drilling a long straight hole

[lathehand]

For todays project I have a 17" long piece of 25mm diameter O1 steel that needs a hole thru it. I used our like new Austrian made EMCO (NOT enco)maximat super 11.I faced both ends, centerdrilled, started the hole with a 15/64 stud length drill, drilled with a long 15/64, reamed 1" deep with a .250 reamer, and then used a 11" long 1/4" drill from each end. When the holes met in the middle it looked like they walked offcenter .02-.03". I drilled in 9 1/2" with the 1/4 drill and the bores blended together but you can see the hole is not straight when you hold it up to a light. Final size of the hole needs to pass a 3/8" shaft.
Any good ideas on how to make the hole straighter as I approach final size?

[Rustybolt]

Getting holes that deep when drilled from opposite ends to meet exactly is usually a matter of luck. A hole that deep is usually gun drilled.
Why does it have to be the same size straight through?

[GadgetBuilder]

Haven't tried this myself, just read about it.

Make a "D" bit on the end of a 20" piece of 3/8 drill rod. Turn the shank down a few thou for all except the last 4 or 5 inches on the business end, following the D.

Harden and sharpen the D bit and polish the 4 or 5 inch section.

Drill the hole out to near finish size, leaving enough for a reamer to finish to size. Ream from one end with a normal reamer, as far as it will reach. Then, use the D bit to extend the reamed section to full length.

The D bit will work as a long reamer with the 3/8 section just behind the cutting edge working to guide it straight. Use lots of oil and go slow, clear chips as needed.

[gsp]

Gun drill.

[japcas]

Depends on where you are doing this. If this is for home, I'd make the D bit reamer. It will straighten up the hole, ream to size and leave a good finish. If I were doing this at work, I'd have them order a proper gun drill to do the job, although the D bit would probably leave just as nice a finish. Just remember to clear the chips often if you use the D bit. Also, finish the hole from one end if you need a straight hole.

[J Tiers]

Don't "approach final size".

I've drilled a few holes like that, 0.236 x 10" deep, etc.

I drilled final size directly. I used a drill with very short flutes, about 1/2" of flutes at the end only. Came out about 0.01 or less off center, which was good enough

Long flutes bend and wander.

Thin drills bend and wander.

Peck drill, about 1/2 diameter at a time.

Gun drills, solid type, are pretty darn good at drilling straight holes, though, and if you can get them, economically, they are the best approach.

[Rich Carlstedt]

Never use a centerdrill or the normal way you start. you want to start the hole as close to perfect, and a drill will wonder right off at the start. You need to use a ball endmill for the first inch or so. A Ball endmill is so rigid, it cuts straight, and gives you the best chance of making it a true hole.
Also if your chips are not exactly the same from both flutes, get another drill
Rich

[Forrest Addy]

There's several tricks. Jerry suggested drills with long straight shanks with short flutes; this is a hellova start.

Also

Grip the work in a chuck or collet and run the far end in a steady rest.

Drill and prebore the first few diameters of hole depth with a boring bar

Drill through a bit underesized to meet in the middle.

Bore to size as deep as you can reach and then ream through.

The surgical risk is highest drilling straingth deep holes. Do the hole first then use the hole to set up to for all subsequent work.

[rantbot]

A reamer, D or otherwise, won't straighten a hole which isn't already straight.

[japcas]

Rantbot is right. If your referring to my post above, my intentions were to drill the hole completely with the D bit reamer. Not go through another hole. Maybe I didn't word it properly. Bore the start of the hole for several diameters like Forrest said and have at it with the D bit. Have fun because you are going to be there for a while. With a hole that deep one might want to relieve the shank of the D bit a little though to prevent it from rubbing and wanting to gall up in the hole. I'd leave a couple diameters of land for it to ride on though as this is what makes it work properly and cut straight. You have to work from one end if you want to be assured that it is one straight hole that a rod of the proper diameter will pass cleanly through. At work, we do a job that gets a 1 inch diameter about 60 inches through a piece of 8 inch 4140. The outside gets turned into a shaft. After the hole is drilled, we always chuck on the end that the hole was started, and run a center in the outlet hole, and recenter the shaft to run with the hole because it always will be out some at that distance. If the outside has to run with the hole, I'd definitely cut it after drilling the hole. We used to gun drill them at work but found a place that specialized in gun drilling that quoted it to us for like $80 a hole. At our labor rate we can't even set it up for that price, let alone drill it.

[Airhead]

How about a different aproach? If I could get away with it, I think I would hog the hole out to maybe 7/16" then bore the ends to take bushings for the 3/8" shaft.

I don't know if this would work for you or not, just throwing it out there.


Rick

[J Tiers]

Thanks for reminding me...... I DID bore the first bit of the holes, now that you mention it. Only a diameter or two deep IIRC.

If it does not start straight, it won't finish straight, because the drill will have a side-force on it pushing it "out". Even if it goes back straight (not very likely), it will bind and have a hump in it.

[Peter S]

For anyone’s interest….I have a book with the following account of accurate deep hole drilling. I have never had to try anything like this so can’t comment. In the book there are diagrams showing the sequence and detail of the reamer mods, but I think it is all pretty clear without the diagrams.

Joseph W. Serafin, “Metalworker’s Benchtop Reference Manual”. TAB Books.


“Regardless of how precisely a drill is sharpened, it is still practically impossible to drill a perfectly straight hole to any greater depth. However, long, clean, straight holes can be made if proper steps are followed. Using properly sized and ground tools, holes more than 3 feet long can be drilled, finished, and running true, to within .001 inch F.I.R. or better.

