Page 1 of 4 123 ... LastLast
Results 1 to 15 of 59

Thread: Pre-Drill & Boring A Chamber

  1. #1
    Join Date
    Jan 2009
    Location
    Ca.
    Posts
    662

    Pre-Drill & Boring A Chamber

    Anyone care to give a tutorial of how they go about doing it. It's something that I have not found being fully laid out with alot of detail.

  2. #2
    Join Date
    Jul 2016
    Location
    Surprise, AZ
    Posts
    270

    Here's how I do it.

    Quote Originally Posted by Louis.J View Post
    Anyone care to give a tutorial of how they go about doing it. It's something that I have not found being fully laid out with alot of detail.
    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

  3. #3
    Join Date
    Dec 2003
    Location
    washington.........STATE that is.
    Posts
    11,033
    Quote Originally Posted by GeneT View Post
    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

  4. #4
    Join Date
    Jan 2015
    Location
    Greenwood, Ca
    Posts
    215
    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.

  5. #5
    Join Date
    Feb 2003
    Location
    Houston, Texas
    Posts
    7,847
    Quote Originally Posted by GeneT View Post
    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
    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.

  6. #6
    Join Date
    Feb 2003
    Location
    Poetry, Tex.
    Posts
    6,671
    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.

  7. #7
    Join Date
    Oct 2019
    Posts
    199
    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

    Quote Originally Posted by Rubicon Prec. View Post
    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.

  8. #8
    Join Date
    Jul 2016
    Location
    Surprise, AZ
    Posts
    270
    Quote Originally Posted by Butch Lambert View Post
    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.
    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

  9. #9
    Join Date
    Jan 2015
    Location
    Greenwood, Ca
    Posts
    215
    Quote Originally Posted by rsmithsr View Post
    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
    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 cant 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 dont, but I can).

  10. #10
    Join Date
    Dec 2003
    Location
    washington.........STATE that is.
    Posts
    11,033
    Quote Originally Posted by Rubicon Prec. View Post
    I have the bore where I want it.
    Which indicator do you use to find the throat to center up to bore a 2.5"-3" Magnum length case?

  11. #11
    Join Date
    Jan 2009
    Location
    Ca.
    Posts
    662
    I have in the past made a double pilot indicating rod to a pre-determined length.

  12. #12
    Join Date
    Jan 2015
    Location
    Greenwood, Ca
    Posts
    215
    Quote Originally Posted by alinwa View Post
    Which indicator do you use to find the throat to center up to bore a 2.5"-3" Magnum length case?
    I dont have an indicator that will do that. I use a different method. I understand the errors inherent in my method so I wont 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.

  13. #13
    Join Date
    Feb 2003
    Location
    Houston, Texas
    Posts
    7,847
    Quote Originally Posted by Rubicon Prec. View Post
    I dont have an indicator that will do that. I use a different method. I understand the errors inherent in my method so I wont 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.
    Well, you have sparked my curiosity.

  14. #14
    Join Date
    Dec 2007
    Location
    Home is Monroe, Maine
    Posts
    909
    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.

  15. #15
    Join Date
    Feb 2003
    Location
    Poetry, Tex.
    Posts
    6,671
    Quote Originally Posted by GeneT View Post
    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
    Gene, If I drill, indicate the throat, and bore to my indication at the throat, don't I have 2 points indicated?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •