[hugh jass]

I need to drill a .30" x 13.5" hole into 4140 stock. Ive been reading up on gun drilling and it seems to be if you're over 20:1 you need to gun drill. If I drilled 7.25 in one end and 7.25 from the other and met in the middle would this work? I'd be at 25:1. If I went SLOW and pecked away at it would I be ok? The hole would be finish reamed anyway after the initial boring. Any advice would be appreciated.

[rowbare]

A reamer will follow the hole you drilled so if the hole isn't straight to begin with, the reamer won't help you. At that diameter and over that length the reamer shaft might as well be made of rubber. If the job is a one-off can you farm it out?

bob

[hanermo]

Impossible to hit the holes ... unless you do it on a lathe.
Where the work rotates, it tends to make the hole straight.

Where the drill bit rotates, the opposite.
This is a near-impossible job - or its easy.

If the hole does not need to be "straight" its maybe easy with a gun drill.
It will not be perfect, but may be good enough.

There are lots of requirements - and you dont mention any of them
? Straightness
? Does it need to hit another feature, hole, position

If something stright live a plunger travels in it, and the tolerance is tight, you will probably have a problem.
Drilling, and double reaming - maybe. Depends on tolerances, a lot.
How big is the part - can it be rotated while drilling like on a lathe. Etc.


Gun drills are much cheaper than getting it done.