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Center punch, center drill or spotting drill?
- Thread starter tominboise
- Start date
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- Jun 7, 2013
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- 10,464
Amen! you will break many less spotting drills than center drills for starting drilled holes.
Spotting drills are short, stiff drills with an angle slightly more obtuse than the drill you follow them with, like 120 degrees for standard 118 degree bits.
Center drills are intended to create a 60 degree hole with clearance at the point to allow a 60 degree center to sit in the hole with good contact all around. The resulting hole is more acute than most drills and is designed to engage on the side of the hole rather than the point at the bottom.
Spotting drills are better for starting a drilled hole. The next bit will engage on the bottom of the hole rather than the edges, which helps reduce catching and wandering when the bit starts. In most cases it doesn't really matter, but spotting drills are cheap so there's really no reason to use a center drill. For carbide bits it's more important since there's a greater chance of damaging the bit if it catches while it starts drilling.
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- Sep 1, 2023
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Well trust me to get it completely arse about face.
It's a quarter inch mild steel. Stop thinking and go make holes.
The real question is, what's your tolerance. If you've got a few thousandths, take my advice above.
If you want to have the best possible precision (far more precise than angle iron it's self is), then make two step holes.
You could do the "right" thing, and use a spotting drill. The more you choke up on the drill and the closer the work is to the spindle, the better.
It's wrong of course, but center drills work just fine to start a hole. Just bury the first (small) cutting edge, don't even let it make any cylinder shaped holes, just a dimple. Kinda like you were using a punch, only no blackened fingernails. Essentially, that foremost tip is just a tiny drill bit, but the rest is quite a bit larger, and more rigid. It won't wander tangibly, and it will mark the hole with the accuracy of your mill table.
Fun thing- I had good center drills LONG before I had a lathe, they were a drill press accessory. If you tease a drill press just right, most any non-split drill bit will kinda find the center of the wobble that (nearly) all drill presses have to some degree or another. Center drills are (typically) "too good", they'll pick up off center a lot easier. If "about the middle of a sharpie mark" is not good enough, I turn to those, and in that use case, they're actually BETTER than a center drill. (Still wrong though). But like I said, I'm not making "holes", just a dimple from the first set of flutes. That's all you need to hold a couple of thousandths. (Well, that and a setup and fixture and material that'll hold a few thousandths, but that's a different issue).
Or center punch, drill 1/16 pilot, then drill 1/16", 5/64", 3/32", 7/64", 1/8", 9/64", 5/32", 11/64", 3/16", 13/64", 7/32", 15/64", and finally finish at 1/4".
I don't know why, but that seems to make sense to most of the internet. Maybe I'm missing something.
There's SO many ways, so many of them don't matter, except in that specific situation where they do matter. The only one that's gonna matter to you, on a stick of angle that's a quarter inch thick, is that the drill point doesn't "walk" while you're engaging it. In a case like that, you could maintain whatever accuracy achieved with your punch marks if you made the holes with a cordless electric drill. Well, presuming you have some practice at it to have achieved a steady(ish) hand, and an OK(ish) sense of straight. There's really not a lot to go wrong with a quarter inch hole in quarter inch material.
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- Dec 22, 2023
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- 88
You will probably laugh, but technically, I am drilling 40 9/32" clearance holes (for 1/4" fastener") on 1" CTC. The hole diameter doesn't matter that much, but the CTC is pretty critical as I am making interchangeable tooling for a shop built finger brake. So I am going to use my Milling machine and the DRO to maintain the 1" CTC and alignment down the axis of the part, and a spot drill to locate each hole. Then come back and drill through at final diameter.It's a quarter inch mild steel. Stop thinking and go make holes.
The real question is, what's your tolerance. If you've got a few thousandths, take my advice above.
If you want to have the best possible precision (far more precise than angle iron it's self is), then make two step holes.
You could do the "right" thing, and use a spotting drill. The more you choke up on the drill and the closer the work is to the spindle, the better.
It's wrong of course, but center drills work just fine to start a hole. Just bury the first (small) cutting edge, don't even let it make any cylinder shaped holes, just a dimple. Kinda like you were using a punch, only no blackened fingernails. Essentially, that foremost tip is just a tiny drill bit, but the rest is quite a bit larger, and more rigid. It won't wander tangibly, and it will mark the hole with the accuracy of your mill table.
Fun thing- I had good center drills LONG before I had a lathe, they were a drill press accessory. If you tease a drill press just right, most any non-split drill bit will kinda find the center of the wobble that (nearly) all drill presses have to some degree or another. Center drills are (typically) "too good", they'll pick up off center a lot easier. If "about the middle of a sharpie mark" is not good enough, I turn to those, and in that use case, they're actually BETTER than a center drill. (Still wrong though). But like I said, I'm not making "holes", just a dimple from the first set of flutes. That's all you need to hold a couple of thousandths. (Well, that and a setup and fixture and material that'll hold a few thousandths, but that's a different issue).
Or center punch, drill 1/16 pilot, then drill 1/16", 5/64", 3/32", 7/64", 1/8", 9/64", 5/32", 11/64", 3/16", 13/64", 7/32", 15/64", and finally finish at 1/4".
I don't know why, but that seems to make sense to most of the internet. Maybe I'm missing something.
There's SO many ways, so many of them don't matter, except in that specific situation where they do matter. The only one that's gonna matter to you, on a stick of angle that's a quarter inch thick, is that the drill point doesn't "walk" while you're engaging it. In a case like that, you could maintain whatever accuracy achieved with your punch marks if you made the holes with a cordless electric drill. Well, presuming you have some practice at it to have achieved a steady(ish) hand, and an OK(ish) sense of straight. There's really not a lot to go wrong with a quarter inch hole in quarter inch material.
Probably overthinking for a home project, but hey, why not. Up until I bought the mill earlier this year, I would have laid it out using a tape measure and dykem, center punched the locations on my anvil and used my old drill press to drill the holes.
That is ALWAYS a fair answer.
So, at 1/32 oversized, you could walk off of center by fifteen thousandths on every position, in every possible direction, and still not care, because each piece could still be bolted into it's theoretical position.
That's a perfect opportunity. Pick your four favorite methods (or however many you want) of making "perfect" holes, and what were you making? 40 or 80 holes or something? Break them up into blocks for each method. (mark it, you'll forget), then when you're done, fish through your metrology department and see how many ways you can check your work to see how you did. Facts and specs can come from anywhere, but this kind of opportunity (and dinking around, frankly) is where true knowledge comes from. If you've got time and ambition to play with that, I'd highly recommend it.
I like spotting drills as well. There are times with thinner material that I'll use one to drill through. I have to drill hundreds of accurately located hole in $5000 worth of 3/16" brass plates. I'll use spotting drills for all of the through holes that don't have to be threaded.
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