Pre-Drill & Boring A Chamber
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GeneT
Active member
Here's how I do it.
An excellent machinist might cut things closer than I do, but I oops'd the first time, so I take a conservative approach. Get your reamer drawing and figure out what diameter and depth you can drill while leaving material all the way around. I aim to leave 0.050" where the drill is closest to what will be the chamber wall. I've gone closer, but been bitten.
After cutting and threading the tenon, I rough drill in steps to the depth calculated (using ordinary drill bits) until I get to the diameter I calculated. then, with a solid carbide boring bar, I set the compound as exactly as I can to the taper of the reamer. I then bore the nascent chamber until I have only .020" to go to case diameter. Note that that's different that 0.020" of forward travel on your reamer. The setup is then ready for reaming, and if done right the bored section should support the reamer as it starts to cut the finishing chamber. It is critical that you not over-drill, nor over-bore. The first time I tried I cut a tiny bit into the shoulder portion, resulting in a score mark at that point - which means I got to do it again... Go slow as you start reaming - most of the initial cutting will still be occurring where the flutes on your reamer are at their most shallow.
That's how I do it, but I've never had much more than the same vague description that you see everywhere else, so I look forward to hearing any tips and tricks others may have.
GsT
An excellent machinist might cut things closer than I do, but I oops'd the first time, so I take a conservative approach. Get your reamer drawing and figure out what diameter and depth you can drill while leaving material all the way around. I aim to leave 0.050" where the drill is closest to what will be the chamber wall. I've gone closer, but been bitten.
After cutting and threading the tenon, I rough drill in steps to the depth calculated (using ordinary drill bits) until I get to the diameter I calculated. then, with a solid carbide boring bar, I set the compound as exactly as I can to the taper of the reamer. I then bore the nascent chamber until I have only .020" to go to case diameter. Note that that's different that 0.020" of forward travel on your reamer. The setup is then ready for reaming, and if done right the bored section should support the reamer as it starts to cut the finishing chamber. It is critical that you not over-drill, nor over-bore. The first time I tried I cut a tiny bit into the shoulder portion, resulting in a score mark at that point - which means I got to do it again... Go slow as you start reaming - most of the initial cutting will still be occurring where the flutes on your reamer are at their most shallow.
That's how I do it, but I've never had much more than the same vague description that you see everywhere else, so I look forward to hearing any tips and tricks others may have.
GsT
Nice writeup Gene, thank you
Rubicon Prec.
New member
On a manual machine and floating reamer holder, I drill then parallel bore about .010-020 under shoulder diameter, as deep as I can while sill allowing decent pilot purchase in the bore.
In the CNC turning center, for most cartridges, I run an 11mm drill in .050 shy of the shoulder, then bore the entire body out, .010 under diameter, .050 short of the shoulder. That reamer is dead nuts aligned and I know the pilot is going to find the bore when it gets there.
In the CNC turning center, for most cartridges, I run an 11mm drill in .050 shy of the shoulder, then bore the entire body out, .010 under diameter, .050 short of the shoulder. That reamer is dead nuts aligned and I know the pilot is going to find the bore when it gets there.
jackie schmidt
New member
I am a machinist, and this is basically the way I do it.
The big difference is the first thing I do is rough out the tenon and then establish the best chamber I can in the barrel, true with pre determined points in the barrels ID.
I then finish the tenon and other operations.
Butch Lambert
Active member
Gene, I do it basically the same, but stick my 504-513 Mitutoyo in after drilling to make sure it didn't move while I was drilling.
Can anybody tell me of the advantages of the "Gordy" method if any?
Yes, I tried it in the old days.
Can anybody tell me of the advantages of the "Gordy" method if any?
Yes, I tried it in the old days.
if you are using a bushing, you have the possibility of it trying to push the reamer off center.
bores are not straight holes.
the whole idea is the taper guides the reamer.
just what I have learned from Jackie
bores are not straight holes.
the whole idea is the taper guides the reamer.
just what I have learned from Jackie
GeneT
Active member
That's a good point Butch, and one I omitted. I check my setup frequently as I go along. I've seldom had anything move and the few times I've had a change in readings I've suspected machining practices rather than moving the workpiece, but who can tell? I just know things aren't as perfect as I want them to be. I went and took a look at the indicators I use, and I think they're the same as yours, but you have the numbers backwards. Mine at least are 513-504. (As I said, I looked, because I don't even try to remember these numbers.) I use the Long Island Indicators extended tips as necessary.
