Monday, May 23, 2011

Pruning Shears Pivot Replacement

My wife was using these shears out in the garden when they came apart on her. The pivot screw and nut promptly vapourized, and drifted off to whatever dimension or parallel universe such things go to when they vanish. A replacement pivot is in order.

The pivot bore is 8mm diameter, with a 'squaring' at one side of one shear. I imagine the idea of the squaring was to hold the pivot screw from rotating when the nut was installed and/or adjusted -- not an essential feature. I'll file[1] away the squaring so there'll be a clear 8mm bore all the way through.

An obvious way to repair this would be to install an M8 screw and nut to act as a pivot, but that's a poor practice. It makes for a pivot with very little bearing surface -- only the crests of the screw's threads. It would be prone to wear quickly, and the relatively large M8 screw head and nut would look awful. A much better pivot will obtain from an 8mm diameter rod bored through for an M4 screw.

- - -

Here's the rod in the lathe having just been squared off and bored to sufficient depth.

(By the way, a 5/32" drill is as close to 4mm as 'damn' is to swearing; 5/32" = 3.97mm.)

It would be nice now if I could just part off the needed length of bored rod, but I haven't yet acquired a parting cutter for this lathe. I'll have to saw off a slightly overlong piece and trim it down to correct length; i.e. just a bit less than the total thickness of the shears' two halves at the pivot point.

- - -

Here are all the components ready for assembly.

A ny-lock nut is ideal here; it will stay put once installed and adjusted. The screw was a standard 25mm long one that I ground down to exact length for this application. The flat washers are oversize No. 6. They just fit over the threaded portion of that M4 screw. I had to bore one out just a little for it to fit the unthreaded shank. The god of even numbers must have smiling on me while I was trimming the rod to length by trial-and-error; its finished length turned out to be exactly 10.0 mm.

And here are the shears sharpened, oiled and back together ready for use.

Quite a neat job, if I do say so myself.

- - -

[1] The steel at the pivot point is quite hard, but not too hard to file with chainsaw files. I could have drilled it through on the drill press, but it would have been an awkward thing to secure for drilling safely, and would probably have dulled a drill badly.

Had I thought of them, I had just the little hand grinder stones I needed sitting in a drawer.

They're chainsaw sharpening stones. I should get all the sizes of them, and keep them where I can see them. I can see this style of stone being quite versatile.

# # #

# # #

No comments:

Post a Comment