Center drills

The purpose of center drilling is to start the hole exactly where intended without the drill point wandering all over the place, yes? This is then
normally followed by a twist drill of the desired size etc. From this concept I would assume that the axes of the drills are concentric, or in other words the hole drilled by the twist drill is exactly concentric with the hole started by the center drill.
This does not seem happening in my case and I am wondering why.
Example: Using my mini-mill, I start the hole with a No.1 center drill and then change to a twist drill (say 7/64"). X and Y are locked. As I bring the drill down it is clearly off centre - today I measured it and it is quite consistent: The drill point moves 0.010" "east" and 0.005" "south" to enter the starter hole. If the full hole is then drilled it is slanted ever so slightly - perhaps 0.001" over 0.25" length. This happened with two different 7/64" twist drills.
I tried a different No. 1, I tried both ends, same result. Looking at the slowly rotating point with a magnifying glass it describes a small circle which is not obvious when I bring it down on the metal. However, there is perceptible vibration of the mill which is absent if I drill with the twist drill. I interpret this that the mill head is doing the circles while the point is embedded. If I had a more rigid set-up the circle would perhaps be apparent.
I tried the same experiment with a No.2 and No.3 - same result.
I thought I'd better find out which is the true center: The "center drill" or the "twist drill" one. This was even more complicated than I expected. I used two centere finders on small punch marks. They both showed center differently! The centre found by the barrel-type coincided with the center drill point, the wiggler type was quite significantly off (I use 10x magnifying glass to get the best accuracy with both).
So the questions at this stage were: 1) Is this a normal behavior? I thought unlikely... 2) Is this because of cheap Chinese center drills? 3) Is this a function of the mill chuck? 4) Is there some other reason?
I was wondering about the way the drills are clamped in the chuck and I tried different degrees of tightening. The last effort involving only light tightening of the chuck both for the centre drill and the twist drill I managed to hit the centre-found punch mark with both the centre and twist drill.
Is it possible that over-tightening the chuck throws things out of kilter? I hope to repeat this with the bigger center drills tomorrow but I would appreciate any insight.
--
Michael Koblic,
Campbell River, BC
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Michael Koblic writes:

Have you checked the tramming of the head?
Have you chucked a dowel pin, gage pin, or other precision cylinder, and indicated the runout?
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    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.
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wrote:

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?
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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
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wrote:
[...]
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: http://www.busybeetools.com/products/EDGE-AND-CENTRE-FINDER.html ) 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
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(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"
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On Wed, 10 Nov 2010 18:04:31 -0800, snipped-for-privacy@gmail.com wrote: <snip>

<snip> see drill blanks http://www.use-enco.com/CGI/INPDFF?PMPAGE&PARTPG=INLMK32 gage pins http://www.meyergage.com/products/individual_class_z_pins.htm http://www.deltronic.com/products/plug-gages.html dowel pins http://www.wttool.com/product-exec/product_id/39646/nm/Dowel_Pins_Brighton_ http://www.wttool.com/category-exec/category_id/12849/page_num/8
Try your local Fastenall store. Only problem is they like to sell boxes of 100. http://www.fastenal.com/web/home.ex
-- 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).
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"F. George McDuffee" wrote:

I have had no trouble buying single items from the local Fastenal stores.
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On Thu, 11 Nov 2010 00:01:45 -0500, "Michael A. Terrell"

=========Must just be my local Fastenall.
You can also use round tool steel blanks. These are ground very accurately to consistant size. see http://www.wttool.com/category-exec/category_id/15887 http://www.wttool.com/category-exec/category_id/15887/page_num/2 http://www.wttool.com/product-exec/product_id/38028/nm/High_Speed_Ground_Round_Tool_Bits http://www.wttool.com/product-exec/product_id/38029/nm/High_Speed_Ground_Round_Tool_Bits
Carbide also available but spendy. http://www.wttool.com/category-exec/category_id/15850 -- 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).
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"F. George McDuffee" wrote:

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. :)

http://www.wttool.com/product-exec/product_id/38028/nm/High_Speed_Ground_Round_Tool_Bits
http://www.wttool.com/product-exec/product_id/38029/nm/High_Speed_Ground_Round_Tool_Bits
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On Thu, 11 Nov 2010 01:16:42 -0500, "Michael A. Terrell"

[...]
I find Fastenal problematic at best.
There are two branches within reach of here. One I will not use at all. The other is more helpful but it tends to be a continuing struggle to get what one wants without having to have 100 units of it.
Furthermore, what it shows in their on-line catalog bears no relation to what is available from or through the local branches at least quantity-wise.
Not so long ago one was able to walk into a branch and pick up common stock like mild steel square rods of several dimensions etc. Now the shelves stand empty, still labeled with the items they previously contained. Yet the manager blames lack of stock on "shortage of storage space". BTW this particular branch is housed in a space the size of a small hangar with a few shelves huddled round the middle. The rest is completely wasted.
Having said that, I had good luck there yesterday, they had all the stuff I wanted including metric lock washers (at $3 I did not mind getting a 100) and a retaining compound they told me last week they would not have.
One has to count one's blessing when one finds them especially now that McMaster-Carr will not ship to new customers in Canada.
Michael Koblic, Campbell River, BC
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snipped-for-privacy@gmail.com wrote:

That branch may have low sales, resulting in lower acceptable stock levels. I have never found an empty slot for steel rod or angle at the store I use.

I buy most small hardware by the 100 package. Only oddball threads or very rarely used sizes are bought in small quantities. It isn't worth the trouble to pick up the exact quantity of something you need, when you'll need more in a couple weeks.

Canada! Well, what do you expect? Those polar bears never where reliable at delivering packages, anyway. ;-)
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On Wed, 10 Nov 2010 23:59:36 -0600, F. George McDuffee

What's the percentage of fat in these, Unka?
-- Education is when you read the fine print. Experience is what you get if you don't. -- Pete Seeger
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On 11/10/2010 10:36 PM, F. George McDuffee wrote:

Find a junk printer - dot matrix, laser, or inkjet. They all have precision ground rods within. That's what I use.
Also, for the best precision in drilling holes with my minimill, I use a collet to hold the drill. The chuck that came with it isn't very good.
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On 11/11/2010 8:34 AM, RBnDFW wrote:

Also remembered another good tool: Piston wristpins from large diesel engines. I have 3 that are about 3" dia x 6" tall. They are accurately ground and make great squares. You can set one on the table alongside the spindle and look at the gap angle between the two.

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RBnDFW wrote:

Maybe a bit large for a mini mill but the rod used on car damper struts could be of use and readily available. Last dampers that came out of my VW I kept and were 22mm OD chromed ground bar and while I could see the wear area by the finish the difference in size from the unworn sections was almost indistinguishable with a 0.01mm graduated non digital micrometer, maybe in the order of 0.001 mm to 0.002mm.
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On Nov 11, 4:56pm, David Billington

IIRC Rollie's Dad used a shock absorber rod.
jsw
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    [ ... ]

    Same thing -- different terminology.
    Enjoy,         DoN.
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