Pre bore problem

F

fourspear

New member
I am having a problem with pre-boring for a 30-06 chamber, the reamer wants to oscillate the instant it starts to cut. I have the compound set to the same taper as the body of the reamer. I have gone in 1.450 to a dia. of .455. As near as I can tell the bushing should able to register in the bore at this depth and dia. This is with all being dialed in to .0002-.0003. On this particular set up I ended up with .0035 runnout in the base. I am useing a pusher to guide the reamer and following commonly used practices with the reaming, depth of cut, cleaning reamer and bore, plenty of oil,etc.
I can feel the reamer oscillate in the handle of the pusher the instant it starts into the bore. I dont know how else to describe it, no chatter, just oscillation.
I am not cutting a taper at the end of the pre-bore, so the neck and shoulder see a 90deg. counter bore when they start to cut, no different than what it would see with no pre-bore at all.
Out of frustration, I cut a chamber with no pre-bore and ended up with .0004 runnout in the base, or at the start of the chamber.
I have been practiceing on old barrels and 4140 round stock to try and identify what I am doing wrong. Thanks to all for any thoughts on what I need to change or check.
 
I'm guessing you set up between centers.

Bores aren't straight.

That said, what means ".0035 runout at the base.". . .????? HOW could you generate runout at the base while preboring?

And is this "oscillation" reading that runout? 3.5 thou is HUGE, you could feel it from acros't the room.

Maybe I'm misunderstanding the term "base?"

al
 
It's hard to tell without seeing what is happening, but, did you also indicate the muzzle end of the bore true? Sounds like you have some whip in the set up.
 
alinwa said:
I'm guessing you set up between centers.

Bores aren't straight.

That said, what means ".0035 runout at the base.". . .????? HOW could you generate runout at the base while preboring?

And is this "oscillation" reading that runout? 3.5 thou is HUGE, you could feel it from acros't the room.

Maybe I'm misunderstanding the term "base?"

al
Click to expand...

How do you chamber "between centers".....????
bill larson
 
"Chambering between centers" means that you set the barrel up by dialing both ends to center. Most often this is accomplished by chucking the muzzle into the headstock end, cramming a center into the bore at the chamber or tailstock end and "truing up" or cutting a cylindrical section to run in a steady rest. Of course there are many modifications to this system designed to make it "better" but the end result is that you've done your best to true two points to the centerline of the lathe. The commonly accepted "best" setups attempt to align the bore at the crown and the bore at the lands to this center, with some folks endlessly nattering about "which center is "best," the center of the BORE or the center of the LANDS..."cuz you could have a high land! (or a deep groove!) or a pickle lodged in the throat"
 
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I'd suggest pushing with a dead center to start the chamber and then switch over to your pusher to finish the chamber.

As to "chambering between centers" that's normally how chambering is done where a dead center is turned in the chuck, a lathe dog is put on the muzzle and the breech end held by a live center in the tail stock. The barrel is threaded between centers. Then the chamber is cut with the threads running in a steady rest. When I did this I cut a flat on the muzzle end and chucked the muzzle end and ran the threads on the steady rest. I've seen photos of guys holding the lathe dog on the muzzle end to the dead center in the chuck with bungee cords. Doesn't seem very secure to me, but can be done like this.

I've never heard of chambering in the head stock and centering the bore as chambering between centers.
 
I still call it "between centers" when one sets up thru the headstock indicating both bore areas to center, but this terminology is just a shortcut to illustrate the method.

To the OP, fourspear, your oscillation....... are you also getting chatter? Or is the tail of your reamer wagging .0035? Please describe your setup better.

I useta' live in fear of chatter back in the steady-rest days! I would never pre-bore to match taper of reamer for instance, but instead step-bore. Now, with thru-the-headstock and hand-held reamer I still step-bore but wouldn't be afraid to taper bore.

al
 
I knew a fellow whose favorite game cartridge was the .280 AI, but he lacked an indicator that would reach far enough down a barrel to indicate in off of the area where the pilot would be as the chambered. This was before the rod that Gordy Gritters popularized)He would dial the barrel in, through the headstock, centered on both ends, cut the chamber using a pusher on the back of the reamer , and then redial the barrel (His indicator could reach the throat of the finished chamber.) referencing the chamber, and then cut and thread the tenon.
 
