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Thread: Pressing wood: pressure vs vacuum

  1. #1

    Talking Pressing wood: pressure vs vacuum

    In the post about "first time veneering"
    I commented that for small flat things, air pressure is often more convenient for me, that vacuum.

    Naturally that resulted in a request for explanation, so starting a new post that can be searched separately.

    Caveats: i started out vac-bagging 3D items and sometimes still do. But my shop is too cramped with machines for a full flat press, and most of my flat press work fits within about 32" x 48" or smaller. If i get bigger flat work, it is almost always in the long dimension. I've evolved a number of pressing options (no pun intended of course) depending on the app. including 3D work & moderate compound work. It will take a while to find photos.

    To start, here is simple flat pressing using collapsible discharge hose with about a 65 PSI rating. It comes in lots of pressure ratings, getting lower as the diameter increases. In diameters up to at least 16" (25" wide when flat). The old firehose presses used literally a layer of parallel firehoses, some of which have 150 psi ratings to raise a platen.
    Anyway, for this project, i was only using about 35 psi. It was more about the convenience of pressing a stack of parts flat.

    You have to understand why this one happened, though:
    Many of you will remember - starting to recover from covid, the war starts, and micro-lam birch plywood becomes unobtainium.
    This project was a very simple bookcase but the specs included no movable shelving, and no back. Have you ever assembled something that size in 3/4" lumber, touched it, and watched it stand there and oscillate back and forth like it is made of rubber?

    There were 2 units including a smaller wider side stand.
    The parts needed to be at least 1" thick both for visuals, and to approach a semblance of structural rigidity.
    The unit had to be delivered down narrow hallways with multiple turns, impossible for the assembled parts or any major assembly. IOW KD.

    There was no more 1" thick microlam anywhere at any price. Virtually no 3/4." Vanishing quantities of 1/2" at exorbitant prices.
    But there was a glut of 3/8" at a price that didn't quite make your nose bleed. After thinking about it, it also occurred to me that laminating 3 layers could facilitate some aspects of the commission including judicious arrangement of some of the cores .

    First photo shows some of the center cores, with cross-grain orientation. 3/8" plywood sheets that have been finger jointed end-to-end, being calibrated in the widebelt. (to make all uniform thickness)

    core splice.jpg

    Next shows dragooning the 92" table on my planer as the flat platen, with 2 lengths of collapsible discharge hose on top. IIRC, there were 3 pressings, of multipart stacks. About as many in each pressing as i wanted to sling glue for in one go.

    planer firehose press.jpg

    Parts out of the press

    laminated shelf ends.jpg

    Nothing to do with pressing, just explanatory photos of "why"


    More later.
    Last edited by stephen thomas; 06-22-2024 at 12:00 PM.

  2. #2
    Join Date
    Nov 2009
    Peoria, IL
    I'm just a big fan of the complete and equal pressure over the entire surface with a vacuum. Pressure depends on the quality of the cauls and number of clamping devices to try and distribute an equal pressure everywhere.

  3. #3
    I found some pix and scans of contour work, where vac was tried, and was too inconvenient, as well as unreliably low pressure.
    Once wood needs bent in compound curves, equal pressure is not always ideal. For many simple one-direction curves, vac would seem ideal, except is is such a nuisance keeping the bag from importuning itself under the part as the vacuum draws down before the part is fully bent. If there are protrusions that need protected with extra peel-ply and bleeder fabric to prevent the bag from tearing, that can be a factor, too. Later posts in this section will show a direct comparison between vac and pressure for the same project.

    First the finished product so the build part will make sense later.

    Brief by designer: "How thin can you make a shelf? It has to look really thin at the front"
    Me: "how thin do you want"
    Her: "It has to be up to 12" wide, and hold up a ton"
    Me: "Literally?"
    H: "Well, they collect glass. Some of the pieces are massive. Like solid shaped blocks of glass with carvings. I want wooden shelves. No metal. No glass shelves. No visible means of support. The work should appear to float"
    M: "Hmmmmmmm....."

    H: "Look, here's a plan view, the shelves get wider as they go up. Here is the elevation. Don't mess with the spacing - it matches exact pieces of glass. Call me back in a couple days"
    M: "OK"
    H: "remember, no visible support!"

    Meeting a few days later:
    M: "made a mock up, i think i can do this with a stressed skin torsion box. What do you think?"
    H: "I like it, the clients will love it"
    M: "well, that's encouraging"
    H: "I did forget one detail. It has to be cut to have LED lighting at every shelf"
    M: "That's going to ruin the torsion box!"
    H: "you'll come up with something. You can have a couple more inches at the back of the shelves if it helps"

    J & M entertainment unit 2.jpg

    J & M entertainment unit 3.jpg

    J & M entertainment unit.jpg

    It is quirky from a fixed view, but moves with the viewer a bit and always has very thin perspectives toward some section, so seems to stay interesting.
    Anyway, on with the pressing, later.
    Last edited by stephen thomas; 06-22-2024 at 9:55 PM.