A few sequential operations must be adhered to in every case. Each step is controlled, leaving nothing to chance.

No doubt, the engine lathe is the machine that is better suited for the long hole-drilling operations. They also can be done nearly as well in a radial drill press. The type of machine would depend on the shape and size of the job and the accuracy desired.

In the lathe, the work is gripped and driven, preferably in the chuck, although it could be supported and driven by other means also. The outer, or free, end of the work must be supported in a steady rest.

The plan is to drill and finish-ream a .750-inch-diameter hole, through a bar of steel 3 feet in length. The hole’s runout must be within plus or minus .001 inch F.I.R.

A The work is faced off and centre drilled.

B Next drill an 11/16-inch-diameter hole, approximately 5 inches deep. If the bottom of the hole shows a runout, as it usually does, it will not matter at this stage.

C Using a small boring tool, counterbore the drilled hole to a .716 to .720-inch-diameter and 1 ¼ to 1 ½ inches deep. This step pilots the following tool, which is a .722-inch-diameter end-cutting reamer ** (see below for description of this tool), giving it a very true start.

D Ream the drilled hole with the .722-inch reamer, down to where a part of the holes bottom is cut out.

E Next, make the centre of the hole’s bottom run true again by drilling into the bottom with the point of a .722-inch drill. However, do not drill into the bottom more than 3/32 to 1/8 inch deep.

F After the hole bottom has been re-centred, go back to the 11/16-inch-diameter drill. The hole is drilled in an additional 5 inches or so.

G Next, the .722-inch-diameter end-cutting reamer is used. Also, cut a small amount from the holes bottom.

H Again, to keep the 11/16-inch drill from straying too widely, the hole’s bottom must be re-centred with the .722-inch drill.

Repeat the 11/16-inch-diameter drilling, the end reaming, and re-centring as often as is required to reach the entire length of the hole.

I Counterbore the front end of the hole to a .746- to .748-inch pilot diameter, 1 ¼ to 1/1/2 inches deep.

J Ream the entire length of the hole with the .750-inch-diameter end cutting reamer** (see below for description of this tool)

Note: Be sure to apply a generous amount of sulphur-based cutting oil to the entire drilling and reaming operations.

** Special reamers and drills required:

For deep hole drilling and reaming, longer shanks are silver (hard) soldered to the drills and reamers. These tools can be any of the high-speed steel types, which are usually found in the shops.

Reamers of the longer, straight-fluted types are preferred, but they must be modified for their intended purpose. These reamers are ground to cut on their front ends only. The outside diameter is spun ground, meaning it has no peripheral cutting edges. Also, the outside diameter should taper back to a smaller diameter by approximately .0001 per inch.

The reamers front end is ground square to its axial centreline, having no angled leading edges (in other words having sharp 90 degree corners). However, the corners are hand-honed lightly to prevent their flaking away under the cut.

For the shank extensions, use either a carbon steel drill rod or a ground hot-or cold-rolled mild steel rod. If you choose carbon steel, do not plunge it into oil or water to cool the hot joint. The rod’s diameter should be a few thousandths of an inch smaller than the diameter of the drill for which it is intended.

Turn the drill or reamer shank down to one-half its diameter. Next, drill the rod using two drills, one for drilling, the other for sizing the hole. The hole needs to be only 1 ½ times deeper than the holes diameter.

Whatever the drilled and turned diameters of the two mating parts, they should fit together quite closely, having no more than .005 to .001 inch of play. Even if no play (clearance) is allowed, upon heating the hollow piece will expand much more than will the solid rod. Also, the thinner the solder is, the stronger will be the joint.
Before joining, thoroughly rinse the hole and the turned shank in carbon tetrachloride or acetone to remove any oil films. Then generously paint both parts with a “Handy and Harman” silver soldering flux. Slip a ring of silver solder over the turned shank diameter before you insert it into the hole in the extension rod.

The rod should be held upright in a vise while a reduced acetylene flame is played around the joint. When the extension rod and the tool’s shank become sufficiently hot, the ring of silver solder will melt, flowing in and around the inside of the joint. Maintain a slight downward pressure on the drill or reamer until it has cooled slightly.

Butt-welded extensions, to small-diameter drills etc. have been quite successfully accomplished using the welding unit of a metal cutting bandsaw. This process is very much like butt-welding the two saw end together.”

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Only very small quibble I would have is when fitting the extended shank – it is usually easier to drill your hole first, then turn the shank to the desired fit.

Also, it is years since I did any silver solder, but when using Oxy-Acetylene, a reducing flame means first getting a neutral flame, then give it a little bit more acetylene – if my memory is correct!

[big-dipper]

Unless there's a reason the hole HAS to be size all the way through, drill oversize and bush each end.

[Rustybolt]

Never use a centerdrill or the normal way you start. you want to start the hole as close to perfect, and a drill will wonder right off at the start. You need to use a ball endmill for the first inch or so. A Ball endmill is so rigid, it cuts straight, and gives you the best chance of making it a true hole.
Also if your chips are not exactly the same from both flutes, get another drill
Rich

I disagree. Center drills are fine.A properly ground drill will follow center every time as long as it isn't crowded into the work. The problem starts when the hole gets deeper and swarf builds up in the lands and wants to push the drill over. Or the drill starts to wear unevenly.my.02.