Lathe hole drilling off center, Why?

[Mr Fixit]

Hi Group,

I have a Southbend 9A lathe that I was drilling some .75" CR round stock that was 2.75" long. I used a center drill in a fixed holder in the tail stock to start a hole.
Then I used a 25/64 drill that was new (manufacturer sharpen) and my Jacobs MT2 mounted drill chuck, then I drilled with frequent cleaning of the chips and cutting oil. The start end is on center but the finish end is off center and I'm trying to figure out why.
This was not a NASA piece but, It was off enough that I could see it without measuring, which bothers me. It will work for the need, but I want to solve the problem if I can to prevent future problems.


TX

Mr fixit for the family
Chris

[chipmaker4130]

How'd the chip look coming out of the hole? Even (approximately) from both flutes? You purged chips regularly, but how hard did you push the drill otherwise? What was the drilling speed? Factory grinds are usually great on US or other major brands, not necessarily on chinese imports. What did the drill point look like after the hole was drilled?

The #1 suspect so far is still the point grind.

[J Tiers]

It can happen if you force the drill too much. Usually you want to go easy, the easier the better if you want the hole to stay centered.

It can happen if the drill is deflected by differences in hardness inside the part.

Using a regular jobbers drill is typically more of a problem. they are quite flexible, and can wander more easily. I like to use a drill with more solid shank, and a shorter fluted section, as a starting drill. They go straighter, and following drills usually do not wander off of an existing hole.

A jobber drill that has unequal lips is bad, because it starts off with a tendency to be off- line. Chances are it may wander off at some point and stay off-center because the flexibility means it does not tend to cut straight as much as a short flute drill

If the drill is stiff, it tends to cut toward center, because the stiffness pushes it that way.

[digr]

What did you have the piece held with, chuck, collet,??

[JoeLee]

I would try piloting the hole first with a 1/8" drill as deep as the drill will go, then work your way up to the 25/64".
Sometimes in "crappy material" like hot rolled trying to drill a hole with out piloting can lead to wander. I like to use the short screw machine drills because they wander less.
Once you get a straight hole started the drill should follow since it has support around it.

JL....................

[Mr Fixit]

I held the stock in a 3 Jaw chuck as that was the last setup. Yes the tip was in the same shape as started and showed no chip or wear on the cutting edges. The chips came out as flakes or small spirals from both sides equally. The drill is a USA drill with the extra relief at the land for a crankshaft or deep hole drill.
Sounds like I didn't let the drill bit do its work, and probably pushed it too fast.

TX
Mr fixit for the family
Chris

[Rich Carlstedt]

You don't say how much drift you had ?
There are many reasons for the drift and some comments made by others are spot on !
Excessive drift is considered to be more than .015" per inch. so if it was ( 2.75x .015) over .040" from center, then it is excessive.
Center drills are for centers , not necessarily for starting drills, although everyone seems to use them that way
A far better way for accuracy is to use a "old timers" technique that gives dead nuts centering , and eliminates the use of a centerdrill.
I don't see guys doing this, but in the old days , job shops did not want to waste time changing out drill chucks and taught this method.
It takes a bit of skill but gives great results.. It helps even more if you have any kind of quick change toolpost ( Aloris ie)
Do this: ALWAYS mount the drill bit so the cutting flutes are horizontal ..Imperative , and a good practice.
Enter the drill bit into the turning workpiece about 2/3'rds of diameter. When you are in about that far, stop feeding and raise your tool bit ( right, the lathe tool bit !) about 3/16" above it's normal position by loosening the post clamp and just lifting the tool block . Now , run the tool bit up against the drill bit and then give it ( the drill) a nudge off center ( visually seen. maybe .010 to .030 or more for bigger drills) .
Now feed the drill bit in ( It will be cutting on one flute only, the rear flute) and as you approach full diameter, back out the tool bit with your other hand. The move together is all of maybe two seconds , and the skill needed is the coordination of feeding and backing out.

Now what happens is this. the drill bit is started into the piece without a centerdrilled hole. It may or may not be on center - no matter !
The tool bit forces the drill off center and thus it only cuts on one flute and creates a conical hole with a tit in the middle. At first it may vibrate if the starting point was not centered--no matter, wait for it to stabilize, and then fed in while relieving side forces. The angle of the flute will allow the drill to follow it's own centered hole form and the tit will be knock out as the conical shaped hole provides support for the drill bit . In other words, the cutting force for the tit is less than that required to remove stock on the larger diameter.

