Tips for collinear center drilling

[implmex]

Hi All:
I wrote in the other thread:
Hi All:
I have read this thread with great interest; here's my take on the subject.
Facing/centering machines like those shown by Emanuel Goldstein have two opposed spindles that are supposed to be exactly co-axial.
The shaft never moves...only the two heads move toward one another.
So no matter how rough or bent the stock is, the centers will always be concentric to one another even if they are not concentric to the OD of the shaft.
This is the desired state for all of the reasons EKretz points out.

You CANNOT achieve the same thing as soon as you move and re-clamp the stock; I don't care if it's on a horizontal, or a vertical mill with a right angle head, or a lathe with a steady.

If your bar is pretty straight and pretty round you can get decently close, but it's ALWAYS a crap shoot.

For most general turning, you can fudge it a bit and do OK enough that no one is going to reject the job.
But if you need to split tenths, you need better than you can get with a bodge, which, as some have pointed out is why facing/centering machines and center grinding machines and center lapping machines even exist, and it's why volume manufacturers are willing to drop millions into this capability.

So in order of desirability:
1) a facing /centering machine
2) A Horizontal big enough to swing the shaft on a rotary so it never has to be un-clamped.
3) A bridge mill big enough to swing a right angle head and still get to each end.
4) All the methods that accept drilling one end, un-clamping the blank, flipping and re-clamping it, and drilling the second side.
This includes lathes, horizontals, drill presses vertical mills, etc etc.
Way way down on the list is a center punch mark and a pistol drill...but in a pinch for a low tolerance part or a shaft that can have the centers re-trued in the lathe with a steady and a boring bar before finish turning, sometimes you do what you have to do.

Having said that, I wouldn't bother centering both ends...If I need really good centers, I always center one end, grab the other in a 4 jaw chuck, turn two spots for a steady, (one close to the chuck, one close to the tailstock.)
I mount the steady and taper bore the center true, then re-set the center and skim the area I had the steady on to make it as concentric to the taper turned center as I can.
I skim the other steady rest area again too, just to be sure all is concentric, and to make the diameters identical (makes setting the steady easier)
Then I flip the bar and dial in the headstock end on the area I had the steady running on originally using short soft pads in the 4 jaw chuck so the part can align itself to the steady without bending.
I support the new tailstock end in the steady, then center drill and taper bore the second center.
I live with the outcome knowing it is pretty good considering I violated a basic rule and flipped the bar to get the second center in.

This is all a lot of fucking around, but it does a pretty good job of making co-axial centers that are properly conical and can support precision work.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[eKretz]

I’ll accept that you are probably aiming for a centerline but you will also need some feature beyond the centers unless the entire work is done between your perfect centers. We’re kinda saying the same thing, maybe.

Not if it's done on a horizontal mill with a rotary table or a facing and centering machine. In the case of the lathe, yes, the spot cuts to run the rest on are those features. They are both cut in one chucking and preferably at the same diameter. I was going to add that about us basically saying the same thing earlier also as well, but forgot.

Very few job shops (more my forté) have I encountered that have double ended simultaneous facing and centering machines. Those are generally more in the realm of very high production shops.

Marcus's emphasis on being sure the part is not set in the chuck with a bind is important and worth noting also. This is especially important on short, very rigid parts and long flexible ones. The short rigid ones can easily be so stiff that they pull a live center around and cause it to run out. The long flexible ones can cause major bow and runout in the center of the shaft.

 

[michiganbuck]

When I want a center to be very good: I 4-jaw the part, indicate its OD to near zero. scalp the center with a tool bit, and then center drill it.

Flip it to the other end and do the same.

That is way overkill and only for a part that has zero stock to take on the OD.
and still, it won't be perfect if the chuck has any wobble because the center drills 60 won't be dead square with the part length (perhaps 6 to 20 millionths off est). That is where the Norton lap makes true center to center for the angle of the 60.
I have never had a perfect center drill and lathe that would get .001 center location.
Agree rocket-science precision nobody would want to pay for.

I have a rocket science friend, Gregory S..and he would laugh at this much overkill.
But have had to do almost that to repair an error made, or a repair precision shaft. spend a half-hour getting center back to the center.

 

[eKretz]

The closest I can get is to run in the roller rest and tool the centers at both ends with the compound. From there if you need it closer you'd have to lap it.