Century Semi-Centennial Stand Repair Question

[Peter De Smidt]

I'm working on getting my Century No. 7 8x10 camera up-and-running. It has one of the Semi-Centennial bi-post stands. I'll have to build a new camera platform for it. I have a question about the threaded rod for adjusting the tilt of the table. Here it is:

http://i955.photobucket.com/albums/ae37/peterdesmidt/Century_Threaded_Rod_zpsnf8rkafq.jpg (http://s955.photobucket.com/user/peterdesmidt/media/Century_Threaded_Rod_zpsnf8rkafq.jpg.html)

The bushing (?) moves smoothly to the left, but it only goes as far to the right as shown. It looks like the threads become less deep there. Is that normal?

[Tin Can]

No. You could take it apart and use a thread file, to fix only a couple tight spots. I don't have threading dies that fit that pitch.

Thread files are very cheap and very useful. I suggest you use mine or buy the 4 common thread files i know of.

http://www.amazon.com/Nicholson-Thread-Repair-Single-Square/dp/B001T4KRHC

Even if the thread file and bad thread are a slight mismatch, i am usually able to fix a thread.

[Steven Tribe]

These are acme threads - or, at least, should be!

[Peter De Smidt]

Thanks, Randy. I'll stop by the hardware store and see if I can figure out what thread it is. The outer diameter of the threads on the left is 0.485". On the right is 0.491". On the left the valleys are 0.420", and on the right they are 0.440". It looks like about 16 turns-per-inch.

[Tin Can]

These are acme threads - or, at least, should be!

That's a good tip, as I didn't know what they were called, but plenty of info on Acme threads online.

McMaster sells new ones. http://www.mcmaster.com/#=wkevzt

[Steven Tribe]

There is an old thread on this subject. I cast the fitments for the titling mechanism in aluminium for a "simplified" bi-post that was missing all the parts. There were plenty of postings about the threads and where to find them.

[Tin Can]

The right guy could put that in a lathe and fix what you got.

The right guy is in Milwaukee area. There are many.

[Peter De Smidt]

I appreciate all of the pointers. I did do a search before asking, to no avail, but now that I knew there was one, I was able to run it down. I would prefer to fix the old part. If I can't, I'll use a new Acme part. Thanks!

[Harold_4074]

Actually, from the looks of it (and the corresponding screw on my stand) that is not an Acme, but a multiple-lead V-thread. I seem to recall that there are four starts, so the measured 16 threads-per-inch gives the same travel per turn as a 4-thread-per-inch screw would.

The suggestion is sometimes made to replace the screw with a piece of single-threaded rod, but this has three disadvantages: the threads in the traveling block would have to be replaced; a given movement of the table would require four times the number of handwheel turns; and the mechanical advantage would be four times a large, raising the possibility damaging the stand at either end of the movement.

Since the stand presumably worked fine when originally manufactured, and most of the traveling block movement was over a short portion of the screw, the problem may be that the less-used parts of the screw are heavily coated with ancient oil, soot, and dirt. The "working" part of the screw on my stand measures a few thousandths smaller than the ends, but that seems mostly to be due to wear; if you thoroughly clean the screw (lacquer thinner and a fine wire brush) you may find that the block will run to both ends again.

If there is actual mechanical damage to the threads, then Randy Moe's thread file suggestion is right on; there is a lot of bearing surface, so excessive clearance is not likely to be a problem.

[Jac@stafford.net]

Glad this issue is being sorted out.
I replaced the whole unit with modern parts.

BTW - is that a Great Pyrenees looking on?
.

[Peter De Smidt]

Yes, Cotton is a Great Pyrenees. He just turned 1 year old.

I'm going to try the wire brush and lacquer thinner today. I had tried a nylon brush and soapy water earlier.