Sherline Laser Alignment: Polarizing Filter

Laser aligner polarizing filter detail
Laser aligner polarizing filter detail

A display across the aisle from the CNC Ghetto at Cabin Fever featured a nice Laser Center Edge Finder with their new polarizing attachment. I played with it for a while and decided that, although my crude lashup gave similar results, I just had to have a polarizing filter, too.

I’d already made a bushing to fit the top of the spindle bore with a small aperture that aids in lining up the laser, so I just added a small recess for a disk of polarizing film. I have, for reasons that should not require any explanation by now, a lifetime supply of polarizing film…

Anyhow, the new polarizing filter sits neatly atop the spindle. The main laser beam lights up the middle of the filter, with junk light spilling on the bushing to the front and rear.

Polarizing film in upper bushing
Polarizing film in upper bushing

Getting a good photograph of the spot size poses some problems, but here goes. This is the original, un-attenuated spot on a scale with 0.5 mm divisions: in round numbers, it’s half a millimeter across.

Normal laser spot size
Normal laser spot size

Cross-polarizing the beam produces this attenuated spot on the same scale: it’s 0.25 mm in diameter, maybe a bit less. Call it 10 mils.

Attenuated laser spot size
Attenuated laser spot size

Obviously, what you’re seeing are overexposed more-or-less Gaussian spots, so their diameters aren’t fixed numbers. But at this level, the inaccuracies of my Orc Engineering lens mount are comparable to the spot size, so reducing the spot any further isn’t going to improve the overall positioning accuracy.

It’s worth noting that the spot size isn’t the same as the positioning accuracy: you can visually align a workpiece mark to less than 1/4 the spot diameter. Claiming 1/10 the diameter would be more brag than fact, at least for me, but somewhere around 2 mils is close. That’s good enough for most of what I do.

I like it!

5 thoughts on “Sherline Laser Alignment: Polarizing Filter

  1. Seeing as I’m going to be building one of these in the near future… their laser edge finder is a thingus that clamps in the mill chuck/collet, with a polarizer, but yours is attached to the ceiling above the mill, shining colinearly down the bore, and now with a polarizer, right?

    1. Yup, that’s about the size & shape of the thing. The key requirement for making it work: mill immovable with respect to ceiling.

      Having actually fiddled with the commercial version, my version’s spot size gives nothing up to theirs. The only real quibble is that they’ve got adjustable centering that might actually stay put within some specified tolerance. Mine stays pretty close, nearly always, without much attention.

      I whup ’em on vertical size (all that matters with a Sherline mill), convenience (spin the lens holder on finger-tight), and price (ignoring the Quality Shop Time to build it).

      Works for me…

  2. O.K., I give up waiting for someone else to ask… what does the polarizer actually do for you? I see that the dot is smaller, but why?
    Thanks,
    – Steve

    1. Turns out that laser light has plane polarization from bouncing between those mirrored ends, so you can put a crossed polarizer in the beam and poof cut down the intensity.

      Given that the beam has (more or less, sorta kinda, insert handwaving here) a Gaussian intensity distribution, reducing the intensity shrinks the visible spot around its center: you’re seeing only the brightest part of the spot.

      That works because the beam is much brighter than it really needs to be, so you can throw most of the intensity away and still see the spot. I suppose if sunlight ever made it into the Machine Shop Wing of the Basement Laboratory, I’d be forced to not-quite-cross the polarizer…

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