One of the most commonly asked questions related to sawmaking is how to drill or punch holes in the blade for the bolts.
The first tool most people turn to is an ordinary twist bit. Unfortunately, a good saw blade has a Rockwell hardness around Rc50, which is hard enough to foil most HSS or cobalt drill bits. You may be able to spot anneal the blade or use a very sharp bit with lots of coolant, but these are workarounds for a tool that is not appropriate for the task. What, then, to try next?
One viable option is a punch. A good one, such as those made by Roper Whitney, will work well, but does represent a substantial investment (unless you luck into a good used one). Depending on the model, they may struggle with some of the thicker plates.
Coming back to drill bits, there are two big requirements. The first is that it must cut a relatively hard material. The second is that it must make a clean hole in sheet metal, and exit without “grabbing” the material.
The first requirement pretty much narrows the field down to bits that are either solid carbide or carbide tipped. The second eliminates ordinary twist drill patterns*.
Looking at what this leaves, I am aware of at least four different options. I have used two of these, and heard of others using the other two with success.
- Carbide tipped masonry drill bits: These are the first bits I tried. Most of them will need to be sharpened, which must be done with a diamond stone or a green wheel on a grinder. Once sharpened, they work well (I have used them in an eggbeater drill). Just don’t overheat them, or the brazing will melt and the carbide tip will fall out.
- Carbide tipped tile or glass drill bits: These are similar to the masonry bits, but I have never tried them. I would guess that they are sharper out of the box, but they may still need to be sharpened.
- Multi purpose drill bits: Made by Bosch and others, these are another carbide tipped bit. I have never used them myself.
- Solid carbide spade drill bit: The cutting geometry is specifically designed for sheet metal, and the carbide will cut through even hardened tool steel. The only drawback is that they really need to be used in a drill press. Yes, I sell these, but only because they are an excellent tool that I personally use.
The video below shows me drilling the blade. I do this with the handle in place, so there is no chance of marking error. The handle itself was drilled in a previous step (these bits will not drill wood).
This second video is just a demonstration of how well these bits cut. The saw blade is 0.042″ thick spring steel (the same as I use in my saws).
*There are some workarounds, such as drilling through a piece of cloth, sandwiching the blade between wood or thicker metal plates, or regrinding the cutting edge (no easy task with a carbide bit). However, you still need to use a carbide bit.
Great videos and thanks for the info. I did have a couple questions. First, does the carbide spade bit leave a burr on the bottom side of the saw plate that needs to be de-burred or stoned to remove it? And finally, I do have access to some carbide spade bits, but only in fractional sizes. If I need to bump up the size of the hole a few thousandths to accept a mating pin (some cold-rolled stock that is unfortunately slightly oversized a bit), can I get away with following up the spade but with a carbide reamer? Thank you.
Yes, it usually leaves a bit of a burr. How much depends on at least a couple of things. The duller the bit and the harder you push, the bigger the burr.
I don’t worry about sizing the hole too closely to the diameter of the bolt. I prefer to leave some room so that future wood contraction and expansion doesn’t cause the bolt to bind in the blade if it needs to be removed. I’ve never used a reamer on thin plate, so I don’t know how that would work out.