Center drills

Have you checked the tramming of the head?

Have you chucked a dowel pin, gage pin, or other precision cylinder, and indicated the runout?

Actually -- that is the purpose of spot drilling -- with a spot drill, not a center drill.

The purpose of a center drill is to make a conical depression with a precise 60 degree angle and a bit of clearance to allow room for the point of a live or dead center to stablize the end of a workpiece in a lathe.

Yes, people do use center drills for the purpose of spotting drills -- but that is not what they were made for.

Hmm ... sounds like a tramming error -- the axis of the spindle is not perpendicular to the bed. And likely the column axis as well. Your drill is longer than the center drill, so you have to move the head up the column. If it is not perfectly vertical, this will drift the center line of the axis to the side -- and front to back as well. The column on a lot of the small mills tilts, so it provides adjustment for the side-to-side part. However, the fore and aft takes a bit more tricky work to get straight -- using shim stock is the usual approach.

Get a machinist's square (a blade with a heavy right-angle piece) and resting that on the bed, check for a slight angle between the blade and a piece of precision ground rod in the chuck. (or better, in the collet, as chucks can introduce errors too.) (Actually -- get *two* machinist's squares, and check them against each other as well. They can come out of square to start with.)

Is yours one of those whose column has a pivot at the bottom, so you can drill holes at an angle? If so, have you checked that the column is truly vertical?

Or is yours one with a head which pivots on the carriage which moves up the column? In that case, that might not be set quite right.

That is either runout in the chuck, or a bent arbor for the chuck. I forget -- does your machine use R8 collets? If so, how are you holding your drill chuck? An R8 arbor, or a straight shank arbor held in a collet? In either case, the arbor might be slightly bent.

If you had a more rigid setup, the center drill would simply make the hole a bit oversized as it swept around. And depending on the orientation of the flutes to the eccentricity, it would either rub hard, or cut oversized like a fly cutter.

Did you try a runout indicator on the shank of the drill? How about with a longer rod (best would be a drill blank -- or an unusually long dowel pin, but you might get away with some drill rod if it has not been bent.

I presume that you started with the wiggler a bit off center, and pressed a finger against the side until it stopped wiggling? *That* is what sets it on center.

Possibly the chuck or its arbor -- possibly the tram of the column and head. More likely, a combination of the two.

The chuck or its arbor will produce the circles.

The tram of the head will produce the shift of the center point as the head goes up and down.

The proper way is to tighten equally using all three holes, one after the other.

Hmm -- the chuck is not a firm fit on the arbor? The arbor is not a firm fit in the spindle?

First get the runout indicator and check for runout on a ground hardened rod (e.g. a drill blank or a dowel pin) both just where it exits the chuck and down closer to the end. If it is better where it exits the chuck, then the arbor may be bent. If it is equally off down the length of the rod, it is likely a bur on one of the jaws in the chuck.

Lots of things to check.

Good Luck, DoN.

On Tue, 9 Nov 2010 18:27:53 -0800, "Michael Koblic" wrote:

First -- center drill and spotting drills are like everything else, you can get cheap ones.

While center drills are commonly used for this purpose, they are intended to drill holes for lathe centers for turning between centers. While 60 degree is the most common because most lathe centers are 60 degrees, 82 degrees and 90 degrees are also available. the 60 degree center drill is the least expensive because it is the most common. for examples see

?PMAKA=369-1830&PMPXNO=2621141&PARTPG=INLMK32 The more correct tool is what is called a spotting drill. These come in several included point angles and should be matched to your drill bit geometry, i.e. 135 degree or 118 degree point. These come in several sizes and if you use one slightly larger than the drill or limit the depth to limit the size of the taper, you will get a chamfered edge. These come in 90 degree, 120 degree and 150 degree included angle. for examples see ?PARTPG=INSRAR2&PMAKA=990-1480&PMPXNO=3007493These drill tend to be stiffer and last longer as these do not have the small "tit" like a center drill that isprone to drift or break off.

for a discussion see

Depending what you are doing, the standard jobber length drill may be introducing run out and is more flexible than a short drill like a screw machine length. I suggest you get one or a few good screw machine length drills with 135 degree split points and give these a try with the spotting drill. Most likely you wont need a complete set, just the sizes you use for you usual tap drills. for examples see The sites are examples only and most any mill supply should stock.

Let the group know what you discover for your shop.

-- Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).

I've seen those problems and think I've traced them to wear and play in the quill and dirt or an accumulation of tolerances in the chuck tapers and jaws. My cheap drill press shifts visibly when I pull on the handle enough to make a drill bite into steel, and the loading on the quill rack changes from supporting it to forcing it down.

I've see a center drill make the mill head vibrate, don't remember which chuck but the fix was clamping the drill in a collet. Possibly there was a chip in the taper?

You can check runout and shift under load by indicating a chucked rod, and quill perpendicularity by chucking the indicator and running it down an angle plate. This isn't the same as tramming it since the spindle axis may not be parallel to the quill.

jsw

A very useful piece of tooling for checking various machine problems is a drill or reamer blank, as suggested by DoN.

