George, if you look at the nose of an ordinary drill bit, you'll notice that the center is web thickness and chisel shaped. This doesn't cut, it just pushes the metal to the sides where the lips can get hold of it and cut it. That takes a good bit of force, particularly in the larger size bits. A split point has secondary cutting lips ground into this central web so that cutting occurs all the way to the center. Less force is required to start the hole, and the bit has less tendency to wander.
Basically, the secondary lips are formed by extending the back clearance grinds to a shoulder halfway across the web so that they meet to form the secondary cutting lips. Done correctly, this can reduce the required drilling force by as much as half.
This is hard for me to describe in words, you really need to look at one to see how it is shaped.
O.K. For that, you want split point (to be covered later).
The only real disadvantage of the Roper Whitney is the inability to reach very far from the edge. I have one, and even use it on flat spring stock from time to time.
O.K. Let's start with looking at the end of a normal drill bit. You have two flats (sometimes curved a bit, depending on the sharpening technique) which meet in the center, leaving what is called a "chisel edge" going across the tip.
A split-point is ground with two intersecting flats on each side, one is the normal one which you find on the drill bits which you already have -- one side of which makes the cutting edge.
The other flat is steeper, and covers about the back half of the original flats. It cuts in to the center, leaving a sharp point in the center, which makes the bit less likely to walk, and vastly reduces the force needed to drill into a given material.
Pick some drill bit size which you use a *lot*. (Probably your
1/8" is a good choice, since it seems that your usage patterns would use those a lot. Pick up your MSC catalog, and order an envelope of 1/8" drill bits with split points.
O.K. Here is one from the MSC catalog, page 40. It is a Cobalt steel, 1/8" 135 degree points, split point, screw-machine length (shorter than the standard jobber's length, but for drilling sheet metal, I don't see a need for the extra length.
Minimum order is a package -- which is twelve in this case (Chicago-Latrobe brand).
Stock # Pr Ea Count Total Pr
81371080 $1.81 12 $21.72
You'll use those up over time, and it isn't that expensive to show you what split point does for you. (As well as the rigidity benefits of the screw-machine length.)
But -- before you do this -- look at your 135 degree bits, and see whether they turn out to be split point style. You should be able to tell fairly easily by comparing one to one of your older ones.
The bits that just came in look like regular chisel bits @ the end but with a different angle than my old bits -- I just went out and double checked them. I'll order some split points in at some point Don. Right now, I pretty much spent my budget and have to wait a bit to spend more. Thank you for explaining all of this.
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