View Full Version : Reboring existing hole off set 1/2 diameter

02-19-2018, 08:15 PM
New to forum, don't really know much about machining or proper terminology. Have a Lathe/mill combo from China.

Anyway, I need to offset an existing .5625" hole in 1/2" thick cast aluminum, one half hole diameter, so moving the hole .28125", while keeping the original hole where is, so as to end up with sort of an ellipse. Can I use a standard 9/16" end mill to plunge drill/cut this new half hole?

Thanks in advance.

brian Rupnow
02-19-2018, 08:20 PM
Yes.--Of course it depends an the material and how deep you need to plunge.

02-19-2018, 08:33 PM
Hi, and welcome to the forum.

Yes, what you propose to do will work. A word of caution, though. Lathe/mill combo machines are not famous for being overly stout, so it might be best to make several small cuts, rather than moving to the new position and plunge milling the material all at once. Be sure to lock the table in position before making the cuts. This is especially true for which ever axis that is not being moved, as only cutting half a hole will cause the end mill to climb into the material.

02-19-2018, 08:35 PM
Keep in mind that you will probably not get a perfect .5625 inch hole from plunging a 9/16 cutter into a spot where there was already a hole. It will be very close, but any flex anywhere in your setup (or in the cutter) will make the hole several thousandths too big. A dull cutter will also change the size or shape of the hole.

I noticed that you are using a specification of ".28125" inches. That's accurate to 1/10,000 of an inch. Generally a mill or lathe imported from asia can be counted on for an accuracy of .001 (1/1000) if you know what you are doing.

But yes, in general, a 9/16 inch "center cutting endmill" can be used to plunge a hole 1/2 inch deep with reasonable accuracy. Snug up all the table locks (X, Y and Z) so it does not move as you plunge the cutter into the work.


02-19-2018, 08:47 PM
Also, an end mill always makes an oversize hole. You might want to hog out most of the material with a 1/2" end mill first, then finish with the 9/16. The end result will look better and be closer to size.

02-19-2018, 09:06 PM
Thanks all. danlb, can to tell me the difference between a standard and a center cutting mill? I got the .28125 by (.5625/2). If I get this operation within .003, I will be a happy camper!

02-19-2018, 09:14 PM
Look at the bottom of the endmill, if there is a hole in the center it is not "center cutting" if the cutting edge touch in the middle it is. With a "center cutting" endmill you can plunge straight down in material that has no hole already in it. With a non-centering cutting you can't because as the name implies, you are not cutting with the center of the endmill. If you have a hole already there that is bigger than the non cutting center you can plunge down or in your case you could move 1/4 the distance and plunge down and then move the other 1/4 and do it again.

02-19-2018, 09:32 PM
.003 is quite doable. I see Oxford already explained the "center cutting endmill" quite nicely.

Make sure that you use a collet to hold the end mill so that it does not spin off center. An even better choice is an "end mill holder", but those take more room and you are more likely to have collets already. Do not use a drill chuck if you can avoid it.

02-19-2018, 10:12 PM
Thanks, I know more already! I need to use the mill in the lathe chuck and move the work into the end mill. Due to the size of the piece, this appears the only way to perform the operation, as it is the only way I can obtain the bore angle needed. The lathe chuck has almost no runout.

02-19-2018, 11:31 PM
I once "adjusted" the locations of a number of 9/16 inch bolt hole pairs in the web of four inch structural steel channel with an ordinary 9/16 twist drill. The trick was two hardened steel drill bushings (https://www.google.com/search?hl=en&tbm=isch&source=hp&biw=1355&bih=910&ei=waCLWsyUNcrszgLWh5cg&q=drill+bushing&oq=drill+bu&gs_l=img.1.0.0l8j0i10k1j0.2732.5754.0.11244.8.7.0. A) pressed into c-bored holes in a piece of oak shaped to a tight fit between the flanges of the channel and clamped in place. A 12-inch Delta bench drill press was all I had and the results were perfect.

David Merrill

02-20-2018, 12:29 AM
It would probably go better to either first move over 9/23" & plunge cut with 3/8" or 1/2" end mill, then finish with the 9/16" end mill. Or, alternatively mill over the 9/32" distance with the 9/16" end mill in a couple of steps.

old mart
02-20-2018, 02:42 PM
Creeping up on it is the best way to go, I have had endmills cut a considerably larger hole than expected. Solid carbide is stiffer and less likely to end up with an oversize hole. Take small cuts slowly.
If you have a spare piece of aluminium to use as a test first, then use it.

Paul Alciatore
02-21-2018, 04:05 AM
A good quality, preferably solid carbide, end cutting end mill will drill a hole that is very close to size. I was making some clamps that utilized split cotters at an angle to the outside surface once and that was the only way I could drill the holes and keep them accurate and on position. Using a drill, even with a milled flat to start it on, produced an oversized hole that wandered off course. Remember; good quality, solid carbide, and end cutting.

Oh, to add to the good description above, an end cutting end mill has only one cutting edge that goes to the center. And unlike a drill bit, that edge goes all the way to the center. The other(s) will stop short of the center, usually by different amounts.

02-21-2018, 10:16 AM
Be sure to use some lubricant when cutting your aluminum. wd40 is what I use for milling, drilling turning. I use wax for sawing and sanding.

Harder alloys and heat treat can sometimes be cut dry but softer alloys can be troublesome and require lubricant.

02-21-2018, 10:51 AM
Nobody has mentioned plugging the existing hole, boring the new hole and removing the plug. something like blue Locktite would hold an aluminum plug while boring the hole, (and reaming if necessary,) and a bit of heat will remove it.

02-21-2018, 11:35 AM
I used CAD to draw the two holes to size and in the relative positions. If you do it in one plunge , it will leave you with a portion of the groove that is only about .487 across. So small steps will give you a better result.

02-21-2018, 02:16 PM
I just went through a similar thing with an aluminum fixture. I had to move a hole over half the diameter of the original (.404 or Y drill) hole. In this case I used an end mill to make the original size hole 1/2" (.500) in diameter. Then I took a piece of aluminum round stock .502 in diameter, cut it to length, put a small chamfer on the lead end, and pressed it into the hole. Then I moved over half the diameter of the old hole. I started with a 1/4" center drill, then followed through the piece with a full length 1/4" drill. I then moved to a 3/8 drill, and finished with the Y (.404) drill.

I disagree with those that say an end mill will make an oversize hole. The only way it will is if the collet or spindle on the machine are lose, or the machine is so light duty it will flex.

old mart
02-21-2018, 03:44 PM
I thought that once until I plunged a 1/4" slot drill about 1/4" deep and ended up with a hole 0.01" oversize. When I tried it with another slot drill, it cut within 0.002".

02-21-2018, 09:03 PM
I have also had endmills go oversize when plunging as well on Bridgeport size machines.

02-21-2018, 10:32 PM
I'm with Duffy..plug the existing hole first. Then use the endmill. Good Luck