...what about making the board out 3 layers of very thin material with the center core being the fiberglass matte (or aluminum, or rigid fiberglass sheet) bonded between the 2 plywood layers. Then Cut on the machine the final shape?

I bet you get a similar modulus of elasticity while maintaining the same section modulus (being just as strong, but less messy)

This would help control the exposure to the substrates.....Just thinking.

Sean

The way I read it with the glass on the bottom is to give tensile strength at the furthest point below the neutral line. If the glass goes in the middle, then that is the neutral line and zero strength will be added by the glass.

Gerald,
Fair enough. I was assuming (i know:) that "stress-skin" effect that the glass was providing would be adequate closer to neutral axis instead of becoming the neutral axis. Good point, Does it then act more like the freebody diagram of a truss, than a simple beam I would guess?

Hi All

Gerald has nailed that one on the head.
To examine the extremes, the top veneers are in compression and provide little strength, that's why the composite is put on the the very bottom.

Regards
Ross

Okay, so I'll wade in here and ask stupid questions in the hope that I'll learn something.

From the photos above, it appears that the material is approximately 12mm thick. Or maybe 8mm thick.

I read Sean's initial suggestion to be a sandwich of 6mm ply with fiberglass in between. Or maybe 4mm ply - basically half thickness of the original material.

Conceptually, then, from bottom to top we have a 6mm layer of wood with its bottom layer in tension, unreinforced, and its top layer in compression, reinforced
by the composite. Then we have the glass composite. Then we have another 6mm
layer of wood with its bottom layer in tension, reinforced by the composite, and its top layer in compression.

While I have no argument that the full thickness with glass on the bottom is the optimal solution for the minimum elasticity, it's not clear to me that the glass in the middle does nothing positive. Questions:

Doesn't the fiberglass layer resist compression better than wood, thus adding some amount of strength, even in compression? Surely it doesn't actively reduce compressive strength, right?

Presuming that the lower wood is mostly cosmetic, isn't the question whether the upper wood is sufficiently reinforced by the glass to meet the need?

Looking forward to some more free education from the MM forum :)

On the composite issue: The separation of the strengthening layers is the key issue. Strength has a cubed relationship with separation, if my memory serves me correctly.

I would not get too tied up in compressive vs tension issues. When a flexible item recovers from a distortion the roles reverse (tension becomes compression).

On a practical note: lay up your cloth at 45 degrees to get best resistance to snapping (alternate layers at 90 degrees). One day industry will catch up and produce 45 degree layup cloth with equal warp and weft.

On a contemplative note: wrap around the rails not only influences stiffness heavily, but it also strength

Oh, yes, and this is MM related.
If you rout channels at regular distances in the core material and lay the laminate in them you will improve resistance to snapping. How far apart? How deep/ Matters for research. There is a relationship between thickness of core and spacing of channels but I can't find the link to it. Try this thread and related ones. http://www2.swaylocks.com/forums/tes...uctions?page=5 and this spreadsheet http://www2.swaylocks.com/files/uplo...4becac7903.xls

Brad, your logic above is to assume that the strength of the board is equal to the sum of the strengths of the individual layers. That would have been true if the layers were allowed to slip over each other. When the layers get bonded to each other, the picture changes dramatically. (take the bending resistance of a phone book, then imagine glueing its pages together....). When there is no internal slip in a beam of different materials, it is typically called a composite beam.

A beam (be it composite or otherwise) experiencing bending load will have the highest tensile/compression forces on the two skins/boundaries. Somewhere in between these skins the tensile/compressive force will be zero. You can happily drill holes in the zero force part, but you shouldn't have even nicks on the skins.

If you drill too many holes, you end up with 2 thin beams that want to slide over each other . . . . . so, what actually happens there in the middle? Even though the tensile/compressive forces there are zero, that is the area of the beam where shear forces are the highest. Thank of shear force as anti-slip. The shear forces on the skins are zero.

