A Couple of Lessons on Chamber Reamers

Forced pilot is damaged

Chamber reamers are pretty complex tools that incorporate all the features of the chamber into one form cutting tool.

At right is pictured the tip of a chamber reamer.  You can see from left to right the shoulder, neck, throat, and pilot.  Note that the pilot appears to be short for the reamer.  It was not short when the pilot bushing was new.

A novice used this tool, the bushing was too tight a fit for the bore.  Proper fit is .0005″ to .001″ under the bore diameter.  That makes for a nice slip fit of the bushing to the bore of the barrel.  The pilot bushing rides on the lands of the barrel.

How do I know the bushing was too tight for the bore?

Evidence of poor pilot fit.

Simple, the bushing was forced back onto the cutting edge hard enough that the throat actually cut the back of the pilot.  Note in the picture at left, the same busing off the reamer.  You can see where the tool cut the bushing.  This portion of the reamer is not very sharp as it was never intended to cut anything.  So that is how I know this bushing was forced into a tight bore.

The primary reason for using removable pilot bushings is so that you can match the pilot to the bore of your barrel.  No need to force things.

You know this guy has no idea how a reamer works…

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The reamer at right was “sharpened” by a “gunsmith”.  The large flat running down the center of the picture has two grinds.  The fat grind closer to the top of the picture us a relief grind, meaning it will never touch the barrel, it is clearance ground to make sure that chips will not get caught behind the cutting edge.

The narrow grind just below the relief grind is where the actual cutting edge is located.  This grind is also relief ground just slightly.  Only the very edge where the grind meets the flute is actually touching the barrel during chambering.  If you stone on the outside grinds of the reamer the dimensions change very quickly because the geometry of the cutting edge and the clearance grind.  Never stone on the outside edge unless you have been trained to do so.

If you look inside the flute on top of the cutting edge where the chips gather during cutting that is the area that you can stone without changing the dimensions of the reamer (in the picture here that would be visible above the grinds we just discussed, of course each flute is one cutting edge).  Again, because of the geometry of the reamer stoning on this inside edge changes dimensions such a tiny amount that it should not create any problems with chamber size if you don’t get carried away.  Normally all that is needed is the cleaning of metal built up on the cutting edge, no real stoning of the reamer itself.

When I look at the picture of the reamer above, I laugh because you can see where the “gunsmith” stoned on the relief grind.  Since this part of the reamer never touches the barrel at anytime it is clear that this guy had no idea how the tool works.  If you don’t know how a tool works, it’s a safe bet you have no business trying to sharpen it.  Send it to the reamer maker if your not sure, its cheaper that an angry customer.

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2 Comments

Filed under Gunsmithing, How To, tools, Uncategorized

2 responses to “A Couple of Lessons on Chamber Reamers

  1. Pingback: Mosin Nagant Custom Sniper Rifle: chatter removal and barrel smoothing

  2. Pingback: Mosin Nagant: chatter removal and barrel smoothing - Gunflicks

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