Are center drills self-centering (in a lathe)?

The face is not flat. The tool that you use to face with should be exactly on the centre line.

Center drills tend to make a larger hole than walk off center. Look at the dynamics of the cutting and you will see how to insure that the drill cuts a larger hole (not a problem as you are interested in the center being center rather than a particular size hole) rather than stresses off to the side, wobbling instead.

-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!

It is a good practice to clamp the tailstock to the ways and partially apply the tailstock spindle clamp when getting a center drill started.

Tighten the spindle clamp just enough to induce some drag and take the play out. I usually do that and then release it after the drill has started.

Yes, I do both. The jumping that I'm talking about is in spite of those measures (and the tailstock barrel is not over-extended either). Obviously there is sufficient "play" between the barrel and the tailstock housing to allow some visible movement. Of course the drill chuck is fairly heavy (5/8" keyless), so that causes the tailstock barrel to droop somewhat prior to the drill contacting the work.

The phenomenon I described occurs on workpieces that are faced quite flat (with no "tit"). It's a matter of the tailstock barrel drooping under the weight of the drill chuck.

And as I stated in another response, this is also with the tailstock clamped firmly to the ways, and the barrel lock almost fully engaged.

What you are experiencing is your center drill not being concentric with the spindle, and it's very common. The source of error can be a number of things, including your drill chuck or how the center drill is being chucked. Assuming that you have faced the part, and your lathe is reasonably rigid, the hole generally will not be off center, but instead tapered as the cutting pressure slowly forces the center drill to dead center. Initially it will wing and cut the hole oversized pretty much equally. The alternative is that you break to point off the center drill, which is very common when using the small ones (#1 and smaller). It can work to your benefit if you have a lot of trouble with this issue to extend the ram on your tailstock so it can more easily move towards center. By their nature, center drills do their very best to run without any wobble, all you have to do is give them the opportunity. It also pays to feed slowly at first so the center drill can establish true center.

Harold

Harold,

Very insightful and informed reply to a difficult question. Your answer covers all the facts, certainly all the facts I am aware of, pertaining to tailstock drilling in the lathe. It is always a pleasure to read your responses. One can read "Harold" with assurance there is no taint of ego and that facts are set forth with "been there done that" accuracy and conciseness.

I'll kick in my 2 cents worth.

My old lathe was real bad at this, just as you state. The center drill was slightly self centering, much better than the lathe anyway. To get it better, I kept a one fluted endmill, just larger than the center drill, clamped in a spare chuck. (broken two flute) Follow the center drill with a very small quick boring job and you're spot on center.

Karl

Although the whole post was right on, I figure that last sentence is

*so* important, I'm repeating it. Discovering that made a *big* difference to my center drilling.

Ted

Yep! This true, I think, because the point of a center drill is designed to cut on its sides much like an end mill. Too fast feed leads to breakage of the small and fragile point.

Bob Swinney

(whoops double post)

Excellent advice, another approach that can be included, which increases the accuracy of a centerdrill, is to use a sharp turning toolbit set up for facing, and make a small conical bore at the center of the part, to give the centerdrill something to pick up on.

A small boring bar can also be used for this approach.

Jim

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For using small diameter center drills in a large chuck such as your 5/8", it might be helpful to make a toolholder for your center drill. This would just be a section of about 1/2" round stock with a set screw to secure the center drill (and a corresponding flat spot on the center drill shank).

The drifting problem I encountered was due to the center drill being short and small enough that when the large chuck was tightened to hold it, more than half of the drill's length was out past the chuck body. This situation wasn't secure because most of the grip, and jaw flex were taking place outside of the chuck (by the tips of the jaws). A normally seated full length drill will be held securely inside the chuck as the jaws are in full engagement with the shank.

WB ................

I'm certainly no expert machinist, but I don't see how the center drill can cut a hole in a piece of spinning stock that is anything other than perfectly concentric with the axis of rotation. Put another way, the center drill HAS to make a hole along the center of rotation (this would be centered with respect to the part if it were chucked properly).

