will make a 3D model of the barrel and e-mail it back to you in a format you can open from any computer. I use SolidWorks.
I will work on it at lunch. My boss is cool but not that cool.
I tried to follow this, but I'm afraid I got confused. Does the slide stop pin rotate? How?
6:00 is normally 180 degrees. 9:00 is 270. 7:30 is in between. I couldn't follow your frame of reference. Where (relative to the gun) is 0 / 360 degrees in your scheme? I apologize if I missed something.
Looking at the gun from the left (non-ejecting) side with the barrel horizontal, the locked link looks to be at a small negative angle (past 0 going counterclockwise). Link rotation is clockwise around the slide stop.
Also, I think the link should be in only very slight, if any, compression fully linked down, see the Schuemann writeup on this.
The barrel end of the link looks to me to be slightly over center (relative to the unlocking motion) in battery - so hence some of the room for the nominal .100 movement before unlock?
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First, we know we want the barrel to "stand" on the slide stop pin in the
in-battery position. Furthermore, we also want things to stay that way while
the slide begins its rearward travel (nominally, for .100" of rearward
travel). That will mean that the lower part of our barrel lugs, just at the
270 degree point, will be flat, or nearly so.
It's not necessary, nor even desirable for the lower lug to keep the barrel lugs solidly engaged until the instant that linkdown begins. The pressure exerted on the upper lugs will do that, up until the bullet breaks free, and the pressure falls off.
Since things are starting to happen fast, once linkdown begins, you want the barrel to fall as quickly as possible. Part of the barrel drop process occurs due to the "Ball on a Tether" effect via the barrel's rearward speed and momentum being abruptly stopped by the link...but gravity also plays a role. Although it's a small one, when things are happening that fast, you want all the help that you can get in getting the barrel out of the slide and out of its way.
Riding the link as the barrel swings up into battery is acceptable as long as it doesn't cause the barrel to swing up too early... which is another pitfall in using a long link to effect a better vertical engagement. Think of when you were a kid, and playing pole-vault. The higher on the pole you plaed your hands, the faster and more violently you got jerked upward when you stuck the pole in the ground. If, during the feeding phase... just when the bullet nose strikes the barrel ramp... it pushes forward on the barrel. As the barrel is pushed forward, it also moves up as it either cams up on the pin... or pole- vaults on the link. As the slide continues to move forward, the barrel is pushed farther forward and farther up, increasing the angle that the cartridge must tranit before breaking over to horizontal. One of the biggest reasons that my pistols are so dead-nuts reliable is because not one barrel is allowed to ride the link... not even a tiny bit. I shoot some pretty cruddy ammo 98% of the time. I have a failure to go to battery maybe once a year in a half- dozen beaters when they get dirty. Once... not once per gun... and I average 50,000 rounds a year.
Early barrel engagement timing can also bring the front corners of the barrel lugs into contact with the rear corners of the slide's lugs. This can lead to radiusing of the lugs, or a solid stoppage if the contact is severe enough.
Ever handled an empty gun that had a slight hitch just before going to battery? Bingo...
So... Timing changes with link center-to-center length works coming and going... not just during linkdown timing. A quick look at lower lugs will provide a clue. The bottom surface is a ramp... not a platform. Compare the differences between a 5-inch barrel and a 3.75-inch Officer's Model. The OM has a steeper ramp. Things happen faster in the chopped versions... and the barrel has a much smaller window of opportunity to avoid a crash with the slide.
OK, then!
GBW, my reference point is the left side of the gun. My circular co-ordinates are the usual one's from geometry where 0 degrees is at the 3 o'clock position. That puts 270 degrees at the 6 o'clock position.
Tuner, thanks for your input. I am intrigued by your comments on riding the link; and that full reliability is realized if link-riding is not allowed to happen. You suggest that riding the link while going into lockup is due to the use of a long link. But, in this particular case, I have created a hypothetical lower lug that has clearance (link in compression) for most of the link-up movement, while using the standard link length. I'll take your word for it that even with the standard link, having the link in compression during link-up is not optimum, but I'm going to ponder that a little. I have a feeling I'm about to ruin a few perfectly good barrels to get the hang of this.
If the s.s. pin ramps up on the barrel's lug surface, where, if anywhere, does the slide stop pin lose contact with the barrel lugs in order for the link to go into compression at full link down?
Finally, during the barrel fitting process, with so many requirements...
You have taken care to insure 100 engagement on the #1 upper lug and at least partial engagement of the other 2 lugs. At the same time, keeping in mind the relative position of the firing pin... insuring that the barrel sits on the slide stop in battery... making sure the barrel drops sufficiently to clear the slide lugs... being certain that the barrel hits the VIS first on link down... I think I'm feeling sick.
