Re: [Cameramakers] Cheapo Helicoid' focuser
Thanks; it always looked so formidable that I was frightened off. If it is hardly worse than a single start thread, it is well worth considering. I appreciate the encouragement, and still worry, but less! Bob At 21:03 24.02.02 -0500, you wrote: Re;Barry Young [EMAIL PROTECTED] The second time you do a multistart thread it is cake. You, Sir, also speak from knowledge. bye, sid. ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Cheapo Helicoid' focuser
Dear Barry, Perhaps you have some advice on getting the depth correct. The obvious solution is wires, for the outside thread (except for those who are lucky enough to have a thread micrometer.) How is the mating inside thread monitored for depth? This is the operation which was my worry when expressing concern. With a single start thread you can advance in small steps, using the outside thread as a gauge. With four starts that is a little trickier; you have to cut all four, test, and if more metal must be removed, repeat. Conversely, if one can measure, there is no big problem in advancing the tool the right amount. Unfortunately, I have never even seen a micrometer for inner threads! You advice will be welcomed by all who might have to face this problem. Bob At 15:22 24.02.02 -0800, you wrote: Hi Robert: Most multi start threads used in photographic equipment use three or four starts. All I have made as replacements have been 4 starts. This is easily indexed using a machined block under each of the jaws on the four jaw chuck. You can do it, only the first one is difficult. The second time you do a multistart thread it is cake. All the focusing helicals I have seen have 60 degree included angle thread flanks. Barry Young ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Cheapo Helicoid' focuser
Hello Robert: Almost all internal thread pitch diameters are measured with thread plug gages. You make one a little big and one a little bit small. When the small one enters and the big one doesn't, Ta da! you are there. For thread form however, almost everybody uses the impre4ssion method. The truly cool use dental impressioning compound, You heat it with a lighter (preferably a Bic lighter) and it softens. When you press the softened impressioning compound into the thread, it cools and solidifies leaving a near exact replica of the thread. Dental compound is dimensionally stable meaning it is the same size when cool as when heated. Amazing stuff, but hard to find and spendy. I recommend the poor mans impression stick. If you have kids it is easy. Steal one of their crayons when they are asleep. If you don't have kids, steal one of those long tapered candles (tapers) the better half uses to lure you over dinner. If you are single, you can apply a light coating of spray on cooking non stick super slick gizmo stuff (Pam) and then fill a thread or three with epoxy. When it sets up, you have a good impression. There is no stealing for the single people though. The really cool thing to do is to slap that impression you made on your optical comparator. Lacking this, you can get at almost any part of the thread by sawing your impression on the band saw. Then you can just use calipers. BTW, since this IS a photographic group, you CAN (and I have) use an enlarger to project your threadform for easy measuring on the baseboard. Like a shadowgraph. I don't want to bore the people here to tears so drop me a line off list. Maybe I can give you even more ideas on where to steal essential shop equipment. Barry Young Ooops, I mean borrow! --- Robert Mueller [EMAIL PROTECTED] wrote: Dear Barry, Perhaps you have some advice on getting the depth correct. The obvious solution is wires, for the outside thread (except for those who are lucky enough to have a thread micrometer.) How is the mating inside thread monitored for depth? This is the operation which was my worry when expressing concern. With a single start thread you can advance in small steps, using the outside thread as a gauge. With four starts that is a little trickier; you have to cut all four, test, and if more metal must be removed, repeat. Conversely, if one can measure, there is no big problem in advancing the tool the right amount. Unfortunately, I have never even seen a micrometer for inner threads! You advice will be welcomed by all who might have to face this problem. Bob At 15:22 24.02.02 -0800, you wrote: Hi Robert: Most multi start threads used in photographic equipment use three or four starts. All I have made as replacements have been 4 starts. This is easily indexed using a machined block under each of the jaws on the four jaw chuck. You can do it, only the first one is difficult. The second time you do a multistart thread it is cake. All the focusing helicals I have seen have 60 degree included angle thread flanks. Barry Young ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers __ Do You Yahoo!? Yahoo! Sports - Coverage of the 2002 Olympic Games http://sports.yahoo.com ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Cheapo Helicoid' focuser
Hi Robert: Most multi start threads used in photographic equipment use three or four starts. All I have made as replacements have been 4 starts. This is easily indexed using a machined block under each of the jaws on the four jaw chuck. You can do it, only the first one is difficult. The second time you do a multistart thread it is cake. All the focusing helicals I have seen have 60 degree included angle thread flanks. Barry Young --- Robert Mueller [EMAIL PROTECTED] wrote: Whether the threads are too fine or not depends on some details. If you are focusing on ground glass it doesn't make too much difference how many turns are needed to reach focus. If you want a scale, exceeding a single turn is awkward unless you invent a trick. The threads on almost all lenses are multistart threads. These give a fast movement (large motion for a single turn) while retaining a fine thread, which is for no other reason, is useful to keep the thread thin and giving lots of contact area while the wall of the threaded tubes uses up little diameter. Unfortunately, you do not easily buy multistart threads; if I wanted one, the easiest source is