View Full Version : Chasing errors again- drill drift

01-16-2009, 12:13 PM
I'm drilling a 3/16 hole in the side of a 3/8 303 stainless rod. I clamp it in the mill vise, drill it with a 5/16 size center drill till the flared part is just under the 3/16 dia. It's centered when I check with caliper. I put in the body drill, and it's 2-3 thou off. How does it drift that far from the centering drill? Is there a better technique?

01-16-2009, 12:27 PM
Is that .002 - .003 off center or is it difference between one side and the other? Either way it is not too bad for a drilling process.

Has the mill head been trammed?

01-16-2009, 01:23 PM
Main suspect is the drill bit. If the flutes or the cutting edges aren't exactly the same depth and width it won't drill straight. I recently drilled over a foot long hole in stainless steel for the spindle of my lathe project and it is straight as an arrow. The bits I used are of high quality Sheffield Steel made in England and properly sharpened.

01-16-2009, 03:20 PM
I spent a month looking at drill bits with a loupe. Does one side need to be offset behind the other via the chip thickness? I think so..

Properly sharpened is a nice term, what exactly does that mean? it means not sharpened by hand. a bit with more edge on one side than the other, a different angle will bend off on a tangent. Deep holes are a pain in the posterior.

Stainless is a pain in the posterior.. Used to be I built control panels from stainless sheet.. TO drill more than one hole? you put enormous amounts of pressure on the hand drill, and just bump the trigger on and off.. it'd throw a curl like it was soft tin.. IF you spin it, you harden it, once it turns blue.. well it is hardened for sure.. GET UNDER the surface was one term someone here used.. on a lathe or drill curl...

Ohh when I built a template and got a plasma cutter.. I could zip a 1 7/32" pushbutton hole in with "locating notch" in a few seconds after clamping the template down..

ON my cnc.. I thought it would drill precision holes.. ahh.. the starter bit.. ahh the driling bit.. and the feed better be enough, but not so much as to arch the drill.. or.. Someone would be throwing things in the shop.. again..

Then alignment to manual punch marks.. First I did a camera with crosshairs.. then a pistol scope with a telescope 45 degree objective mirror.. THE camera worked great once I figured in the offset and left that rascal on the mill head.. I even cut a sears lasertrac apart and mounted it on the mill for quick holes. (now tied into the usb 5vdc)

Best thing is the uni-bit in the cnc to make the punch marks, then.. well it works most the time.. If these guys tell you they never mess up.. well experience means you can hide your mistakes better..

I recently drilled 140 1/4" x 1" deep holes in the cnc with a Harbor freight drill bit.. Now.. considering the quality of most their stuff that is a record. It's the new bed for the shaper/rotary.. All the hole patterns lines up with the existing southbend table pattern. That amazed me too.

01-16-2009, 04:03 PM
The best way I do it is use a center drill as you described and then a stub drill. It still may be off some and I doubt if it will ever be exactly "0" as you you want it. The effect of drill into a round part is a problem and even on a flat plate it can drift.

01-16-2009, 05:53 PM
So I guess I'm too idealistic, huh? The 2-3 thou is the off center distance. I'm using a factory sharpened stubby, I doubt my Drill Dr would do better. I understand deep hole drift, but this is into a 1/8" centered pilot, how off could it get?!

This stainless rod goes into a 5/8 lovejoy universal joint bored for it and gets a #10 cap screw as a lock pin. the actual pin drill hole is #13, I just got a reamer that size today from McMaster. Custom fitting parts together feels like failure to me, but I'll put them together and run the reamer through before pinning them.

I've had failures of this joint in the past, so I'm being extra anal. I had previously pinned it with the stainless tapped 10-32 and the U-J cleared, but ham fisted stagehands of one show in particular managed to torque the thing enough to keep shearing a hardened 10-32 cap. The original prop, on Broadway now for 5 years, has NOT sheared the pin and has needed zero maintainance, but that's being performed by an actor rather than a stage monkey.

