A few days ago I wrote about fixing up a Starrett combo square that I found at a garage sale. It had all three heads - 45/90, protractor and centering - but I could only get the centering head to clamp the rule securely. Turns out it was user error and I'll explain a bit here about that.
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| Here's the 45/90 head with rule in place |
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| The clamp screw, spring and nut |
You can see at the left end of the clamp screw, there is a raised tab that fits into the groove in the rule. When the clamp nut is tightened, the tab pulls the rule tight in it's slot in the head.
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| The groove in the rule |
Here's where I went wrong. Note the little nub on the side of the clamp screw, roughly centered along its length.
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| The nub in question |
This nub's purpose is twofold: to keep the clamp screw from sliding too far down in it's hole and to keep the clamp screw from rotating in the hole. But there's a catch. It turns out on older Starrett combo squares (and accessory heads), there's a small recess cut on the side of the shaft into which the clamp screw goes. This recess extends from the bottom of the slot (in which the rule slides) towards the clamp nut for about 1/8". I tried to get that in a picture, but it's very tough to photograph due to its location.
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| Looking down into the rule slot of the 45/90 head The camera is actually a bit to the right of being straight above the hole (the round hole is for the clamp screw) |
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| Closer-up pic showing the recess for the nub |
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| An even closer pic - if you use your imagination, you can see the recess. We're actually seeing the bottom of that recess. |
So it turns out that the clamp screw only goes into the head one way. The nub has to go into that recess. If it's turned 180 deg, the nub gets caught on the bottom of the rule slot and you can't tighten the rule enough. Turned the proper way, the clamp screw goes 1/8" deeper in its hole and you can clamp the rule tightly.
I'm glad I figured this one out - I was considering filing off the nubs on two of the clamp screws!
Now, here's a newer Starrett combo square for comparison. This was one of the only high end tools I bought new when getting started, and like all Starrett stuff it's been great. It's probably 10-12 years old now.
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| The clamp screw, a special washer, spring and nut |
There is no nub on the newer clamp screws. Instead, there is a flat milled on opposing sides of the clamp screw and the washer's hole has two flats that match the flats of the screw. There are two tabs on the washer that fit into recesses in the head casting and that is the mechanism that keeps the clamp screw from turning in its hole.
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| A flat milled on the clamp screw (the washer is in place, but isn't easily seen in this pic) |
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| A tab on the washer up into its recess |
So the clamp screw can go into the head two ways. There is no nub to require it to fit only one way. I hope this clarifies the way these squares clamp the rule and that it helps someone who is up against the same issues.