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Thanks to all for the responses. Al and others; I began this to get experience with pre-boreing and to see how deep and to what dia. I could go to for a 30-06. I started out with a piece of 1 1/4" 4140, 3" long. Chucked this up in a 4 jaw, dialed in to 0, faced it off and used a N (.312) bit drilled a hole through it. I then drilled in with a 27/64 (.429) and went in with this to a depth 1/8" short of where the neck would be. Proceeded to pre-bore with the compound set to the same angle as the body of the reamer. I pre-bored to 1.4 deep and .455 dia. at the base or start of the chamber. I indicated at the start of the pre-bore and had .0 runnout.
I set up to ream the chamber and the instant it started to cut, I could fee oscillation in the handle of the pusher. Several attempts at this led to the same results with some tweaking to see if I could identify the problem. On one of these that I cut the complete chamber on, I ended up with .0035 runnout at the start of the chamber where the base of the cartridge would be. I never got any chatter, just oscillation.
I changed my set-up and tried using a short section of .30 cal barrel. I set this up in a cathead and used a gordy rod to get the alignment. Dialed in to get minimal runnout at the area just ahead of the throat and at the start of the chamber. Drilled and pre-bored same as above and ended up with .00035 in the lands just ahead of the throat and .00025 at the base or start of the chamber. Some of this I would attribute to not having a bushing larger than .3008. This was a used barrel. Several attempts at this yielded similar results, such that I am comfortable with this. As I mentioned in the original post, I cut one with no pre-bore and it was much improved over what I was getting with the round stock. Might be the lesson is forget the round stock and just experiment with scrap barrels.
I have encountered this same oscillation in one other project and was able to minimize it by changing from a pusher and going to a dead center in the tailstock. It didn't occur till the last 1/3 of the chamber was being cut. Hinnant, in his book on chambering, mentions this oscillation and seemed to think it was no big deal. I mention this only to point out that it seems to be somewhat of a problem. . I would have to think that it is an alignment issue.
I would be amiss not to thank all who post on this forum. You have done more with your advise, procedures and method to stimulate the economy with equipment and tool sales than all the stimuless plans.
ever dreamed of.
 
fourspear
It sounds like your bored hole is not true with the hole in your practice material. I wanted to see how far I could chuck on the taper of a barrel the other day and it slipped. It did the same as yours.

Glenn
 
As I understand it, you are preboring using the compound slide to achieve your purpose, and experiencing oscillation as soon as the reamer is engaged. I don't know if your problem is related to the use of the compound slide, but I will say that I have discovered on my lathe, a decent Chinese version, the compound screw runs out and wants to make the slide oscillate when the screw is turned. If I tighten the gibs enough to minimize the oscillation, the screw is too hard to turn efficiently. So being aware of that situation I use the compound slide sparingly. I am suggesting that you try Al's method of step boring using the cross slide and power feed and see if the oscillation of the reamer disappears. Again, don't know that our problems are the same, but that might be a way to check. Rich
 
I'm still somewhat lost as I'm missing some pieces of the alignment puzzle in my head, for instance HOW do you really align or adjust short pieces of stock?

That said, two items come to mind..... the slippage mentioned by Glenn, and uneven hardness in the steel stock if it's just shop stock.

al
 
Another idea here... is it possible, could it be, that somehow when you start the reamer into the prebored area, you are managing to deflect it some, or perhaps it's not starting straight and true with the barrel axis (you could probably start it using a dead center) due to being not "on center" on whatever pusher you're using? So that by the time it's maybe 1/2" into the prebore and it's being supported by the sides of the cut area, it has been moved off center somehow. I realize that "a reamer follows the hole," but this can be subject to some error...

With this in mind, you might try this: Get the throat area and muzzle indicated in. Make sure the setup won't slip. Drill to your usual depth, then prebore (I start at the drilled shoulder and feed out) to between .010 and .020" less than the reamer shoulder diameter. Check with indicator to make sure throat and bored section both indicate properly. THEN, at the rear of the chamber where the reamer will start, bore maybe 3/8" to 1/2" in, to .002" less than reamer shoulder diameter. Carefully and lightly feed the reamer into this portion, making sure that it's not off center, etc. This will get the reamer started straight. Feed the reamer after this as you usually do. Each time after you pull and clean the reamer and relubricate, feed it slowly and carefully up to the cutting face making sure that it doesn't bind until it actually begins cutting at the shoulder.

Also, try just taking the pilot off the reamer and not using it at all. If all the above is done well, the chamber body will provide plenty of support when the neck area enters the bore. And if you think about it, since the pilot works up to maybe an inch ahead of the throat area and depending on how much runout the barrel has fore and aft of the area you indicated, a close fitting pilot can actually move the throat area OFF center and undo some of your careful setup with the indicator..

Just some (hopefully helpful) thoughts...
 
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