Questions about twist drills

[Rich_Carlstedt]

Am I correct in thinking that when the drill is stationary and the part turns it will theoretically bore a near perfect hole?

No. Because the drill is likely several times its diameter in length, to say nothing of its reduced strength due to the creation of the flutes, the drill can, and will often, deflect. The hole may be round (or not), but it may not be on true location. If that feature is critical, it's wise to take a light boring pass simply to round it and relocate any deviation from the intended location. .......

Sorry Harold , with all due respect , That comment is not correct.
Gun Drillers will tell you as well. A stationary drill will always drill straighter .
The reason is as follows
When a drill bit is turning , the speed of the flute tips is the same as they revolve around the AXIS of the drill bit
That means the chip load is balanced
When the work piece is turning , and the drill bit is introduced , the turning axis is the piece part, and if the stationary drill try's to deflect , the flute moving outward sees a greater SFPM than the other flute which see a smaller cutting diametr and thus a lower load. The drill flute with the greater load, deflects or shifts towards the center, thus keeping the drill on a more accurate course.

I agree Harold with your roundness comments 100 %
Rich

[John Hasler]

When a drill bit is turning , the speed of the flute tips is the same as they revolve around the AXIS of the drill bit
That means the chip load is balanced
When the work piece is turning , and the drill bit is introduced , the turning axis is the piece part, and if the stationary drill try's to deflect , the flute moving outward sees a greater SFPM than the other flute which see a smaller cutting diametr and thus a lower load. The drill flute with the greater load, deflects or shifts towards the center, thus keeping the drill on a more accurate course.

I agree Harold with your roundness comments 100 %
Rich

Sit on the spinning work piece in your lathe and watch the spinning bit drill into the stationary part. Hang from the spindle on your mill and watch the spinning part rise up to contact the stationary bit. Hold the spindle of your lathe still and spin the lathe: looks like a bizarre drill press. Hold the spindle of your mill still and spin the mill: looks like you're drilling with a weird lathe. It's just a matter of frame of reference.

One small difference: when the bit is spinning and it deflects a bit centrifugal force will tend to deflect it more.

I agree that drills make crooked, egg-shaped holes, though. But then, *all* holes are crooked and egg-shaped. It's just a matter of degree.

[RSG]

My last question in this regard is - What dia should I be boring the hole to for a .625" reamer to follow? perhaps .010 thou under?

For a reamer of that size, a maximum amount to ream would be roughly .015". Less is good, especially if you have bored to create proper location. Depending on the nature of the reamer, you should be able to achieve success with as little as a thou remaining to be reamed.

Harold

Thanks for the recommendations. Despite having success over the years with the one drill then reamer I have come to realize it's just a matter of time for something to fail so I have changed to drilling a smaller hole then boring to .615" prior to reaming. So far the outcome appears to be the same but now I am confident it will be spot on. I produced 5 journals today using this method and I didn't really see any noticeable amount of extra time required to do it.

[Patio]

Harold,

Is there a commercial bit sharpener that will accomplish the job on bits from say 1/16 to 1/2".

Gregg

Yes there is, I have one. It is a Darex, and is the parent company of the "Drill Doctor", although this is a commercial version. It is very easy and fast to use, which is it's main attribute. It will do up to 1 1/4" drill with the right attachments.
With that said....It does not always get it right, as can be seen by the two chip loads while drilling sometimes, and anything over 3/4", one needs to change the set up, which takes time. For large drills or if exacting needs are called for, I think one is better to do it by hand.
Chris, if you would like to come by the shop someday, and have a look. you are welcomed here. Bring a couple of drill bits and you can have a go at it too.

[BadDog]

I missed that question. Like Patio, I have one that will do a good job in that range, but I use an SRD (aka TRD) rather than a Darex. It's simple and to the point, super fast and consistent in use, and is easily configured for various angles as desired. However 1/16" is too small to do without swapping wheels. For this sharpener it would be more accurate to say 1/8" to 5/8" on the standard wheel. But I have no collets to deal with, and it's a much more compact system. And with the small wheel I think it can likely do a very good job down to 1/16", though I'm not sure what the stated range is. I have a small wheel for mine but have never tried using it.

