I poured a mold of a chamber that was cut with a boring bar (a new customer brought it to me wondering if the chamber was good) and it’s running roughly .004 total (.002 per side) oversize vs. the actual reamer print. I always cut chambers with a reamer so I’m looking to see if this is a safe chamber or not. Headspace is good and it’s within .0005 all the way from base to throat, just has a little more wiggle than I’m used to. What say the experts?
Actual reamer cut dimensions
- Thread starter Kck
- Start date
I'm curious what it's chambered in. Cutting a chamber with a boring bar is no easy task, even with cnc and it gets much harder as the chamber gets longer. I do have to wonder who can do this very challenging job, yet couldn't maintain tighter tolerances than .004" It doesn't really add up. If the chamber is .004 big anywhere along the case body, it really makes me wonder about the throat, with such a small boring bar, especially extended for a long cartridge chamber. Hmmm.
I had these questions as well. Chambered in .338 Lapua. I didn’t go into it since I didn’t cut it as my thought is to tell him all I can do is just cut a half inch off of the barrel and re cut the chamber with a reamer and that’s if the current chamber is on center. The work is nice work but cutting a chamber that deep with a boring bar is tough. However, like I mentioned, the mold is within .0005 all the way down through the neck, just .004 total bigger than the reamer. I’ve made up my mind on the deal to tell him that I’ll cut off enough of the barrel to rechamber with a reamer. However, I’ll have to use a long enough range rod to verify that the chamber is cut on center. If not, he gets a high dollar paperweight that I won’t bother with.
If it is only .004 large (over minimum), it is usable. How much larger is it than new brass? WH
DaveTooley
Gold $$ Contributor
It's been done for years by a few people with success. They liked the challenge. Special carbide boring bars are used.
As far as being usable I'd bet it is. Some are successfully taking 8X68S brass, .520" at the base and using it in 300 PRC chambers.
.002" per side isn't much. The issue may be with a FL die. 7/8-14 dies don't have much wall thickness with a Lapua case and could crack.
As far as being usable I'd bet it is. Some are successfully taking 8X68S brass, .520" at the base and using it in 300 PRC chambers.
.002" per side isn't much. The issue may be with a FL die. 7/8-14 dies don't have much wall thickness with a Lapua case and could crack.
^^^
Cut with a boring bar, not a reamer. A LOT can be wrong...
I get that inserts are a lot cheaper than reamers, but I wouldn't buy a barrel with a chamber cut with a boring bar.
With a reamer (assuming proper setup), you know the chamber will be cut to the dimensions of the form tool- and buying from a reputable source you have high confidence that it will be ground to the specs, you expect.
Even with a casting, IMO there's relevant dimensions that can't be precisely measured with mikes and calipers. Cool as a DIY for those that can, but as someone that still struggles sometimes holding a thou with a boring bar due to tool flex- I don't have confidence that even with a highly accurate CNC that it can match the form tool.
JMO, YMMV.
Cut with a boring bar, not a reamer. A LOT can be wrong...
I get that inserts are a lot cheaper than reamers, but I wouldn't buy a barrel with a chamber cut with a boring bar.
With a reamer (assuming proper setup), you know the chamber will be cut to the dimensions of the form tool- and buying from a reputable source you have high confidence that it will be ground to the specs, you expect.
Even with a casting, IMO there's relevant dimensions that can't be precisely measured with mikes and calipers. Cool as a DIY for those that can, but as someone that still struggles sometimes holding a thou with a boring bar due to tool flex- I don't have confidence that even with a highly accurate CNC that it can match the form tool.
JMO, YMMV.
Sorry, I kinda went in a wrong direction based off of my title of the thread. I guess my real question would be what actual dimensions does a reamer cut as opposed to the actual measurements of the reamer? Since it gets plunged in multiple times one would think that, even though it's most likely getting guided in by a bushing, it would cut a couple thou over actual dimensions? Consensus on my end is to lop off threads and fix the issue. Customer has a range rod that I'll have to use (I'm not a range rod guy as I have only indicated off of the bore with an extended arm indicator).
How is that relevant?
Boring bars are not precision instruments or tools. Make one on a grinder by "eyeball" and maybe an angle gauge. Nothing even needs to be measured.
No matter what the business end of the cutter measures, it all becomes the same zero when you touch off- and all of them flex.
Maybe Chad will come across this and comment on CNC chamber boring, and tool flex.
I can all but guarantee he's looked at it.
carlsbad
Lions don't lose sleep over the opinions of sheep.
If I cut a chamber that is .0005" over spec, then I feel I have failed. As you say, it is not uncommon with poor setup to cut a chamber a couple of thousandths over the reamer size. If you are getting oversize chambers, then you need a better setup. --Jerry
Most chambers I cut are less than .0002" over reamer diameter. Very easy to keep it tight and true with a good set up.
Dusty Stevens
Shiner
On a tapered case chamber im just wondering how you measure diameter at a point on the print to the ten thousandth?
shortgrass
Silver $$ Contributor
Opitcal comparator /'shadow graph'?
Dusty Stevens
Shiner
CMM?
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