As a mental exercise, intentionally offset the tailstock by some significant amount. Now run the center drill into the stock. The center drill is acting like a boring bar and cutting an oversized hole that is surely concentric with the rotation axis. Now, depending on how the center drill is oriented, it may be more or less eficient in the stock removal process. Even if you chuck a piece of driveway gravel in the tailstock and shove it into the stock, the resulting cavity should be "centered".

Based on the other comments in this thread, my understanding of this process is certainly flawed. Please help me figure out what is really going on here.

Thanks

Bruce

Well, almost, anyway!

What you have to say is pretty much dead on. The variable being someone that is ham handed and quickly starts the center drill in the face of his item. It is possible to get centers that are not concentric, in fact, very easy. You must consider that if the center drill is started quickly and forced, and the tailstock quill is easily moved, you will generate an eccentric hole. The best way to learn to properly drill center holes is to do all your practicing with miniature center drills. They tell no lies. If you force them prematurely, they reward you by breaking off. When you can successfully drill with them, I suggest you use the same technique with larger ones, especially something like a #5 that you just know can't be broken. That way you'll drill center holes that are as close as possible. The idea that center holes will automatically be "perfect" paints a cute picture, but in practice you'll find that that is not the case. There generally is error, albeit very small. Center hole drilling is actually a very critical operation, taken much too lightly by most folks, laboring under the assumption that it doesn't matter.

It does!

Harold

It can drill non-concentric, because the rigidity of tailstock rams tends to be less than perfect.

The difference between the flex at the tip of a centerdrill, and the flex that appears at the end of a short toolbit in the toolpost is what tells.

This is why I like to centerdrill for the centerdrill, using a tool (boring bar, or very pointy facing tool) in the toolpost to give a true, concentric starting point for the centerdrill.

Jim

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A boring>The center drill is acting

Easy. Flex or spring.

A boring bar doesn't necessarily cut a concentric hole, either. Try boring out an off center hole. The first pass will likely be off as well. Not as far off as the bore was originally, but still off. Each subsequent pass will be more accurate.

John Martin

In a perfect world...

Drill bits have a stall point between the two cutting edges that is, for a practical purposes, not a cutting edge but a chisel. It is called the chisel or chisel point sometimes. So the initial pressure forces the chisel into the metal deep enough to let the cutting edges get started. That stall point is the primary cause for the pressure you feel when you feed a drill.

Once the chisel edge is forced in deep enough, the cutting edges can get started. Unless the material and drill are perfectly parallel and concentric, the chisel edge is entering the material slightly off center and wants to be dragged around in a circle as that happens. That is the jump or circular motion seen at the initial contact.

At that point, if you're feeding the drill with a lot of pressure, the tip of the center drill can break. If you're using light pressure the center drill will hammer around in a small circle and finally create a hole that is slightly larger than its diameter but fairly well centered. As you continue to feed, the beveled cutting edges will seek a point that balances the load on the two cutting edges and you get your "perfectly centered" hole.

I would love to be able to watch some of the things that I have been doing for years under very high magnification to see what is really going on there.

As you describe it, that center drill is history if it is run into the material with any force. And the cutting edges on a drill and a boring bar are very different, a drill point does not make a good boring bar if presented as it would be for drilling. And boring bars are not started "blind" but in a (drilled normally) starting hole.

Only if the piece of gravel happened to have a centered point or spur on it. If the point or spur was not perfectly centered the material would want to drag it around in a circle.

Hope this helps some.

It is because tooling is not perfectly rigid. Drills are especially flexible -- center drills less so -- but the tailstock and lathe bed (of many small HSM lathes) flex significantly also. Thus if (I.e., when) the tip of the bit starts or ever wanders astray from the lathe centerline, then the tooling bends to keep the bit in the hole, even though it's off- center.

Jim