“Have you ever wanted to lay out the frustum of a cone on a flat surface?”
“A what?”
“A frustum, the part of a cone that is left when you lop off the top.”
“Oh, yeah. If I had a nickel... sadly, I’m still poor.”

It can be a little intimidating if you’re not familiar with the concept of how to go about it. Our draper came to me wondering how she could accurately lay out a dress based on an actor’s measurements. (Apparently the traditional layout method wastes fabric.) So I came up with this simple excel sheet that does the calculations based on the following measurements: waist (small circumference), hem (large circumference) & inseam (height of the frustum.) It gives back the details needed to lay the cone frustum out on a flat surface. The diagram below gives the general gist of what’s what. There is a larger version of the image in the excel document.

Frustum.xls
From fabric to sheet goods, I hope this calculator finds some use.

Found out a couple of pitfalls when building mirror flats with mirrored acrylic skins. Our usual acrylic adhesive is a two part methacrylate based adhesive. It works incredibly well when bonding acrylic to just about any substrate (steel, wood, other plastics...) Some formulations cure to a relatively clear finish, which is quite the selling point. Unfortunately, we found that methacrylates will distort the mirror coating on mirrored plexi. Whoops. You can see the distortion in the picture below. It’s probably better to stick to liquid nails next time. As a side note, 3/16” mirror acrylic does not look great with toggles on 2’ centers; I recommend a thinner acrylic sheet contact cemented over a plywood skin to realize a flatter surface.

Here’s another choice for edging a deck and other scenic elements: paper. We recently did a production of Bad Dates and the designer wanted a smooth black finish for the “cut” edges of the walls and deck. Since the audience was sitting only four feet from the set, we experimented with wallpapering with a 50# black kraft paper. The advantages were as follows.

• the paper is thin enough to make the edges very clean
• the texture was very flat, smooth and consistent - much like bristol board.
• there was no nap to deal with
• one roll of 12” x 750’ paper clocked in at 14 bucks. (without shipping)
• the paper allowed us to effectively disappear the seams of the planking and luan skins

All in all I was very pleased with the result. I think it was a better, more consistent finish than if we’d puttied, sanded and painted. If you go this route, be sure to do samples first! I found the paper at Quality Paper.

File this under great hardware finds. - I needed low-profile, high capacity wheels for an effect and stumbled onto these. They are load wheels for a pallet jack (the wheels pictured are Crown part number 44506 / McMaster 2670T58. The yellow caster mounts are shop built.)

44506 Load wheel specs:

• 2500 lbs load rating!
• 3” diameter x 3 7/8” wide
• Roller ball bearings
• Shore 90A polyurethane tread
• Slightly oversized 3/4” shaft (about 25/32”)
• $30 each from Mcmaster
• $45 each from a Crown distributor (price will undoubtably vary)

What makes these attractive? Comparably rated caster wheels tend to be much larger in diameter, cost more, and don’t usually feature roller ball bearings. (Mcmaster’s “High Capacity Nylon Wheels” come close, but they are a Shore 80D. That’s as hard as a hard hat!) Crown lists these wheels as “load wheels”, McMaster lists them as “Polyurethane-Tread Pallet and Lift Truck Wheels”.

Here’s a link to Crown’s wheel and tire catalogue (PDF.)

If you want to do your own research, a quick list of some pallet truck manufacturers: BT, Crown, Hyster, Lift-Rite, Multiton, Prime Mover, Raymond, Rol-Lift, and Yale.

Danger Will Robinson! The cast iron wheels pictured below are the ones that were spec’ed in Keith Cornelius’s 1997 Tech Expo article An Inexpensive, Human-Powered Roll-Former. (Grainer part numbers are #3G262 and #3G263.)

Obviously they’ve changed a little bit since 1997; the new “feature” is a crown to the surface of the wheel. Unless you’re looking to mangle your steel whilst bending, I suggest you find flatter wheels.

I suggest finding flatter iron wheels through Mcmaster-Carr. Their 4” wheels are comparable in price, though Mcmaster’s 5” wheels are around $30 while Grainger’s 5” are around $13.

This is what a rotary milling table looks like after disassembly. Sweet.
Click the pic for a larger version.

How much force does it take to bend 3”x16ga box tube to a 5’ radius in one pass? What’s the largest steel shape you can jam in a typical scene shop-built manual roll bender without cracking a roller? A week ago I didn’t know the answer to either one of these questions, now I’ve got a pretty good idea.

