Tap for Tripod Thread(and general repairs)

I've been mulling over the state of your monorail mounting block Ben. It doesn't look good, does it?

Given the overall state of it; if it was mine I'd be tempted to grind or mill the bottom flat and open out the hole to take a complete replacement base part with a 3/8" thread in it.

Some work involved, yes, and access to a lathe or CNC miller would be nice, but that mangled block is all that's stopping your camera crashing to the ground again. It's got to be worth considering a better long-term option than a puny adapter barely holding onto a few mangled threads... and forcing you to use a tripod bolt that's really one size too small.

I'm with Joe on this, should be able to find someone who can run up something more substantial on a CNC for a few beers.

I'd happily do it for you, but postage from France may be a bit excessive.

I'd go with a plate that covers the whole of the base and is fixed via four small bolts. Forget the tripod screw (though you can add one if you wish), just make the whole thing an Arca-Swiss dovetail.

Thanks guys.

If my work weren't shut down, this is the sort of thing our machinist could knock together for me for the cost of materials and possibly a few cutters to cover wear and tear on what he'd be using.

I like the idea of directly having it fit an Arca-Swiss clamp.

And a camera that size and weight really should use a 3/8" mounting screw.

I think that a lot of people tend to underestimate the strength of screws. I made some very rough estimates of the load one of those view cameras would put on the mounting screw. It looks to me like there is a safety factor of somewhere around 50 to 100 times of the maximum static load that camera could put on a 1/4-20 screw. (That's with the camera pointing down vertically.) So Calumet seems to have been pretty safe choosing that size.

Not to say that there's no use to having a larger screw - a 3/8-16 screw should give a bit more than double the strength. But it's usual in professionally designed structural parts of equipment that everything is in the same ballpark stress-wise. So if you drastically increase the strength of one part then something else becomes the weakest link. In the Calumet camera, is something else barely as strong as the mounting screw? I dunno, but maybe worth looking at.

As a note, the camera has about a 2" wide foot on the base. So there is abou a 1" lever effect working on behalf of the camera. If you were to make a new base plate, one that is twice as wide, this would cut the tensile load on the tripod screw in half. So another way to get much of the benefit of a larger screw without losing tripod compatibility.

Ps, I'm a bit bothered by the way Ben's insert is broken - an outer part torn off. I'm sort of suspicious that there was not much thread engagement - possibly only one thread, or a bit more? If so, might be worth examining the end threads of the tripod screw with a strong magnifier.

It's still pretty astounding to me that it broke like that. Unless the camera had a long history of short thread engagement with lots of stress applied.

I'm doubtful that I could tear off a flange like that in an install, even with an impact driver. But I dunno; probably won't try it either.

At first I thought it was some sort of low-grade material that was really brittle, but if it's OEM it's probably a decent grade. But who knows? I'd still put a strong magnifier on the tripod screw; if the thread pattern is deformed...?

I think that a lot of people tend to underestimate the strength of screws. I made some very rough estimates of the load one of those view cameras would put on the mounting screw. It looks to me like there is a safety factor of somewhere around 50 to 100 times of the maximum static load that camera could put on a 1/4-20 screw. (That's with the camera pointing down vertically.) So Calumet seems to have been pretty safe choosing that size.

 

Not to say that there's no use to having a larger screw - a 3/8-16 screw should give a bit more than double the strength. But it's usual in professionally designed structural parts of equipment that everything is in the same ballpark stress-wise. So if you drastically increase the strength of one part then something else becomes the weakest link. In the Calumet camera, is something else barely as strong as the mounting screw? I dunno, but maybe worth looking at.

 

As a note, the camera has about a 2" wide foot on the base. So there is abou a 1" lever effect working on behalf of the camera. If you were to make a new base plate, one that is twice as wide, this would cut the tensile load on the tripod screw in half. So another way to get much of the benefit of a larger screw without losing tripod compatibility.

 

Ps, I'm a bit bothered by the way Ben's insert is broken - an outer part torn off. I'm sort of suspicious that there was not much thread engagement - possibly only one thread, or a bit more? If so, might be worth examining the end threads of the tripod screw with a strong magnifier.

You obviously know a lot more than I do about structural engineering and are probably right that a 1/4 " screw should be strong enough, but my 9 and 12 lb. Toyo 4x5s have always been attached to 3/8" screw fittings on my tripods. Watching my first Toyo start to head for the ground because a tripod head rated by the manufacturer for 2X the weight of the camera couldn't hold it at more than a couple of degrees off level sent me off to find something a lot more substantial--a Gitzo Ball 4 that did the job until I got the 12 lb. Toyo. Then the offset design of that head likewise threatened to dump the camera and tripod and I went for a Bogen 3038 that pivoted from the center and worked, if not as smoothly as the Gitzo.

I think that a lot of people tend to underestimate the strength of screws.

Yes, theoretically screw threads are the strongest mechanical devices for their size, but they can be overloaded, so they must be kept within their tensile strength specs to be safe. However, overall strength will be greatly reduced, for example, when a steel screw is holding aluminium parts , the union between the two is only as strong as the soft aluminium. The number of "working" threads is another determining factor for strength. The more threads doing the work, the stronger the joint will be because there's more metal to shear away, which requires greater force.

I need to look at this a bit closer.

TBH, I haven't used the camera a ton, and it's been sitting in the corner mounted to the Tilt-All for a while. In moving a few weeks ago, I folded up the tripod with the camera still attached and set it up in the backseat of my car(there was some other stuff around it). That probably wasn't my smartest moment, but I was also trying to get as much moved as I could. I found it broken like this when I went to take it out of the car.

I've never been SUPER happy with the amount of thread engagement this particular tripod offers, and it was probably made worse about a year ago when I recovered the platform. I used the thinnest sheet of automotive cork gasket material as I had, but it still gave even less engagement.

Someone upthread mentioned that these were originally supplied with a long screw that could be trimmed to length. I have another massive studio tripod around here where, rather than trimming, you can use a secondary collar to change the length of engagement. I probably should take a look the screw on my faithful Tiltall. At one time, I feel like the manufacturer had stuff like this available for purchase, but I can't find it in a quick search. The Tiltall has been my go-to LF tripod for a while, and I feel like an old Marcioni Bros. one is worth fixing vs. buying a new one.

I think that a lot of people tend to underestimate the strength of screws. I made some very rough estimates of the load one of those view cameras would put on the mounting screw.

Be that as it may. Every large format, and some medium format, camera that I've owned or used has come with a 3/8" mounting hole. The larger thread wouldn't exist if it wasn't thought to be needed.

It's not just a case of tensile strength. The camera-tripod interface has to resist twisting and loosening forces as well, and a larger thread diameter affords more angular leverage, such that the screw can be tightened down harder with less effort than a smaller thread. The same could be achieved using a finer thread pitch, but that would tend to bind easier and not offer as great a thread-depth or wear resistance.