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Tripod remodel

Old tripod with updated hardware

Having a metal lathe is pretty sweet, even if it means having to remake all the things I made when I wished I had a lathe. The cobbled together spikes on my tripod is one such thing, made from a mix of plumbing parts and lag bolts, so I made some new ones from .75″ stainless round stock and some off-the-shelf hanger bolts. They weigh about the same as the old arrangement.

New spikes. Socket side of spike was tapped for hanger bolt.

My ER32 lathe chuck showed up, which is great for removing the stock for operations in other machines without loosing concentricity when you bring it back to the lathe. I was able to do each operation for all three parts at one time without having to do a bunch of repetitious setups. I turned shoulder first, then the 15° taper, cut the parts to length, took the parts to the mill to cut a bolt head pattern in a hex collet block, back to the lathe to bore the socket on the other end and tap them for the mounting bolt.

I cut the shoulder on the taper to reduce weight a bit and still leave enough diameter to cut a 1/4-20 internal thread for a dual-thread stainless steel bolt. The machine-thread side taps into the socket, and the wood thread taps into the tripod leg. The bolt head pattern is to easily install and remove the spikes for maintenance without resorting to a pipe wrench. I’ll thread lock the machine threads into the spike, then screw the whole thing into the tripod leg with a box wrench (I first coat the wood leg tenon with some silicone to keep moisture out).

The carbide insert tooling left an ok finish on the lathe, it was difficult to push to the insert hard enough to leave a better finish. Carbide seems to like a really aggressive cut, which can be a problem with such a tiny lathe. It’s powerful enough, but with the proximity of the tiny feed screws to the bed ways and the other feed screws it’s kind of a knuckle buster. I need smaller hands. No issue on the mill, it has much larger handwheels and plenty of room. I recently got some new end mills from Lakeshore Carbide and really like them, even the 3 flute 3/8″ mill that I got for aluminum cut through the stainless steel like it was Delrin. Using a quality end mill is quite the experience. (Another recent revelation- the Irwin/Hanson taps available at hardware stores are flat crappy and best used for coring apples. High speed steel taps available from places like McMaster Carr or Fastenal are actually cheaper and vastly superior as well. When I first used a decent tap, the hole threaded so easily that I thought I’d way over-sized the pilot hole.)

February 3rd

I made new clamps for the leg stages. Same overall cam design, but tweaked the components. The clamp brackets are made from Delrin, and wrap around the legs instead of using a small flat of 3mm aluminum. Also increased the size of the clamp posts up from 4mm hollow stock to 6mm solid stock- I noticed while taking the old clamps apart that the old posts had bent slightly from the clamping pressure. The new posts are countersunk into both the clamp brackets and cam shaft, and the cap screws are countersunk as much as space allows. The cam shaft is now 10mm, up from 8mm.

Exploded view. The clamping posts are partially engaged by shallow grooves in the tripod legs themselves, which keeps them in place and square to the bracket and cam. The posts were bored on each end to a depth of 12mm and threaded to about 10mm.

Fabrication was pretty quick- working with Delrin mostly- but drilling the cam shafts for the screws and counterbores was a little tricky. 3 different bits and 6 holes per shaft. I didn’t want to keep swapping out the bits for every hole so made a couple of registration blocks from aluminum, drilled a 9mm hole through, then cut a clamping slot into one side. Both blocks together are exactly as long as the shafts, so it was easier to set up a stop block and flip the work to drill the other side, the flip the blocks upside down to counterbore for the posts on the bottom. I found I could remove the shafts from the fixture and still reinstall them without loosing registration. The clamps for the lower leg stage are narrower, so I cut the fixture blocks down to the length of those shafts when I was done with the upper stage.

Also made new cam seats out of Delrin, the ones that came with the quick change skewers were chrome-plated pot metal and have started to rust. One end was counterbored for the cone spring on the lathe, then parted off to final length. Then the other end was cut to match the cam lever profile, using a collet block horizontally on the mill.

I was going to cut two grooves in each of the pivot shafts for snap rings to keep the cam levers centered, but made some spacers out of Delrin instead. They have to be cut to the minor diameter of the lever’s cam  or they will negate the clamping action altogether.

Still need to deburr and buff everything, Delrin throws up a lot of fuzz. Turning it on the lathe is pretty crazy, the chips don’t break so I ended up with a quarter mile of it wrapped around the live center on every pass.

I also really need to refinish the legs, scuff sand everything an apply another few coats of SeaFin, but it’s been so rainy and humid I will probably wait until spring and keep using the Gitzo for now.

February 8th

I took off the aluminum plate to refinish the cherry hub, but it was in pieces. It had been screwed to the aluminum plate, so no telling when the glue had failed. Apparently the glue wasn’t even really necessary- I never noticed any weakness. I considered reusing it, but I wondered if I could come up with a simple but accurate method for making one from a solid piece of wood. I made a solid one from ash on the very first version of this tripod design many years ago. I cut the spokes holding the hub vertically on the table saw with the blade set at 30º, and that wasn’t very accurate or pleasant to do.

[Most of the following is academic since I ended up repairing the old hub.]

