Monkey Press Construction
This is a detailed overview of my pneumatic press, complete with part numbers where applicable.
The press cavity is 92″ long x 16″ wide x 12″ high. Total press weight is right around 2,000lbs including the molds. There are two 5″ fire hoses which operate between 50-80psi depending on board surface area. There are two heat blankets, one on top and one on bottom, for a total of 3.5W/in^2. The blankets are 14″x85″, 240V for a total draw of 17.35A. It takes about 10 min to heat a board from 70F to 175F.
The mold is adjustable for different effective edge lengths, and different nose/tail lengths and rises. I tend to stick with a single camber block that has yielded good results for us over a broad range of shapes and lengths. I have an ever growing selection of nose/tail blocks. If a new block shape is needed for a particular build then I’ll design a new one and whip it up on the CNC machine.
The press is designed to handle boards up to 200cm long with ease. It’s built out of steel W12x40 I-beams that are 12” high, 8” wide, 9’ long and weigh 40lbs/ft. There are two beams side-by-side on the top and bottom for a cavity width of 16”. The top beams are supported by two 16” long beams turned on their sides for a cavity height of 12”. That puts the total weight of just the I-beam material in the press at 1,547lbs.
I have used a very common press design, and a frequent concern with this design is how much it deflects under pressure.
The short answer: 1.1938mm (0.047") in the center @ 50psi.
That's essentially how much you'll lose in camber on your board or skis due to deflection in this press. I consider this an acceptable tradeoff for the simplicity of the design, both in construction and operation, and if you care it's easily factored into your mold design. I honestly doubt anyone has actually ever cared, given that there's always some difference between the final camber in a board or ski and the actual mold.
Deflection was measured with a dial indicator set at the center of the bottom beam between the floor and the top flange. Deflection of 0.047" @ 50psi, 0.0405" @ 40psi, and 0.033" @ 30psi. The caliper was checked for square with the flange before measurement, and all measurements were repeatable.
I also took measurements from the top with a mount for a dial indicator screwed to the wall behind the press. Measuring the center, and the ends, resulted in a center deflection within 0.003" of the measurement taken from the bottom. The measurements on the ends allowed me to see the lift of the upper section as a whole as well, which was on the order of 0.45mm (0.0179").
The press is assembled with 1/2″ Grade 8 nuts and bolts. This is a simple, cost effective way for anyone to put a press like this together and get a reliable joint. Welding is another option, but it takes a very skilled welder to produce the proper joint to withstand the pressure generated by the hose. If you’re a good welder, or know one, then that’s a reasonable way to go, and is a lot less drilling. But if you’re not, then a weekend’s rental of a mag drill (less than $100, including the drill bits you’ll chew thru) and $35 for the nuts, bolts, and washers is the solid way to go. You also end up with a press that comes apart, which may be useful depending on your workspace and whether or not you ever need to move the press.
I torque the frame bolts to 40ft lbs to ensure that all of the bolts are under pretty much equal tension once the press is pressurized. There is nothing scientific about the choice of torque, it is just something that felt reasonably tight using a normal crescent wrench. The only goal is that they are all the same.
A little time spent on the air system pays off in simplicity and reliability in the long-term. There’s a little bit of soldered copper piping in the system, but that’s really just for the long runs, and it’s completely optional. The rest is made up of standard brass fittings put together with a little Teflon tape. (Tip: get the yellow stuff designed for natural gas applications. It’s a little thicker and seals more easily.)
Airflow to the bladders is controlled with a hand-operated lever air control valve, 4-way, 3-position, closed center. Part number 3368K26. It’s hooked up so that when the lever is in the center, no air flows. When the lever is to the right, air flows into the bladders, and when the lever is to the left air flows from the bladders out the exhaust.
The exhaust is muffled with a simple sintered bronze exhaust muffler, part number 4450K2. It’s about $2 and completely worth it to save your hearing.
Overall pressure to the system is limited to 90psi with a simple brass pop-safety valve, part number 48435K72.