The blanks I have are Cleveland brand, and are fairly precisely round, straight and have a conical point on one end. Several sizes of blanks will facilitate checking many different conditions of chucks, and different sizes of collets.

Clamping a blank in a chuck or collet can reveal several aspects of center-of-axis and runout, and likely to be very helpful in indicating a condition of direction of travel of the inaccurately placed drill in this situation, shown clearly by using a square or angle plate as recommended.

Some folks may recommend a rod from a printer, which may be ground fairly accurately, but may also have worn spots (probably near the center of the length) from many cycles of the printer's head assembly.

The chattering can be expected for the stated drilling situation, as one cutting edge of the drill is very likely catching metal before the other cuting edge, beginning the rapid rotational skipping/flexing of the drill. After the centering alignment issue is corrected, chatter may still occur since the 60 degree sides of the center drill's hole don't match the angle of the drill's point. One method that eliminates chatter is lightly contacting the drill point with the workpiece before the power is turned on. When the power switch is flipped on, adding moderate down-feed pressure will generally cause the drill to cut without chatter.

I prefer to use a straight-drilled pilot hole, that is approximately the same size of the web section of the larger drill. This eliminates the need for the chisel point of the web in the large drill to displace the metal directly under the web. Split-point drills generally don't need a smaller pilot hole (with or without a center punch or prick punch mark), since they begin to cut as soon as the points contact the workpiece, and continue to cut without having to force the web of a conventionally ground twist drill into the workpiece.

As mentioned elsewhere, your mill needs to be trammed. If your drill bit does not project (stick out) the exact same length as your center drill, they will not hit the same center of the work piece. The offset will be proportional to the tangent of your tram angle error.

Ivan Vegvary

Very informative as usual Don.

***** Wiki Center drills, numbers 1 to 6Center drill bits are used in metalworking to provide a starting hole for a larger-sized drill bit or to make a conical indentation in the end of a workpiece in which to mount a lathe center. In either use, the name seems appropriate, as the drill is either establishing the center of a hole or making a conical hole for a lathe center. However, the true purpose of a center drill is the latter task, while the former task is best done with a spotting drill (as explained in detail below). Nevertheless, because of the frequent lumping together of both the terminology and the tool use, suppliers may call center drills combined-drill-and-countersinks in order to make it unambiguously clear what product is being ordered. They are numbered from 00 to 10 (smallest to largest).[9] *****

Hmmm... Would it not lend itself then to the thought that even on a lathe you should first spot drill and then center drill?

In my experience a #2 center drill is stiff enough to start a hole at the spindle axis even if the punch mark is off, while a 1/8" spotting drill will shift slightly into the punch dimple. Which one is better varies with the job and the layout accuracy.

My personal preference is a #3 center drill 4" long, to avoid cranking the table up to spot and and down to drill.

jsw

[...]

Windows Live Mail has done it again. Not only did it swallow my original post but also your reply. Fortunately I now have two other news readers.

That's the one. Last time I did this it was within 0.001" from end-to-end and front-to-back. I will do this again - but first have to clear things off the table. Oh, joy! The opportunity to re-align the vise. Again :-)

[...]

Before I read your post I checked the run-out just under the chuck. No dowel pins or drill blanks here so I used the drill shanks. I did two just incase. Both (3/8" and 9/64") showed TIR 0.0005" about 3/8" under the chuck. BTW my spindle is MT3. I have never had to take the chuck off so I am not sure how it is attached at the other end. I just change the whole thing for a collet chuck when I need to.

I was out of town today and took the opportunity to look for dowel pins etc. No luck.

I shall go through the drawers and see if I have any unusually long drills or anything else which might do the job.

Yes, but then you have to bring the point over the mark. I find that challenging without the magnification. Even using the other center finder (this one, so there is no confusion:

) the alignment of the two parts of the barrel are tricky and much easier with a magnifying glass.

BTW I thought that the wiggler should be more accurate. However, it was the other one that was consistently agreeing with the center drill entry point.

[...]

I would go round al of them 2-3 times. With the No. 1 the feeling is of never really tightening enough. With, say, 1/4" drill you go around and you know when the thing is tight. There is a definite "stop". With the No. 1 there is a sort of springy feeling even after you have been around 3 times. That is what made me wonder about the possible distortion caused by the chuck. I thought this was supported by the fact (yet to be confirmed) that if I tightened only lightly the center drill run straighter (see the last effort where the center finder, center drill and the twist drill all managed to coincide).

[...]

Tomorrow is a good time. After 11 AM...

Michael Koblic, Campbell River, BC

Thank you very much for the references.

I recall some time ago someone here mentioning spotting drills as being distinct from center drills. For some reason I did not investigate this further.

