Pre boring chambers
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    Default Pre boring chambers

    Hi all,

    For those that a pre boring chambers before reaming . . . .

    What process are you using? Pre drill to within 20tho of diameter to shoulder, set compound at chamber wall angle, take passes at XX depth to within XX of chamber diameter?

    If your pre drilling how do you ensure / prevent a burr forming at the end of the drilled hole which would prevent a reamer pilot getting to the bore for guidance.

    What tooling are you using to drill and bore especially for 223 diameter cases or long magnums?

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    Pre drill under the case size by one full drill size. Depth is drill point to just before the start of the shoulder and then a .001-2" boring pass to make sure it is true and burr free. Have not done anything under standard .308 bolt face size cases. Long magnums follow the same procedure but drill under size more and take a couple of boring passes to stay concentric. After that, let the reamer do the work.

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    ^^^
    I don't pre-bore, but if I did, I would do it this way.
    No reason to over-complicate it.

    I would think any burr raised would easily get pushed down the tube by the pilot, and then either flushed out the back with a flush system or blown out the front when cleaning between passes without one.

    "Theoretically", if you had a perfectly aligned, and extremely rigid setup you wouldn't even need the pilot. The reamer is going to follow the bored hole, period. If your bore isn't perfectly aligned- or at least to within a couple tenths (assuming you're using a live pilot of correct size), the pilot ain't going down the tube. But since perfectly aligned manual lathes are pretty much non-existent we use floating holders and a snug (but not tight) pilot helps a great deal to prevent chatter IMO.

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    Quote Originally Posted by dwebster View Post
    Pre drill under the case size by one full drill size. Depth is drill point to just before the start of the shoulder and then a .001-2" boring pass to make sure it is true and burr free. Have not done anything under standard .308 bolt face size cases. Long magnums follow the same procedure but drill under size more and take a couple of boring passes to stay concentric. After that, let the reamer do the work.
    What type of boring bar and inserts do you use and how much are you taking off per pass roughly?

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    He's talking about a single pass- it's all you need to true the hole to spindle/bore axis. Since the bore is already dialed in perfectly concentric, the only runout is from the drill bit/and tailstock misalignment which should be very minor on a decent machine.

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    I predrill standard and magnum chambers with an 11mm carbide 140* drill at 1588 rpm, .008" ipr, and never get a bur at the bore. I do check it with a tight fitting bushing on a rod before I send the reamer into it.

    I don't feel the time spent setting up a manual machine to taper bore is worth it. What are we talking about, at most .020" of taper on the diameter? If you parallel bore .010" under the shoulder diameter, the reamer isn't going to care if the bore was tapered or not. The reamer is following what is in front of it, the tapered part it just cut. I do taper bore on the CNC because it doesn't take any longer to program or machine. I program my prebore .020"-.030" under reamer diameter.

    In the CNC, I drill .050 short of the shoulder and bore to that point. I rough ream about .020"-.030" short of final HS. I check the HS and program the final peck.

    On the manual, I drill as deep as I can and still have the pilot make purchase in the bore. I parallel bore to that depth, visually .010" or so under the shoulder diameter. The exact diameter isn't critical, you are just making the hole straight/parallel to you spindle axis.

    I’ve got a couple .223AIs to do today. I’m debating even bothering with drilling and preboring them. May just run the chamber roughing cycle, do the counterbore, the the final HS peck.

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    As I've mentioned many times on this forum, I predrill about .020 under shoulder diameter about .060 short, and taper bore with as short a solid carbide boring bar that will work. I use a pusher on my reamer and will not use a snug bushing or any at all. As mentioned above, your reamer if pushed will follow your prebored hole.

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    Quote Originally Posted by Butch Lambert View Post
    As I've mentioned many times on this forum, I predrill about .020 under shoulder diameter about .060 short, and taper bore with as short a solid carbide boring bar that will work. I use a pusher on my reamer and will not use a snug bushing or any at all. As mentioned above, your reamer if pushed will follow your prebored hole.

    Interesting.
    You use a floating holder, an unpiloted reamer- and don't get chatter?
    I would've assumed the reamer would need to be in a rigid setup with no pilot.

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    Quote Originally Posted by tobnpr View Post
    Interesting.
    You use a floating holder, an unpiloted reamer- and don't get chatter?
    I would've assumed the reamer would need to be in a rigid setup with no pilot.

    I don't use a floating reamer holder. I have a pusher style similar to the Bald Eagle. Bald Eagle Floating Reamer Holder - Pacific Tool and Gauge The difference is my holder looks the same, but has a "pointed" radius on the back that a flat tool in the tail stock pushes. I guess that I've been lucky on the chatter. Bear in mind that I only do my own BR rifles and hunting rifles. Though I've been doing this 20+ years, I've not had as many opportunities as some to have that problem. I chamber in the headstock with catheads on each side. Using Deltronic pins I indicate both ends. I predrill and then indicate the throat area before taper boring to the indicated area in the throat. I want my reamer to follow my prebored hole without the influence of the bushing.
    Hope that clears it up.

    A lot of my BR friends use a version similar to this. A lot of ways to skin a cat and this is what suits me.

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    Quote Originally Posted by 300sniper View Post
    I predrill standard and magnum chambers with an 11mm carbide 140* drill at 1588 rpm, .008" ipr, and never get a bur at the bore. I do check it with a tight fitting bushing on a rod before I send the reamer into it.

    I don't feel the time spent setting up a manual machine to taper bore is worth it. What are we talking about, at most .020" of taper on the diameter? If you parallel bore .010" under the shoulder diameter, the reamer isn't going to care if the bore was tapered or not. The reamer is following what is in front of it, the tapered part it just cut. I do taper bore on the CNC because it doesn't take any longer to program or machine. I program my prebore .020"-.030" under reamer diameter.

    In the CNC, I drill .050 short of the shoulder and bore to that point. I rough ream about .020"-.030" short of final HS. I check the HS and program the final peck.

    On the manual, I drill as deep as I can and still have the pilot make purchase in the bore. I parallel bore to that depth, visually .010" or so under the shoulder diameter. The exact diameter isn't critical, you are just making the hole straight/parallel to you spindle axis.

    I’ve got a couple .223AIs to do today. I’m debating even bothering with drilling and preboring them. May just run the chamber roughing cycle, do the counterbore, the the final HS peck.
    I have a Colchester master 2500 lathe and while it's nice it isn't even close to CNC rigidity so methods need to be adapted to compensate for that. I'm going to be pre drilling using the tailstock wound in by hand so getting a perfect burr free bore may be more good luck than good management.

    My thought on taper bore would be that the reamer would start cutting on the full flute length and be fully supported / aligned rather than just on the corner of the shoulder / body junction but I see your point. If I bore to 20tho under and there is only 20tho taper in the case then none of the reamer is supported when the cut starts anyway

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    With a decently ground drill, you shouldn’t be pushing a bur into the bore, regardless of the machine. Still, check it with an appropriate busing before shoving a reamer in.

    The reamer is tapered, so the only thing you can really change by the prebore is how much the face of the shoulder cuts. Everything that follows has already been cut by the reamer due to the shape. Now, if you were going to taper bore so the breech end is larger than the reamer shoulder, that’s different. I don’t feel that is a good idea because the front of reamer should cut. This method would probably result in a less than ideal surface finish, and possibly chatter. You are also running a much higher risk of the reamer not completely cleaning up the chamber.


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