Internal tapered bore

[Paulc]

How best to bore a tapered hole in steel?
Trying to match the taper of a civil war cartridge (Burnside) in the breech block.
Lathe plus boring bar or ...?

[whateg0]

Yes, lathe plus boring bar. To get best results you need to have an original that you can measure the taper in. Adjusting the compound to match. Or if using a taper attachment match it with that. I don't know how steep the tapir is so I don't know if a taper attachment is appropriate or not

[whateg0]

The maybe more important question is how will you measure its depth?

[whateg0]

I actually just dealt with something similar where I needed to turn a tapered groove in the middle of a shaft. In my case, it wasn't critical where the groove ended up, but I still wanted to duplicate the location as closely as I could. So, I used an 1/8" dowel and an L-shaped piece of metal to measure its distance from the end of the shaft. When making a D1-3 adapter for my lathe, I used 3 ball bearings spaced around the perimeter but locked in place at a fixed depth to make sure the new one matched an existing backplate.

[pete]

Depends on how your lathe is equipped for that best method. Ideally as Whateg0 said a proper taper turning attachment would be about the best method without the correct chambering reamer. Most TT attachments seem to have about + - 10 degrees per side for angle setting, and while I don't know the taper for that cartridge then using Google Images it seems like it would be well within what most TT attachment could do. I'm guessing you don't have one or you wouldn't be asking. For setting the angle and using the top slide on something that could be highly critical for it's precision then pre-setting it to the known factory specifications for the angle will only get you very close. Setting with a sine bar and gauge blocks for the very best accuracy there's still enough small clearances within even a brand new top slide to throw the angle slightly off due to the cutting loads taking up the slack to change the angle a fraction. Static adjusting without that load then it won't be the same angle as with them. That might be splitting hairs but it's enough to be easily measurable.Turning a male taper first as a way to check the angle won't really work either because the cutting loads between male and female tapers load the top slide in two different directions with conventional turning. Better would be turning your male test piece until it's correct then turn the female taper with the boring bar upside down and cutting on the back side of the hole with the lathe spindle turning in the normal direction. That way the cutting loads on the top slide are as close as possible from the same direction.

It can also be properly set by cutting female test tapers to creep up on the adjustment of the top slide to the correct angle and measuring that it is correct by using this method. https://www.mathscinotes.com/2014/09/ta ... age-balls/ until the measurements agree with what they should be. Yes it's a complex and slow way to finally get what you want, but it's how I'd probably do it for factory produced empty cartridges and off the shelf reloading dies without that correct chambering reamer. And it takes those gauge balls and a depth mike that's known to be accurate. If your having to machine your own dies and fire form brass from something else as the parent cartridge or machining your own from solid brass that changes everything since getting close enough on the top slide setting would be good enough as long as both the chamber and dies replicate the same angles for both. So the best method for you and your equipment requires a bit more information about what have you got and what are you doing to come up with re-loadable brass? There are a few gunsmithing businesses that are set up to rent out chamering reamers, but that's a pretty obscure cartridge so I doubt that's an option.

[Harold_V]

The idea of a reamer may be a better choice, as the typical cartridge length versus its diameter is likely to require a bar size that is too limber to work well. It would help if it was made of solid carbide if you have no alternative.

H

[SteveM]

My dad made a reamer for an oddball cartridge (he was using it to make a resizing die).

If you make a reamer, you can measure it to make sure that it matches the cartridge before you ever take it to the breech block.

Don't remember what the cartridge was, but he called it "a cannon shell necked down to a phonograph needle".

Steve

[Harold_V]

I strongly suspect the cartridge in question is not of that design, and may, indeed, be a rimfire type cartridge, which was common in those days. It would be of tapered design, but without a neck and shoulder. Such cartridges don't have headspace issues, as they orient on the rim, unlike shouldered cartridges.

If what I've stated, above, is not correct, please post a correction.

It would be nice to know the cartridge size so we could provide more information.

H

[Paulc]

Cartridge looks like this

54 caliber. Fires from percussion cap igniting cartridge powder thru small hole in base.
I'll try to get some measurements today of the new brass i have.
How about a d reamer?

[Harold_V]

Wow! Totally unlike anything I had expected. Thanks for posting the image.

H

[liveaboard]

Mount a cartridge on the lathe, centered between 2 formed rods [drill a hole to hold the nose and round off another rod to fit into the indentation at the rear].
Then put a dial indicator on the top slide and run it up and down the tapered side of the shell, adjust the angle until it's right.
Then you can bore the taper with a suitable boring bar.

[John Hasler]

Mount a cartridge on the lathe, centered between 2 formed rods [drill a hole to hold the nose and round off another rod to fit into the indentation at the rear].
Then put a dial indicator on the top slide and run it up and down the tapered side of the shell, adjust the angle until it's right.
Then you can bore the taper with a suitable boring bar.

I've done this (though not with a cartridge). Works well, but runout could introduce error in the taper angle since both the bullet and the indentation may not be very precisely formed. You can reduce that error with the following fiddly procedure:

Find the maximum and minimum runout at the large end and find the point halfway between. Mark it and note the topslide dial reading. Repeat at the small end. Go back and forth between these points adjusting the compound until the readings on the DTI match. You are now a bit closer to the actual taper.


I'd use a center in the nose. Holding the cartridge while drilling the nose may be tricky.