Saturday, November 6, 2010

Truing a Portable Circular Saw

My old Sears Craftsman portable circular saw is a tool that I seldom use; I don't do much in the way of construction carpentry. When I do use it, it's often with a sawyer's straightedge clamped in place to guide it as I shorten a door or whatever. The saw's behaviour with a straightedge guide led me to take a closer look at the machine's construction. Here's a photograph of the subject saw. Note the elevation pivot point just in front of the front handlebar. That will be key to some of what's to come later.

For a saw to work properly with a straightedge, the saw's blade must be dead parallel to the edge of the saw's baseplate. If the blade is skewed slightly towards the guided edge at the front, the saw will want to forcefully cram itself against the straightedge, to the point of deflecting it. If the blade is skewed slightly away from the guided edge at the front, the saw will want to drift away from the straightedge. Either case makes for a difficult-to-guide saw.

This saw's blade is skewed towards the guided edge at the front by almost 0.5mm, so it wants to cram against the guide. Here's a slightly annotated photograph of the underside of the baseplate to help illustrate what I'm on about.

What it boils down to is that dimension 'A' ought to be exactly equal to dimension 'B'. On this saw, dimension 'B' is less than dimension 'A'. There is no adjustment provision.

That aside, what's with that concavity in the saw blade that I can see now that that paper rectangle's straight edge is up against it?

I loosened off the saw's spindle bolt, and the concavity went away. Something must not be right with the parts that clamp the blade to the spindle. Let's have a look.

On the left is the flanged bushing nearest the motor that the blade is clamped against. in the middle is the outer clamp washer that the bolt tightens down on.

The washer is undercut away from its periphery. That does make sense. The idea would be to concentrate the washer's clamping force out around its periphery, where the clamping force's moment arm is the longest.

That would have been a great idea had they followed through with the correct and obvious thing to do -- i.e. make the bushing's flange the same diameter as the washer. But they didn't do that.

The flange's diameter is smaller than the undercut washer's diameter, so it comes as no surprise at all that when tightened, the combination produces a concavity in the saw's blade on the motor side of the blade.

Here's a quick and dirty fix for that; flip the washer so its chamfered outside will face the blade.

I'll lose the good effect of the undercut, but I'll no longer have a concavity in the blade. The clamping effect of the washer should still be quite adequate.

Returning to the matter of blade/baseplate-edge parallelism, here's how I went about actually gauging it so I could determine that it was off and by how much.

That small level is a magnetic one. I placed it there to give me a crisp, dead-straight reference edge to refer to, rather than the pressed up lip of the saw's baseplate. That's the depth probe of a vernier caliper that I'm using to get the dimension. It's a delicate bit of business, but it does work to give you quite an accurate idea of a saw blade's parallelism to the baseplate's edge.

Anyway, having determined that the saw's parallelism is off, what's to be done about it? That question brings us to the elevation pivot point that I mentioned much earlier. Here's a close-up of the pivot point from above that will give us a clue.

Note the gap at the right side end (up in the photo) of the 'hinge'. I'll force the saw's body over rightward to shift the gap to the left side end.

With the gap shifted over, I remeasured the parallelism, and it was much improved. But the saw's hinge is inclined to bias itself over to the left. What's called for is a shim washer that will force the gap to be taken up at the left side.

The pivot pin, a 1/4" diameter roll pin, will have to come out and be replaced by a 1/4" - 20 bolt.

And here we are after a bit of struggle with the roll pin; it really didn't want to leave home. Also in the shot are an example of the bolt used, and the fine adjustment tool for tweaking the shim-improved parallelism to perfection. (I'm not kidding.)

That's a 2 1/2" long bolt that I cut down by 3/8" for this. That length of bolt gave me an unthreaded shank length great enough to serve as a pivot pin. To have put a full-length threaded stove bolt in there as a pivot pin would have been sinful; having things pivot or rotate on lengths of screw-thread is a poor practice.

With the shim washer in place, the parallelism was much better, but still not perfect. A couple of whacks at the appropriate corner of the baseplate with that two-pound hammer brought it as near to perfection as I ever expect to get it.

Now if I go out and buy a new blade that has all its teeth on it, I'll have a half decent saw.

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