View Full Version : How to mount Gem lenses?

Mark Sawyer
7-May-2017, 20:29
Gem lenses, for those unfamiliar with the term, are lenses set in a simple barrel mount, no flange or other provision for mounting them on a lensboard. I know of some ugly-but-utilitarian methods for getting one on a lensboard, but wonder if anyone has come up with a functional-but-aesthetically-pleasing manner of mounting a Gem lens?

7-May-2017, 20:58
Slide it into a felt-lined tube to which a flange is affixed. Or wrap the lens with the fuzzy side of velcro and slide into a tube.

8-May-2017, 00:40
I have made flanges for barrel (projection) lenses from wood, metal, plastic (plumbing fittings) that are simply bored through the host material with the right dia for a snug fit, and some kind of lock (screws, a cut to provide a clamping action with a screw across the cut, goo, rubber, etc... If you have a lathe, mill, drill press with a fly cutter, router with a circle attachment, etc, you can make one as ugly or nice as you like...

Not hard, and something simple that can "disappear in plain sight" will do the job...

Steve K

Steven Tribe
8-May-2017, 02:43
Many Gem lenses do already have a thread on the barrel - at least the BF & Co. Darlot I have does.

There was a surprising amount of standardisation of barrel widths for the small plain projection Petzvals. These were often sold in sets of different focal lengths along with a mount which had a pressure fit. Unfortunately, for every 2/4 projection lenses, there is only a single mount available. I show a typical mount and - guess what! - the Darlot Gem fits perfectly. This must be because of standardisation of barrel widths.

Of course, this is an awful lot of brass to do a simple job. Diameters of the mounts can be reduced by the old method of velvet strips.

8-May-2017, 05:24
The ones I have have a lip at the outside element. They would drill a hole (or 9 holes, or 12 holes) in a lensboard, and just press them in. Mine are from the 1850s though. Kind of like this later mail camera. http://www.antiquewoodcameras.com/roy-mail.html


Later, they had them threaded. The threaded part screwed into a brass plate from the inside, which had threads. Your lens is much later, and probably was in a projector of some type, mounted in a metal tube with felt. I would try something like that, for focus.

Note the ad text.

8-May-2017, 11:59
If you have a drill press and small enough fly cutter (I'm not familiar with these lenses, but you should be able to get down to something a bit over an inch in diameter) you can make snug fitting holes in two pieces of hardwood or aircraft plywood, and then carve or file a bevel one of the inside faces. Put a snug-fitting o-ring (thick enough to slightly more than fill the bevel) on the barrel between the two pieces of wood, and then screw them together until the lens is firmly held. (This is the plane-surface analog to the old-style twist-grip tripod legs.)

If you make one of the pieces small enough, and put it on the back side of the lensboard, you will have a pretty secure mounting where the lens just seems to have been grown in place :)

Fly cutters aren't precision tools, but with enough patience they can make pretty decent holes. It is important that the wood be held rigidly, so the best technique that I know of is to screw it down to a piece of scrap that is large enough to be clamped to the drill press table. It's a cute trick to put three or four screws inside the lens bore diameter, three more inside the cutter path and a few far enough out to keep the scrap in place. The three innermost ones will keep the pilot hole for the fly cutter intact, and the others will retain everything else until you are done.

After making the lens hole, reset the cutter and turn the part with the lens hole into a ring, complete with pilot holes for the eventual mounting screws!

Mark Sawyer
8-May-2017, 12:48
Thanks, everyone! I'm going to try the fly cutter on the drill press. If nothing else, perhaps I can make a "beauty ring" to hide the ugliness underneath... :)

9-May-2017, 05:12
If the sizes are close, one of those hole saws (like the ones used for drilling doors for a lockset) can also be used... But as Harold 'sez most of these boring operations develop considerable torque, so make sure the workpiece is very well secured, and cut a little, then back off, as this produces much friction and can burn wood and cutting tools can loosen from the excessive heat... Use the slowest speed on the drill press as possible... Also look up "boring bar" and see if one of the machine tool sizes might work on your drill press...

Sometimes fly cutting can produce a tapered hole if the area around or above the cutting tool can make contact with the cut area as it goes deeper through the material, so make a test cut first on a scrap...

If you have a router, do consider getting a Jasper circle attachment for it... Makes a REALLY nice round hole, but is set for 1/16" increments... (You will never look back if you get one!!!)

Steve K

9-May-2017, 05:47
since it is small, you can always bore a hole in a board and put a rubber gasket ring in it and make
yourself "friction mount" ... i sometimes do this with barrels ... or a "universal iris" works too.

11-May-2017, 13:53
Sometimes fly cutting can produce a tapered hole if the area around or above the cutting tool can make contact with the cut area as it goes deeper through the material, so make a test cut first on a scrap...

In machine-tool parlance, the necessary clearance away from the cutting edge is called "relief" and can easily be added with the aid of a grinding wheel.

Both of the fly cutters that I have owned use standard 1/4" square tool steel lathe bits, and I have re-ground bits from time to time to make chamfers, rounded corners and cuts deeper than the length of the fly cutter pilot.

For cutting disks, a tool ground with the leading point on the inside (and appropriate clearances) works beautifully, and for very thin stock grinding away most of the bit tip to make a very narrow cutting tip is a good idea.

Fortunately, even high speed steel lathe bits are fairly cheap, and with some foresight you can get four useful cutting edges at the same time on one bit!

Jim Andrada
13-May-2017, 23:03
I don't like working around the rotating fly cutter so I spin the board in my woodworking lathe and mount the cutter in the tailsock so it sits still.

15-May-2017, 08:11
Yes, anyone who is "comfortable" working with a fly cutter should probably find an inherently safer way to do the job. That is one reason why I favor mounting the workpiece to a piece of scrap which can be c-clamped to the drill press table. And always figure out, before turning on the motor, which of the three quill handle knobs will be "up" when the cutter is in the work!

I like the idea of using the wood lathe, but mine at least has so much play in the tailstock that getting it locked down on-axis and parallel to the spindle would be a chore. For a tricky job, it might be worth it, particularly with a boring head in place of a fly cutter.