Archive for January 21st, 2013

Slitting a Ferrite Toroid

The object of the game: cut a slit into a ferrite toroid that will accommodate a Hall effect sensor. Those doodles showed that an FT50 (half-inch OD) toroid would be about right for the cheap AH49/EH49 Hall effect sensors on hand and those doodles shows that the permeability of the ferrite mix doesn’t make much difference. Not being quite sure how this would work out, I figured I’d start with the simplest possible setup and complexicate things until it worked…

A fold of cereal box cardboard cushioned the brittle ferrite in the Sherline’s clamp and the vacuum hose in the background collects airborne grit. I touched off X=Y=Z=0 with the wheel at the center of the toroid’s equator:

Slitting ferrite toroid - first pass

Slitting ferrite toroid – first pass

The first pass went swimmingly, with the diamond wheel far more concentric than I expected, using manual jogging along a 0.5 mm deep cut. The wheel is slightly over 0.5 mm thick, measured on the grit, and showed no sign of strain on a 1 mm deep cut at 100 mms/min, so I used manual CNC to run the wheel back and forth along the cut.

After clearing the slot, I moved the wheel upward to + 0.5 mm, repeated the passes with a 1.5 mm depth of cut, then did the same at -0.5 mm. The end result was a nice slot with parallel sides:

Slitting ferrite toroid - complete

Slitting ferrite toroid – complete

The actual gap measured 1.72 mm, not the 1.5 I wanted, which means the flux density will be lower than the previous calculations predict. Assuming the Z axis backlash compensation works as it should, then the kerf is 0.72 mm. Of course, that also assumes the arbor runs true and the wheel cuts symmetrically, neither of which I’d put (or, heck, have put) a lot of money behind. On the other paw, the sensors are 1.5 mm thick (just under the datasheet’s 1.6 mm spec), so +0.1 mm clearance on each side works a whole lot better for me than, say, -0.1 mm.

All in all, there was no excitement, no muss, no fuss, no chipping, no breakage:

FT50 ferrite toroid with slit

FT50 ferrite toroid with slit

Talk about beginner’s luck!