After taking a good close look at my guns, I am pleased to report that I own one example where the barrel actually does stand on the tips of the lower lugs in linkdown. It is a Kimber Gold Match, suprisingly enough, with a Kart barrel - about 10 years old. I was very impressed to discover it.
Quote:
If the s.s. pin ramps up on the barrel's lug surface, where, if anywhere,
does the slide stop pin lose contact with the barrel lugs in order for the
link to go into compression at full link down?
That's the tricky part. Riiiiiiight at the final couple thousandths before the barrel hits the bed. The other part of feed/RTB reliability is directly related to the height of the bed... and hence the placement of the barrel when it's fully linked down in relation to the incoming round. If stopping the barrel's drop with the link in comporession... don't sweat it. As long as it hits the VIS first... has adequate clearance at the top... and then drops to the bed, it's not an issue.
For absolute, as near-to-100% feed/RTB reliability that can be attained... you want the feeding round to exert downward pressure on the barrel as it enters the chamber throat, in order to counteract the tendency for the barrel to move forward when the round starts to push forward. That means that the nose of the round has to contact the throat well above center... but it still has to make that contact in order to maintain full control of the round. (This is how Tripp's magazines manage to provide smooth feeding, though in some guns, the angle of entry is too straight... and control of the round is lost.) Sometimes this requires lowering the bed... something that Kuhnhausen says is a no-no... but sometimes the specs demand it. Other times it also requires a little clearancing in the slide to get the barrel to drop that far... along with a very light clearancing cut on the sides of the barrel at 4 and 8 O'Clock. .003 inch maximum, and stay clear of the junction of barrel and lower lug. Ya don't want any sharp corners there.
The other tricky part is in recognizing when the mods are required... and which ones... and when feed reliability issues can be resolved by other, less invasive means. The above modifications are to be used as a final resort.
Happened to examine my 1957 Colt commercial 1911 today. No telling if the barrel is original, but it does say Colt on the hood. The link is standard length at .278". At NO place along the lower lugs has the bluing been rubbed off. In other words, the entire surface from start to finish on the lower lug radius is untouched. Guess that qualifies as "riding the link", huh? Fortunately the upper lugs have not been clobbered.
This gun is going to George at EGW for some ramp repair because someone "had his way" with a dremel on the feed ramp. Typical, typical.
Thanks Mr. Lazarus. I went to engineering school too - So far, so good, should have quit there. Then pilot training, so now and for ever more I'm afraid, I tend to think of 0 as north. Sorry for the confusion.
And to Mr. Tuner - thanks again for all your help - to us beginner builders it's wonderful.
One additional clarification for us class dunces, if you don't mind...
"...The pressure exerted on the upper lugs will do that, up until the bullet breaks free, and the pressure falls off. "
What is the source of the pressure on the upper lugs? I understand radial (well, spherical) pressure from the expanding gas inside the barrel, but I'm missing why this would tend to hold the upper lugs locked.
Quote:
"...The pressure exerted on the upper lugs will do that, up until the
bullet breaks free, and the pressure falls off."
What is the source of the pressure on the upper lugs? I understand radial (well, spherical) pressure from the expanding gas inside the barrel, but I'm missing why this would tend to hold the upper lugs locked.
When the bullet is passing through the bore, it does so under high frictional resistance.
Of course, as the bullet accelerates, the resistance decreases, but it's still pretty high.
This tends to hold the barrel hard forward.
The force vector that drives the bullet and barrel forward is also driving the slide backward in recoil. The locking lugs are engaged horizontally, and the barrel stays put until the bullet exits and the pressure drops.
Quote:
No telling if the barrel is original, but it does say Colt on the hood. The
link is standard length at .278". At NO place along the lower lugs has the
bluing been rubbed off. In other words, the entire surface from start to
finish on the lower lug radius is untouched. Guess that qualifies as "riding
the link", huh? Fortunately the upper lugs have not been clobbered.
As long as the barrel doesn't lock... stand... on the link, and the link closely follows the shape of the lug through its operational arc... and the lug geometry is within-spec... a couple thousandths won't hurt anything, other than put a little extra stress on the .154 pin hole in the lug. It's when the slidestop pin and the front radius of the lug show an acute line of departure that feeding gets erratic. Simply put... If the slidestop pin maintains a constant distance at all riding points, all will be well. When it does gain distance as it swings toward the full linked down position... that's where a slight elongation of the top portion of the hole really helps. It doesn't affect linkdown timing and gets the pin closer to the lower lug at the "pole- vault" point. It can, however, have an effect on the barrel being stopped by the link in compression... but as noted before... this isn't necessary as long as the barrel stops on the VIS before it hits the bed.
Alright, someone help the new guy along. What does VIS stand for?
Seriously... Vertical Impact Surface. The vertical wall at the rear of the area that the lower lug and link fit into. That's what stops the barrel's rearward travel during linkdown.