  4. #4
    Now the messy part. How sausage is made.

    I've done 3D/compound curves on large pieces (curved church pews) with vacuum, but the layers were solid wood thin laminae which does give and compress.
    This compound curve is maybe deeper than it looks, and already a 1/8" plywood skin. I thought about how to react vacuum and not have the top curve to meet the bottom.
    Despite the random chaos of how the photos look, it was fairly well planned, and planned to minimize wasted material and effort.

    For the strongback form/table there is a W4 x 16 (Steel "I" beam) under the long run, and wooden beams under the short wings. All covered with 2 layers of 3/4" plywood worked and handplaned flat.
    The top surface of the structural shelves is 1/2" baltic birch with the edge beveled down to about 1/4". The ribs are 3/4". Any sort of cheap convenient material is used for reaction sheets against the bladders.
    The top shelf & second shelf were each made in 3 pieces connected with zip bolts to facilitate installation.
    The lower shelves were made as single units.



    DSCN0020.jpgglue up 3.jpgglue up 4.JPGglue up 5.jpg

  5. #5
    The single piece shelves take some interesting bladder arrangements & the contour is the most risky - the 1/8" sheets are pre-scarf mitered for strength, and will take the deepest compound bend in one shot.

    DSCN0014.jpglast clamp.jpgsttealthflight2.jpgDSCN0005.jpg

    Next/Future date: direct comparison alternate curved lower doors by vac, then by pressure.


  6. #6
    Richard -
    There are lots of times vac will be easier. Much depends on individual shops, specific projects aside.
    Both are tools to be applied when whichever is most convenient - or foolproof.
    I'll get to the flat presses for small work later, which was the comment that led to this.
    I have found that for myself, after years of using vacuum, my laziness impulse inexorably leads me to pressure when possible.

  7. #7
    Stephen Thomas, That is very impressive work! I spent many hours in church pondering how curved pews are made (while I waited for the clock to turn). I remember reading about and being amazed by techniques for laminating curves shown in Wooden Boat Magazine. Wooden Boat showed a Constant Camber dingy being made with wood veneer using vacuum. It is an impressive article and a very neat boat.

    I can't find the constant camber dingy on the web. This shows the concept.
    Last edited by Maurice Mcmurry; 06-23-2024 at 7:25 AM.

  8. #8
    Years ago I was in a piano rebuilding shop and saw a setup for glueing bracing on soundboards using fire hose. Simple and quick.

  9. #9
    That's very cool. Stephen. Similar to the Eames chairs It's often said that wood cannot be bent to a compound curve but it ain't necessarily so. I must say my own laziness impulse leads to vacuum pressing whenever practical as it is so simple and quick to set up and avoids pulling out any clamps, but your pneumatic technique makes more radical bends possible with the higher available pressure.

    Before "cold molding" with epoxy became popular autoclaves were used for forming boats and airplane parts, a more elaborate and capital intensive version of your methods. Note the careful attention to personal protection equipment in the video. With this method a vacuum still needs to be pulled on the work between mold and membrane so it might be thought of as vacuum pressing with a higher ambient air pressure.
    Last edited by Kevin Jenness; 06-23-2024 at 12:55 PM.

  10. #10
    The curved doors for the lower cabinets seemed to be a perfect fit for vacuum bagging:


    The big nuisance is dealing with the peel and bleeder plys and manifold. Then managing to get it all in the bag and sealed. I've been mostly a one man shop for the past decade, so things like that are a factor especially as work gets larger. (Work i did not think was large 20 years ago now feels like it is, too. ) If you do a range of sizes, fabric is constantly being consumed, or needing storage.
    But it works well, so long as the fabric layers are kept from intruding between the form and the lay-up as the vacuum is drawn down.


    Pressure, OTOH, takes a cover sheet, some creative clamping at times; and might benefit from side guides at times. But overall, especially for multiples, it always feels faster and less stressful to me



    Next/Later - pneumatics & platens for phlat stuff

  11. #11
    Kevin - that boat link is great! Just Friday night, a bunch of us were at a steamshop meeting of machinists, pilots, boat builder, and woodwhackers, and discussing WW2 - 50's era "plywood" moulded boats. I'm going to send your link to one of the boatbuilders, whose dad got them a moulded plywood boat back in the 50's. (We are all aware of the modern cold moulded types esp Gougeon.)

    I'm more familiar with airplanes - Fairchild built some really big airplanes in the early 50's with a similar autoclave process, but they used sheets of glue between the layers. That melted and then set - it was thermoset, not toy thermoplastic. Wood is an exceptional material for airplanes in many ways, but it just got too scarce, too expensive, and too costly for the intense level of skilled labor.
    The Loughead brothers (Later "Lockheed") method for blowing Vega fuselages was interesting and impressive. They built a concrete form as a female half-mould, lined it with the layers of laminae, put a bladder in and a lid on, and pumped it up. IIRC, mere casein glue in the early 20's, too. The ones that failed were not due to glue, though, they made early ones a little to light for the forces later experienced by some pilots, and added another layer or so in the makeup.