01-16-2009, 06:40 PM
Center drill or spot drill? Center drills are for lathes to match the taper if the center. When starting a drill bit in a center drilled hole they will catch and often drill off center.

Use a spotting drill with the same point angle of the drill you use.

01-16-2009, 06:42 PM
Maybe your calipers are off

01-16-2009, 07:11 PM
A question I might ask .Is how are you locating the center of the shaft before you center drill it . Some times picking up one edge is not good enough. Pick up both sides and take half the number and move to center.

Uncle O
01-16-2009, 07:17 PM
It would be best to tram it in, edge finders are not good enough.


01-16-2009, 07:30 PM
It would be best to tram it in, edge finders are not good enough.


I got close with an edgefinder, then zeroed in the center drill with test holes in a piece of stock. The stock was set so the top was level with the jaws, I then tested by caliper from the outer edge of the vise jaw to the rim of the hole, first zeroing it on one side then measuring the difference. The jaws are identical thickness, it's a pretty accurate method.

The center drill vs spot drill is a good point. I wish I had dropped one on my M-C order!

01-16-2009, 07:36 PM
what machine are you using to drill it with .

all the best.markj

Uncle O
01-16-2009, 08:28 PM
While I would agree thet your method is "pretty accurate", it allows variables to be introduced. Calipers just aren't as accurate as a DTI, if you tram in the part you know that your spindle is over the centerline of the part. Now when you drill, if your hole is off-center you know some other gremlin is raising it's head. The bit is poor, the spindle is flexing, the table isn't locked......


01-16-2009, 08:35 PM
the spindle is flexing, the table isn't locked......


Thats why i asked what machine he was using

Those cheap Chinese drill presses or even worn out ancient camel backs etc, have slop in the spindle ..what can happen is you can put your centre drill right on the mark and your centre drill will drift over as your drill because the quill extension is so sloppy a fit in the bore....the more you extend the quill the worse it gets .

so your original centre drilled hole is off ..without you knowing .

all the best.markj

01-16-2009, 08:58 PM
Do everything with your quill locked. There is "clearance" which will cause "float" in the best of quills.

Use the knee or square column - depending on the type of mill - to raise/lower the job on a knee mill and the milling head in the case of a square column mill.

If I wanted that "start" be as accurate as I could, I would have a centre-drill in an adaptor in a boring head and "bore" the 60 degree "lead" with the centre drill (one side of the centre drill only - obviously). A centre drill is about as short and rigid as any drill. But as it will have zero back rake, take it easy.

Drilling may do for general hole sizes but for positional accuracy "boring" is required to either start the hole (position) and/or to finish it (size and alignment).

Raw drilling and its associated potential errors are a large part of having such large "clearances" in holes that have to be aligned with each other and/or for a bolt to pass through.

01-16-2009, 10:57 PM
I'm going to be perfectly Frank with you even though my name is Carl. one or two thousandths out with a drilled hole is damn good. The only way you MIGHT get it closer is drill undersize and bore it to size. I guess you would have to make a little boring bar for your boring head.

Think about it, your drilling a 5/16" hole in the side of a 3/8" shaft so the posibility of runout or drift is huge. I am surprised you get it that close the way you did it.

01-16-2009, 11:26 PM
It's true there are lots of variables, but some of those brought up in the most recent post aren't relevant, since I said the center drilling was right on, it was the drill that somehow drifted out of the center hole. I'm doing this on an 80's vintage JET 16 mill/drill with a 6" brand new Phase II Kurt clone. It's a round column, so quill is all I got. I'm the 1st owner of the mill, and I don't do production work.

What about an undersized drill, then a slow cut with an 11/64 end mill, followed by a #13 reamer? Wouldn't the end mill correct the drill's drift? I'm not actually going to do this, but it sounds good.

What do you guys mean by "tramming it center" if not the trial and error center drilling I described?

01-16-2009, 11:38 PM
Drill drift???
I thought you were talking about an actual 'drill drift'...

01-16-2009, 11:47 PM
Drill drift???
I thought you were talking about an actual 'drill drift'...

Did you think I was having problems "chasing" some decorative details onto my drift?:D