Pre drilling/Pre boring chamber

In the case that you would predrill or prebore a chamber I'm assuming that the reamer must still enter "pilot first" or the drill/bore will be guiding the reamer. In that case aren't you simply drilling or boring a fairly shallow hole (should correspond to the distance of the back of the pilot to around 1/2 way up the shoulder in most cases) that would be opened up by the back of the reamer once you are at full depth? Or are most people dialing it in enough that they are comfortable boring the majority of the chamber and just finishing with a reamer being guided by the prebore?

Does any of this change if you are working in a steadyrest rather than through the headstock?

Thanks for the help.

Milo,
I am trying to gist what you are asking. I will just tell you some things which come to mind. You are using a lathe and pre drilling a hole for a bore finish or a reamed finish hole. The way to do a reamed hole regardless of steady rest or not is to Spot the hole then drill the hole leaving it smaller in size than the size the reamer will cut to. The reamer will finish the hole to size yet it requires a smaller hole so that it can form the dimension. Per side of the hole it may be .006 less or more depending on the overall size. Someone may give you a good rule of thumb I just look at each case when I have a reamed hole to do and select what I feel will be right depending on the setup and the material. When I say .006 per side for example if you ream a .500 finish size using that size reamer then you want a drill bit to drill say at around .490 . Not looking at a chart now you might look at a chart and find a drill. The key is to have enough material to remove yet not too much. It would probably be fine to drill the pilot hole to .025 dia undersize. Center drill/drill pilot hole/ream.

For boring you would only need to perhaps spot (maybe not) drill and then rough bore then finish bore. Keep your boring bar over hang not too long yet long enough to cut to the depth that you need. In other words if the boring bar hangs out twice the length of the bore depth it is hanging out too much because when you have only a little to clear your bore depth then you have the tool much more stable or rigid. I hope this helps a little.

Spinit, thanks for your help. My main concern is depth of the drill hole or bore hole. Is it acceptable to bore a chamber (undersized as you say) but so deep that the live pilot of the reamer doesn't engage the bore until after the shoulder of the reamer has begun cutting the remainder of the chamber? I wish I could animate things, it'd make things easier

I would think it would be a brave man who would have the reamer body cut before the pilot is securely engaged in the barrel bore.Theoretically,if all units of your setup are centred ,straight,square,and tight,it probably wouldnt matter.Lacking this ideal situation,the best way to chamber is between centres with the muzzlebush one centre, and the centre in the reamer the other.This guarantees that no external sideload can be applied to the reamer,causing an oversize chamber.The pilot must be solidly engaged in the bore to support the setup.It is obvious that any predrilling or boring must be concentric and parallel with the bore,or sideload might break the pilot .Pressurized oil feed is also pretty near essential.I hold the reamer from turning by hand with a tap wrench,which instantly frees up a jam.Obviously,never turn a reamer backwards.Regards John.

milotrain said:

Spinit, thanks for your help. My main concern is depth of the drill hole or bore hole. Is it acceptable to bore a chamber (undersized as you say) but so deep that the live pilot of the reamer doesn't engage the bore until after the shoulder of the reamer has begun cutting the remainder of the chamber? I wish I could animate things, it'd make things easier

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I set my compound to the body taper on the reamer. Dial in the bore as close to the throat area as I can reach with my DTI. Drill out the bulk of the chamber stay about .100" short and at least .050" under the should dia on the reamer. At this point I can usually reach in farther with the DTI so I recheck the throat area. Bore the chamber with the compound stay at least .050" short on depth and .020" or so under reamer dia. At this point the reamer pilot will usually engage the bore. From this point I have reamed chambers with and without the reamer bushing installed and dont find any difference in the end results. In most cases I just install the correct bushing before I ream

If you correctly drill and prebore, why do you need a bushing? You might think that this is sacrilege, but if you have any machining experience you will find out the truth. If you indicate the throat and taper bore to that point your reamer will follow your prebored hole. I do not want a bushing to influence my reamer. Think about it. You want your reamed hole to be coaxial to your bore at the throat.

Since you know that the hole is not perfectly straight you want the cutter to be coaxial to the hole, the rifled hole, not the one you thought was right. The rifled bore may take a circuitous path to the muzzle but the chamber needs to be coaxial with the bore at the point where chamber ends and rifling begins.

Butch Lambert said:

If you correctly drill and prebore, why do you need a bushing? You might think that this is sacrilege, but if you have any machining experience you will find out the truth. If you indicate the throat and taper bore to that point your reamer will follow your prebored hole. I do not want a bushing to influence my reamer. Think about it. You want your reamed hole to be coaxial to your bore at the throat.

