ATM's have long known of the "pencil test" described in the oldest of the ATM books available.  The pencil test is used to determine when you have good contact between the mirror and the tool during grinding.  In this test, you simply draw a cross-hatch grid onto the surface of the mirror using a pencil.  You then either start a normal wet, or if you're quite brave, you very carefully mate the mirror and tool and very lightly stroke them together for a few strokes without grit or water.  In either method, the pencil marks will begin to disappear and by watching the rate of smudging and disappearance across the mirror, you can easily deduce where the mirror and tool are in close contact and where they are not.

    In recent years a variation of the pencil test using a Sharpie® permanent marker has been widely used.  The test is performed by drawing the grid onto the mirror's face with a Sharpie marker and proceeding to do one or more normal grinding wets.  The Sharpie test works somewhat better than the original pencil test because the marker is waterproof, more vivid thus easier to see, and lasts longer into the wet than pencil marks.  The Sharpie test can also be used for marking pits on the surface of the mirror that need to ground out.  While doing a normal inspection of the mirror and noting pits, you just mark the pit with the point of the Sharpie pen.  The ink will stay in the pit for a few wets or until it is ground out.

    I had been thinking about an idea for some time for improving on the Sharpie Test.  At the same time, I was looking for a way to solve the larger problem of reliably detecting all pits during fine grinding.  I finally got around to trying out my idea and am happy to report that it works better than I had ever hoped.

    The idea is use "Dykem®" machinist's layout fluid in place of the Sharpie marker on the surface of the mirror.  Dykem is a very thin paint or dye that is brushed onto metal parts to allow one to draw on the metal with any sharp scribing tool.  Machinists scribe the layout of cutting and drilling locations (& etc.) directly onto a metal part before machining.  Once applied, the Dykem looks (and smells) a lot like the ink in a Sharpie pen.  Being very thin, it easily fills the all the pits and scratches on the surface to which it is applied.  It is quite water resistant.  You could think of it as "Sharpie ink in bulk."

    Dykem is the best known of several brands of machinist's layout fluid.  I purchased mine locally from a machine tool supply house.  You might try looking in the phone directory under "Machine Tools", "Machine Shop Supplies", or "Machine Shops" for a supplier near you.  If there are no suppliers nearby, you could try asking at a local machine shop.  While a local machine shop may not normally sell you retail supplies, they may sell you small quantities or at least they could tell you where they get their own supplies.  The other alternative is to mail order Dykem.  Suppliers like MSC (ph# 1-800-645-7270), J&L Industrial Supply, and MacMaster-Carr will carry the item.

    The Dykem is relatively inexpensive - I paid just over $4.00 for a 4 oz bottle.  Locally, it is about $30 per gallon.  That would last a very long time, but I would probably recommend purchasing in smaller quantities since it is so volatile, it may not last long after being opened.  Dykem is available in brush-on form in cans (which I used) or in spray cans at a slightly higher price.  It comes in dark steel blue and dark red colors.  I'm playing with the blue now, but also purchased some red to experiment with later.

    In use, I simply paint the whole surface of the mirror with Dykem, let it dry thoroughly, and then grind for a couple of wets.  During the first wet, you can easily observe the wear pattern of the tool on the mirror just as you would when watching a Sharpie grid marked on the mirror.  The Dykem easily shows exactly how good contact is during the wet.

    The nice added benefit of using the Dykem was that even after as many as six wets, the dye remained in the bottom of the deeper large pits that had previously escaped detection and still needed to be ground out.  It was very easy to put the washed mirror on my light table (in which I had installed regular fluorescent bulbs) and look for dark dye marks remaining on the surface.  In fact, it worked even better to dampen the surface of the mirror so that the ground surface seemed to disappear and then just look for the dark blue specks which the marked the bottom of the still-dyed deeper pits. These pits readily stood out as small polka-dots all over the otherwise translucent surface.

    After going through two iterations of this, I am convinced that this is a fairly fool-proof way of checking for pits before going on to the next abrasive grade.  Simply paint the mirror with the Dykem, grind two quick wets and then check that all traces of the Dykem are gone.  The technique is also very useful to see just how many wets it takes to completely regrind the surface using any given abrasive size.  While I usually painted the whole surface of the 8" mirror I was experimenting on, you could use as much or as little as you desire for a larger mirror.  Painting a grid as in the sharpie test would work just as well for establishing contact.  The method really shines though in searching out remaining pits before changing grit sizes.

    This technique works best early in fine grinding at #120 and #220 grit sizes.  The usefulness of the technique begins to decline with smaller abrasive sizes and don't think I will ever use it past #400 grit. By the time that abrasive size is reached, the pits are small and uniform enough to pose a lesser problem.

    I'd be pleased to hear from others who try this.  It seems to work so well for me, I am adding it to my normal operating procedures for all future projects.

John D. Upton

Dykem is a trademark of ITW Fluid Products Group.
Sharpie is a trademark of Sanford Corporation.