[Harold_V]

Am I correct in thinking that when the drill is stationary and the part turns it will theoretically bore a near perfect hole?

No. Because the drill is likely several times its diameter in length, to say nothing of its reduced strength due to the creation of the flutes, the drill can, and will often, deflect. The hole may be round (or not), but it may not be on true location. If that feature is critical, it's wise to take a light boring pass simply to round it and relocate any deviation from the intended location. .......

Sorry Harold , with all due respect , That comment is not correct.

Not a problem, as I disagree with your philosophy, so it's likely we'll be at odds over this issue.
However, the point I tried to make is that a drill is NOT rigid enough to behave as a boring bar, thus it is very capable of producing holes off location, not round, not straight, and whether the drill turns or the work turns make no difference.

When a drill bit is turning , the speed of the flute tips is the same as they revolve around the AXIS of the drill bit
That means the chip load is balanced

You're assuming both lips are identical (same angle, and identical in length), that both are sharp, and that the drill is cutting on location. In that case, it makes no difference which turns, as the drill will do what it does---it will drill a hole.

When the work piece is turning , and the drill bit is introduced , the turning axis is the piece part, and if the stationary drill try's to deflect , the flute moving outward sees a greater SFPM than the other flute which see a smaller cutting diametr and thus a lower load. The drill flute with the greater load, deflects or shifts towards the center, thus keeping the drill on a more accurate course.

Want to give that a little more thought? Whether the drill turns, or the work turns, the lips of the drill, if moved off center, experience the same thing in regards to surface speed. It's not a function of which one is turning, it's function of being closer to or farther from the center of rotation.

Face it. Twist drills are not precise instruments. They may or may not create a hole on location, and it may or may not be round and straight, which is really a matter of interpretation, as what one considers a straight, round hole may not be straight and round by another's definition. I see that all the time----my "good enough" is often not the same as the other guy's..

Harold

[earlgo]

When reaming from the tailstock of a lathe, it's often desirable to allow the tailstock to float, and feed the entire tailstock instead of using the handle to advance the reamer. Harold

When we were at the CST there was a lot to be said for using a floating reamer holder to keep things like rifle chambers aligned and try to avoid the bell mouth. I bought a used one from the old Travers Tool Company and it has worked fairly well over the years. Any thoughts on this?

--earlgo

[Harold_V]

Any thoughts on this?

Yep! Good thoughts, and the real reason why it's better to ream with the tool rotating than stationary (assuming the hole is concentric with the reamer at the outset. If it is not, it makes no difference).
The tailstock of a lathe is almost NEVER on the centerline of the spindle. Even when new, they're (expected to be) high, and that they are in line front to back is adjustable, and is often not aligned. A floating holder permits the reamer to seek center, avoiding having to bend, as it does under rigid conditions. I've never owned a floating holder, but to allow the reamer to seek center, I grip by a short amount of the shank, and do not tighten the chuck much, allowing the tip of the reamer to move easily, finding center. That, however, still presents the reamer to the bore at an angle, albeit very slight. Not an ideal situation, which is generally improved by the use of a floating holder.

There's a lot of myth involved in drilling/reaming, as we've seen by the comments tendered in this thread. Much of it is nothing more than nonsense---things thought to be true, but aren't.

Harold

[Rich_Carlstedt]

When a drill bit is turning , the speed of the flute tips is the same as they revolve around the AXIS of the drill bit
That means the chip load is balanced

You're assuming both lips are identical (same angle, and identical in length), that both are sharp, and that the drill is cutting on location. In that case, it makes no difference which turns, as the drill will do what it does---it will drill a hole. ..... I disagree with your philosophy, so it's likely we'll be at odds over this issue.Harold

You completely miss the point Harold . I am explaining the rotation about the drill bit axis versus the part axis when it rotates!
I was trying to impart knowledge from 10 years experience in deep hole drilling in dies and thousands of holes upto 48 " in depth.

