I usually do this with a die grinder and abrasive disc, cause that’s how I learned. But it’s sketchy, and although can be quick and accurate, it’s well, sketchy…and loud. (I avoid using the air compressor at all costs. But it has it’s uses — blowing/drying out tubes being it’s best use IMO). I like to think I always have a steady hand, but messing this slot up with the die grinder is disastrous after the entire frame is all done! Many builders just use doubled-up hacksaw blades and then finish it up with a skinny warding file or jewelers file. But I like learning new techniques so I tried something on the mill.
Having seen the many other framebuilder’s Flickr sites and studied how others do this relatively simple procedure, I decided to try it out for myself with my cross frame. (My frame = test dummy. I also tried brazing on the canti-brake studs and that went really well — pictures later after it’s done taking a bath).
It’s really easy. I first mounted the frame by the seat tube using a tube block in the vise, leveled with the digital angle-finder (most used tool I own) and then centered and slotted the tube. Not much room on my mill table but i made it – just barely! Luckily I have a belt-driven machine because my current (cheap) slitting saw has a very low tooth count. It caught and stopped the mill halfway through the cut because my feed-rate was too high. I upped the RPM’s on the mill, then fed REALLY slowly and it was smooth sailing from then on.
For those of you that are interested in doing this but haven’t bought the saw yet, don’t get slitting saws with a low tooth-count (i.e., 30 teeth). Get “Jewelers” slotting saws from somewhere like here. Mine are on order and should help with a smoother cut and they have every variation in width and diameter (as well as a MUCH higher tooth count — 220 teeth for the same diameter as a Grizzly saw!). Also, at least a 3″ diameter saw since otherwise the mill head gets in the way of the binder (if it’s forward facing like mine).
Next, I’m going to try slotting the chainstays for tab dropouts with the mill…should be fun! The hacksaw technique is pretty easy, but I’d love to get it done cleaner and more accurate on the first cut. The hacksaw method requires some filing and fitting so getting a more streamlined process will help me in the long run. In order to hold the rear end of the chainstay, I took the chop-saw to a 5/8″ Paragon tube block to make it narrower. There is less contact with tube but holds just fine and there’s more flexibility with tapered round tubes. Forgot to take a pic of that…it’s not pretty though.