View Full Version : Would this work?

10-28-2005, 03:51 PM
To find a small bend in a tube such as a rifle barrel. Mount it in a four jaw chuck and indicate the OD. Crown end forward. Mount a laser source in the toolholder and indicate it parallel with the spindle and on center to the spindle bore.Now place a target(a piece of paper) behind the headstock. Moving the cross slide in and out to coincide with the tube wall while rotating the spindle, observe the dot on the paper to see if it's distorted.It won't tell where in the barrel the bend is, but I think it can tell you how badly it's bent and measure it. It would also tell if the bore were concentric to the OD, at least at the crown.

10-28-2005, 07:50 PM
It might, but it's an awful lot of work.

Put a razor blade parallel to the muzzle but not touching it, with the edge horizontal, on the centerline. Rotate the barrel while watching the reflection of the razor edge along the sides from the chamber end, and note the points of distortion.


[This message has been edited by Leigh (edited 10-28-2005).]

10-28-2005, 08:22 PM
Maybe I miss understand what you are doing, but couldn't you just put the barrel between centers and then rotate it while running an indicator along top dead center, that would tell you exactly where the high point in the bend was, of course I guess this would only be accurate if the bore was concentric with outside diameter.

10-28-2005, 08:33 PM
I'm checking if the bore wanders between the muzzel end and the breach.Or if it's just bent. Someone is claiming that some barrels were drilled wrong and not concentric between the muzzel and the breach. I don't think that a gundrill can regain center after it wanders.It's my contention that the barrels were turned wrong after the were drilled.With the laser it can be determined if the bore is straight. Just measuring it between centers will only tell me that the OD is out of round.

Paul F
10-28-2005, 08:54 PM
In my experience, the easiest way to tell if a bore is straight is this...

Take a piece of brass for that chamber. Punch out the primer.
Put it in the chamber.
Look down the muzzle at a brigth light behind the brass.

You will see a series of shadow-rings down the bore. If they are all concentric with each other, the bore is straight. If they 're not, it's not.
It's also important to note that it DOES NOT MATTER if the hole in the primer pocket is perfectly centered. All you need is for the hole to be about the size of the primer flash hole, somewhere in the general vicinity of the middle of the case.

It's amazingly accurate.
Someone who examines telescope mirrors with a light and a razor blade can probably describe WHY it works.. the optics descriptions of it give me a headache...
It works.
And with a little practice, you can tell about HOW bent a barrel is.

Hope this helps.
Paul F.

Ron LaDow
10-28-2005, 09:13 PM
Paul F
Without any math, the reason the optical approach you mention works is the fact that light travels in a straight line and the angle of incidance equals the angle of reflectance.
Raking light, like noon on a 'vertical' wall tells you if if is flat even if you look from the side. You're doing the same sending a very raking light down a bore and adding the fact that you're looking into the light source.
The corolary is if you're machining optical gear, you gotta be real accurate...
(hope this is clear)
Ron LaDow

10-28-2005, 11:23 PM
I think the laser idea would work- even one of those pocket laser pointers. Mounted on the crosslide, you can use two adjustments, one for parallel to the spindle, or slightly off parallel, and the other for setting the laser to graze the wall of the bore.

Another idea, mount a small mirror at a 45 so you can see along the length of the bore from the laser end. This mirror would have the laser beam just clearing one edge of it, so no laser light is deflected in any way from the mirror. It could be mounted on the toolholder with the laser, or it could be mounted separately, say on a steady. Some careful adjustment of the laser would give you a pretty good indication of the runout down the bore.

I like Paul's idea, though. That sounds like a tried and true method, and it's pretty easy.

I once made a light stick for a gunsmith so he could insert it all the way down the barrel and check the light as it reflected back up the bore. This was to check the need for cleaning, and I used an led for the light source. I think a laser pointer would show up faults a lot better, but you definitely wouldn't want to be looking towards the laser, or have any way that the beam could enter your eye. Looking down along the beam should be ok. Just don't reflect it back.