Deep hole drilling “large” stock on mini lathe?

[TTD]

Hi guys, I have another newbie question for ya…


Note: By “large” stock I mean larger diameter than spindle bore of my particular lathe, which is quite small…just have to put things in perspective…


First, a little background on the project I’m getting ready to tackle - I designed a trick side lever breech for my .25 cal airgun (another hobby of mine) and have pretty much figured out how to go about machining almost everything using only my 7 x 12 mini lathe, but admit that I’m stumped on how to perform the very first step...boring a full length (i.e- 6.5” long), 7/16” diameter hole in some 1” aluminum hex bar.


Now, I know on a bigger lathe you would normally just mount the hex bar in the lathe chuck, put your extra long 7/16” bit in the drill chuck in tailstock and have at it (after center drilling first, of course), but my wee lathe only has a ¾” spindle bore and 10.5” of carriage travel so there goes that idea out the window. I could (theoretically) do the operation in reverse - mount the drill bit in lathe and hex bar in milling attachment and feed the bar into the spinning bit, but I’m afraid the hole will end up veering off-center, effectively making it nothing more than a paperweight with an off-centered hole.


Any ideas, providing it’s even possible on such a small machine???


Worst case scenario (and the direction I think I'm heading), I have a machinist/gunsmith buddy who lives about an hour from me who can perform the operation quite easily and quickly on his much bigger 12 x 28 (1.5” spindle bore) so I’m not really stuck, was just hoping to be able to do 100% of the project on my own for a change…kind of a “personal milestone” sorta thing for me…lol

 

[MarkStephen]

Another newbie here and I am going to hold off on saying on how I would do it as it may very well be wrong. What I will mention is some thing I'v been told by "them that know" about drilling long holes, though your hole is not that long. The first thing I was told is, drilling long holes never come out straight. That is to say the exit will always be off axis from the entrance. The longer the hole, the more off axis it will be. Standard twist drills are good at doing this. To reduce this error, the gun drill was invented - GunDrill - for drilling those real long holes. After the hole is drilled, the work is placed between centers and the OD is trued up with the ID so both are on the same axis. So if your OD Must be 1" or real close, use a bit larger hex or round stock to give you enough material to turn it down true with the axis of the drilled hole.

Now to you more informed readers out there, was this info given to me accurate, or is this in error?

Mark

 

[Wreck™Wreck]

Now, I know on a bigger lathe you would normally just mount the hex bar in the lathe chuck, put your extra long 7/16” bit in the drill chuck in tailstock and have at it (after center drilling first, of course), but my wee lathe only has a ¾” spindle bore and 10.5” of carriage travel so there goes that idea out the window. I could (theoretically) do the operation in reverse - mount the drill bit in lathe and hex bar in milling attachment and feed the bar into the spinning bit, but I’m afraid the hole will end up veering off-center, effectively making it nothing more than a paperweight with an off-centered hole.

What makes you think that holding the drill in the chuck and the part on the crossslide is any less accurate than drilling from the tailstock?

 

[TTD]

What makes you think that holding the drill in the chuck and the part on the crossslide is any less accurate than drilling from the tailstock?

Just the way I was (briefly) taught, I suppose. I'm pretty much green as grass with lathes/milling machines so still LOTS of learning to go yet. I did work/train at buddy's shop last winter for a couple months while I was laid off, but I tried to cram so much info into me in those 2 months that I think a bit leaked out...

Anyways, I was taught (right, wrong or otherwise...) that I would have less problems with a "wandering" bit by having it (bit) held stationary in tailstock and your work piece spinning in lathe. Something about "allowing the bit to follow the path of least resistance", which in my mind made perfect sense so I've been running with it ever since.

I am certainly open to (and appreciate) any and all suggestions, though!

 

[mattthemuppet2]

one option - drill undersize from both sides after indicating your stock as true as you can (gentle taps with deadblow, tightening different chuck key holes etc), then make yourself a D bit from drill rod to drill the rest to size (this is from what I've read, never done it myself!). You could try a reamer, but they tend to follow the drilled hole, so that might not help. Lots of peck drilling either way to clear chips so your drill tip doesn't wander.

Does it have to stay hex? If not, the advice above about drilling then mounting between centers to turn the OD concentric to the bore would be the best way.

 

[Andre]

Could you show some pictures of your design? I would like to make a sidelever breach for my Benji Disco.

 

[TTD]

So if your OD Must be 1" or real close, use a bit larger hex or round stock to give you enough material to turn it down true with the axis of the drilled hole.
Mark

If I was using round stock that is exactly what I would do, but if it can be helped I really don't want to tackle that chore with hex.

Regardless of what method I end up using, I will be indexing off the bore for the .875" radius to be cut along entire length of bottom as well as the 3/8" dovetail rail up top.

 

[TTD]

Could you show some pictures of your design? I would like to make a sidelever breach for my Benji Disco.

Probably won't happen til after Xmas, but when I do actually get at it I will start a new thread with all the details in the more appropriate "gunsmithing" section of forum. Here is a couple pics to tide you over til then...btw, breech is for the 22xx/Benji Disco series...

 

[12bolts]

Todd,
Is your lathe big enough that "if" the hex is chucked up, you can still get the carriage far enough to the right to get a boring bar in the end of your work?

Cheers Phil

 

[Andre]

You do some good sketchup work! If you ever build full out plans with dimensions, (which I think is a great idea!) please let me know!

Which piece of the bolt cocks the hammer? Is it that pin sticking down from the bolt? Kinda backwards, but a great idea. I like it!