Showing posts with label magnetic stand. Show all posts
Showing posts with label magnetic stand. Show all posts

Sunday, May 1, 2011

DTI Mag-Base repairs

a while ago I discussed a broken mag-base I salvaged from being tossed out. That article discussed the dis-assembly of the mag base.

In an effort to get the "To Do List" a little smaller I finished off the repairs to the indicator base.
I used the spindle from a discarded tap to make up an actuating system for rotating the mag-core. I simply filed the required square shape into the section which used to hold the jump valve.
The threads which actuate the original tap were turned away, and replaced with a parallel section.


Figure 1 - Tap spindle filed to square

A plastic bush was turned to locate the mag-core inside the void in the mag-base, and to provide support to the spindle. The plastic was from a sheet of 25mm (1") thick plastic (nylon I suspect) that I rescued from a bin. A suitable square was cut from one corner, and a 10mm hole drilled through it. A 10mm bolt and nut were inserted, tightened up, and used as an arbor for the turning. Whatever the plastic was, it certainly was "stringy" in the swarf.


Figure 2 - Commencing turning the plastic bushing

A brass indicator/ handle was made from an old brass fitting, and some sheet brass, and soldered together.


Figure 3 - Actuating knob and pointer prior to soldering


The spindle was designed to pass through the front plate which was made from some brass strip. The front plate is shown on the RHS of the exploded view below (Figure 4)


Figure 4 - exploded view of mag-base

In the above exploded view, everything to the right of the magnetic core, and everything above the magnetic base were made from salvaged materials.

The magnetic core was filled back to remove the old damaged paint, and engraving markings. A few minor dings were cleaned up, and then the base was primed with cold-gal paint (Zinc-it) then followed up with a couple of light coats of Silver Hammer finish paint.

Upon assembly it was deemed too difficult to re-drill the existing holes to their proper spots, so new holes were drilled for securing the face plate.



Figure 5 - the magnetic core in the base with the spindle and bushing in place.




Figure 6 - The mag-base assembled

One of the salvaged lengths of steel from a gas strut was used to make the mast on this base - although a brass socket and washer was turned from an old extinguisher part (CO2 nozzle) to stiffen the joint instead of simply using the M8 thread.



Figure 7 - The  mast on the mag-base made from gas strut

An offcut of the gas strut remains, being about 125mm (5") long which will most likely be used for making one of the connecting rods.




Figure 8 - The completed mag-base next to the Taig lathe.

I've possibly made the mast too tall, but standing next to the Taig lathe, it has sufficient height to ensure access to anything I put in the lathe, or the vertical slide. I've yet to make the other smaller poles, joiners, and DTI connectors, but that should be fairly easy to do over the next few weeks.

That said, guess who's added more projects to the list... You'd think I'd have learnt by now, or maybe it's my curse (or is it blessing) to always have more jobs than hours to complete them...


I did up a quick explanation of how a mag-base works for someone, and decided to include it here in case anyone else needs to know how they work. I've since cleaned it up and converted it to a JPG.


Figure 9 - Theory of operation - magnetic stand

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.