Centre drill are often used as spotting drills in the lathe but there's a pretty wild mismatch between center drill point angles and the the usual jobber drill that follows. Special spotting drills are produced for NC use at about three times the price of similar centre drills.
How do the special features work and are they worth the premium price? Some seem to be single flute designs and the included angle is usually less thqn 118 deg jobber drill angle.
What's the experience in using these items and what's the reasoning behind the design differences?
Jim
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You can get spotting drills in both 90º and 120º angles. The main advantage is the robustness of the drill. Longevity far outweighs the cost especially using TIN coated spotting drills. I love em! :-)
I use them a lot on the mill, seldomn on the lathe. I like them! The keep long, because you won't drill deep holes, and you can't and should not drill holes. Just the conical part. Mine are two-fluted. Buy them!
I have never used a spotting drill - and don't know what their construction is. However, just an alternative idea for pentagrid: have you tried stub drills. I almost always use these for starting holes, especially in small sizes. They are about half the length of a normal jobbers drill bit, and hence much stiffer, but only cost about the same or a little more. I do often spot with the very tip of a centre drill, but I'm not sure it is necessary.
You can buy whole drill sets (w/ the case) in the stub length, even the 115 pc set. A little pricey, tho. Stub drills can often save you a tool change, altho I'm sure the purists out there are clenching and grinding their teeth. :) The silver lining to breaking a jobber's length: Now you got a stubby!
What David said, but insist in a split point to insure free cutting without deflection. The web of a drill has extreme negative rake and doesn't cut worth a damn. By splitting the point, the negative rake is eliminated, replaced by a cutting edge that cuts instead of plows. The difference in starting and drilling is remarkable.
Folks that came up in the shop the old way did *not* routinely use spotting drills. That's what center drills were for, along with actually drilling centers. Don't ask me how I know this-----it's hard enough getting old without someone bringing it to your attention.
Spotting drills are really a tool of the CNC age, where it's important to have a device that will start a hole where it's desired and not worry about breaking the tip off, which is common in center drill use, particularly when you get down to a #1 or smaller. By virtue of the grind design, spotting drills have virtually no web, so they behave much like a split point drill. Their short length provides rigidity and insures that the hole you drill will be where desired, and the grind permits easy evacuation of the chips that are generated. The tip is far more robust than that of a small center drill.
Because of my training, and the era in which I spent my time in commercial shops, I do not use them, nor have I ever.
As others have said, I like spotting drills a lot. I use them in a variety of CNC machines - mills, lathes, and screw machines. I used to pay the big bucks for expensive NC spot drills, but I recently ordered a few 3/16" NC spot drills from Western Cutting Tool for $3.86 each. They actually look pretty good, and I have had no problems with them.
I've been looking for a supplier for these for decades, ever since I tried some a friends father had. None of the drill vendors I've spoken to have even heard of them. I even checked with several of the major US manufacturers at the Eastech machine tool show a few years back.
Black and Decker do the sort of drills Andrew is describing, under the Piranha name. They work well in sheet metal (at least up to about 10mm) but are expensive. Our workshop guys at work call them crown-point drills. They are similar to the lip and spur drills used by woodworkers. Same basic idea - cut the rim of the hole before removing all of the support.
After reading this thread I bought some spotting drills to try (from ebay), and I have only one comment - wonderful!!
Centre drills are good for drilling centers where you need a pilot hole and a 60 degree "countersink" for the center, but for getting a hole in the right place without hassles or the drill bit wandering all over, spotting drills are unbeatable.
I just drilled a cross hole in a bit of hard shiny 8mm round stainless with a rickety old pillar drill, and I didn't even use a centre pop to mark it - nae fuss nor bother, and it just started in the right place!
I don't know how they work, because they don't look too different, but I am convinced - apart from centers, I'll never use a centre drill to start a hole again.
OT, how does one spell the word? Is it optional, or another case of two countries divided by a common language?
You spell centre, I spell center. You say tomato, I say tomato. It all works. I do find it interesting that so many words, and so many accents, that should be the same are so different after a couple hundred years. Even though we have been talking to each other the entire time. Not just England and the USA. England, Australia, Canada, and the USA. All different. And across each country big differences. Not just class differences in the way we speak, but even in the same class. Blue collar workers sound much different in different parts of the country even though they use much of the same slang. ERS
The one big difference is rake angle. Center drills are zero rake, do not cut to center, and have little chip relief. Those conditions do not lend themselves to easy machining.
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