Of course this is not a solution to all the drill ills, but is a darn faster way of getting the drill centered and in the proper location and is really quite simple when you get the hang of it.

when you look at your drill bit , choose the following point for you lathe support. Drills have different helix angles so there is no magic number . Since the cutting edge is horizontal , chose the next horizontal point of the drill- which is back 180 degrees from the cutting flutes to place the tool bit support. This is the most even point that will not slip due to the helix angle. If you do not want to use a tool bit, any block/rod will work. If you use carbide insert tooling, just raise the tool holder high enough to not allow the carbide to touch the drill bit. When done, loosen the clamp and let the tool slide down and you are good to go.

Rich

[RichR]

In high school the teacher showed me how to use the tool bit to create a centered dimple for starting a drill bit.

[J Tiers]

If you start the drill. but go slow for a bit as you come up to cutting full diameter, you will end up using the drill as a sort of "single point tool" if it is off-center. If you let it cut until it just stops cutting and is about to rub, then feed in a little, you will end up on-center after a few of those cycles, assuming it is fed on-center. Do that until you have it drilling at full diameter.

It does not take too long, and it always ends up on-center for me. Does take a sharp drill. but dull drills tend to wander because of excessive pressure..

For that technique, I prefer the cutting edges more vertical so the swarf falls away and cannot collect on one edge and possibly cause a force to make it go off center. That may not be necessary, but it does not seem to hurt.

[darryl]

I do pretty much as Rich described. It works. In my experience you can never rely on a drilled hole being concentric as many factors contribute to the wander. But this method gives about the best result for me. Depending on the final size desired, if you can bore after drilling you'll be good. If the hole size doesn't allow for that, then you can always make (or buy, presumably) some reamers or D-bits.

Getting the hole started perfectly on center is your best bet if you are going to drill only. The best way to do this is to use a tool in the tool post to actually machine the starting point. Using a stub drill bit or a center bit in the tailstock is no guarantee that you'll get perfectly centered, though it helps a lot.

If you're going for a fairly large hole, you might start with a pilot hole, then bore that out some. It may be quicker to then follow with the final drill bit, but at least you have a full well-centered through hole to guide it. I do this quite often if the hole is less than an inch deep or through, using a boring bar that will start working in a 1/4 hole. I drill with a 1/4 inch bit, then use my solid carbide 1/4 inch boring bar to clean that up, which is making the hole about 5/16. A 1/2 inch bit will follow that nicely, and it's quicker than boring the rest out. Of course you can always drill undersized and finish by boring if you need the best finish and accuracy.

JT added another insight here, that of taking off the rake angle at the very edge of the cutting lips. I do this for brass and plastic, and it allows for a more precise hole in addition to it's benefit in the other materials in that it doesn't grab.

[J Tiers]

.....

JT added another insight here, that of taking off the rake angle at the very edge of the cutting lips. I do this for brass and plastic, and it allows for a more precise hole in addition to it's benefit in the other materials in that it doesn't grab.

No such thing. Never mentioned it, did not intend to in this context.

But it is a good plan for some brasses etc where the drill may dig in.

[CCWKen]

Maybe the tailstock isn't centered or the extension isn't parallel to the ways.

[BCRider]

Maybe the tailstock isn't centered or the extension isn't parallel to the ways.

I'm rather surprised that Ken is the first to suggest that the lathe should be checked to see that the tail stock is axial to the headstock in all respects. I'm thinking along the same line as well. It may be worthwhile doing some testing with a dial indicator and a slug in the chuck that is cut to a size close to the diameter of the tail stock ram to use as a reference.

[J Tiers]

When the OP said (IIRC) that the drill entered on center, it appeared less likely it was off. Usually they droop, and end up off center low.

BUT, mine is to some extent, yet it drills on-center and does not drift much in even deep holes. Even with the droop, I have drilled a quarter inch hole 8 1/2 inches deep, and was off less than 10 thou in that distance (making a new spindle for a small lathe). I am not convinced that it is a really strong cause of drift if you get the drill started on-center.

[macona]

Using a center drill can cause drills to start off center. Use a spotting drill instead and just make a small indentation.