The forum discussion is most valuable and I bookmarked it. I read about half of it and will finish it in peace. Then I will read it three more times to make sure I understand it fully :-)

The drift seems to be that a) spotting drills are the correct tools for what I have been doing, b) they can be ground from old twist drills, c) the center drills are self-centering if used on a lathe as intended, and d) one could therefore conclude that their behavior in a drill press/mill such as I describe is entirely possible.

I do have a set of machine screw drills which I use exclusively on my mill and in the Taig, one of the reasons being the space limitations. I should add that they behaved similarly to what I described with jobber drills.

Still, all of this is a good excuse to go through the tramming, run-out-etc. tests again.

However, I have some broken 1/4" drills lying about so I shall have a go at converting them into spotting drills.

Michael Koblic, Campbell River, BC

.

A 120 degree spotting drill makes it easy to calculate the spotting drill depth if you want to pre chanfer the hole. The chamfer diameter will be twice the drilling depth of the spotting drill. If you are drilling and tapping a 5/16 rhreaded hole you would drill half the 5/16 depth plus the added depth for the chamfer.

John

[...]

I started to repeat the tests on my drill press to see if it behaves differently from the mill. I did not do enough to come to any conclusions. I have to check if my drill spindle is MT3 also which would make it simple to exchange the chucks.

Michael Koblic, Campbell River, BC

(snip)

When I use the pointy end, I feel the joint with my finger nail. It is easy to tell if it is overhanging or under-hanging a small fraction of .001"

see drill blanks

pins pins Try your local Fastenall store. Only problem is they like to sell boxes of 100.

-- Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).

I have had no trouble buying single items from the local Fastenal stores.

Hmm ... not from Microsoft I hope? :-)

[ ... ] [ ... ]

And with the head moved up and down?

:-)

Pretty close. What happens to the runout when you shift from light to tight tightening of the chuck?

BTW Drill shanks are soft, and the chuck's jaws can bite into them so they won't slip. End mill shanks are hardened and ground smooth so they work well in collets or end mill holders, and are *not* best used in a drill chuck. (The same applies to the center drills, FWIW.)

The soft shanks on drill bits also means that it is possible for the shank to bend, so the tip will describe a circle even if the chuck is prefect.

And -- you are unlikely to find a long drill bit which has a shank long enough to allow runout measurements. (Aircraft drills are the exception -- but they may be too long for your machine.)

O.K. So R8 is not in the game. It is possible that the arbor (MT-3 to whatever Jacobs taper the chuck uses) is bent -- but less likely with that 0.0005" TIR. This then suggests that the jaws in the chuck either have burrs in them, or there is wear in the body of the chuck. Different brands of chucks have different quality levels. The Albrecht keyless chucks are very accurate and can handle a long service before they start showing problems. (But they are larger, so they leave less space between the spindle and the table, so check for that too.)

What happens if you use collets for the center drill and the plain drills? (Assuming that the plain drill is of a size which matches a collet which you have.)

They are hardened and ground -- but fairly short usually. The best thing is drill blanks, which are hardened and ground to precise diameters -- but much longer. (And quite expensive, which is why I don't have an index full of the standard sizes. :-) But that length makes it easier to check for the drill chuck holding the drill at an angle, thus causing the circle. Measure runout just below the chuck, and close to the end (but before the diameter starts to fall).

[ ... ]

Aha! This is what I was answering somewhere above. You are unlikely to find a drill whose smooth shank is long enough. Usually the extra length is in the flutes.

[ ... ]

So -- use magnification -- and a small LED flashlight for illumination.

And if the axis of the chuck is not parallel to that of the spindle, you will still have the point wandering in a circle. One of the reasons for the sliding offset is so you can push it while spinning slowly until the point stops moving in a circle. That is the center of the spindle axis -- but perhaps not the center of the chuck's own axis.

But both of those are longer than the center drill, so you will have had to move the head up to get them in the space. And, BTW, it is possible that your column is truly vertical, but the head is slightly off so its axis is at a tiny angle to the column. Another reason for measuring with a machinist's square and a drill blank.

Yes -- as someone else suggested, hardened steel rods from dead printers (or some older dead disk drives) can be used in place of the drill blanks -- if not badly worn.

Give in the chuck somewhere -- a jaw with a chip under it, or something similar. Try another chuck. Or check with collets.

Your No. 1 is not fully deep in the chuck, so you are only holding it with the tip of the jaws -- and they may be flexing in their ways. Try another chuck. Yes, more money for a chuck *does* mean a more accurate one.

Yes -- flex of the jaws in the body when you don't have a shank contacting the jaws the full length into the chuck is a possibility.

Again -- try collets.

Good Luck, DoN.

========== Must just be my local Fastenall.

You can also use round tool steel blanks. These are ground very accurately to consistant size. see

Carbide also available but spendy. Unka George (George McDuffee) .............................. The past is a foreign country; they do things differently there. L. P. Hartley (1895-1972), British author. The Go-Between, Prologue (1953).

It could be. But then, I've only ought from them for a couple years and some things might be by the pacakage only.

That is a comon purchase for me at Fastenal. :)

Two birds with one stone:

jsw