Based on a mention in a project article I read I decided to pick up one of these “unibit” (this is Irwin’s term, there may be a more generic one) for drilling thin steel cases.
IRWIN Unibit 10231
In particular, the article mentioned that these bits help to make cleaner holes that require less deburring. I didn’t exactly know why this might be, and I’m saving up for a nice Cobalt Steel set of bits but I was at Fleet Farm anyway and knew I’d be drilling some of this steel that same afternoon, so I decided to give it a shot.
The only brand I could find was Irwin (maybe they are the only ones making these bits?), which is fine because I’ve been satisfied with the other Irwin bits I’ve purchased, although had I known the bit was available in Cobalt maybe I would have ordered the bit online and waited (probably not, since I wasn’t sure how well it was going to work yet). There were three bits available on the shelf, the #1, #2 and #4. I chose the #2, which has 13 steps and can drill (cut?) holes between 1/8″ and 1/2″. The #4 looked cool, but was a lot more and the #1 with only 6 steps, wasn’t much cheaper.
As I found out, having more steps is better if you can swing it and when I buy a Cobalt version of this bit, I’ll opt for the 20-step bit.
First I tested the bit on an Altoids tin that I keep specifically for testing this operation. This makes it easy to compare each method/technique. For the drill I used a Chinese bench-top drill press that got from my father. Instead of a vice I used this cool little “fence” that he made for it to keep everything stationary, this is important because in the past when I’ve used regualar twist-bits I’ve had these little boxes ride right up the flutes and take off like a helicopter.
I opted not to use a block behind the hole. I usually do so, because it allows me to use more pressure on the drill without denting the steel, but I wanted to see what this bit could do unassisted.
Taking my time and watching the material, the first test (to the first stop on the bit) went very well. I took the box out of the press and examined the hole. The denting problem was minimal if not non-existant. I’m sure that this won’t be the case on a larger piece, but it’s good to know. Overall the hole was very clean, with some burring on the exit side but better than a regular bit. I think the deburring action of the bit is due to the cutter of the next step up beginning to bite which removes a little material from the entry side of the hole.
Next I took the bit down another step and then another. The sound changes and the following holes require less effort than the first (something to note if you’re only trying to go a few steps). By stopping the drill it’s easy to see what size the hole is and what’s next using the legend on the bit.
Once the testing was over I used the bit to drill the speaker mounting holes in a lunchbox that I’m turning into a guitar amplifier. Overall it worked very well, I didn’t even “dimple” the steel to guide the bit (although this can be attributed as much to the use of the press as to the bit itself). The holes required little cleanup afterward and overall I was very impressed.
Next I tried the bit freehand as the holes for the amp’s controls were going in the side of the box and it was too big to fit in my press on its side. For these holes I used an awl to dimple the steel slightly to prevent the bit from wandering and chucked the bit up in my 18v Black & Decker cordless.
Using the variable speed control of the drill, I kept the speed low until the bit penetrated the steel. Controlling the pressure of this drill is a lot tougher than the press and I didn’t want it to shoot through and accidentally make the hole too big. Once the first hole size was cut however I increased the drill to maximum speed and cut the hole to size for the gain control without problems.
Drilling the remaining holes I noticed a couple of things about the bit. The main thing is that the larger the hole, the less deburring you have to do. If you drill just to the first stop, there is usually a big chunk of burr on the exit side of the hole, but if you keep going this falls away and leaves a very clean hole.
The second thing I noticed happened when I was drilling the many holes for the speaker grill. I drilled the holes to the first stop but allowed the next stop to just touch the entry hole to deburr the face of the amp and while this worked, it does dimple the hole a bit. For this application the result is favorable, giving the grill a sort of “bullet hole” look, but I could see this being a problem if you are going for flat, smooth holes.
Also I tried using the bit on the reverse side of some of the holes to try and deburr them the way the entry holes, but this didn’t work all that well and I wouldn’t recommend it, at least not in light gauge material such as this.
Overall I think this bit is a great addition to the toolbox. It was a bit more expensive than I would have expected at about $17.00 and I imagine the Cobalt version (mine was just high speed steel) is probably twice that, but since it would get a lot of use in hard material (and I have no idea how to sharpen it), I would recommend going with the Cobalt, as well as a model with more “steps”, if you can afford it.
Jason J. Gullickson 