Originally Posted by Josh1158
Im a machinest I make over engineered medical medical parts every day. I have to hold +/- .00015 all the time. Im having a hard time understanding why so much barrel work is done with reamers. I feel that a normal boring bar would do fine. Can someone please explane this to me.
Josh, I don't know what kind of machine work you do, but here is the skinny on reamers. Straight flute reamers leave a finish that will almost always be better than when using a boring bar. They will also maintain tighter tolerances and reduce work time by a significant amount. A good reamer, used in a rigid machine and setup will easily hold tolerances of .0005 and less if used correctly.
Since you specifically asked about barrel work, I will focus on using a boring bar mounted on the tool post of a lathe. The dials on every lathe I have ever seen are incremented in thousandths. makes it a bit difficult to hold tolerances of less than .001, which a reamer does easily. There is an interesting trick for working in smaller increments than .001 on a lathe though. You can swing your compound feed to either one and a half or three degrees, depending on whether it reads diameter or radius, and set up 10 to 1 ratio by feeding with the compound rather than the crossslide. If you check the setup carefully by using a test indicator that reads in ten thousandths, this method works very well, but it is mind numbingly slow working in such fine increments, and if you are not very careful, tool wear can bite you in the ass.
If you are talking about boring a chamber, it gets even worse. Many, if not most cartridges do not have straight walls, but are a tapered case. Now you have to set up your taper attachment, and carefully indicate it into the proper taper. Another time consuming task. And don't forget abut the shoulders of the bottle neck cartridges. They vary widely in angle and radius at the transition from case body to neck. Your boring bar would have to be carefully ground to match this radius and angle. All this, when a reamer is easily capable of holding dimensional tolerances of .0005 or less. If you would like to take on the challenge, be my guest. I will stick with reamers for all of my barrel chambering and throating needs.
Now let's assume that you want to chamber a rifle barrel for a specific cartridge, and wanted to do it with a boring bar. After all of the previously mentioned setup is done, you carefully install the barrel in your trusty four jaw chuck, and indicate it to perfection. You then proceed to carefully bore the chamber with your custom ground boring bar. When you think you are done, you remove the barrel from the chuck, thread it into the receiver, place a headspace gauge in the newly cut chamber and go to close the bolt, only to find out that you are still just a couple thousandths from the "go" gauge seating with just the slightest hint of pressure. Now you have to reinstall the barrel in the chuck, indicate it to center again, and then try to move the shoulder of you chamber forward that very small amount to make it perfect. If you were using a roughing and finishing reamer set, it would simply be a matter of inserting the finishing reamer and giving it a half turn by hand and then rechecking your headspace, and repeating until you get the proper chamber depth.
Chambering, throating, forcing cone and pretty much any other reamer I have ever tried have always functioned flawlessly for me, leaving a great finish, proper dimensions and proper operation.