I turned some 40mm cherry down on the lathe to a 125mm blank, then laid out and cut a hex pattern on the mill.  I tried to orient the hex pattern so none of the spokes would have a lot of weaker short grain. When flipped, this pattern provides two parallel clamping sides for milling each spoke. My vise wasn’t big enough for both a v-block and the large diameter round, so I clamped some 45º blocks to a plywood scrap to keep the blank from spinning or shifting. To register each side of the hex pattern, I used a 1-2-3 block registered against the outside lip of the vise jaw along with a thin parallel bar as a gauge between the block and the layout lines. Pencil lines aren’t very accurate or reliable- at least mine aren’t- so I went around once cutting a millimeter outside the line, then took a final pass on each side without changing the depth or position of the cutter, with the parallel bar now acting as a registration stop for each flat of the hex.

Forgot to take a picture of the setup, but to layout the spokes, I first took the intended width of each spoke and used that dimension to scribe the inside circle. I marked them  slightly oversized just in case there was any layout error. Once clamped on the on the mill I found the center, setup a stop to register the blank’s location in the vise, then cranked the Y axis up to the outside edge and marked the center of the spoke. I rotated the blank 120º to find the center of each of the other spokes. With the same compass setting I ticked off the outside edges of the spokes, then joined those to the circle with a straightedge.
After drilling a test hole to see how cleanly my 12mm end mill would plunge cut, I drilled a hole in the crotch of each spoke. Using a mag indicator stand as a vise stop isn’t ideal, but will have to do until I make something better. Some of the holes appear to be inside the lines, but I trust the vise stop more than my layout.

Cutting the spokes using Y feed. I cut one side of each spoke first, the moved the table and stop for the other side. Once I was satisfied that each spoke was symmetrical and parallel, I locked down the X feed and quill and cut each one to final dimensions.

Finished blank after cutting away the hex pattern. Turned out very well, all the spokes are within a tenth of a mm, and the carbide end mill left a pretty finish with no burn marks in the cherry. The hardest part of all this was turning such a large chunk of wood down on a tiny metal lathe. Still needs some sanding and edge work- not to mention some holes. Boring the center for the head mounting screw will be easy on the lathe, but still pondering how to bore the spokes for the leg pivots.
Note the old hub in the background, showing the glue line failure. But despite this I think this may be the better method, mostly because of the long straight grain in each spoke. It’s also quite a bit easier to make, just cut a double miter on each spoke, then drill each of the leg pivot holes before glue up. For drilling the head mounting screw, the intersection of the miters makes finding the exact center pretty much foolproof. The end grain isn’t a great gluing surface, but if it’s attached to a solid top plate it’s pretty strong.  And it certainly wastes a lot less wood. I used a 150 x 150 x 40mm chunk of cherry for the solid hub, this method only required a 50 x 250 mm piece 40 mm thick. It might be worth playing around with the failed hub, maybe make a horizontal spline to rejoin the spokes. This would provide a lot more gluing surface, and once cured I could re-bore the center.

New aluminum plate for the head. The press-fit Oilite bushing will protect the wood from the head mounting screw thread. I also turned some  6mm Delrin to use as a bearing surface. My Ries head doesn’t rally pan unless you loosen the mounting screw, so this may allow for a little smoother panning.

February 9th

Blind spline. Pretty simple to make, just clamped the parts inline on the mill and cut a 6mm slot, being  careful not to cut past the corners. I used some Baltic Birch ply for the spline, which should be stronger than a small piece of hardwood. I bought a few of these strap clamps on sale a few years back but this is the first chance I’ve had to use one. I also sandwiched the hub in a few scraps of melamine and clamped them together to keep it flat.

Since I already had so much work in to the other one, went ahead and bored the holes, radius’d the corners and finished it with teak oil. Also made a new knob for the head mounting screw.

It was tricky center drilling and counterboring the hub  on the lathe, had to use some brass shims on the 3 jaw chuck to get it somewhat centered again. I swept the outside edges of the spokes with a dial indicator and tweaked it for 20 minutes but the best I could do was 0.25 mm runout.  This was more for practice than anything, the hole needs to be oversized anyway to keep wood movement from clamping down on the head bushing and splitting the hub into kindling.

Drilling the leg holes square to the spokes was simpler than I thought it would be, I clamped the hub on a pair of parallels, then checked registration with another parallel on top of the vise.  I’ve been using R8 collets instead of the cheap drill chuck that came with the mill. A set of 30 was cheaper than a decent  chuck. Collet runout is about one hundredth of a millimeter, and they give you more headroom on the Z axis, but the set does take up a lot of shelf space.

The knob was turned down from  40mm aluminum square stock.  I center-drilled and tapped the block on the mill first, then threaded in a 3/8-16 stud and jamb nut and put it in a collet for turning on the lathe. I should have cut out some of the waste first on the mill, it took a while to cut the shoulder on the lathe. Once this was cut, I put it in a square collet block on the mill and scalloped the sides with a 50mm face mill. I cut a little step in the top, then back on the lathe I domed the top using a shallow taper on the compound slide. Just need to get a 3/8-16 stainless steel stud and Loctite it in the knob.

February 11th

Put everything together this morning. I ended up reusing the old hub-  the outside diameter of the new hub was just big enough to interfere with the stops that locate the fully retracted position of the legs (metal rod between the outer legs, just below the hub). Not a difficult fix, just need to make a deeper edge radius on the bottom of the spokes,  but it would mean more teak oil and waiting for that to dry. I’m ready to be done with this.

Done, except I’ll probably make some soft spikes for indoor use. Since the spikes can be unscrewed from the hanger bolt mount, it would be easy swap them out with some rubber feet. Instead of using silicone to seal the wood leg tenon, I just used some O-rings to keep the moisture out.