Glass fibre, without resin, has great tensile strength. Adding resin gives it compressive strength (or else the glass threads just buckle). The resin is a good glue and has high shear strength. If you put that in the center of a beam, the resin makes a glue, but the tensile strength of the glass is put in a part of the beam where there is no tensile force.

Seems like both Red and myself have paid for some expensive broken boards before :).
Swaylocks for those non surfers is a treasure trove of tec info about foams, resins, fibreglass as well as natural fibers.
Some great craftsmen have visited that site.

Attachment 12177

Why this 12mm ply fails is that each alternate layer is at right angles to the previous face.
Look carefully and you will see the first and second layers at right angles.
Effectively this 12mm ply only has 3 layers doing any work to control the flex and at least one of those (the top one) is in compression.
Those other layers keep the sheet flat are but passengers controlling flex.
All of a sudden it is not as strong as one might think at first glance.

Skateboard decks are typically made of high quality laminated Baltic Birch veneers that are bonded together with a non creeping glue such as Titebond 3 or Selleys Aquadhere PVA + (the crosslinking PVA's).
PVA or white glue never really sets hard so is not suitable. Often all the veneer fibers run longitudinally and the hard glue holds the shape of the sheet.
A seven layer laminate of BB with hard glue is way stronger than a standard sheet of plywood.
It is important to note that how I have made this deck is not optimal just practical.

Fiberglass as Red points out, is axial, that is, it has fibers that go in both directions at 90 degrees to each other.
One layer of glass therefore provides strength in both directions.
Reds comment about laying the glass at 45 degrees on alternate layers forces any fractures to mimic the cracking pattern of a brick wall when the footing subsides.
Any failure is forced to run along a greater distance than a simple straight line crack, zigzagging through the path of least resistance.

Brad, I'm not sure if I can say for sure what is going on if the glass is in the middle because I don't actually know.
I am thinking of the thickness of the board as a continuum of compression at the top to tension at the bottom. In the end it depends on the materials properties.

Update I trimmed the glass from the deck with a Stanley knife then used a ball bearing guided straight cutter and a round over bit to tidy up the excess.
It required only a slight touch with the sandpaper to finish (still had the suit on though).

I'll post a final pic once the clear coat dries.

Regards
Ross

Glass cloth can also be of the unidirectional type where the strength is in one direction only.

Anybody use honeycombs for boards?

Hi Gerald

The mind of the engineer, you have provided a very competent explanation of what is going on.

I used to unpick squares of woven cloth to make unidirectional cloth :), called rovings in the surf industry.
It used to be for glassing fins direct to to the board but replaceable fin systems have changed all that.
At last count the surf industry is flogging quad axial cloth, I remain quite confused on that one !!

Honeycomb, no. I try to keep up but sometimes you got to know when to stop.

Thanks for chipping in.

Regards
Ross

I never get the chance to do any Fibreglass stuff... just happened that way...
I did lots of reading on the subject nevertheless. In engineering term, fibreglass outer layer with core material in between is actually stress skin. stress skin is strongest when the distance between the skins. the main purpose of the core is to provide space between the skins..
Plywood got its strength from spreading the stress to alternative directional veneer grains. that is how they are suppose to do.
I'm wondering will applying fibreglass skin on both sides of a shuttle ply (before cutting) will make them stronger & hence better for Fleishman's chair stuff.

You could also try cutting the decks at a 45 degree angle in the base stock. Also take a look at cabinet grade plywood it has more layers.

http://en.wikipedia.org/wiki/File:Spruce_plywood.JPG

http://en.wikipedia.org/wiki/Plywood

Thanks, all, this has been very enlightening! :)

I think it will make the ply too stiff , and you wont be able to torture the ply into shape.
Done a bit of Ply / epoxy boat building and its amazing the additional stiffness some reinforcement creates.

