Chamber run out issue

A

AZLarry

Guest
223rem chamber has run out of .0015" at the last half of the chamber. But the neck and lead areas look good with no run out.

Q: What caused the back half of the chamber to have run out and how can it be avoided?

Details:

a) Indicated barrel bore using long stem indicator. Looks good the length of case.

b) Drilled then straight bored chamber to .347" diameter for a depth of 1.3" (that is the longest carbide boring bar I had on hand)

c) 223rem chamber was cut with a removable pilot reamer held by a floating holder similar to the bald eagle type. Chamber looks good with no chatter or obvious cutting issues.

d) Used through the lathe headstock method with spider and 6 jaw chuck. 125 rpm spindle speed.

Thanks, Larry
 
Larry

I peronally never liked the floating reamer holders. That may have been what caused your problem. Check the diameter of the chamber nearest the head of the case to see if it is oversized as compared to the reamer. That may give you a clue as to what is happening.
 
Larry

Did you measure for run out after you bored the hole? Even though you used a carbide boring bar it is a small diameter and the boring bar may have been pushing off. With a floating holder the reamer is only going to follow the hole.
 
The bored hole was running true.

What about technique on holding reamer and stopping the lathe while reamer is still in the bore????

Do you leave the reamer in the bore and back off on the pressure while stopping the lathe spindle?

Or just pull out the reamer while the lathe is turning?

Thanks for the reply's....
 
Larry

.0015 is a lot of runnout considering you did pre-bore the chamber.

The only thing I can think of is your reamer pilot encountered a spot up in there that did not run true with your initial indicated spot, influrencing things to where you ended up with the runnout.

I have had chambers run out as much as .0004 after everything is done, but that is no where near as much as what you are experiencing.

As a little tip, when this happens, (sometimes it does despite all of our best efforts), re-indicate the chamber exactly in the midle of the chamber, before you finish your tenon. That way, you are "splitting the difference" with the runnout, and the chamber will at least be as close to alignment in the action as possible.............jackie
 
The floating reamer holders I have are all a disappointment. I don't use them. They all seem to have problems. Its just better to allign the tail
stock. Once alligned, dowel pin it and recontour barrels elsewhere. It
doesn't take much to put a dead center back in the tailstock on a chip
blown out of the chamber , attention to detail can't be taken to far.
 
Just a thought. Because only the out board half of the chamber was out of round the spider screws might of "moved" causing the other end of the barrel is wobble???

I should of checked run out before the chamber was to finished depth. Also don't finish the barrel threads before the chamber. Lessons learned.....
 
Eccentricity versus runout

It is not odd to me that the back end of the chamber runs out more than the neck. When measuring run-out from what is believed to be a properly bucked in bore at the throat-you are measuring angularity. That is what Jackie was referring to when he said the bushing must have followed part of the bore that did not align with center of lathe spindle. The farther back from that point you get, the more run-out you have.

It would be mighty difficult to have a consistent run-out from back to chamber to throat-unless the entire barrel was dialed in eccentric.
 
get rid of the pilot

I haven't used a pilot since I can remember. I indicate throat, rough drill, angle bore then finish ream. If you have indicated the throat and forward of that (where the pilot rides rides) is out, as most of them are, all of the other work is a wasted effort. The reamer is going to follow the pilot to some degree even with single point bored hole. Bore scope the throat when you are done. We are reaming the throat @ about 1.5 degrees, any misalignment will stick out like a sore thumb. This is the business end of reaming for accuracy.
Regards...Herb Coates
 
HCP said:
I haven't used a pilot since I can remember. I indicate throat, rough drill, angle bore then finish ream. If you have indicated the throat and forward of that (where the pilot rides rides) is out, as most of them are, all of the other work is a wasted effort. The reamer is going to follow the pilot to some degree even with single point bored hole. Bore scope the throat when you are done. We are reaming the throat @ about 1.5 degrees, any misalignment will stick out like a sore thumb. This is the business end of reaming for accuracy.
Regards...Herb Coates
Click to expand...

Interesting...

Thanks Herb
 
Thanks for the input.

I agree about the reamer bushing alignment causing issues. Will try without the bushing.
 
