While I was working on a project to do Rotary Inlay (coming soon) I needed a way to make my own rotary carving bits so I could make inlays shaped the way that I wanted instead of just being limited to shapes that are sold such as sphere, cylinder, ballnose, flame, etc... I was intrigued by grinding points, but the thought of using a grinding bit on wood appeared to be very slow and was likely to leave burn marks on the wood especially if the wood was fine-grained hardwood. It turns out that there is a simple solution to make these grinding bits cut much better. And I also discovered that I could make my own bits shaped however I wanted for very little money. I wish I had discovered this before building up my collection of bits! This page has the instructions for making your own.
Tools you'll need:
1. Drill press. A small drill press is needed to drill accurate holes square to a surface. I suppose it could also be done with a hand-held drill, but the accuracy and balance of the finished bit might not be as good.
2. A grinding wheel. The cutting material of the home-made bits that I make come from a white vitrified aluminum oxide 1" x 8" grinding wheel I bought at the local Harbor Freight, at the time of this writing they cost $9.99. This is a 60/80 grit wheel and I am lucky that it seems to be ideal for this use. I also tried another wheel that I thought would be better simply because it was a coarser 36 grit wheel, and it was gray in color. It didn't perform anywhere near as well as the white wheel! Even after dressing the wheel it did not cut very well, and just feeling the surface with my fingers, the grit did not feel sharp. I don't know why the grit wasn't as sharp, but it was probably made in a different way. I'm guessing that the white wheel was made of grit that has been broken to form sharp edges, and maybe the other was not fractured?? I just don't know for sure. Get the white wheel from Harbor Freight.
3. Diamond hole saws. Sounds expensive, right? Not really, I bought a set of these on Amazon. These are electroplated and need water while cutting through the grinding wheel. Luckily the grinding wheel soaks up water like a sponge, and holds enough to keep the bits cool so it doesn't need to be sprayed or submerged while cutting it. These hole saws are just long enough to cut a plug from a 1" thick grinding wheel. The links on this page are affiliate links, and I will make a small commission from your purchases using them. Thank you for your support!
4. Small diamond hole saw. These are needed for drilling the hole for the shank of the bit. I bought a 10-pack of the 3mm size. You'll also need a 6mm or 1/4" bit if you'd like to make larger 1/4" shank bits. I don't drill the shank hole all the way through, I stop about 1/4" or so from going through.
5. Gel type superglue (CA). To bond the grinder wheel plugs to a shank I found that this adhesive works great! A thinner viscosity would just soak in to the grindstone moving away from where it is needed to bond the shank when it is inserted (and probably affecting the cutting ability of the bit too). This thick viscosity prevents the glue from soaking into the wheel. You should use just enough that a small amount squeezes out when the shank is inserted.
6. Diamond dressing bit. This is important, you want the coarsest diamond bit you can get in order to make the grinding point as sharp as possible. This 60 grit vacuum brazed diamond cylinder does a good job. Smaller diamonds make the grindstone too smooth and it will not cut as well.
6b. Diamond dressing bit. Another option for a diamond dresser is this sintered diamond disc. It is intended for carving granite and has diamonds embedded throughout the disc instead of just on the surface so new diamonds are exposed as the bit wears down. In my experience this bit has worn down faster than I expected. Cutting aluminum oxide grinding wheel material is extremely abrasive on any tool! This is the same sintered technology used in diamond wheels used in angle grinders and larger concrete cutting wheels.
I recommend using 1/8" steel from welding rods to use as shanks for the bits. This steel is higher quality and more round than using a nail for a bit shank. I use discarded butts from welding rods for my bits. Clean off any leftover flux gently with a hammer and spin the rod in a hand drill against a wire wheel to clean off any rust that might coat the steel rod. Slightly roughen the portion of the shank that will be in contact with glue. Round the corner on the end that goes into the collet to prevent damage to the collet.
I do not recommend using this construction method for large diameter bits - especially not thin cutting wheels. The aluminum oxide wheel material might not be strong enough to withstand the centrifugal forces produced at the speeds that some rotary tools are capable of. I am not liable, use safety glasses and a face-shield and proceed at your own risk. Keep the maximum diameter of any bit you make under a half inch (12.7mm), and keep the rotary speed below 20,000 rpm for safety.
The secret to making a grinder bit cut well is the way that it is dressed. Before attempting to carve with a bit, it must first be trued concentric by spinning it against a diamond dressing bit (preferably a 60 grit or coarser vacuum brazed bit spun with a drill press). This will remove any wobble from the shank hole not being perfectly centered. Make sure the dressing bit has removed material from all around the entire grinding bit. The axis of the bit should be approximately at a right angle to the axis of the dressing bit. Do not let the bit dwell in one spot, make a single light and quick pass over the bit with the dresser to sharpen the bit. Any more than that just wears the bit down faster. What happens is the diamonds on the dresser bit are randomly spaced and create angled streaks as they pass through the grinding bit. There will be some tiny "teeth" created where the diamond does not touch the grindstone. If the dressing pass is done too slowly, then there will be more of the angled streaks and fewer teeth between resulting in a smoother less aggressive wheel that doesn't cut wood as well, or is more likely to leave burns on harder wood. When dressing a grinding bit, run the dresser bit at higher speed, and the grinding bit at medium speed. After dressing the bit, blow the dust out of it using compressed air while it is spinning. The rest of the process is pretty much self-explanatory by watching the video below.