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Archive for January 14th, 2010

Sears Craftsman Radial Saw Elevation Knob Handle

Broken Knob

Broken Knob

Mary’s folks visited us for Christmas and her father brought along a shelf that needed cutting; their apartment doesn’t have room for his shop equipment, alas. I cleared the crap off the radial saw, grabbed the elevation knob to crank the blade up to get it set for ripping, and … the handle broke off.

That’s not the first time this has happened, so I wasn’t entirely surprised. The knob is large enough that I could complete the mission just by grabbing the rim, but it was a near thing.

The handle is made of some wonderful engineering plastic that doesn’t solvent-bond well with anything in my armory, although Plastruct had enough bite to make me think it would work. That repair actually lasted several years of admittedly low-duty-cycle use, but obviously this couldn’t continue.

Stress raiser

Stress raiser

The problem seems to be built into the handle design. This pic shows that the fracture spans a high-stress part of the handle: between the inside right-angle corner (upper left) that rests on the outside of the knob, across the handle’s web, to the corner of the recess in the flange at the bottom of the picture.

The red hoodickie is the latch that secures the handle in its deployed position, wherein it sticks out at exactly crotch height for average human males. That accounts for the fluorescent red tape around the handle.

Broken surface

Broken surface

You can see how the latch recess triggered the crack: that notch where the latch wraps around must be the highest-stress part of the handle. I suspect the original design didn’t have the latch (or had something different) and the fat web near the round feature on the left extended all the way to the angled flange on the right.

That would work!

I epoxied a pair of rectangular brass tubes across the fracture inside the web, where they fit neatly below the latch. I roughed up the web with an awl to give the epoxy more surface to grab.

Incidentally, this is one of those cases where you might think a cyanoacrylate adhesive would work. It won’t: too much shock, too much pressure. I used it to hold the parts together while the epoxy cured, but that’s about as far as I’d trust it.

I’d like to add something to the notch, but I’m not convinced a right-angle brass flange and some epoxy will have enough grip to make any difference. It would certainly require changing the latch, perhaps by thinning the left side, which would make that weaker. On the other paw, I can probably eke out a miserable existence without the latch.

Brass internal reinforcement

Brass internal reinforcement

The picture shows the clamping in operation. A snippet of polypropylene (from some random consumer packaging) under the tip of the clamp prevents it from becoming one with the project; the clamp tip is slippery plastic, but you never know.

Perhaps this fix will last for a few more years…

Y’know, I’m beginning to believe that finite-element analysis will be the death of us all. Obviously this handle was modeled to a fare-thee-well, with only enough material to meet the expected stresses in the expected directions. Unfortunately, the real world doesn’t cooperate: the forces are always larger, the conditions always worse, and the materials always weaker than the design anticipated. A “safety factor” of three or four or maybe even ten just isn’t enough!

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