Come On, Guys

Let's look at this from a Machinist Stand point.

Any time you are dealing with matching machine work with areas that do not run exactly true, then you are faced with the concept of "splitting the difference". In the real world of Machine Shop Practice, this means you do the best you pssibly can with what you have towork with.

I did a barrel this week for my new Rifle that had spots than ran out as much as .001 within a 1 inch length. That means no matter what you do, you can only indicate in one spot. Anything one side or another from that spot will exibit runnout.

I pre-drill, indicate the bore as close to where the throat area is as possible, bore the chamber to run dead true with this, and then use a pilot. What I am doing is splitting the difference between what I could indicate and what the pilot might encounter up in there. Usually, I will have less than .0003 anywhere in the chamber. This is acceptable. And as I said before, if I do come up with a little runnout, I will indicate the middle ofthe chamberdead true, once again "splitting the difference".

The bore scope is a very good way to visually check yourself, because as was said, with that shallow 1.5 degree lead,anything much over a couple of tenths will show up. .001 inch will show up like a sore thumb.

Remember, if a hole is not dead straight, all you can do is indicate one spot. If you determine where the ideal position of that spot is, every thing will come out within the accepted tolerances of this type work. If you are consitantly getting chambers that show NO RUNNOUT what so ever, then you probably need to get a better indicator.........jackie
 
Dumb questions..
If the pilot is a good fit, and the chamber prebored on a taper, then what would make any part of the chamber "want" to deviate form the CL of the headstock, which I presume is the runout reference? Is it that the chamber is forced off as the pilot is advanced down a wandering bore, and then the reamer wants to stay with the new heading?

If a center was adjustable, so that it could be indicated in to the headstock axis, would there be a need for a floating reamer holder? Is the rationale for a loose pilot fit that the center of the bore should be determined from the grooves rather than the lands, and that while this can be done while indicating in the barrel, pilots only see the CL of their land contact?

Also, a fellow once told me that for longer chambers, like the .280 AI that he prefers for hunting rigs, lacking an indicator long enough to reach where the throat will be, he indicates the barrel using two indicators and a precision fit rod at the breach end, chambers with a floating holder, and then re-indicates the barrel on the chamber before cutting the tenon and shoulder. Thoughts?
 
That would probably be fine, given a near perfect indicator rod, that
has a near perfect fit and is of such length as to reach ahead of where your going can be tested. It also must be long enough to give you meaningful
readings outside the blank. Its a phalacy that floating reamer holders can
accomodate errors in 3 plains at once. Looks good in your hands, but when torque is applied along with tailstock pressure, things happen.
 
Jackie said it best...

We have to split the differences when necessary. Bores are not perfectly straight so we have to deal with it.

Boyd..The answer to your first question is yes, that is why I don't use the pilot. you are just adding another variable to errors that you will encounter. Gordy Gritters has a method too get this area running true by adjusting the outboard end of the barrel too correct it. Another way to skin the kitty.

As for your second question...adjustable tail stocks centers are available from MSC, or a least they were. If you can't find them, I'll check in the shop tomorrow. If your center runs true then don't worry about it. Most can vary from day to day therefore I use one and indicate it in before reaming each time. Takes a grand total of about one minute to do. Be careful when sweeping the tail stock center with the indicator. If your indicator is extended very far it WILL give you a false reading. Remember we are only talking about a tenth and the indicator can "droop" that much. If you don't believe me attach your indicator with a magnetic base to what you think is solid, zero it and turn it upside down, surprise. I use an Interapid attached to the barrel hanging out of chuck, no more. As for floating reamer holders, don't use them, they can cause more problems than they cure. Get your tail stock pushing straight with the axis of the lathe spindle and forget about it.

How much of a difference does all of this make, I really wish I knew. I'd be tinkled pink to have loaded rounds that sit behind this chamber to have a run out of less than a tenth. I just want to have that chamber to run as true as I can. Feels good to check all of your work (hours) and see the needle on an Interapid barely wiggle. But, I've seen more than one less than perfect chamber take the wood home.

Such is life.

Regards... Herb Coates
 
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