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Rifles, Reloading, Optics, Equipment
Reloading
Runout?!
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<blockquote data-quote="NesikaChad" data-source="post: 218106" data-attributes="member: 7449"><p><strong>Neck turning</strong></p><p></p><p>I should have read the whole thread before responding. Lets look at this from a machinist perspective.</p><p></p><p>We have a barrel with a chamber machined in the back of it. We want to believe that the theoretical centerline of the chamber is on the same centerline as the rifle barrel bore. This means the chamber and bore are very concentric with one another. The "DATUM" or point of origin on all this is that theoretical bore centerline. Everything is based off of that.</p><p></p><p>Ok, skewelsz out, go play.</p><p></p><p>Not so fast.</p><p></p><p>Now we take a piece of brass formed into the shape of that chamber with a 40 ton press. If we put the brass on rollers and then place an indicator on it, it has very little runout telling us that the outside is indeed pretty round.</p><p></p><p>Now chamber that brass. The OD of the case is pretty concentric to the bore because that is what it registers on when the gun goes into battery. The few thousanths of slop between the case and the chamber take some of this away, but its not much.</p><p></p><p>I'm interested on what goes on on the inside of the brass. Short of splitting cases in half and measuring wall thickness, we just don't know, and this is where you may be finding some of the problem. The other part may be in your inspection equipment.</p><p></p><p>If your using calipers to measure wall thickness on a round (tubular) surface your wrong. The caliper jaw on the outside runs to the tangent of the arc however the inside jaw is catching the edges and throwing the dimension off. Smaller the case neck, the more pronounced the error becomes due to the tighter radius.</p><p></p><p>A micrometer built for measuring wall thickness on tubing is what should be used. Starrett makes a nice one for less than $200 dollars.</p><p></p><p>The typical neck turning tool uses an arbor to locate off the inside of the case neck. We assume that the inside of the case neck is concentric with the outside, but if it isn't all we are doing is making the neck a uniform wall thickness with no consideration as to how it relates to the rest of the cartridge.</p><p></p><p>This is why (and I realize its not in everyone's ability to do this) I use a tool room lathe with a 5C "emergency collet" "chambered" to the cartridge I am neck sizing. The case is supported on the outside and the necks are turned so that they stay concentric to the outside body of the cartridge. It's a real, real, real pain in the arse to do it this way, but it does do a pretty nice job. </p><p></p><p>Make sense?</p><p></p><p>I realize I really haven't answered your question and may have only added to the confusion. Lapua brass is renowned for being top shelf stuff, so I would certainly invest the time to ensure the other things mentioned aren't going on before thawing out the Visa on a 20K Hardinge tool room lathe.</p><p></p><p>Good luck, tell us what you come up with.</p><p></p><p>Cheers.</p><p></p><p>C</p></blockquote><p></p>
[QUOTE="NesikaChad, post: 218106, member: 7449"] [b]Neck turning[/b] I should have read the whole thread before responding. Lets look at this from a machinist perspective. We have a barrel with a chamber machined in the back of it. We want to believe that the theoretical centerline of the chamber is on the same centerline as the rifle barrel bore. This means the chamber and bore are very concentric with one another. The "DATUM" or point of origin on all this is that theoretical bore centerline. Everything is based off of that. Ok, skewelsz out, go play. Not so fast. Now we take a piece of brass formed into the shape of that chamber with a 40 ton press. If we put the brass on rollers and then place an indicator on it, it has very little runout telling us that the outside is indeed pretty round. Now chamber that brass. The OD of the case is pretty concentric to the bore because that is what it registers on when the gun goes into battery. The few thousanths of slop between the case and the chamber take some of this away, but its not much. I'm interested on what goes on on the inside of the brass. Short of splitting cases in half and measuring wall thickness, we just don't know, and this is where you may be finding some of the problem. The other part may be in your inspection equipment. If your using calipers to measure wall thickness on a round (tubular) surface your wrong. The caliper jaw on the outside runs to the tangent of the arc however the inside jaw is catching the edges and throwing the dimension off. Smaller the case neck, the more pronounced the error becomes due to the tighter radius. A micrometer built for measuring wall thickness on tubing is what should be used. Starrett makes a nice one for less than $200 dollars. The typical neck turning tool uses an arbor to locate off the inside of the case neck. We assume that the inside of the case neck is concentric with the outside, but if it isn't all we are doing is making the neck a uniform wall thickness with no consideration as to how it relates to the rest of the cartridge. This is why (and I realize its not in everyone's ability to do this) I use a tool room lathe with a 5C "emergency collet" "chambered" to the cartridge I am neck sizing. The case is supported on the outside and the necks are turned so that they stay concentric to the outside body of the cartridge. It's a real, real, real pain in the arse to do it this way, but it does do a pretty nice job. Make sense? I realize I really haven't answered your question and may have only added to the confusion. Lapua brass is renowned for being top shelf stuff, so I would certainly invest the time to ensure the other things mentioned aren't going on before thawing out the Visa on a 20K Hardinge tool room lathe. Good luck, tell us what you come up with. Cheers. C [/QUOTE]
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