Easy method to re-drill a blank?

[daveg]

So, I just drilled three blanks at the wrong diameter. I had an 8mm bit in the drill press & needed a 3/8". Visually they're close, but obviously the brass for a 3/8" kit isn't going to fit in an 8mm hole.

Is there a relatively easy method to re drill the blanks to the required size while maintaining concentricity with the previous hole? My first guess is no, but I thought I'd ask.

Fortunately this isn't something I do to myself very often, but when I do, it'd be nice to save the blanks. Right now, I'm getting ready to drill three new blanks and I'll reserve the other three for an 8mm kit in the future.

 

[skiprat]

I would have thought that the difference between 8mm and 3/8 is large enough not to be too concerned about concentricity. Can you at least remember or tell which end of the blanks you drilled into? That should help a bit to keep them aligned. Good luck:wink:

 

[1080Wayne]

Shouldn`t be difficult as 8mm = .315 and 3/8 is .375 . Assuming you are clamping the blank in a vise of some sort and drilling on a drill press , just make sure that your 8mm drill goes freely into the hole , clamp your vise in place , change to the 3/8 , and re-drill .

 

[KenV]

Successive drilling is a technique used by machinists on a regular basis. The hole guides the next bit.

Most of our drill bits tend to be a bit aggressive at the start of entering the existing hole, especially if the blank is not oriented exactly as first drilled. Just go up in size in smaller increments to re-center the bore, and redrill to the desired size.

 

[Dale Lynch]

Refresh the edge on the 3/8" bit before you drill.Cleaner rentry.

 

[randyrls]

When I drill blanks, I mark a cross on the center of the blank. When drilling, I clamp the vise to the table and put the line on the right side of the drilled blank. If I need to enlarge the hole, I can easily do so.

Wayne's suggestion is a good one!!

 

[daveg]

Thanks for replies & ideas. I decided to drill new blanks simply so I could get this order out the door. I still have the undersized blanks & can attempt to drill them out tomorrow.

I'd imagine it's better to drill them out with a standard bit instead of a brad point, and I've got an index I can work up through.

I've also marked the 8mm drill bit with some red model paint so it's easy to tell them apart. I know that I have astigmatism pretty badly in both eyes which makes it difficult to judge parallel lines & I should have taken the 15 seconds it would have taken to check, but I was in a hurry. Always time to do it again if you don't do it right the first time, huh?

Edit: I use a small table top drill press with a machinist's cross slide to drill my blanks. It should be fairly easy to set up & drill them again, just a bit time consuming to do so.

 

[plantman]

:rain::rain: David; This is my method. Put the 8mm bit back in the drillpress. Slide the blank all the way on the drill bit. Put your vice or whatever holding device you are using on the blank while holding the drill press down. Clamp the vice down. If you are not binding, the drill bit should come out of the blank with little or no resistance. If not reset the vice. Replace the bit with the 3/8" bit and drill away. Do this with the second blank and they should match up perfectly. Do not use a bradpoint drill for your second hole, it has nothing to center on. As stated above, the larger drill will follow the smaller one and center itself. Also, if you are not in the practice of sharpening a drill bit or do not have a sharpener, any alteration of the center point will throw your drill bit off and it will not drill straight. Jim S

Edit: The cross slide vice should make this very easy.