We’re talking about making a roll bender at work, and are waffling over whether to build a manual or motorized bender. I realized that we didn’t really have a handle on the forces involved in bending various steels to various radii; these numbers are instrumental when making this decision. (Not to mention during the design process.) Earlier this week I sat down and created an excel file to shed some light on the situation. Figured I might as well share it with my peoples.

Snag the files here: XLS or PDF

The majority of the worksheet calculates for:

1. The force required to bend a variety of steel shapes to various radii in one pass (à la motorized bender).
2. Adjustments for three different roller/die distances: 12”, 16” & 20” apart.

The manual pass sections are intended to show how much force would be needed when making the first pass in a manual roll bender. It calculates:

1. The force required to reach the yield point of each steel shape.
2. The resultant deflection and radius achieved in the first pass.
3. The above calculations adjusted for three roller/die distances (12”, 16” & 20”).

Big effin disclaimer!
Lets be honest folks, I’m not an engineer. Confidentially (just between you and me) I am confident that the calculations are fairly accurate. But under no circumstances will I claim that they are 100% accurate. The reason is this: the last thing I need is a lawsuit because someone too lazy to verify the math took it in the eye from a flying bit of steel. Don’t you dare cut corners.

This worksheet is only intended to give a general idea of the numbers involved when bending steel. It cannot be counted on to give precise data concerning specing and designing of a specific machine. There are no allowances for the inherent springy-ness of mild steel, dodgy steel quality and varying manufacturing tolerances.

Bending steel is indeed a fudgy art. But now you’ve got some numbers.

Quick thought on the porta-band stand described in an earlier post, mounting the saw to the vice-mount takes a couple of minutes. You could make a smaller custom foot that always stays on the saw and “dovetails” into the vice mount. A single machine screw could be used to secure the small foot to the mount. This way the saw could go from case to vice (and back again) in under 30 seconds.

Ian made this custom foot for the Porta-band that clamps into the vice this past week. Its 3/8” cold roll plate with a bit of scrap 1x box tube welded to the underside. A spring clamp on the trigger acts as a low tech on/off switch.
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There’s a great instructable on how to TIG weld on the loose. Those who’ve already mastered any other welding process will probably enjoy it. While you’re at it, check out Cheap Welding for Punks.

While in grad school I became a fan of what I like to call Past & Punch Templates. When you need to fabricate a part that is particularly funky or needs to be exceedingly precise, you print a full sized drawing from CAD and past it onto the material that needs to be machined. Fairly simple right?

For years I’ve used spray 77 to adhere the template to the part, but then you have to deal with removing the adhesive from the part when it is going to be painted or moves against another part. A step I feel too lazy to take at times.

Last week I got to thinking how ideal Post-it glue would be for this kind of work. Post-its don’t mar other surfaces, and it is a hell of a lot less hazardous than spray 77. If only they sold the glue without the Post-It. Well they do (kinda, see here). You can find the adhesive at Amazon. It comes as a glue-stick that couldn’t be easier to apply (added bonus: it brings back memories of kindergarden.)

Perhaps this could be used to afix routing templates as well. I haven’t tried it yet, but it sounds good in theory. I’m ordering a stick this weekend and I’ll let you know how it goes. So as to not sound like a 3M shill, you can also Google “repositionable adhesive” to look for similar products.

Fun links:
Electron microscope image of Post-It glue.
Wiki link on Post-it notes.
Re-stickable Glue Stick MSDS

Update for “Pivot Puck” post. -- We installed the pivoting masking wall today, and it was a smashing success. Pivot pucks are definitely going into our stock inventory.

We need to pivot a few masking walls for our upcoming production of Hughie. So this little UHMW bugger was born; I like to call it a pivot puck. Screw the puck to a surface and place a shaft in it (our shaft is welded to plate steel that bears on the top surface of the puck) and you’ve got an instant pivot point! (Just make sure the screw heads are countersunk below the surface.) It’s so sexy that it may become a regular stock item in our inventory. I will post an update after we use them for the show.

Continuing in the pivoting vein, these are the pivot points we are using for the top of the masking walls I mentioned above. If you haven’t seen the half cheesebouroughs before, they are hard to beat when attaching stuff to pipe.
edit: Oh yeah, you can find them at BMI.

Last summer I was at a constuction site and saw these little buggers. Brilliant. Head on over to Mcmaster-Carr (or your favorite supplier) and check them out. Sold as slotted shims and horseshoe shims, they are available in a pleathora of thicknesses, sizes, and materials. They're color coded, standard thicknesses, and reusable. Perfect for leveling a show deck, layout work, or (heaven forbid) getting fugly platforms to bolt together tightly. Also, you can find them in variety packs. (Images were unscrupulously ganked from Mcmaster.com)