Speed Indicator

Last Updated: 13 July 2007

The speed indicator was non-functional.  It's belt was missing, and when the driver was spun manually the entire mechanism made an awful racket and  the needle wouldn't move.  This gauge functions exactly like any ordinary mechanical speedometer, which means it uses a flexible drive cable, so I assumed that cable was broken.

Notice the glass is also broken.  It's mounted in the saw shell (just above the blade welder) using four bolts as shown.  The bezel (painted red) is simply pressed into the gauge case and is pried out using a screwdriver (gently, so as not to bend it).  Once the bolts and bezel are removed, the entire assembly can be removed from the shell.

I was very happy to find the drive cable unbroken and in remarkably good shape.  It was probably saved by whomever removed the drive belt from the gauge driver.  It's not clear how long the saw was being operated without a functional indicator, but based on the condition of the oil in the driver bearings it looks like quite some time.

On the one hand, it's good that the cable isn't broken.  On the other hand, that means the problem is in the gauge itself.  Having never repaired an instrument like this before, it took some time to figure out how it works.  Here's a couple pictures of the gauge internals, partially dismantled.  The first is a face-on shot looking at the front of the gauge mechanism with the face removed (not the clearest picture ever, but the best my camera can manage).

I could spend several boring paragraphs attempting to describe how this works, but even with pictures it would be virtually incomprehensible to anyone who hasn't dismantled one before.  The basic principle is a rotating magnet on the input shaft which spins inside a steel "drum" to which is connected the gauge needle shaft and a tiny clockspring.  The delicate interaction of that rotating magnet with the drum, needle shaft, and clockspring causes the needle movement around the gauge.  An ingenious mechanism, but extremely fragile.

Remarkably, I think I managed to repair this thing.  Of course, during the repair I completely uncalibrated it.  But it now indicates a speed when I spin the input shaft with an electric drill, so after the saw is back in working order I can recalibrate it (there's actually a simple adjustment mechanism within the gauge that acts on that tiny clockspring I mentioned earlier).


Next is the gauge driver, which is really just a simple bearing with a pulley on the end to transfer rotation to the flexible cable.

Although it appears the shaft can be removed from the bearing, it can't.  Of course, that didn't stop me from trying to remove it by force (5 ton puller, lots of hammering).

Unfortunately, the bearing must be replaced as a complete unit and I can't see any way to properly service it.  It needs service - it's binding through 1/2 its rotation.  The good news is this is an exceptionally simple part, and I could probably make myself a replacement using a couple standard bearings.

I soaked it in parts solvent for a few days in the hopes of loosening accumulated oil sludge.  Then I soaked it in SAE 5w-20 detergent motor oil for several months.

In checking the manufacturer of this speed indicator, I was surprised to find the back of the gauge housing engraved "AC Spark Plug Co. Flint, Mich.".  Obviously, AC Spark Plug Co. has no business making gauges for machine tools.  I can only assume, therefore, that their efforts in this case were applied as part of the war effort.  They must have been pressed into service making parts for machine tools.

Actually, they probably only made the gauge housing.  It's obviously a steel stamping, and AC must have had a press capable of turning these things out, so they were either pressed into doing so by the government, or they worked a contract from DoAll in order to keep their machines busy since domestic automobile production was basically curtailed during WWII.

There's really not much to be shown for the reassembly, except the gauge installed. Suffice to say all the parts in the above photos were cleaned and polished to like-new condition. The bearing in the driver case, after soaking for months, isn't perfect but far better than before.  Good enough to consider serviceable, anyway.

Regarding the calibration of the gauge, I haven't been able to do so.  Note in the "after" picture, with the saw at rest the gauge doesn't quite indicate 0.  This is because of where I've chosen to position the needle.  The calibration as it stands is completely "dialed out", meaning the spring which returns the needle to 0 is running as slack as possible.  Despite this, the gauge needle never makes it all the way around to read full-speed when the saw is run at full speed.  At most, it reaches the 325 fpm reading, but I believe the saw is running faster than that.  The gauge moves to various positions smoothly as the saw speed is changed, but until I carry out a thorough measurement of the actual speed I won't know how far off the gauge is.

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