Deep Hole Drilling

[LJP]

I need to drill a 5/16" dia hole, length wise through a piece of PH4140 steel. The material will be 3/4" x 2" x 12".
My plan is to mount it in a 4 jaw and a steady rest in my SB13" lathe. Begin with center drill and drill as deep as possible with (new) 5/16" jobber length bit.
Then mount a 5/16" hardened bushing at the hole (make a fixture to hold it) and continue drilling with a (new) 12" long 5/16" dia parabolic bit. Remove chips every 1/2" or less, lots of oil.
Do I have any chance of getting a straight accurate hole through 12" on center, at the other end?
Any comments or suggestions would be appreciated.
If anyone has the proper equipment to drill this hole, would you be interested?
Thanks, Larry

 

[rgray]

That's a job for a gun drill.
I think your chances of being within .010 are slim other wise.
I see them on ebay but have not managed to get any yet.
Very expensive new from what I could find.
A good setup would run a booster pump for the drilling fluid at a high pressure(10,000 psi)
I have known of people using them with patience with low pressure fluid.
Was looking at specs for parobolic drills and seeing them good for 8x dia range(2.5in on 5/16)

 

[fredybender]

The parabolic flutes are good for chip evacuation, but poor, for guiding the drill, for hole position.
A gun drill has a "pad" surface, opposite the cutting edge, in order to guide it.

Gun drill can be used on a lathe, and no, you do not need hi pressure to use, them, but you get better results with hi pressure.

What is called hi-pressure, is usually above 300 psi.

What is critical with a gun drill, is volume, not pressure.

You can buy a gun drill from a Sandvik Coromant distributor, as Sandvik has just started stocking inch sizes in Kentucky.

To use a gun drill properly, hole size must be bored I would say, within a few tenths 1XD (pilot hole)
Then use the manufacturers speed and feed calculations. Just be shure to have the point engaged, before the OD on the gun drill.

I have mold people around here, and the coolant holes need to pass at a precise location in injection molds, and this is done on older boring mill, using gun drills at slow speeds.

HTH

 

[Terrywerm]

Larry, drilling deep holes is almost an art by itself, but you may want to consider using what is called a D-bit, which you can make yourself from a length of drill rod. Commercially they are called gun drills and usually have lengthwise holes in them to provide for cutting oil pumped in and for chip removal. A D-bit will drill a flat bottomed hole, and will drill very straight, but requires a starting hole of the same size. Attempting to start a hole with a D-bit is a recipe for disaster. D-bits can be made very long, but when drilling the bit must be removed from the bore frequently as chip removal is not the best. D-bits can be made from either W-1 or O-1 drill rod. Only the cutting end needs to be hardened and tempered. Once hardened, it should be tempered to a dark yellow if I remember correctly. I have included two views of a D-bit sketch, you should be able to get a good idea on how they are made. One word of caution: the cutting end, or half-diameter portion of the bit should not be greater than three times the diameter, and in small diameters should maybe even be a bit shorter.

 

[stevecmo]

I'm curious about your set up. From your description you're dealing with a piece of rectangular stock? How are you using a steady rest?

 

[LJP]

Guys, thank you all for the responses and the ideas!
Wermie, I am intrigued by your suggestion. I will dig further into a "D" bit. It sounds like a solution that may work for me. The bushing would probably not be necessary with this set up. Thanks
Steve, I am using rectangular stock. I will make a spider, to spin within the steady.
Larry

 

[Terrywerm]

Larry, have a look at this thread on d-bits. http://www.hobby-machinist.com/showthread.php/6209-quot-D-quot-Bits

I have made a couple of them in the past, and it is pretty straight forward. One of the key bits is to make sure that you machine away about .001" less than half of the diameter of the parent stock - but no more, maybe a couple thou less. Get it as close to perfect as you can. If you cut it down too thin it will cut undersize. If need be, machine away a little less, then touch up with a file to the proper dimension before you harden and temper. Using a d-bit does do away with the requirement for a drill bushing too. One big advantage to a d-bit is that you can make any contour on the end of the bit that you want as long as you provide relief on the other 'side' of the cutting face. A flat bottom hole is not a requirement, but should work just fine.

 

[stevecmo]

Steve, I am using rectangular stock. I will make a spider, to spin within the steady.
Larry

:thinking: Now why didn't I think of that? Thanks, Larry! As I get older, sometimes the obvious seems more elusive.

 

[george wilson]

With a "D" bit like Wermie's illustration,I made out of 01 drill rod,I drilled a hole 16" deep and 9/16" in diameter through a real tough piece of propeller shafting. It was only .002" off center where it came out the other end.

I have to say that my 9/16" D bit was made .005" OVER half the diameter,rather than less than half of the diameter. I can't remember my source. It could have been Holtzappfel(sp?) from the 19th. C.. I have all 3 volumes. It worked fine,though.

To drill with a D bit,you must first very carefully bore a hole about 3 diameters deep with a single point boring tool. The accuracy of this starting hole will determine the accuracy of the hole the D bit will make. You can drill only a short distance between withdrawing the D bit to clear it. It is a time consuming job. But,if you are careful to never crowd the D bit in too fast,and clear it very often,you can drill quite an accurate hole with one. Of course,I used cutting oil,but had no way to use oil under pressure.

The pressurized oil serves to clear the drill,but a gun drill has a different configuration from a D bit. The D bit has no way to carry the chips out with oil pressure. You must do that patiently.

 

[Terrywerm]

George is correct all the way and has pointed out an item where I was a bit unclear. The thickness of the stock that is left to make the business end of the drill needs to be slightly greater than half the diameter. The dimension is not super critical, anywhere between .001" and .005" over half the diameter will work fine.

In my earlier post I said to remove slightly less than half the diameter, which was not real clear. Thanks George for the clarification!