Saturday, May 1, 2010

Indicator stand - magnetic - disassembly

I obtained a very damaged indicator stand (magnetic) which was to be thrown out. The damage includes the lack of the main post, all clamps, and the magnetic base is damaged by the loss of the front plate, switch lever, and some dings.
Below is a picture of what a complete stand should look like, with an indicator on it (picture taken from the Amazon listing for the Grizzly indicator/ stand set). I will rebuild the damaged one back to this level of usefulness, and use it with my existing indicator.

Before I can repair the damaged unit, I need to disassemble it, determine what needs fixing/repair, and then proceed from there. This page will cover the disassembly, another page will be written up to cover the rebuild once I've done it.

Since the front panel was already missing, I simply removed a brass lock screw from the side, and tried to remove the magnetic core from the internals of the base. I found the only way to achieve this was to tap the corner of the base on a block of wood, with the open end downwards, and use momentum of the core to pull it out (similar to a kinetic puller if you're into reloading) Once out, the pieces were laid out as shown below...

The pieces are: (from left to right) remains of actuating lever, magnetic core, spacer ring, base.

The base is a metal component, a singled piece, but it's made of four sections. All of these sections are visible in the photo below. The two sides (bulk of the base) are made of a cast iron, separated by a brass section about 5mm (3/16") thick. The final section is a zinc/epoxy shield which covers the hole at the back.

The brass separates the two iron halves, and effectively "breaks" the magnetic circuit between the two halves.

The magnetic core resides in the hole in the middle. The core comprises a strong magnet, with some pole pieces to direct the magnetic flux across the axis of the core. Some bonded plastic flanges are located at each end, and these plastic end pieces have a square hole molded in for turning the core.

The above photo shows the core with it's endpieces (the one on the right has a spacer ring fitted), whereas the photo below shows the square hole in the end for turning the core...

The spacer ring is to prevent the magnetic core actually contacting the interior of the base during normal operation. The diameter of the core is measured at 0.8mm less than the diameter of the hole it rotates in. The spacer ring is measured to centre the core in this hole, and this gives a rotating clearance of less than 0.5mm between the magnetic pole pieces, and the base.
The small gap is designed to offer minimal attenuation to the magnetic flux, but still permit rotation of the core. The base is designed (with the brass strip) to offer two flux paths from one side of the core to the other...
One path is through the iron side, through the magnetic material contacting the base, and then back through the other half - this path is the path which "grabs" whatever the base is sitting on.

The other path is where the flux flows within the same iron half - the flux path is vertical, and splits, with half of the flux circulating in the left hand iron side, the remaining flux in the right hand side.

That pretty much covers the internals of a magnetic base, and roughly how it works.
To repair this stand I'll be doing the following:

a - Machine a front spacer ring
b - Build a new actuating lever
c - Build a face plate to hold the lever
d - Build a post including mounting threads, washer, locknut, etc
e - Build the clamps, bars, etc to connect the indicator to the post.
f - Paint/ finish the components

As always I'll photograph as I go, and document after it's done with comments and findings.

No comments:

Post a Comment