Drilling accurately

A

abpro

Joined
Sep 20, 2006
Messages
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This may sound like a stupid question but, how do you go about drilling a hole EXACTLY where you want it? I am starting to build my belt grinder and need to mount pillow block bearings on 1/4" plate. I will be tapping the plate to accept the 1/2" bolts, and these holes need to line up well so my shaft through the pillow blocks will be where I want it . I always mark and punch for the hole, but with the larger size drill bits, getting the hole started right on the punch hole seems to be a problem. Any hints guys?
 
You can start the hole with a smaller drill that has the same angle point as your larger drill. Do not drill past the depth of the point. You want to have a cone shaped dimple. Then, seat your tap drill in the dimple and drill your hole.

You should also have some clearance in your pillow block holes to allow float, so you can align your shaft prior to tightening the bolts.
 
I would scribe my lines and punch it then start with a smaller bit like rj martin said. I have done this in the past and it works quite well.
 
1) locate hole.
2) Take punch, strike moderately with hammer to leave indent in the material to be drilled, your metal plate.
3) Drill pilot hole with small bit such as 1/8th inch bit
4) Drill hole to size with larger bit

Use coolant such as water or cutting oil, and add as you drill the hole. It will not only make the drilling faster, but will keep the bit sharper longer between sharpenings.
 
use a center drill after punching a starting location . a center drill will not "wander" on you ,,then a small drill bit then finally your final drill bit

to mark your hole(s) you can also use transfer punches, these are punches of various diameters,,so you can clamp down your pillow block where you want it to be located,,get the largest transfer punch that will fit in the bolts holes without getting stuck. then transfer punch all your mounting holes ,,then they will all be in there proper locations relative to each other
hope this helps..shaker,,aka Terry
 
abpro, RJ & ritz are telling you right. Spot drill with the smaller bit, 1/8" works well for picking up a center punch spot. It has enough flexability to move into the punch mark if you're a bit off. As RJ said, don't drill the spot drill deeper than the point. The reason being, you'll end up chipping the edge on the larger drill as it contacts the square shoulder created by going past the point and getting into the full drill diameter of the 1/8" spot drill . Also, as RJ said, generally, the standard for a "clearance hole" such as on the pillow blks should be 1/32" over the nominal size (the 1/2" threaded hole) which would mean a 17/32" hole (.531) in the pillow blk. That should give you about .030 to work with for alignment. Just for the record, many tap drill charts will list 27/64" (.421) drill for a 1/2"-13 tap. If the hole isn't for a structural attachment you can safely go up one tap drill size to 7/16" (.437 ) and still be well within the acceptable thread percentage limits. The .016 larger hole makes a huge difference in the ease of tapping and is what I would do in the same scenario. EDITED TO ADD: Bufford & Shaker, no flame meant, I just type way to slow. You hadn't posted yet when I was reading and then went to post. ;)
 
Shaker has a good idea with the transfer punches.

I use an optical center punch to mark my holes exactly where I want them to start. It lets me magnify the point I need to drill then I can use the punch in the alignment fixture. It places the divot in the right spot every time. I then use a small bit to drill a pilot hole and go up from there. I don't get bit wander any more with this technique.
 
To determine what smaller sizes you should use, look at the larger drill bit first. There is a cutting edge on the tip that runs perpendicular to the length of the drill. You're next size down should be a drill diameter equilant to the length of that cutting edge. If you drill a large enough hole, you might have to use three different sizes to work up to the final size. To determine the drill size for the smallest (starting) hole, look at the middle size cutting edge on the end of the drill bit to determine your starting size.

If your starter hole isn't large enough your drill will wander. If your starter drill is too large, the larger drill bit may not center properly.
 
The old school way was to center punch, start the hole with the correct size drill but no go so deep to get past the cutting edge then see if it is exactly where you want it, if it is continue drilling, if not take a cape chisel to adjust the position of the hole. The cape chisel is used to gouge a slight groove on the side of the started hole you need to move it. The drill will want to move that direction and the process can be repeated untill the cutting edge of the drill gets below the surface of the material being drilled.
 
Exactly? Like dead nuts?

This is a smart ass response, as it is probably beyond your current set up, but some may find it fascinating. And you already have good advice.

First your drill it as accurate as you can, but undersize. Then you thread the hole and mount a "button". You then use your granite surface plate and indicator and tenth's blocks and indicate that button in dead nuts and tighten it down. Then you mount in your lathe/mill/jig bore and indicate in on the button with something like a Blake coaxial indicator or whatever. You then bore it out with a single point tool, then ream.

That is how you do it exactly. Or a few tenths (.0001= one tenth) anyway.
 
Nathan the Machinist said:
This is a smart ass response, as it is probably beyond your current set up
Click to expand...

Ya think :rolleyes: ;)


abpro , do you have a mill or will you be using a drill press exclusivly ? As you can see there is surely more than one way to skin a cat. Pick the method that will work with the machine tools you have to work with and keep things in perspective. You're not building an injection mold or a balistic missle and should be able to make it work with one or a combination of the suggestions here. Let us know how she works out for you and if you have more questions just ask :thumbup:
 
Thanks for all the input guys. Nathan....whoa!! I have a ten inch drill press that I will be using. I suppose my original question should have been,"how do you stop your drilll bits from wandering whilst drilling? This really is the problem, in the proverbial nutshell.
 
abpro said:
This may sound like a stupid question but, how do you go about drilling a hole EXACTLY where you want it? I am starting to build my belt grinder and need to mount pillow block bearings on 1/4" plate. I will be tapping the plate to accept the 1/2" bolts, and these holes need to line up well so my shaft through the pillow blocks will be where I want it . I always mark and punch for the hole, but with the larger size drill bits, getting the hole started right on the punch hole seems to be a problem. Any hints guys?
Click to expand...

Marking and using the punch is the right way. But when you start your drill, come down on the mark and put a little pressure to hold it in place. This will help stop the bit from moving around. This is normal for bits to do this. Do this and you will be ok. It works for me and it will for you.
 
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