Lens Caps
A Nice Lens Cap
A nice lens cap can be hard to find.

Good fitting, lightweight, impact resistant lenscaps ought to be fitted
to any valuable lens. You don’t have to put up with second rate or
“found” caps, jar lids, or wrong sizes.

We make perfect lens caps.

  • Good lens caps can be made from solid acetal plastic. Contrary to popular notions plastic is not an inexpensive material. This piece of generic black acetal plastic costs about $18.00. The same thing in Delrin (Dupont’s trade name for acetal) would cost about $45.00. Three or four lens caps can be gotten from this piece, making material a substantial part of the price.
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  • First, the outside dimensions are turned to the end of the plastic billet. By using a Computer turning lathe (CNC) a crowned end is made; this looks traditional and makes a stong, lightweight cap with an ergonomic grip. The center will not press in and contact the surface of the glass as is a problem with the ordinary “blow molded” caps usually supplied with lenses.
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  • After the formed end is cut off it is turned over and a milling machine end mill is used as a drill to make a flat bottomed hole.
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  • After the flat bottomed hole is bored a regular boring tool is used to remove the plastic from the interior of the cap. Notice how the front outside edge of the cap is configured to fit the recess of the three chuck jaws.
  • The trick to having the cap be lightweight and thin involves making the last cut heavy and accurate. The remaining (yet to be cut) plastic causes the work to be strong enough to support the cut. Once cut it cannot be easily trimmed, being too lightweight to support a further cut. This technique allows one or two caps to be made economically without the need for special tooling or fixtures.
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  • The “Goldilocks Principle” needs to be applied to ensure the fit is not too loose and not too tight. If the fit is too tight the cap can stick and bind–a dangerous condition which can cause the lens to be accidentally dropped and damaged. If the fit is too loose it will be an irritant and fall off and be lost. To be just right the base diameter is made a few thousandths too large and an ordinary electric soldering iron used to make some grooves to provide good friction for fit and allow air to escape easily. A really precise close fit will cause a pneumatic action which can cause the air to be trapped and dust to be driven between the elements.
    Best fit is when we have the lens at hand to get it right

Lens caps made by this method using black acetal in smaller sizes and grey PVC in larger sizes are from $25.00 to $300.00 for diameters as large as 250mm.
© 2016 S.K. Grimes
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