Drilling: very basic question

I also like to mark out the finished circle size. When the hole is noticeably off center well before it has vertical walls, it is still possible to move the hole by using a chisel to indent the side of the cone that has the least radius being cut. The drill will remove more metal from that area. That method has saved my bacon on numerous poor starts of holes that needed to be right on. That method also seems to help on a hole that starts "lobing", drill making a three sided cone.
 
The great thing about center drills is that you can buy really small ones. Spotting drills, AFAIK, only come in 1/4" the smallest.
So if you have really small holes to drill, the combi Csk/drill works. I have to try that cross mark layout w/o the punch mark trick.
I've missed the mark too many times trying to hit the mark with a punch. Even with an Optical center punch! For large drills, I just
peck a spot first, then follow through to drill the hole. For small holes, I occasionally use small Guhring 140° carbide drills. 3-5mm.
Try all of the variations of spot/drilling to see what you like and what works best for you.
 
Doug I don't know if it has been mentioned but if you aren't drilling deep holes then stub drills can be useful and since they are so short they are stiffer and can help with the accuracy.

I normally use 118° bits. and I have 120° spotting drills. Yesterday I was trying to accurately drill some 1/8" holes which I first spotted with a 120° spotter and followed with a 1/8" stubb bit. The bit would jump around when entering the divot. What the heck? This morning I went and check the box of stub bits...they are 135° ! Not sure where I got them. I don't have a 135 spotter so did find a large 135 stub bit and it works perfectly now to locate the 1/8" bit. I struggled all yesterday.

David
 
Thanks for asking about this. Between this post and the other post linked above I just learned a lot about drilling I didn't even know, I didn't know.
 
Thank you guys, some really great comments from you all. Some thoughts:

I just finished watching the Tubalcain videos, which were Machine Shop Tips #55, 56 & 57 (#57 was actually an addendum to #56). I thought they were pretty interesting, and for me they were enlightening. They pointed out to me some things to consider...

1. How I used to accurately locate & drill holes actually had some good points but I now think could be improved. I had a nice little benchtop milling machine from Little Machine Shop with an R8 spindle, and I used a centering microscope to position the quill dead-nuts over the spot I needed to drill. Unfortunately I had to sell the milling machine because the wife & I just moved across country, and I didn't want to lug it around. It was the centering microscope that allowed me to see the spot with perfect clarity. But marking off the initial placement for the holes was always my weak link - possibly getting the right tools would help (starting with a new milling machine!).

2. Tools I have: I already have a nice granite surface plate. Also have jobber-length fractional drills, mechanic-length (stubby) fractional drills, A - Z drills (jobber length) and numbered drills (jobber length), hole punches, layout fluid, combination drill/countersinks, and some layout fluid. ALL OF MY DRILLS are 135-degree split point geometry. I have to quit using the drill/countersinks for initial hole locating.

3. Tools I DON'T have: a height gauge, an angle plate, prick punches, the optical center punch (seems pretty cool, probably not as good as a centering microscope but might do the trick until I can replace my milling machine), and 140-degree spotting drills.

4. Of course... learning how to use the tools I don't have yet.
 
I think all too many people do not pay enough attention to the sharpness of their punches; if the prick punch is sharp, you can literally slide it along one line and feel when it drops into the cross layout line (this is done with the punch at an angle while dragging it, then erect the punch and hit it lightly with a hammer. This technique can have results that are literally within a few thousandths.

I would pay close attention to this quote. A sharp prick punch has a narrower included angle than a center punch does and it is a simple matter to feel the intersection of a cross mark. It is not so easily done with a center punch but it works, too, if it is sharp. The thing with punches is that the divot it makes usually has uneven edges and that can throw off a small drill. One thing that helps is to leave the point of the center punch in place but swing the punch around in a circle and make multiple small taps; this helps to even out the hole. This will usually get you within a few thou of dead on.

I own an Optical Punch and it works okay but it is prone to parallax errors. If you are not looking directly down on the cross mark, it is too easy to punch off center. Then you have to use your prick/center punch to move the center of the mark anyway. I don't use the Optical Punch often anymore. I keep my punches sharp and feel the lines instead.

When I need to hit the mark dead center I use a magnifier and a sharp center drill run at high speed so it cuts immediately. Then I just touch it to the cross mark until I get it dead center and then drill deep enough to allow the work piece to center itself under the drill. Then a spotter and then the main drill.

One thing that might help the new guys is how to mark a cross mark. There is a tendency to make deep marks using multiple passes with a sharp pointy thing, and the lines will usually be longer than necessary. It works better to use layout dye or a Sharpie to give contrast, then use either a blade or sharp carbide scribe and make one light pass to make each short line. A sharp prick punch held at about a 10-15 degree angle will easily fall into these lines and you will feel when you hit the center of the cross mark. Then raise the punch until it is vertical and lightly tap the punch; you are making a preliminary mark, not a deep divot. Follow with a sharp center punch held vertically and make a single firm (not hard) tap straight down. This creates a divot that will allow just the tip of a sharp center drill to center the work piece under the spindle. Clamp the work piece down and then you can use a spotter or your main drill.

When sharpening your prick punch, the angle is about 40 degrees. A center punch is usually about 60 degrees or so.
 
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