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tdmidget said:

The rifled bore may take a circuitous path to the muzzle but the chamber needs to be coaxial with the bore at the point where chamber ends and rifling begins.

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Excellent. I thought that during my research I encountered two different ways of preboring. One just was shallow, or was a bore ream bore ream process, the other is as you state. Ensure that the prebore is concentric to the bore and use it to guide the reamer, the pilot becomes un-necessary. I wanted to make sure I was thinking about it correctly. I'm just a hobby machinist and I have more time available to me on a computer than I do in my home shop. It creates fairly large gaps, that can only really be solved by making chips.

Thanks for the help!

People who do this work professionally have arbors,called range rods that exactly fit the bore,and allow both ends of the barrel to be set perfectly concentric with the bore.You cannot assume that the external of the barrel is concentric with the bore.Since borescopes have become commonplace,you would be amazed at how many people bring back a new factory rifle complaining the the chamber is not concentric with the bore.Its easily seen.This is not grounds for returning a rifle,and if it meets accuracy criteria,it will not be exchanged,nor rectified.Regards John.

When I was doing it on a manual machine, with where the finished throat will be dialed into near zero TIR, I'd single point bore to diameter about .010".020" under the reamer shoulder diameter (I'd just eyeball this dimension) to a depth that just allowed the pilot to make purchase in the bore when the reamer contacted the breech (usually about 1.1"-1.2" for my common cartridges). I'd ream the chamber to about 1/2" of final depth. At that point, the reamer is well established in the chamber so I then remove the pilot for the remainder.

My chambering strategy is much different now.

A lot of times when on a lathe before reaming the drill could walk a little and also the hole may not be drilled round. It happens little yet one can if they wish bore to the size under your reamer size to allow your reamer to follow a true hole. The reamer does have a natural angled lead which moves to the hole. A reamer should be put in with enough lenth to allow movement and alignment to the hole. Not chocked up too much. Now if you are doing a blind hole where there is a shoulder at the bottom or no through hole then perhaps programming the bore to do the work of the reamer would be best. When this is done then you need to have a acurite way to measure the hole and sneak up on the id finish diameter by backing up your x offset and take a cut undersise perhaps as much as .04 at first and then step it in .010 then check it right to see how taking just .01 does this allows you to understand what adjustments need to be made and then you just move it in about the same way and check each step while getting closer to see that it remains repeatable. I hope this helps you a little not standing right there sort of will make some of what I say confusing to you. Overall if you think about what I have said at least it applies to things in general and is one way of doing things.

Sea Sick Steve said:

I set my compound to the body taper on the reamer. Dial in the bore as close to the throat area as I can reach with my DTI. Drill out the bulk of the chamber stay about .100" short and at least .050" under the should dia on the reamer. At this point I can usually reach in farther with the DTI so I recheck the throat area. Bore the chamber with the compound stay at least .050" short on depth and .020" or so under reamer dia. At this point the reamer pilot will usually engage the bore. From this point I have reamed chambers with and without the reamer bushing installed and dont find any difference in the end results. In most cases I just install the correct bushing before I ream

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Question, if there is about .016" of diameter difference between the chamber diameter at the counterbore and shoulder, what advantage do you feel "taper" boring is giving you over straight boring, especially if you are 20 or so under reamer diameter? I can not see the time involved in matching the taper exactly creating any benefit over simply straight boring 20 under the shoulder diameter. Just something to think about. I've done it both ways and decided the time involved was not worth it.

300sniper said:

Question, if there is about .016" of diameter difference between the chamber diameter at the counterbore and shoulder, what advantage do you feel "taper" boring is giving you over straight boring, especially if you are 20 or so under reamer diameter? I can not see the time involved in matching the taper exactly creating any benefit over simply straight boring 20 under the shoulder diameter. Just something to think about. I've done it both ways and decided the time involved was not worth it.

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I use a floating pusher so I feel better about "locking" the reamer in over a greater length along the reamer body as opposed to just having contact at one point on the body at the start of reaming. I might feel better about it if I used a good floating reamer holder I just havent sprung for one yet. I do it as a hobby for myself so I have the luxury of not being concerned about how long it takes and I actually consider my time spent in the shop as therapeutic LOL. I just did a chamber for a 50 cal 338Norma based cartridge, it has very little body taper. I did not taper prebore this one and the results were perfectly fine.