There is a pressure gauge and pressure regulator on the input side and a gauge on the bladder side to monitor the actual bladder pressure when the valve is closed.
There’s a quick connect on the input side that matches the rest of the air tools in the shop, so a standard shop air supply hooks up to the press. There’s also a similar quick connect from the press frame to the flexible hose for the bladders to allow the bladders to be removed when necessary. This also hooks directly to shop air, just in case.
The single best guide I’ve found for how to solder copper pipe is Soldering and Brazing Copper Tube and Fittings, directly from the Copper Development Association, Inc.
People usually have a lot of trouble constructing leak free bladders. They usually seem to end up with a lot of “goop” involved in an effort to stop leaks at the ends and at the through couplings. The bladders in my press are essentially leak free to 90psi with only Teflon tape.
The ends are held together with standard 1″ angle iron from Home Depot using seven 3/8″ Grade 8 bolts: two on the outside ends, one in between the hoses, and two through each hose. They’re torqued down pretty snug, but we didn’t kill ourselves tightening them. No silicon sealant, plumber’s goop, etc., and no leaks.
The through couplings were more of a challenge. We went through a few tests before settling on what you see below. If you look closely at the full sized image you’ll see a little soapy water around most of it. This picture was taken with the bladders at 90psi. Again, no goop. The secret here is the combination of the three kinds of washers, and the simple fact that the nut for the panel mount coupling is on the outside of the bladder. The washer configuration is repeated on the inside. The 1/8″ thick rubber washers are against the bladder, then the steel, then the Aramid/Buna-N between the steel and the brass nuts. The result is a great seal with the bladder, and a great seal to the nuts. This is not super-tight… just snug, with a little deformation in the rubber washer visible during assembly.
The steel washers are the standard 3/4″ washers from Home Depot. Here are the other parts and the McMaster-Carr part numbers:
- Med-Pressure Extruded Brass Thrd Pipe Fitting 1/4″ Pipe Size, Panel Mount Coupling, 50785K273
- 3/4″ Screw Size, 2″ Od, 1/8″ Thick Large-Od Extra-Thick Reinforced Rubber Washer (10), 90131A106
- Aramid/Buna-N Washer 3/4″ Id, 1-1/2″ Od, .0625″ Thick (5), 93303A317
These parts aren’t super-cheap, but they’re completely worth it.
All aluminum used for mold skins is 0.032” 5052-H32 aluminum that I get from Alaskan Copper & Brass Company here in Seattle. From top to bottom I have the following:
- top MDF mold and 2×4 fillers.
- 1”x16” steel cat track bars
- top heat assembly
- one aluminum skin
- top heat blanket
- one aluminum skin
- board assembly
- aluminum top mold skin
- aluminum base mold skin
- bottom heat assembly
- one aluminum skin
- bottom heat blanket
- one aluminum skin
- MDF mold
This allows for easy insertion of the board into the press after wet layup.
Each assembly is held together with a very simple system: 4 holes are drilled near the edges of the aluminum skins, two on each side, and the two sheets of aluminum with either a heat blanket or the laminate are held together with small lengths of 12awg copper wire, or with zip ties. You can see these poking out the sides in some of the pictures.
Cat track suspension
The cat track is suspended with simple bungee cord, bought in bulk from McMaster-Carr. 8858T21, 100’, red.
In this design the flanges of the I-beams are bolted together, and placed under tension. This means that the flanges of the beams are being used to support the load created by the bladders in between them, and this is not how I-beams are designed to be used. There is a risk that the flange could separate from the web under sufficient pressure. However, with large I-beams this design has proven to work well for me and others over many, many years and press cycles.
Nevertheless, this is a fair criticism of the design, and I encourage anyone who wants to build their own press to consider this, and consult with an engineer to validate the design of your own press.
There are alternative designs which do not suffer from this flaw, and more properly support the upper and lower beams from the outside, rather than holding them together from the inside.