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Alan in Vermont
03-12-2011, 09:02 PM
OK, this is terribly basic but it's something I have a hard time getting right. What's a good way to get a hole right spang on center through a round workpiece? I'm faced with having to make new pushrods for some small hydraulic cylinders and need to drill through the rods for crosspins. Holes willl need to be clearance for .500 and .5625 through 1.00 and 1.125 stock respectively. I've got a few options, drill press with loose vise, Smithy combo machine using the mill setup with a drill chuck and vise mounted on the lathe carriage, and a Bridgeport with a super solid, bolted down, vise and DRO.

The Smithy and BP aren't mine but I have free access to them. While I've done some crude crossdrilling it has never been anything that really should come out right on the money. All suggestions appreciated.

portlandRon
03-12-2011, 09:24 PM
Use the BP. Mount work piece in vice, horizontal with the table and with end that you want to cross drill hanging past the end of the vice an inch or two.
Mount a drill chuck with a piece of drill rod in it. Lower the drill chuck until the rod is just below the bottom edge of the stock to be drilled. Move the cross slide until the drill rod makes contact with the stock. I use a piece of shim stock between the drill rod and the stock moving it until it's just pinched to determine when contact is made.

Set the cross slide a to "0". Now add up the diameter of the stock you are drilling and the drill rod. Divide that number by 2. This gives you the distance to move the cross slide to be dead center on the stock. NOTE: if you use a piece of shim stock to determine contact you need to include the thickness of it in your calculation.

Arthur.Marks
03-12-2011, 09:27 PM
My vote: use the Bridgeport to make a top/bottom vee fixture to fit drill bushings* to guide your twist drill on the drill press. Similar to this example, made by Heinrich: http://www.heinrichco.com/drill_jig_hand-op.htm

*http://www.mcmaster.com/#drill-bushings-and-liners/=bepht7

Arthur.Marks
03-12-2011, 09:28 PM
Also note, given portlandRon's suggestion: if you are going to drill without a guide bushing (as above), you MUST use a spotting drill first. A twist drill will wander on a curved surface. Spot. Then drill.

Rich Carlstedt
03-12-2011, 09:35 PM
First , you need a V Block that is locked to the table somehow, and the drill bit be centered on the bottom of the V. (use your quill stop just short of hitting the bottom). Once set up, never move it
2nd, Clamp the rod so it does not move once you start
3rd, start your hole with a ball mill (least deflection of any cutter) or a spotting drill, the same size or smaller than the finish drill. Center drills also work, but are not as accurate as the first two. Never use the drill bit to start on a curved critial surface, except it is OK after punching through the top
for the tube bottom as the surface is concave. Do not predrill the bottom undersize, unless you spot drill it first.
I think this should work for your.
No freehand drilling !

Rich

wooleybooger
03-12-2011, 10:42 PM
once i have x and y set and dial settings Written down, i mill a flat across the top using the Y feed. use a mill equal to or slightly larger than your finish drill size. youre looking for a square,flat spot on the top of your shaft.

v860rich
03-12-2011, 11:09 PM
If the DRO has a 1/2 key just edge find one side of stock, zero that axis, edge find other side of stock, press 1/2 key.
The reading on the DRO will be the exact center of your stock, so just move the table until DRO reads 000.
Drill hole.

THANX RICH

People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!

darryl
03-13-2011, 01:42 AM
I did a setup on the mill at one time that seemed to work pretty well. Instead of a pair of V blocks, I used a pair of shop made L blocks. Each has one flat surface with a vertical tab rising from it. The rod to be drilled is set against the tab and clamped. The edge of the tab gives a reference surface when you're dialling in the mill table. I found it easier to set up, especially where you don't have much vertical range without cranking the head up or down, and probably losing your alignment at the same time (round column mill owner here). The L blocks were made from angle, with one side cut down to a manageable height, and the other side previously milled to be flat so they could sit flush on the mill table without rocking. I milled them in place, clamped to the table side by side. I wanted to key them to a t-slot, but didn't take the time to do it.

You can easily align the blocks to the x axis by using the reference sides of these blocks. If this is done, you won't have to worry about losing your center alignment point when you crank the x axis to position the setup for the actual drilling of the cross hole. You can also set up a stop on the mill table to help keep holes an equal distance from the ends of multiple workpieces.

I have a few precision rods that I can mount in a drill chuck or endmill holder- I use these in combination with zig zag papers (.001 thickness) when picking up an edge. Drag the paper between the rod and the reference surface while cranking the table over- when you start to feel the drag on the paper, you know you're 1 thou away. Set dial at .999, work out the math, crank the table over the right amount to center the workpiece, remove rod, mount cutter, etc.

