Measuring Finished Chamber Dimensions

[DaveTooley]

This has been a very interesting thread with an experienced smiths and machinists contributing. I am at the beginning of the learning curve when it comes to running a lathe. My question as a beginner is: If you have measured the reamer and are confident it is sized within tolerance and it is a piloted reamer with a bushing that fits correctly, wouldn’t you just need to measure the diameter of the chamber at the .200 line with a gauge pin to detect if the reamer was inserted concentric with the bore in a stable setup? If the gauge pin hits the number at the called out dimension at the .200 line doesn’t that tell you the chamber was cut to size and not oversized as long as the pilot keeps the front of the reamer centered and snug?

We all started somewhere. For a long time I was dependent on the bushing keeping the reamer centered. That was before bore scopes and an even simpler tool, a jewelers loupe. There are tolerances in everything we use. Bushing and reamers included. So there is no such thing as a perfect fit. The simple key to a concentric chamber is having a concentric hole for the reamer to follow. That's radially and axially . Do that and you don't need a bushing. Then a simple pusher to push the reamer. It really is that simple.
By chambering this way the chamber is cut to the size of the reamer and there is really no need to check the chamber. Errors in reamers most frequently happen in the throat area. I have never heard of a problem with the body of a reamer. Maybe if you get to cute tightening up the dimensions. Very seldom are there problems with two manufactures we use, more frequently with another manufacturer. I stopped spec'n out the body of reamers a long time ago. Reloading dies have to work so I base everything off of SAAMI dimensions.

 

[LVLAaron]

Very seldom are there problems with two manufactures we use, more frequently with another manufacturer

 

[Skedaddle Shooter]

We all started somewhere. For a long time I was dependent on the bushing keeping the reamer centered. That was before bore scopes and an even simpler tool, a jewelers loupe. There are tolerances in everything we use. Bushing and reamers included. So there is no such thing as a perfect fit. The simple key to a concentric chamber is having a concentric hole for the reamer to follow. That's radially and axially . Do that and you don't need a bushing. Then a simple pusher to push the reamer. It really is that simple.
By chambering this way the chamber is cut to the size of the reamer and there is really no need to check the chamber. Errors in reamers most frequently happen in the throat area. I have never heard of a problem with the body of a reamer. Maybe if you get to cute tightening up the dimensions. Very seldom are there problems with two manufactures we use, more frequently with another manufacturer. I stopped spec'n out the body of reamers a long time ago. Reloading dies have to work so I base everything off of SAAMI dimensions.

Thanks Dave,
I have been starting the chamber with a piloted drill, then come back with a boring bar to true any inconsistencies in the pre drill. Then the finish reamer and so far that has worked well for me. I appreciate your input.
Joe

 

[Skedaddle Shooter]

How do you accurately determine where the .200 line is? And how do you measure it?


To answer some of your questions with other data... Some chambers are oval. Some are octagon shaped. If they're oversized they're probably an odd shape. How would you measure those?

Just some food for thought.

Good points, not having used any of these methods I can’t speak from experience. It seems to determine the position of the .200 line you could turn a pin to the dimension of the chamber at the .200 line, hold it in your tail stock. Run it in until you touch the back of your go gauge and zero the tail stock. Account for the difference between the length of the go gauge and your chamber (go + .002”?) and advance the pin forward and see where it contacts the chamber walls. If it exceeds the .200 line you may have a problem.
My original thought was how will the OP determine if the chamber is cut oversized. I assumed if it were over sized it would be a result of his reamer being off the centerline of the bore, leaving an oversized but symmetrical chamber. I have no idea how to measure an octagonal chamber, that would be a tough oneI was just thinking on how to confirm you had bored the chamber to the correct size, but you are correct, an odd shape boring could still contact the pin at the .200 line in a could of spots giving you the allusion of a properly reamed chamber.

 

[Tod Hendricks]

How do you accurately determine where the .200 line is? And how do you measure it?


To answer some of your questions with other data... Some chambers are oval. Some are octagon shaped. If they're oversized they're probably an odd shape. How would you measure those?

Just some food for thought.

Wouldn’t sweeping the chamber with a dti post chambering confirm an odd shape?

 

[LVLAaron]

Good points, not having used any of these methods I can’t speak from experience. It seems to determine the position of the .200 line you could turn a pin to the dimension of the chamber at the .200 line, hold it in your tail stock. Run it in until you touch the back of your go gauge and zero the tail stock. Account for the difference between the length of the go gauge and your chamber (go + .002”?) and advance the pin forward and see where it contacts the chamber walls. If it exceeds the .200 line you may have a problem.
My original thought was how will the OP determine if the chamber is cut oversized. I assumed if it were over sized it would be a result of his reamer being off the centerline of the bore, leaving an oversized but symmetrical chamber. I have no idea how to measure an octagonal chamber, that would be a tough oneI was just thinking on how to confirm you had bored the chamber to the correct size, but you are correct, an odd shape boring could still contact the pin at the .200 line in a could of spots giving you the allusion of a properly reamed chamber.

I think a ball bearing would be better. But you'd also need someone who's better at math than me to tell you how to measure it