Making Skins #74 - Brisbane Australia

[smreish]

Little trick about the plugs I have used.
Set up the 3d profile of the plug to cut a "small cone", with about 2 deg sides. Makes a nice tapered fit (and room for the glue)

Nice Job!

[Surfcnc]

Thanks Sean

Used the "barrel bung" thing before on timber floor and wall penetrations that needed to be repaired but never thought to cut them that way in the first place.
I actually discussed hand sanding the plugs to that shape with my mate.
Will have a play tomorrow and see if I can cut a few as you have suggested.

Regards
Ross

[smreish]

Ross,
It's easier to get the proper shape if you use a BALL NOSE rounded bit. This way, the tangent of the ball shape cuts the side wall. Or, I have used a V-bit (5deg) taper, but not as good of results.

Good luck.
Sean

[Surfcnc]

Hi All

I set my Mechmate a new task today to cut out some longboard skate decks.
Love watching the machine work away but have yet to run a toolpath I am totally happy with .
This one was too slow as the experts on the forum will no doubt agree.

This time I played with the idea of cutting at .2mm allowance using a climb path and then coming back with a conventional toolpath right on the vector.
Danilo recently discussed this on Kobus' thread and I like the idea of the cleanup cut.

Also used a 4mm end mill for everything and chained the tool paths together.
It was press start .... wait a little, then a skateboard appears.

Skateboard Deck Finished.jpg

IMG_2233.jpg

http://www.youtube.com/watch?v=KQNwAW5akeE

Regards
Ross

[Kobus_Joubert]

Nice Ross, and how did the CLEANUP pass work for you on the plywood ?

[Surfcnc]

Hi Kobus

It worked well, particularly on the inside cut out for the trucks.
The two cut method will be my prefered option as I like the machine to do most of the work if at all possible.

Broken one in half already testing to see how strong they are.
The decks require 2 or 3 layers of 4 oz fibreglass cloth on the underside to control the flex if anyone is planning on making their own.

Regards
Ross

[Surfcnc]

Broken

IMG_2235.jpg

Reinforced underside with 3 layers of 4oz fiberglass cloth and white tinted epoxy resin.

IMG_2234.jpg

Regards
Ross

[rival]

Hi, Next time could you do the underside re-inforcing on your uncut sheet. Save a lot of trimming of messy resin/cloth.

Be interested to see how it cuts as this is something I want to do, when I get my machine finished .

Andrew

[bradm]

A quick google search shows that "dried" epoxy dust is sensitizing, just like the resins themselves. You would need to treat all the resulting dust with the same care (no breathing or skin contact) that you treat liquid epoxy.

[Surfcnc]

Hi Rival and Brad

Rival - exactly right the machine will cut the cured resin and cloth and that is future project. Good advice.

Brad - yep it's nasty stuff. I use gloves, a sanding suit and a respirator to limit my exposure.
Then an immediate shower after the job is done and all up the exposure is minimal.
It is the resin of choice for my boards as polyester resin literally melts the newer EPS foams on contact.
Some of the early users of epoxies in the surf industry were pretty haphazard with the stuff and have paid the price of becoming sensitised.

Regards
Ross

[smreish]

...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

[Gerald D]

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.

[smreish]

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?

[Surfcnc]

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

[bradm]

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

[Red_boards]

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

[Red_boards]

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

[Gerald D]

Originally Posted by bradm

........ a sandwich ..... with fiberglass in between. ....... 4mm ply - basically half thickness of the original material.

Conceptually, then, from bottom to top we have a 4mm 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.......

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.

[Surfcnc]

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.

IMG_2238.jpg

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

[Gerald D]

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

Anybody use honeycombs for boards?

[Surfcnc]

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

[KenC]

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.

[MetalHead]

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

[bradm]

Thanks, all, this has been very enlightening!

[rival]

Originally Posted by KenC

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.

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

[KenC]

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...

[Gerald D]

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.

[danilom]

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.

[danilom]

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.

[quadtech]

Originally Posted by KenC

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...

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