# Magnetic Autocollimator Target Mirror



## Alexander McGilton (Dec 1, 2018)

Hello everyone, This is my first project posted here so I hope this catches your interest. Since getting my Autocollimator I was wanting to make a tool that would enhance my ability to make square fixtures and also have a more universal use then the standard surface plate target mirrors. This type of magnetic target is available from a few specialized manufacturers, but given that used ones sell for over a thousand I went about making my own.                



 
Of the three main components the outer body is made from O1 with a lip that covers the front rim of the mirror and has a thread in back for the locking ring to press the assembly together. The mirror is a 50mm 1/4 wave length flat glass that I had aluminized, still available at surplus shed. The locking ring is made from plane steel and has five holes. two for spanner and three for magnets to be inserted.


 
After rough machining and heat treat, the body was ground on both sided. The inner lip was left in machined condition with only a light sanding of the scaling, four points were measured and the outside was ground accordingly for parallelism to the tenths of a thou.  In assembly I found that the mirror rattled in asembly ever so slightly. Not to a surprise so I added a sheet of polystyrene as a spacer to make up for the rough side of the glass, also for the thread becoming out of alignment in the heatreatment    



For the final finishing and for the optimal parallelism the assembly was lapped as a whole. In the image below, the autocollimator is set perpendicular to the angle plate with the mirror in line. By rotating the mirror in four steps the high and low points can be identified then brought to the lap, with more pressure on the high side the mirror assembly can be honed into parallel. This can be accomplished even if the angle plate is not to the utmost perpendicularity, as mirror is rotated the high and low points are measured then the autocolimator's leveling feet are adjusted such that the recital is in the center of the travel, thus bringing the scope itself into perpendicularity to the face of the angle plate regardless of the granite. After several cycle I got the mirror to repeat to under three seconds of ark when revolved, at this point I was getting to the limits of the flatness of the hand scraped angle plate and lapping passes that would otherwise bring closer to parallel were getting it farther. It's still suburb at the moment and I can always further Lap it in should I make a more precise angle plate. How this mirror can be used to square angle plated is to flip the piece over then take a reading, the error in seconds is half of what the autocollimator shows. The autocollimator doesn't need to be necessarily parallel to the the granite, as it could be zeroed to one of its positions then measure the error.      


Finlay here is some close up images of the mirror and the case made for it


----------



## dlane (Dec 1, 2018)

Looks good , what is it used for , what dose it do ?


----------



## Alexander McGilton (Dec 1, 2018)

The autocollimator or the magnetic mirror?
 The autocollimator I a devise that can measure differences in small angles over great distance, It projects a beam of light to a mirror or another autocollimator and in the eye piece you observe the error of the receiving beams angle as a recital over a ruler or micrometer scale. This can be used for measuring flatness of surface plates, the parallelism of machine ways and so on.

The magnetic mirror is a target mirror that can be placed anywhere ferrous and measure it's angle as is stands or over distance.


----------



## Optic Eyes (Apr 12, 2021)

Target mirrors are usually adjustable, the Mirrors on my Jig Transits are as well, I have a Davidson with magnets and a screw in cylinder.


----------

