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5×7 diary, part 3

After almost 4 years of use, I thought I would make a few improvements to my shopmade 5×7 camera.  One thing that always bothered me is the front standard. The gimbal swing is great, and the movements work well, but the rise and fall stanchions are a little anemic, should have used 3/16″ stock instead of the 1/8″, and should have made the anti-torsion bar that joins the stanchions at the bottom a little more burly as well.  There’s too much flex in the standard.  Also, the front shift platform is awkward. I stupidly economized on aluminum plate stock, getting only a 12″ piece of 1/4  bar stock for both the front and back shift platforms, and by the time I was done with the back the piece I had left over was about 1/2″ too short for the front. So instead of just ordering another $5 worth of material from McMaster-Carr I went ahead and used the short piece. Usually I’m not this cheap- maybe I’d already milled the dovetail slot before I realized it was too short. Regardless, the front standard pivot brackets overhang the platform, which not only looks goofy, but there’s also nothing to indicate a neutral position by feel, you have to eyeball the dovetails being flush with the platform. I thought about installing a detent, but due to the location of the dovetail slide and shift slot there’s no good place to install one. The furnace is on the fritz in the house, so as long as I’m cold I might as well spend some time in the shop.

Undersized, underthunk

I had ordered some 1/4″ precision ground aluminum plate for the scanner carrier project that I never used, so milled some new parts for the front platform from that. Towards the very end of milling the new dovetail slot I sheared all the teeth off the bit. (I’m still a rank amateur at metalworking, but the metal 45° dovetail bits don’t work very well on slots that are only as wide as the bit’s diameter. I know enough to always hog out the throat cut first with a straight bit, but I think that the slot needs to be wider than the diameter of the dovetail bit in order to take light passes and not have to cut the full dovetail on both sides of the slot in a single pass*. It may be ok on a massive mill with a good coolant and chip clearing system, I don’t know, but on my rusty micro mill in a dark corner of the shop with me hovering over with a shop vac not so much.) I tried an old wood dovetail bit on the ruined part as a test and it worked surprisingly well, so milled a new blank and started over. The slope is a lot less than the metal bit, 14° vs. 45°,  but this made the mating pieces easier to fit and there was a lot less stress on the bit, and the slope is more than adequate to resist pulling forces and keep the slide tight to the platform.

autocrank
Ersatz power feed

I’m almost glad I broke the bit because all the tedium of cranking the mill table to make a ruined part made me finally try something I always wanted to do- remove the crank wheel on the X axis and chuck a cordless drill onto the lead screw. I almost cried it was so awesome. Tasks I usually dread like squaring long edges and milling long slots are now just merely dull instead of Sisyphean. Also, using a motor makes it possible to snug down the gib screws to further reduce X/Y slop on the table, which further reduces vibration and makes for better cuts. It’s a bit of a hassle to swap back to the crank for precise positioning, but totally worth it for long slots. I may replace the x-axis lead screw for one that can be adapted for a permanent power feed.

I got some 3/16″ stock for the new stanchions, and used the existing stanchions for a template. Encouraged by the dovetail bit’s performance, and the newfound ease of taking a million light passes, I tried a wood bit to mill the torsion bar as well, but the metal-working bits do work much better for general milling.

 

 

sideby

So far so good, but if I screw this up I can retreat to the old parts for reuse. Hope I don’t have to though, the difference in sturdiness is night and day, even after adding another wooden strut to the old one. I used 1/2″ x 3/4″ stock for the anti-torsion bar, and milled some fall clearance for the swing knobs on the bottom of the gimbal frame. The hard part was tapping this to attach the stanchions, had to set up a vise to hold the bar on end at the drill press. It would be nice to have a right-angle head for the mill or a lathe for horizontal boring.  All in, the new parts make it about 4 ounces heavier than the old set up. I may drill some holes in the torsion bar to reduce weight a bit.

roomy

Rough fit. Plenty of extra material on the shift platform. The loose-looking bottom screw on the torsion bar will be the pivot point, and will attach through the pivot bracket. I have since added another screw to each side to strengthen the connections, since the pivot screw can’t be fully tightened down.
Still need to clean up the slots with a file and diamond stone. The end mill bits I’ve been using aren’t great at cutting both sides of a slot at the same time. Something I just learned about milling in general- I should be using an end mill smaller than the finished slot to rough it out, then slowly expand the slot on the Y-axis by taking very light climb cuts on each side until the final slot width is reached. Climb cutting produces a nicer finish than conventional milling, but must be done in very light passes, the direction of the feed can cause the table to lurch if taking too much material in one pass. This causes the bit to deflect and produces a pretty crappy finish, and may even spoil the work piece.

tapjig

Front shift platform and pivot brackets get clamped together upside down to a registration block, then moved over to the mill to pilot the holes in both pieces at once. I forgot to take a picture of the actual setup until after everything was done, but this procedure worked pretty well, so reenacted this shot for future reference. At the mill a clamp goes on end to end to hold the brackets tight to the block, and the mill gets clamped to the X table. The pivot holes aren’t drilled until after the brackets are tapped and test fit onto the shift platform.

