Monkey Press Construction

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This is a detailed overview of my press, complete with part numbers where applicable. All part numbers are for McMaster-Carr unless otherwise specified.


The press is designed to handle boards up to 200cm long with ease. It’s built out of steel 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.

Many question the use of I-beams turned 90 degrees as the separator between the two layers, and yes, in the limit, there is the possibility of minute deflection allowing bowing of the upper and lower beams. In practice, however, this is simply not an issue.


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. Part numbers: 91257A748, 94895A825, and 98023A033.

I put the press on casters, and although the final weight with everything in it is right around 2,000lbs, it’s easy to move. Part numbers 22955T42 and 22955T36.


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.)

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.

Bladder connections

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. These bladders 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. Right click the close-up and open in a new window for a full size view. 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.


I press the average width board at right around 50psi. This presses the laminate completely into the mold, and results in good squeeze out without removing too much resin. I compute the fiber fraction of every board I build and carefully track this to ensure I’m getting the ratio of glass to resin we’re after. For especially narrow boards I drop the pressure proportionally. Pneumatic presses are capable of amazing pressures, but too much pressure is just as bad as too little. Too much pressure results in a dry laminate that is weak and prone to delamination.


All aluminum used for mold skins is 0.032” 5052-H32 aluminum that I get from Alaskan Copper & Brass Company here in Seattle. From bottom to top I have the following: the MDF mold, one aluminum skin, the bottom heat blanket, one aluminum skin, the aluminum base mold skin, the snowboard, the aluminum top mold skin, one aluminum skin, the top heat blanket, one aluminum skin, the 1” steel cat track bars, the bladder, the top MDF mold and 2×4 fillers.

Put another way, it’s the mold, the bottom heat assembly, the board assembly, the top heat assembly, the track, the bladder, and the top filler. 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. 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.