Glue Joints & Laser Cut Parts

[Leatherhead]

All,

I wasn't sure where to post this.........

If the laser cutting process leaves a charred surface, doesn't this have to be removed to provide an acceptable gluing surface? According to the FAA publication "ASA-AC43.13B, Acceptable Methods, Techniques, & Practices", glue joints should not be sanded...........as a matter of fact, I will quote the publication.

From Page 1-5, Section 1-6...

"Sandpaper must never be used to smooth softwood surfaces that are to be bonded."

Specifically, I am pondering how to make my rib webs. The webs are 6mm spruce. So, how do I remove the charred residue if I make the webs with a laser cutter. Is laser cutting not an option? Must I router all the webs?

Thanks in advance.

[Butch]

Leatherhead,
Do it the old fashioned way. Make a template of the rib web. Bolt the template to the web material (or use carpet tape), and run it across a pattern bit in a router. Or cut and sand them individually. Time consuming and less accurate but any variances will average out over many parts.
Pete

[Sreiko]

Laser cut give very precise cut but in the area of cut material properties are more downgraded. Most clean way is water jet cut and the best. If you use epoxy cement there should not be any problem about the fit. Maybe to sand but slightly surface with 180 grit sandpaper. More sanding can not be done as well this will change the geometry of the part. This sanding is actually surface preparation and does not belong the sanding to shape surface- just preparation.

[womenfly2]

Can I add my two-cents worth.

The preferred way to prepare wood (not plywood here) for bonding is to scrape the surface with a steel scraper with the grain. Then wet the wood to re-raise the fibers. When dry, then glue ASAP.

Sanding leaves residue between the fibers, it cuts them, and lays them down flat. Even vacuuming them does not remove all the wood and sand particle residue from the surface and depth. Hence scraping is the best.

The bond (adhesion) of wood to glue depends on surface area and encapsulation of the wood fibers themselves. That's why wetting the surface after lifts (stands) up the wood grain (microfibers) and lets the adhesive encapsulate them and fill in all the valley too.

Laser cutting burns away the wood fibers reducing the surface area of the adhesive bond-line.

If you Laser cut I would scrape the surface to virgin material, wet it, dry, then glue. You should be fine.

But always do a test sample first ..... your life may depend on it!

WF2

[Spadmaker]

Let me gently, nicely, and politely, with my usual understated emphasis, add my opinion to this:

DO NOT LASER CUT WOOD!!!!!!!!!!!!

I've been around and around this issue for years with people. You cannot (even with epoxy) get an acceptable gluing surface with a laser cut edge. Even if you sand afterward (not recommended anyway AND which ruins the dimensional accuracy), the internal wood fibers will still be charred, brittle, and contaminated with carbon.

With all due respect to WF2, while scraping is acceptable, it is actually PLANING that is preferred, (scraping sometimes does not go deep enough).

Gluing a sanded edge is not allowed. It says so right here in the bible. See? There are so few exception to this rule, that I'm not even going to get into it.

Pete is right: the only good way of gang-making parts is with a template and router. It's fast, accurate, and provides the ideal surface for gluing.

And a small side note: Even for metal parts, water jetting is preferred over laser cutting, as it does not alter the metallurgy of the edge of the part.

Water jetting a wood part should not even be thought about for fairly obvious reasons.

gently and nicely,
Bill

[Sreiko]

Also it is good to secure fittings, joints and corners with composite fabric. On the image here is presented some of material from my workshop. First on the left is the E-glass 70 g/m2, in the middle is the S-glass 125 biaxial fabric and on the right side and black color is three axial carbon 250 fabric. Use it in regard of the position which have to be straightened.

[womenfly2]

...... scraping is planing, just at a slower speed. That is also what a router cutter does ...... if its sharp.

With all do respect.

WF2

[Der Grüne Flieger]

Question to all,

Has anyone considered to use an X-Y routing table to make multiple pieces?
Spadmaker, with all those machines you are making this could be something that appeals to you?

Phil

[Laserlloyd]

Leatherhead:

Laser cutting is a thermo cutting process and will leave a edge as if you are heat searing it and should not leave a charred edge. in most cases one would have to lightly sand the edge to open up the pores.

Their are two way one could laser cut wood. One is to use Oxygen as a assist gas but this would have a more heavy charring affect on the edge. The second is the more desirable method of using Nitrogen as an assist gas. N2 is an inert gas and in metal cutting will not oxidize the edge but in wood cutting the edge is effected by the heat. You will get a brownish edge with no heavy charring. The secret of cutting wood is to cut it as fas as you can to minimize the heat effect. Now because you would use N2 the gas presser is much higher then of O2. Since your gas flow is much higher you use more gas, more gas increases the cost of the part.

I have laser cut wood and have achieved a really nice edge. You have to find a Laser Job Shop that supports wood cutting. Most likely it will be a Co2 laser. It will have to be a Non-Capacitance cutting head (for non-ferus material). try to locate as many as you can ad submit a sample of the material. have each one cut sample parts to determine which one will do the best job. There will be a difference form each one because of their system and expertize in this process. It would be nice to supply them with the same DXF of a sample part.

If you have any question on laser cutting just ask me. I'm a laser applications engineer for a laser manufacture and have been laser cutting for over sixteen years.

I hope this is of some help...

Lloyd...

[Butch]

Let me rephrase what I posted earlier and paraphrase Der Grune Fleiger; have you considered using your hands?

In my case I have very little experience with metal work. I have a mental block against it. Call it writers cramp. In order to make the sixty or so fittings for the SPAD I have fallen asleep trying to learn autoCAD. I have priced services to scan and cut the fittings. Etc. I just bought a band saw for $165 and am about to start cutting metal. Had I done that last year I would be finished now. And it is a hell of a lot more satisfying and probably more efficient since I only have one airplane, not an assembly line.

Technology is great, but there are old skills that it is important to be proficient in. Cutting wood and metal are two of them.

Pete