By the way, this is not my "Philosophy " of operation, but I can understand where you are coming from.
Most every machinist I have ever met, does not understand the machining process of deep hole drilling that has been going on for over a 100 years, nor the requirements of it.
And yes , before you say it..... " gundrills" are different than twist drills , BUT the PHYSICS is the same !
Holes will always be straighter when the workpiece turns ! ( I never said they were perfect ! )

Please read this old copy of Machinery's Handbook
https://books.google.com/books?id=nloPA ... &q&f=false

Perhaps it explains it better than I have. The man that taught me did it for over 50 years
here are the statistics I was taught AND Observed during my work !
Twist drills when turning can drift .015" per inch
Gundrills when turning can drift .0015" per inch of depth
Gundrills when the work is turning can drift .00015" per inch
These are all with properly started holes , and you do not do that with a center drill !
Rich

[Rich_Carlstedt]

....................................

There's a lot of myth involved in drilling/reaming, as we've seen by the comments tendered in this thread. Much of it is nothing more than nonsense---things thought to be true, but aren't.

Harold

You said it !
Rich

[Harold_V]

When a drill bit is turning , the speed of the flute tips is the same as they revolve around the AXIS of the drill bit
That means the chip load is balanced

You're assuming both lips are identical (same angle, and identical in length), that both are sharp, and that the drill is cutting on location. In that case, it makes no difference which turns, as the drill will do what it does---it will drill a hole. ..... I disagree with your philosophy, so it's likely we'll be at odds over this issue.Harold

You completely miss the point Harold . I am explaining the rotation about the drill bit axis versus the part axis when it rotates!
I was trying to impart knowledge from 10 years experience in deep hole drilling in dies and thousands of holes upto 48 " in depth.

Not really, Rich. I understand the difference between rotating the part as opposed to rotating the drill, but in both instances, it's very important that the drill be started on true center, otherwise the problems are similar. I'm not suggesting, not for a moment, that one may not be superior to the other, just that they are both troublesome. I've been down that road countless times, although I confess that the deepest hole I've drilled was likely only about ten inches, maybe a little less. And it was more than enough trouble, done on a lathe.

And, back to the real point---the topic was a drill behaving as a boring bar, which is true to a point, but virtually NEVER enough to correct for errors introduced by less than optimum pointed and sharpened drills, or by misalignment. That's really what the topic was about, and that's the intended purpose of my response.

I agree--gun drills should not be compared to twist drills. They share nothing in common aside from the fact that both create holes. Gunsmiths, using less than great lathes, have (successfully) drilled deep holes for a long time now, thanks to that technology. In fact, if you care to do a search on this board, a gentleman built a Sharp's rifle just recently, from stock. He drilled and rifled the barrel on his lathe, which is the only machine tool he used to make the entire rifle, action, trigger assembly and all. It goes without saying, it's a work of art.

Harold

Edit:
Remembered the thread in question.. Here's a link: http://www.chaski.org/homemachinist/vie ... hp?t=99914

[earlgo]

Ha, when Mr. Carlstedt mentioned gun drilling it reminded me of a part of the last job I had in industry. A division bought drill collars and made and installed the steering electronics for deep hole oil-field wells. The drill collars were about 18ft long, and were typically made in 2 long pieces and a short one then welded together. The electronics were installed in pockets in the sides and connected with wire tunnels that were gun drilled. Some of those wire tunnels were 6' to 7' long, and an inch or so from the surface of a 8" dia drill collar. (I am sure Mr. Armstrong has seen these in his travels.) The material was a special Swiss made stainless steel famous for its abrasion resistance, and tougher than a (insert metaphor here.) I am just adding to the truth that these holes were straight and gun-drilled. I forget the straightness spec on the holes but they had to be in the right location throughout or the stresses in the drill collars would vary and if the holes were out too far, lead to premature failure. At the time, if I remember, it cost about $250K to retract a drill string so it was very important to have a good product.
--earlgo
(who is constantly reminded of stuff during these discussions)