Which aspect of the 'Gordy method' are you asking about? I dial my bore in as Gordy describes. It's darn difficult to 'prove' that one technique is better than another as most techniques have produced great shooters at times, but my understanding of the Gordy method (lining up two points in the bore that correspond with the first little bit that the bullet sees) just makes sense to me. My workholding is different because my lathe headstock is too long to access the muzzle end. I can't find any rational explanation that dialing the breech and muzzle end of a barrel into the same line would be of any advantage if you assume that the bore is not perfectly true.
GsT
Rubicon Prec.
New member
I have the bore where I want it. I bore the chamber body to match that. The pilot is just along for the ride, but I can’t see any reason it would try to push anything off center.
My reamer is aligned well enough in the turning center that I can rapid the -.0002” pilot into the bore at 1200 ipm. (I don’t, but I can).
Which indicator do you use to find the throat to center up to bore a 2.5"-3" Magnum length case?
Rubicon Prec.
New member
I don’t have an indicator that will do that. I use a different method. I understand the errors inherent in my method so I won’t make claims of runout that can never be proven. I can say, and backup the fact that my rigidly held reamer with a pilot less than .0002” under bore diameter will go right in without a crash.
jackie schmidt
New member
Well, you have sparked my curiosity.
If you compare the readings you get from running a long stylus in there, to fitting a PTG range rod. You will get two different deviations but which represents the bore? The range rod is referencing the lands, and is a "middle man" and the stylus loses resolution due to its length. I could make a comment on my opinion of the long magnum cartridges, but that might get some people riled up.
Butch Lambert
Active member
Gene, If I drill, indicate the throat, and bore to my indication at the throat, don't I have 2 points indicated?
GeneT
Active member
That's a great question Butch, and I've reversed my thoughts on it a couple of times as I sit here pondering it. My answer above, was really addressing the Gordy method vs chambering between centers (or the equivalent vis a vis indicating both ends). On one hand, your chamber should definitely be axially aligned with where you indicated, but I'm not as certain that it guarantees lineal alignment with the start of the bore. If the barrel was tilted, with the axis of tilt right at the spot you indicated, then when you bore a chamber, the chamber would be linear with the indicated point, but might the point not necessarily be linear with the start of the rifling? If the bore was way out of alignment, you'd 'see' that because you'd be indicating an oval cross-section, but a practical misalignment probably wouldn't be noticeable. Does that make sense? Am I missing something?
At the end of the day if you can stick a borescope in there and see that the leade is uniform across all the lands you've probably done all you can with respect to alignment.
Regards,
GsT
Edit: Butch, in re-reading what you wrote, I may have misunderstood entirely. Did you mean indicate the throat and the bore or indicate the throat and then perform a boring operation? (I initially interpreted it as the latter, but am now unsure.)
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Butch Lambert
Active member
Gene, let's start again. I use Deltronic pins to indicate muzzle and breach. I am in the headstock with a cathead on both ends. I do warm up my headstock bearings before final indication. I predrill and reach in with my Mitutoyo indicator and indicate the throat. I taper bore to the indicated throat. I want my reamer to follow my prebored hole and not be influenced by a bushing beyond the throat.
Gene, If a barrel has a bore that makes a RH or other turn beyond that, it should have been scrapped. A Gordy rod cannot straighten a wandering bore.
I can take the time to do this as I only do my own work. Most of this .0001" stuff is BS anyway! Remember the thickness of a brown human hair is about .003"
GeneT
Active member
Thanks for the clarification Butch. Yes, you have two points dialed in. In theory the advantage of the Gordy method is that you're dialing in the first bit (let's say an inch) of the bore, rather than end-to-end. Bores *do* wander - of that much I'm certain. If you look at commercial gunn drilling machines it seems the common boast is no more than 0.004" from true in 36" (an odd figure, but I think 36" might be the capacity of the tool). So, if the bore wanders 0.004" from truly straight the _in theory_ aligning the bullet with the first little bit of the bore makes sense (whether it makes a practical difference or not is another matter). But there are lots of valid questions: does that tiny deviation from straight actually have an effect on anything? Does the Gordy method actually compensate for it? The answers of course are "I don't know". IMO, there are just too many variables in putting shots on target to definitively say. So, a related question is: do you think the Gordy method hurts?
GsT