A few thoughts about pivots. A frequent mistake I make is is to drill the pivot points into the base tilt brackets first, then attach them to the shift platform afterward. It’s difficult to ensure that everything parallel, square, and folds up correctly without stressing the parts or restricting the movements. For this new front standard I made a wooden block that’s the correct distance between the brackets, then rabbeted a recess on each end for the bracket bottom flanges. That way the shift platform and brackets can be clamped together upside down at the mill for drilling all the pilot holes in both parts at once. After the holes are tapped in the shift platform, the holes in the bracket are countersunk for the screw heads. After the brackets are attached to the shift platform and checked for registration and symmetry, the brackets are removed, ganged together, and then as the last step the pivot holes are drilled through both parts at the same time for the base tilt pivot and the brace pivot. It’s always a good idea to drill both sides of a pivot point- or for that matter any movement- at the same time, this helps ensure the parts are symmetrical and that there is no misalignment when installed. But doing this last, after everything is checked for registration, makes the process a lot easier and gives much better results.  Even if it’s a off 1/64″ it can make it can skew the movements and even cause stress to the parts.

Most pivots on field cameras are also attachment points. For the pivots themselves, I plan the hole size to accept a sleeve bearing. Plain screw thread makes a really bad wear surface, and will chew up and enlarge the hole over time, and it’s also impossible to get the fit tight enough to prevent slop without also destroying the thread. If the pivot hole is even slightly oversized, the whole standard will feel loose even when tightened down. I use nylon screw insulators (bearing sleeves with flanges), they are available for every screw size.  They make decent wear surfaces, and are easily replaced if needed, and they never damage the pivot hole. I get the next size smaller (4/40 for 6/32 screws) for the pivot I’m using, then tap the bearings with the screw itself so there is absolutely no slop in the fit. A little dab of dry teflon film lubricant at the hole will allow the bearing to turn in the hole. A little thread lock on the screw will prevent the actual use of that movement from unscrewing the connection.

The exception to this is the base tilt brace for the front standard. Since there is really no room for a nut between the bracket and the standard stanchion when closed, I use a stainless steel rivet for the pivot point. There is a washer between the brace and bracket, and the excess rivet sleeve is ground away after the rivet is cinched down. Once filed down to about 1/32″, the sleeve is flared with a punch to lock it in place. Again, a little dry teflon lube at the pivot makes it spin nicely. This has always struck me as an unnatural use for a rivet, but I’ve been doing this for many years waiting for inspiration for a better method to strike, but it has served well so I keep doing it on every new camera. The only downside is it the rivet has to be drilled out if you need to take the connection apart, but it’s easily re-pinned, and I’ve never had to take one apart unless I was recycling the parts for a different camera.

rivet

rivet2

rivet1

Pivot rivet. The thumb screw and nut in the top picture are fed through the other pivot hole only to keep the tilt brace stable and parallel to the bracket during riveting and filing. Once cinched, the rivet ball gets knocked out with a punch, then the excess sleeve is filed down and rounded over. Finally a few more taps with a tapered punch flares the sleeve to lock it in place.

detail

Detail of the finished standard. Shift platform is now flush with dovetail for easier centering. I was going to install a ball detent to locate the neutral position for shift, but will save that project for warmer weather.

folding

Brackets all line up nicely when closing.

I was going to mill some slots in the torsion bar to reduce the weight a bit, but blew the fuse in the mill while practicing on a test piece. Not wild about the look of the bar itself, should at least ease the edges and corners more. But it is really sturdy, especially with the new thicker stanchions. It’s a nice improvement.

scuffbar

folded

Also added a scuff guard to the rear standard frame so the wood doesn’t get chewed up by the  pivot brackets when closed. It’s a piece of 1/4″ aluminum channel that’s recessed into the wood for clearance under the latch.

I finally added some neutral position notches for both the front and rear standard base tilt. I’m always reluctant to do stuff like this because it’s pretty easy to screw up the layout and then you end up fighting the notches to find true neutral. I took some time to level the stone slab, I found plumb on each standard by using a level as well as a large drafting square registered to the slab and the camera to make sure they agreed with one another, then carefully marked the struts on each standard for the notch. I used a triangle file to start each notch (flat of the file registered against the opposite side of the strut slot and file’s vertex cutting the side to be notched), then switched to a 4mm round file (roughly the thread diameter of the locking screws) to cut the notches to a depth of about half of the locking screw diameter. Filing too deep creates too much slop in the neutral position. Works well, checked the results by repeatedly opening the camera and checking the position of the standards with the square and level.