By the way, if you need two or more holes in the same piece, but want to keep the mill table axis clamped during the whole process, you can insert a close fitting rod into one drilled hole, then use that with a 123 block or similar to assure an accurate vertical there so that subsequent holes are in line with the first.

There's one more thing I thought of at the time I made these L blocks- because I was drilling right through the diameter and didn't want to muck up my fixtures, I had the workpiece overhanging the block. A support block under the very tip would have been a nice way to alleviate the possibility of the workpiece tilting from the drilling pressure. The support block would have been machined at the same time as the L blocks so the flats would be at the same exact height. I did not take this step at the time, but I wished I had done so. I might just make up another kit of these, making it complete at this time. Thanks for the reminder-

SGW
03-13-2011, 07:51 AM
These days how I do it is clamp the rod in the mill vise and use my DRO's centerline function. Indicate the back jaw of the vise, then the front jaw, and the computed centerline is the centerline of the rod.

Before I had a DRO I used a long-reach Starrett wiggler with its 0.50" edge finder attachment to indicate one side of the rod, then I computed how far to move over to be on the rod's centerline.

jack3140
03-13-2011, 06:47 PM
Use the BP. Mount work piece in vice, horizontal with the table and with end that you want to cross drill hanging past the end of the vice an inch or two.
Mount a drill chuck with a piece of drill rod in it. Lower the drill chuck until the rod is just below the bottom edge of the stock to be drilled. Move the cross slide until the drill rod makes contact with the stock. I use a piece of shim stock between the drill rod and the stock moving it until it's just pinched to determine when contact is made.

Set the cross slide a to "0". Now add up the diameter of the stock you are drilling and the drill rod. Divide that number by 2. This gives you the distance to move the cross slide to be dead center on the stock. NOTE: if you use a piece of shim stock to determine contact you need to include the thickness of it in your calculation.
thanks i needed that i tried it this morning and it worked fine at last i can drill a hole in the center of a round rod one more lesson learned thanks jack

mark61
03-13-2011, 07:19 PM
Clamp the rod/round stock to your drill press table. Using you Tri-square with the round centering head and the right angle head with a level-place the round centering head in contact with the rod. move the level head as far up the scale as possible. Now drop the scal end down to contact the rod move everything so the level is LEVEL! Scratch a mark. Close enough for mill work! Easier as the rod diameter gets bigger than with small stock.

mark61

darryl
03-13-2011, 08:29 PM
Using only a drill press- it can still be done. You need an adapter that will slide down the shank of a pilot drill bit and center itself over a round workpiece. Actually, you let the workpiece center itself under the adapter, while the shank of the pilot bit guides the adapter. The V on the bottom of the adapter must be centered to the pilot bit hole, and perpendicular to it at the same time.

The pilot bit would ideally have a long shank, and the length of flutes very short. The hole guiding the bit would then give support as close to the start of the hole in the workpiece as possible. If there's play at this point, you wont be able to rely on getting a well centered hole drilled. Sometimes it might come out good, other times it will have walked off center a bit and followed off center. A properly made adapter though will work quite well.

A couple of things first-you will want to know that your dp table is square to the spindle. If it isn't, you could get a hole that starts well centered, but is not perpendicular to the workpiece. Unless you have a 2000 lb drill press from hell, just the pressure from drilling could throw the accuracy off. Much better to do the whole operation on a milling machine. It would seem that what you're doing requires some good degree of precision, and you will not normally get that from a drill press.

R W
03-14-2011, 05:47 AM
From a recent experience while drilling a 3/8" hole through a piece of 5/8"
round, the 3/16 pilot went through OK but the larger drill started to run off
badly this was rectified by repositioning the workpiece, in future in this situation I would try using a 1/4" or slightly larger bit before the 3/8".

darryl
03-14-2011, 10:31 PM
Yes, have had that problem also. The only good way to keep positions and hole sizes within close specs is to use endmills, etc. Shortened and modified drill bits can be ok too, but I've come to realize that you don't rely on drill bits to do precision operations. Also, in the case of cross-drilling, you don't ever want to let the workpiece move before you've done all operations on it that need to be synchronized in some way. It's too easy to get two holes slightly out of line with each other. You can use temporary pins and setup blocks to keep the alignments close, but ideally you keep the workpiece clamped until you have no choice but to move it.

A case in point would be two cross holes drilled at right angles to each other.