    Per Maurice's note about the church pews, I'm still not sure how they did it from the end of the 19th c though the very early 1920's. It seemed undoubted that the ones i replicated were glued with hot hide glue. So it must have been one of the legendary high temp rooms guys worked in to extend the open time. But i don't have a handle on how they were pressing 3 or 5 layers, 11 - 12 ft long, with a curve for the back, and a radius curve for the amphitheater seating. Plus every row, the radius changes.

    I made an oversize inside strong-back form, sealed it really well with a couple coats of epoxy sanded out, and used epoxy and vacuum to lay up the plies. (Single sided, sealed to the form)
    I was lucky in that my 6 pews were each side of the last 3 rows. By that far back, the radius change effect on the actual arc was small enough that by making the form to the radius of the second pew in that series, it was still flexible enough to adapt to the first and third. The seats were 3/4" strips on edge in a 2" thick blank and then moulded (machined) to shape for the butt scoop & edge roll-over. I laid all 3 for each side up at once to one form at the front of the smallest, with a few extra spacers between each to gain some radius. With slight spring-back, the result was close enough.

    I have a folder of some slides my dad was taking to document it, but he caught cancer and was too sick at the end to complete it. There are some pix of me spraying the finished pews, i'll try to dig up.

    Thank you for the notes and nice words!
    Last edited by stephen thomas; 06-23-2024 at 11:50 PM.

  12. #12
    I hear you about work getting heavier.

    I have used bleeder fabric a few times and never have used peel ply. I have used fabric to cushion the membrane/bag from sharp corners. For curved work usually a grooved platen under the form and a top sheet of waxed 1/8" bending ply or p-lam suffices to distribute the pressure over face veneers. A cover sheet of 6 mil poly protects the bag from squeezout. Bleeder fabric or screen when the form is really big or the work is bagged outside the form. Large radius work, less than 4" high, is done in a frame press, so much easier to load than a traditional bag.

    It certainly is true that large cumbersome layups can be a pain to load in a bag. I have used a bag seamed on one edge and sealed/clamped on the other three edges to make it easier, and have used a single ply sheet sealed to a base platen with dumdum tape for work larger than the bag available. If I were starting out with vacuum pressing I would look hard at the zipper bags offered by Vacupress.
    Last edited by Kevin Jenness; 06-24-2024 at 9:08 AM.

  13. #13
    Once again, (mostly) finish pix and description first, or the process would make no sense at all.

    A lot of hardwood flooring jobs passed through this shop, mostly for replication/restoration, or for uncommon applications.
    Many modern commercial space floors are concrete paver systems on chairs or standards with all the building's functional systems underneath.
    Typically, either carpet is rolled over the pavers, or they are matched to carpet tiles adhered to each one.

    At the EEOB, it was determined that hardwood should be used in a re-built/re-adapted space, to be consistent with all of the legacy hardwood flooring throughout the building.
    It should be appreciated that if a pattern is included, there is virtually no leeway for non-"perfect" consistent size and geometry. At least government buildings are climate controlled.
    It is really difficult to fudge panels on site, or it all goes to heck rather rapidly. This floor included about 1200 sq. ft. of removable 2' x 2' parquets, and a similar area of fixed border and field hardwood creating inter-locking visual patterns.

    This is 1/2 of the floor center medallion mocked up on the panel saw as an easel:


    Some of the other field parquets, including ones drilled for HVAC difusers:


    Some of the parts:

    Last edited by stephen thomas; 06-24-2024 at 10:59 PM.

  14. #14
    Since the tooling would be passed along to the installer/contractor to fit and adhere the wood parquets to the concrete pavers on site; i made a set of 3 steel boxes all exactly the same, each with a set of liners for each app to prevent glue adhesion to the master tools, and to center parts.
    Pressing was essential both to squish the trowelable urethane flooring adhesive to a uniform thickness, and to keep every parquet the same size and square, as it was pressed.

    This was messing with fairly high pressures - i did a lot of calculations with large safety factors before committing.
    The bladders are Firestone Firestroke actuators bought in the early 80's for a different project. They are designed for the purpose, and can also sometimes be seen under 18-wheelers as helper springs.



    Employee at the time demonstrates emptying a brick of 5 parquets from a form:
    (Kurt if you are out there, i'd love to hear from you!)


    Kurt & I organized loose assemblies of parts and piled them in order on a cart.
    Then i troweled plywood backers on the bench, and placed one in the bottom of each form.
    Kurt assembled the wood pieces on top, one of us laid plastic separation sheets over the wood, and then another square of troweled plywood.
    While a batch was setting, we'd take a break, clean and organize tools, cut more parts, & layout the parts for the next 15.


    de-molding parts:


    Later: more basic pressing with actuators.
    Last edited by stephen thomas; 06-24-2024 at 11:02 PM.

  15. #15
    that shop looks like a fun place to visit.

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