A long skinny drill

Keep it sharp with a perfect "point". I mean the point should be centered without one cutting edge being longer than the other. Peck drill often pulling chips completely out of the hole. Use lots of lube/coolant. Start the hole with a center drill than a stub drill of the same diameter then a jobber length before using the long one. This will start the hole as straight as possible due to rigidity and help guide the long drill later. The hole will probably still wander some before you're through. Randy

Yup, I have some of those. They were sold for use in model construction, where a hole must be drilled in an otherwise unreachable location, not necessarily for drilling deep holes. They are very handy for drilling holes in formers for pushrods or pushrod guide tubes.

Precision deep hole drilling is normally done with gun drills as I recall (never seen it done, just read about it).

Pete Keillor

Is it a through hole? If it's possible you might want to drill from both sides and meet in the middle if the idea is to have both ends in the correct spot. Otherwise the other end of the hole will be off by a lot or a little depending on a lot of things. For best results, drill slightly undersize with a shorter drill, commonly know as screw machine drill say about 1/2 inch deep, then in the same setup, use a 3/16 endmill to chase the hole making it perfectly straight and on size. Then in the same setup, drill with your long drill using that hole as a pilot to guide it straight. Use short pecks and moderate pressure.

Fred

Start with a center drill then go as deep as practical with a short 3/16th. If all the right stars are in alignment it will probably still wander off - but then again it might go straight - stranger things have happened. Mild steel (and, I assume, a part with minimum time invested) - go for it. Little to lose.

Hope this helps. Ken.

Be extremely careful to peck drill for chip evacuation as other posters recommend. Also, use very low pressure, because long, skinny drills will start whipping about their axis. When this happens, they are liable to go off in any direction except the center of the hole.

The short answer is YES, it will probably wander.

Are there any optoions available to you to correct the situation if it does wander off?

For example, can you drill the hole as a 1st operation, then run between centers to finish other features relative to the hole??

No.

Some thoughts.

12" x 3/16 is certainly a problem and it's not whether it will wander but whether the wander can be kept within acceptable limits.

This sort of hole normally calls for drilling by a single flute gun drill. This is very different from the drill you have but gives pointers to useful techniques.

A drill of this length has pretty well zero sideways stiffness so wander is totally reliant on its point geometry and the accuracy of its initial guidance.

Centre drill starting is not good enough because it relies on the mismatched angles of the drill and the centre drill successfully guiding the lands of the drill flutes to be accurately concentric with the headstock axis. It is much better to positively locate the flutes by first single point boring at precise drill diameter for at least the first

1/4". This gets the drill entered truly concentric with the headstock axis.

With the point of the drill now directly supported by the flutes the effect of minor point asymmetries is greatly reduced. Asymmetries in both the main cutting edges and in the central chisel edge can produce sideways forces which can initiate wander but it is very poor cutting geometry of the chisel edge that produces the largest and most dangerous forces. A good split point grind would be ideal because it greatly reduces these forces but it's not easy to get good symmetry with a freehand grind. Thinning the central web gives some improvement and is easier to carry out.

Patience and frequent pecking is the watch word for the actual drilling. The lighter the drilling forces the better the chance of the flute lands keeping the drill accurately cutting in the right direction. It is essential to avoid any build up of chips in the flutes that could force the drill off course. Plenty of lubricant - preferably a sulpur based cutting oil.

Good luck!

This is good advice, the only thing I can add is that after boring the first

1/4" that you don't go to the 3/16" x 12" drill but start with a drill of the screw machine length, then go to a drill in jobbers length and use progressively longer drills finishing with that 12" drill

This can help, but only if you can be certain that the earlier drills do not produce even marginally oversized holes. The long skinny drill is totally reliant on the cutting edge guidance provided by the immediately adjacent drill lands and even a tiny amount of clearance can initiate the start of wander.

Jim

snip

If you have the equipment, gun drilling is certainly the way to go but this is a long way from "D" type cutters that are easily home brewed for special jobs.

shows both the shape and the grinding profile and these are very different from the useful and familiar "D" type profile.

The gun drill stem is hollow and it is essential to pump lubricant in at about 1000 psi to the tool tip to forcibly flush out the swarf.

The end cross section is about 3/4 of a circle and the position of the bearing lands is carefully chosen to provide the optimum tip support geometry for the material being drilled.

The cutting edge directly intersects the drill centre. This eliminates the usual chisel edge and the deliberately asymmetric grind produces a controlled net sideways force which provides guidance by keeping the bearing lands located on the prevously drilled hole surface.

They can produce superb results but both the drills and the necessary associated equipment are pretty expensive items.

Jim

You want a "gun drill" It is a single cutter "D" type bit.

Ill make someone a real deal on an Eldorado Gun drill.

Gunner

Liberals - Cosmopolitan critics, men who are the friends of every country save their own. Benjamin Disraeli

They're aircraft drills, probably picked up by the hobby shops. Aircraft metalwork often requires the use of really long drills to reach deep into recesses or to make sure holes in widely spaced flanges are aligned.

Dan

How big/heavy is it? How about that RAID array we discussed?

There are two units to a gun drill. The actual drill unit, which in this case is about 5' x 32" wide, and the power coolant unit containing th coolant tank and the hydraulic pumps needed to supply coolant at 1500 psi. Its about the same size. Weight of the two is actually pretty light, perhaps 1000 lbs for the pair.

I believe this particular unit will drill 12 or 14" deep.

Id like to get $5k or best offer for it. Includes slushbox, some drills (new in tubes) Buck 6 Jaw chuck and whatnot.

Need some serial numbers, model numbers etc?

Gunner

Liberals - Cosmopolitan critics, men who are the friends of every country save their own. Benjamin Disraeli

Nope, no room in the shop for something of that size. Didn't know the scale of what you were talking about...

It will drill very straight. Keep your speed just a little below normal for that size drill in that material and use lots of a good cutting oil. Most important is to romove the drill from the hole frequently to remove chips. keep it sharp and use enough pressure to keep it cutting.