This one time, I bought a relatively expensive holesaw arbor to have around as a spare I could use with an older holesaw kit. I don't know maybe it wasn't that expensive. Seemed expensive at the time. Regardless of the price, it was the cheaper design with the locknut instead of the design with two dowels that support the holesaw better under rotational torque. I already had one of those, but that wasn't the point, I only had one.

I don't use holesaws that often, but when I do.... that cheap arbor broke the second the teeth hit the metal. Like literally, (insert the sound from the moment the very tip of the saw tooth comes in contact with the lightest of pressure against steel), Boom. Done. It wasn't even worth fighting out of the carded plastic packing nightmare. Then I realized that the only thing holding this thing together was about ~ .040 of the edge of the hex where you chuck it into the drill. From the outside it is this beefy hex and, even solid up in the top, but right where it meets the body of the arbor, hollow.

What I should've done was drive that thing right back to the store I purchased it from and returned it. At the time I did what any good hoarder, I mean resourceful person, would do and put it on a shelf to fix it or do something with it at a later date. Then I went back to the one arbor holesaw show for awhile. Since then, I've learned my lesson and now own two good quality holesaw arbors and...the broken one in the picture.

Before anyone asks, the holesaw pictured was not the holesaw size I was attempting to use when it broke. It was much smaller so I doubt that I exceeded the design limitations. I also was using a drill press, which in my experience is the safest way to use a holesaw, otherwise it can be your wrist that break instead of the arbor.

The holesaw pictured is however the one I want to use now, and the entire reason this got turned into an Instructable and removed from the shelf. I plan to dedicate this setup to cutting holes in plastic as 3.5" is a popular size for 3" pipe bulkhead fittings. Seeing as plastic doesn't have as much cutting resistance as steel, I figured this was as good a time as ever to attempt fixing this thing.

Step 1: Thinking and Drilling

There are, or were, about three different ways you could attempt to fix this that I came up with.

Attempt to just weld the hex right back on there. (risk misalignment and the same fate potentially)
Drill through the existing hole enabling me to stick the pilot drill bit through the hole, just leave the hex bit off, weld that mess together, leaving the pilot bit to chuck to. (second picture, decreased chuck surface)
Drill through the existing hole enabling me to stick the pilot drill bit through hole, attempt to drill through the upper hex bit, roughly machine the end of the bit down with the drill/ bench grinder lathe to provide alignment for the upper hex, slide the whole mess together for a welding extravaganza. (lose some of the pilot bit length as well as pilot bit change-ability, and hope that when it cools it still rotates somewhat concentric)

Obviously the only real choice here was #3. The hardest choice.

Following that logic, the first step was to drill out the top of the existing hole left from breaking because the pilot bit wouldn't go any further than where it meets the set screw that normally holds it from rotating in position.

I just chucked it up in the drill press on a 1x4 using the holesaw as the holding fixture. This hole is just clearance for the bit to get through from the lower section. For me the pilot bit and clearance hole size required a 1/4" bit. It doesn't necessarily have to be centered because the bottom bore will center it fine within the arbor, as long as you don't go too deep with this hole, only enough to clearance so that the pilot bit can make it through. I did try my best to center it as good as possible but I doubt it was too perfect setting on that 1x4. The pilot bit fits up through the bore perfect now.

Step 2: Drilling the Hex

Once that was done, I could attempt to drill the hex out in order for it to slide over the pilot drill bit now extending itself all the way through the arbor up through the top.

In order to do this, I needed my drill press vice. I first chucked the hex in the drill chuck, then lowered the hex down into the vice jaw area. Very carefully, I snuck up on tightening the hex in the vice. When the vice jaws were good and solid on the hex, I secured the vice down to the table. With everything locked down, I loosened the drill chuck releasing the hex in the vice centered with the drill press arbor.

I could now chuck in the correct size drill bit and drill down through the hex. I based this drill bit size off of the existing size of dimple left in the hex when the company originally bored it. For me this was about .200 so I went with 13/64 ~ .2031, I figured 3 thousandths wouldn't be important. Everything turned out pretty good with my set up and I had a nicely centered hole in the hex piece.

Step 3: Weld Time

Not pictured, because I got wrapped up in the moment but I chucked the pilot bit in backwards into my cordless drill and used that in conjunction with my bench grinder to carefully "lathe" down the end of the bit to the size of the new hole (~.2031) that I drilled in the hex. It is not too complicated, the drill rotating keeps the grind rate consistent while I consistently stop and check with a micrometer so that I didn't go too far. A proper lathe would be better for this but then again, so would a proper holesaw arbor.

With all that done, I was in the home stretch. I could reassemble everything making sure that I liked the pilot drill height setting with this holesaw because it definitely wasn't changing after this. I could still use the arbor with other holesaws but the pilot drill bit was going to be set at that height. I locked the set screw down tight to set the pilot bit, and then slid the hex section over the "machined" area.

I tried my best at aligning everything as close as I could, hex wise and then tacked it. Upon inspection to make sure nothing moved I continued to just weld the rest of the hex section. I finished it off with a plug weld in the end, just to make sure.

Step 4: Grind Time

After welding it up, I summoned my inner grinder finesse and proceeded to grind all of the welds down as smoothly and equally off of the hex section without taking too much. It never fails, a grinder almost always takes a little too much. It ended up pretty good though.

I hastily chucked it into a cordless hand drill expecting it to wobble around with ridiculous run-out. To my surprise, it was on par with a normal holesaw on a good arbor.

Now, a year later after the initial break, I finally have my cheap spare arbor. As I stated in the intro, I will use it for plastic and plastic only. I think it should work fine with other materials as well but I guess I see it more like, as long as I stay dedicated to plastic it will stay useful a long time.

I hope this was helpful in one way or another. Now when this happens to you at late night/morning hours over the weekend and you don't have another arbor to break, you may be able to get through a project:D

Thanks for checking this out as always!