At work we recently made some curved roof linings for a motorhome conversion , using 3mm ply with a laminate bonded to one face. Amazing the stiffness just laminating the two together .


Andrew

Andrew， thanks for the heads up. My issue is too many voids within the plywood & the cut job is too flimsy to my liking, even with 18mm shuttle ply. Maybe the additional stiffness helps, maybe not.... I really have no idea...

We used to have a company out here that specialised in impregnating standard ply with resin and there were some good results. Havn't been in that field for a long time and maybe they aren't around any longer.

By the look of the crack and inside layers the ply is of very low quality. It looks exactly as some waterproof kind I got to cut some time ago. Not that its more NOT waterproof than the plain russian birch BB/CP/CC grade I use, but its very easy to break, has great number of voids and those dark layers.
Look for canadian or Russian birch and BB/CP grade without waterproof glues. I bet that I could not even bend the 12mm thick and 20cm wide board (for skateboard) and cracking comes out of question. Look at the sides of boards they should be all almost same color some just a little bit darker and with absolutely no voids whatsoever.
Will post a picture of some items made with it.

Here it is. Hope that its not clogging the thread. These boards come only in 1525x1525mm size it may help in finding them. And the price here is about 35$ for 15mm board or 30$ for 12mm board.

http://www.cnc.rs/mechmate/birch/birch_plywood1.jpghttp://www.cnc.rs/mechmate/birch/birch_plywood2.jpg
http://www.cnc.rs/mechmate/birch/birch_plywood3.jpg

Ken, try marine grade ply - it usually has fewer voids and of better
overall quality.

Near Enough to Finished
 

Hi All

Quality ply, nice stuff there Danilo but mostly a victim of the race to the bottom on price from the mega barns here in Australia.
Not complaining and I do know where to source the quality stuff if I drive far enough.

Top Side - Complete with a DIY beach sand grip surface

Attachment 12179

Bottom Side - Trimmed up and looking the part

Attachment 12180

Regards
Ross

One for you Gerald - Honeycomb Wooden Boards
 

A week or so ago here in Queensland we had the 3rd annual wooden board day.
I missed the date somehow but have attended the previous two meetings.

Upon reading the wooden board blog as a catchup, I was impressed to find that a New Zealander flew across the ditch and showed us what they could do with their CNC machines - WOW.

http://woodensurfboards.blogspot.com/

Scroll down to "Mike Grobelny from New Zealand with a couple of samples of his high tech approach to wooden boards"

Example image from the site and more if you follow the link.
Attribution to Grant Newby who also organises the equally creative and successful Alley Fish Fry.

Attachment 12189

Regards
Ross

Those are amazing boards!

Do those holes fill with water?
some parts are covered but it seems like aesthetic feature?

Very nice!

There is a longboard manufacturer that uses the same honeycomb shapes but in aluminium:

http://www.silverfishlongboarding.co...s-catalog.html

My understanding is that they are all covered, they have cutout the top layer only to show the inside.

Ross

Very cool aproach to a wooden board :cool:, beats trying to chamber a solid balsa board by hand with a router.

Did you get a chance to pick those up, what's the weight and flex like? I imagine pretty stiff compare to the traditional framed approach.

Hi Danilo Jed and others

Danilo, my guess is he seems to have a bit if a mix of fully skinned to partially skinned. The pockets would hold water and make turns slower if they were exposed.
My vote is for fully skinned but as with any honeycomb design a leak is a disaster if it happens and is not detected.

Jed, us surfers are just so predictable aren't we. I did not get the chance to pick up one of the boards as I missed the event.
At previous events, picked up "every" board and tested for weight.
Some are far too heavy and are really made for the living room wall.
Others were marginally heavy, while some are surprisingly light from the talented makers who had an eye on the overall weight in the design criterion.
I have chatted to some makers about their weight control and a few trade secrets are required for the lighter stuff.