buying an old lens, or try making it myself, a thought which frightens me more than a little; getting the precision needs some work when you have maybe 10 starts on a thread, but it is not unthinkable (yet; I haven't tried it so I am still able to imagine the troubles away.) Incidentally, the helicoid threads I have encountered have sloped sides and serve both as bearings and to provide the linear motion. Roughly 60 degree sides seems like the best choice. Acme (29 degrees) is probably inferior. Anyone know what the thread pitch is on a medium format helicoid focusing lens? This is something there is little need to ask. One computes the difference in distance from lens to film for infinity and for the closest subject you want to include in the focusing range. That difference is the required motion for the lens being considered. If you want long motions the rack and pinion with a bellows is probably the easiest way to go. In this regard, look at linear ball bearings and a fork on another track to prevent rotation. (Unless you are quite good, I cannot recommend two, parallel rods each with a linear ball bearing on it unless you include some sideways motion to take up errors in the precision of placement of the rods.) With a proper choice, plastic bushings could compete well with the linear ball bearing for smoothness and you would be rid of lubrication and rusting, both of which argue against linear ball bearings outdoors. (Delrin can make nice bearings, nylon is probably also good, and Teflon should move nicely but probably will flow with time, leaving a bit too much play. I have seen plastic running on the tracks in chart recorders and it holds up well, giving smooth motion over long lifetimes.) One problem that you'd have with most pipe fittings is that the thread is tapered and only so long--certainly not two inches. the tapering tightens as you screw into the fitting. You could tap your own and go farther but I don't know if it would be worth it. Plus your lens is spinning around with all your shutter cocking, release and f-stops in a different place every time you change focus. Cutting you own thread with a proper pipe threading tool is easy work and making it long is also easy. Saw off the tapered part! You can deal with the too easy rotation by adding a bit of friction. One trick is slitting the nut and bending in the metal between the threads to make reasonable resistance to rotation. This also reduces wobble from the clearance between the low precision threads. Consider making the threads on a lathe, and you may well decide the pipe thread is not so difficult after all. Having the lens rotate is not very nice, but I can live with it; after all, when it is upside down it is harder to read the speed setting on the shutter, but maybe the f number will be easier to set if the scale is opposite the speed scale and is suddenly up. There is another kind of helicoid. It has been used commercially. It is just a helical slit in the tube with a pin protruding into the slit. This is not trivial for most of us, but if you are remounting a lot of lenses the construction of the attachment might be worth the effort. You would need a drive coupling the table drive screw of a milling machine to a rotary table. This could be done with chains, gears, drive belts, or even electronically via stepping motors or servo motors (this solution gives you immense power for certain applications). A similar procedure would work on a lathe with a grinding attachment, provided this could run slowly enough for a milling tool. If my memory is not too bad, there is a description of this
Re: [Cameramakers] Cheapo Helicoid' focuser
The threads on helicoid focussing mounts are generally of the multiple start variety. For example, a coarse pitch thread such as 4 TPI is started 4 times around the barrel of the mount but cut to the depth of a 16 pitch thread. This gives the desired long travel with minimum rotation combined with the support and precision of the finer pitch thread. Cutting multiple pitch threads is relatively easy for someone who knows how to do it on a standard screw-cutting lathe. bye, sid ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Cheapo Helicoid' focuser
Re;Barry Young [EMAIL PROTECTED] The second time you do a multistart thread it is cake. You, Sir, also speak from knowledge. bye, sid. ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
[Cameramakers] Cheapo Helicoid' focuser
Threads on sink compression fittings are probably too fine for focusing. they are also from 1 1/4 to no more than 2 1/2 in diameter. Anyone know what the thread pitch is on a medium format helicoid focusing lens? I would guess it would be closer to an Acme thread type pitch (like what you would have in a shop vise) like 8 turns per inch. Metal pipe thread is also too fine me thinks. ABS drain pipe? Probably too jerky even with lubbrication. And again I would think the threading is too fine. 90mm Scheinder Super Angulon focuses at about 100mm this is roughly 4 for Infinity. 1:1 would be 8 and too close for what we're all interested in here. So I would guess for a 90mm lens you'd want to beable to travel about 2? One problem that you'd have with most pipe fittings is that the thread is tapered and only so long--certainly not two inches. the tapering tightens as you screw into the fitting. You could tap your own and go farther but I don't know if it would be worth it. Plus your lens is spinning around with all your shutter cocking, release and f-stops in a different place every time you change focus. I think though an acme rod as a drive screw for focusing on a slider would work but the acme rod is steel and heavy. Rack and pinion are still probably the way to go. Will ---William Nettles [EMAIL PROTECTED] Nettles Photo / Imaging Site http://www.wgn.net/~nettles Find out what's happening in Echo Park: [EMAIL PROTECTED] From: [EMAIL PROTECTED] Reply-To: [EMAIL PROTECTED] Date: Fri, 22 Feb 2002 12:07:08 -0700 To: [EMAIL PROTECTED] Subject: Cameramakers digest, Vol 1 #348 - 7 msgs Depending on how much focusing you need, you might be able to do something with the compression fitting for a sink drain. ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
