How do I drill a centered hole in the side of a turned piece?

[Tony Wells]

Using an edge finder like John does, except I just zero the DRO on the first side, and go across and whatever trips the EF, divide it in half and go to that figure. Center. If the part diameter is so large I can't get to -Z- center, I just use a stop on the quill and go to the same point in -Z- on both sides. Using only the "corner" of the EF still works the same. I've worked to within a couple of tenths this way, when pressed. That accuracy is never required on a drilled hole, however. Reamed, maybe, bored, perhaps.

I think though, that the original question was about doing it in a lathe. Those cross-drill attachments are a slick way to do that. Still have to set them on centerline of the machine, but once that is done, it shouldn't change appreciably. I wouldn't say it would be good for much better than 0.005 though.

 

[jgedde]

The way I would likely do it is to make a drill guide from a round piece turned in the lathe the same diameter as the round part to be drilled. Could be a small piece parted off when turning the original part. Drill a small hole through it say 1/8" in the lathe. The piece doesn't have to be long, about 1" will do. Set your part to be drilled in your vice supported so that it is horizontal to the jaws and set your guide on top of it where you want the hole drilled and tighten your vice. Now just align the piece under a 1/8" drill bit in the chuck and slide your vice around until the drill will slide up and down through the guide. You are now dead centered with no fuss. Once you have this pilot hole drilled just loosed the vice enough to remove your drill guide being careful not to move the piece being drilled, retighten the vice and finish to what ever size you want.
Nick

Clever! I like it!

Your suggestion is an old-timer's trick for sure! In the days of CNC and DROs, these tricks are being lost to the world.

I also love the idea of a cross slide mounted drill attachment... I wonder if one could be designed universal enough to be a group build? It certainly could for those of us who use AXA QCTPs. The driving shank could be 1/4 or 3/8" to allow any hand drill to be used... Now that's a tool we would use!

As far as my DTI method (I'll call it the "Manning Manouever" in honor of the guy who taught it to me)... I'm so used to being in an environment (at work) where the guys routinely work to 0.0005" (and sometimes as tight as 0.00007" - yes, I meant, 70 millionths to 0.0002) that I get hung up on precision techniques and lose the obvious. But, now the method is on record for those who may need sub 0.001" location of a center hole...

John

 

[jpg366]

Would it make sense to use a small end mill to make a flat, once the alignment is done? Then drill?

 

[george wilson]

I have never used that tailstock crotch drilling piece. It just looks dangerous to me!! IF drilling brass with the crotch,I'd DEFINITELY grind vertical edges on the cutting edges of the drill bit. If it were to grab,which is almost certain with brass,it could get pulled out of the crotch and twirled ,breaking the drill if it hit the bed,or messing up your hand if it could rotate 360º. In fact,I'd grind the drill vertical no matter what material I was drilling. But,I'd only use the crotch if there was absolutely no other way to drill the rod. Even then,I'd figure out some OTHER way to drill it!!!

 

[pdentrem]

The crotch center is no worse than drilling on the DP.
As long as the piece is longer than the distance from the hole to the ways it will not spin. I always brace against the DP post or a block, vise, hold down bolt, vise grip, c clamp etc what ever is attached or can be to the drill table works for me.

 

[HSS]

Since we're drilling holes, I try to explain how an old machinist showed me today. We were putting a grease fitting into the end of a 12 X 2 round pin that holds a thumb on the stick of a mini-excavator. I drilled down the center of the pin 7 1/2" with a 5/16 drill bit, bored the same end with a 3/4" drill to counter sink the grease fitting and moved over to the DP. I needed 3 holes located 2", 5" and 7" from the end to meet the center hole drilled down thru the center of the shaft. The way he showed me was to center punch the shaft where you wanted the holes, (they don't have to be in a straight line) and start a hole with the bit you want to use. After you've drilled in a little way, back the drill out and check the depth of cut on each side of the hole. If you are drilling inline with the center hole the cut on the sides walls of the hole you are now drilling will be equal. If one side is deeper than the other you rotate the shaft toward the shallow side and drill a little farther. check again and if the sidewall of the hole is the same, drill thru, if not readjust. We hit the center on all three holes drilling them this way.
Patrick

 

[Phils69]

Like the others said, If you are doing it in a mill use an edge finder and this should get within .001 if you have some practice on it AND are using a DRO. Find the first edge, set Y to zero, find the opposite edge, divide that value by two and move to that #. Once you are where center should be set your dro to Y zero. I ALWAYS go back and find each side then to verify that the #'s are the same in Y- and Y+. If you are doing a hole that requires extreme accuracy once you have the Y zero where you think it should be use the indicator methods described above to load the indicator to zero on each side of the workpc to verify the Y's are the same in Y+ and Y-. The best way to verify,if there is room in the setup, is to move over in the vise and drop down where the part is not in the way and keep the Y stationary. Then swing an indicator touching the front and rear jaw just rotating the spindle. Do a slight adjustment to where the indicator is reading zero on each jaw. You can also plunge end mill a little flat then center drill it prior to drilling. Do realize though that the bottom of a standar end mill is not flat but it's a lot flatter that round material.

If you do not have a dro it is a different beast. You don't want to move the Y in two diffent directions when you are finding a zero on a part using the graduated hand dial. The backlash in the ball screw will screw you every time, pun intended. If possible use the drop down method above and indicate the Y until you are centered on the opening. If that is not possible hold a gage block against the rear jaw and move the Y until the edge finder touches the gage block. Zero the handle at this point. Mic the part then move over half the part and half the edge finder and that will put you there. Drill one pc then adjust as needed. Remember when doing this only move the Y in one direction or your ball screw free play will give you an error.

 

[Brandon]

The way Jgedde described it is how I've always done it. It's also one of the best ways to cut a pocket keyway. It takes time, but if you want it dead nuts that's the best way to go. Like I've always said, if you ask 5 machinists how to do something, you will get 30 different answers. That's the beauty of this trade.