For those of you that looked at the full link you would have seen that some of the boards use an aircraft wing type design.
These internal struts being more frequently being cut by CNC these days. Some makers are every inch hand tool sort of people so it will always be a mix.

Regards
Ross

Pablo - Great link thanks.

Regards
Ross

I'm no surfer, never had & doesn't look like I'll ever be. but I don't get it, I'm very puzzled over the water tight concern. Looking with a pure engineering angle, If there isn't skin on the honeycomb board then what is the purpose of having honeycomb. Also if the honeycomb does not adhere to the skins fully (water tight sealed) than the strength is severely compromised....

Ken, the honeycomb pattern just reduces the weight in comparison to the solid wood shape.
Can't really comment on the significance of the top skin with respect to strength.

Regards
Ross

My guess looking at them is that they are going to be pretty damn stiff, no idea about weight but really like the idea, one for the memory banks.

Lamination Tests
 

Glue laminated 2 layers of 200 x 5mm vertical bamboo stock then epoxy glassed the underside with 4 x 4oz cloth and the top with 2 x 4 oz cloth.
Initially the Mechmate was "supposed" to cut out the pre-glassed laminate but a series of comedic errors lead to some holes being drilled in the wrong place and the spoil board moving under the cutter.

The composite was now only good for some destructive testing so the pics tell plenty if anyone is interested.

Attachment 12212 Attachment 12208
Attachment 12209
Bottom was undamaged expecting pin point loads from an the exposed aggregate driveway
Attachment 12211
Top has obvious compression failures, a delamination on the inner layer and a set of compression "waves" on the outer layer of glass.
Attachment 12210

The epoxy was uncured so some improvement in compressive strength could be expected.
The conclusion here is where the glass is in the composite sandwich makes a lot of difference in its perfomance.

Regards
Ross

..I love going back to science and engineering school. Skate board & Science would keep the kids attention for sure! Great sharing of information.

Foodsaver!
 

Ross,

I see the bulk clampage use... I laminate balse onto foam cores for my R/C airplanes and use a Foodsaver.

The poly bags that come with the Foodsaver do not adhere to epoxy, can be made any length and are great for appying even pressure for this kind of work on a budget. The only limitation is the width of the bags available.

Possibly an alternative to the clamps?

Dean.

Hi Dean

I have started to build a venturi vacuum system that uses an old 40l compressor tank as a reservoir.
It will be based upon a Joe Woodworker V2 system.
Baby steps on the vacuum system but thanks for noticing my many clamp strategy and suggesting an earier alternative.

Regards
Ross

Ross, you can run the compressor as a vacuum pump, reverse the inlet & outlet, then plunk in a differential pressure switch. good volume & very good vacuum head.
Cost near to nothing & good project to play with.

Fridge Vacuum
 

No problems Ross,

I did some research when I was looking into vacuum clamping and found a few DIY systems using an old fridge compressor. They were all a bit of overkill for what I needed at the time.

Good luck with the project! I've been reading with interest :)

Dean.

That's one of the uses I have my machine tagged for. Thanks for the link. Interesting stuff.

Nice flex on your laminates. Innegra cloth might be good in this application (or it may be too "sloppy")

Ken,
When using a compressor for vaccuum you end up sucking oil out of the compressor. I have dealt with this by having a 2m pipe vertical from the inlet. This acts as a reservoir to prevent the oil ending up on the floor. But the compressor is aging rapidly because of diminished lubrication..

Ross, You will be very happy with the Joe Woodworker V2 setup. I built mine almost a year ago and my only complaint is I wish the vacuum switch was a little more sensitive.

You can make you own reusable vacuum bags very cheaply with thick contractors grade garbage bags. Use modeling clay to seal the bags up. Once done you can peel the bag apart and recover clay and the bag. When you get the bags down look up leach fields and vacuum impregnating. You will never spread resin around by hand unless it is a small job. If you have any questions just shoot me a message. My real job is working on military helicopters doing composite and structural repairs.