Composite Gantry

[Karlf]

Has anybody used comosite materials to build the gantry? I would think bi-directional fiberglass would make a great gantry. Its strong, rigid, light, and it lends itself to being hand made very well. Isn't that what we need?

[mxtras]

Yes but composites also make good springs! Not a desirable trait for a cutting machine.

I suppose if the structure was designed heavily enough in the right manner it would be feasible. In cutting machines, mass = rigidity.

Scott

[High Seas]

Cheap?
Known skill sets?
Probably why you don't see much in composite material use here.
With a bit of engineering and right choices, some patience a very nice, rigid, lightweight, etc, etc is possible.
I'd use some core materials to up the mass properties and get some beam effect - or maybe a bit of CF wound on a mandrel -- Humm, is that why I made that 4th axis?
Welcome aboard!
Are you planing a composite Gantry? There are some newer highbuild products you can get that are more appropriate maybe than Bi-axial.
Jim

[Geof]

Here you are; 120+ pages with 2400+ posts discussing using a composite material for making machine parts.

Epoxy-Granite machine bases (was Polymer concrete frame?)

[Karlf]

Thanks for the link. It appears as though they are talking about casting a composite part. I'm talking about cutting the part out of foam and then appliying multiple layers of bi directional fiberglass. I'm planning on building a composite airplane this way. I'm hoping to use the cnc router to help speed the build up.

As for the spring effect. This will happen with any material. If you make it flex and don't take it past it's yield strength. It will spring back. If you build it strong enough not to flex then it won't spring back.

[Geof]

Originally Posted by Karlf ......As for the spring effect. This will happen with any material. If you make it flex and don't take it past it's yield strength. It will spring back. If you build it strong enough not to flex then it won't spring back.

Except for your last sentence this is correct, but it is impossible to make something strong enough so it will not flex; everything flexes under a load. The problem with your idea is that you will have a lightweight but strong and relatively stiff structure and what happens here is that when it flexes and springs back it does it quickly. Think of a thin and tight guitar string, you get a high note. In general high speed vibrations in a machine structure are undersirable because it is easy to excite them. Preferably you want a heavy structure so any resonant frequencies in the structure are low and not so easily excited by the action of the cutter. Ideally you want a structure that does not have any resonant frequencies which is part of the goal in the epoxy granite composite structure. Part of the reason this goal is attained to some extent is that in this composite there are two or more components with radically different strength, stiffness and density. This means they have different flexural properties and there is no common resonance mode so they do not readily vibrate in harmony. Fiberglas layers and rigid foam are closely similar in stiffness and density and a structure made of these may be strong and stiff but it has a tendency to ring like a bell. You may be able to deliberately incorporate vibration dampening by loading some of your glass layers with dense fillers which would give a density gradient across the structure and introduce competing rersonances but then you are starting to lose the goal of simple fabrication.

[Regnar]

I went to a school that taught us some real basics for composites. We used honeycomb material with 3 layers of woven fiberglass. Vacuum bagged it overnight and our piece was strong enough for a Nissan Titan to drive over it with no damage done to the piece. I cannot speak for bending because we laid it on the ground. On another note. Some helicopters whole tail sections are made this way where they are designed to counter act the head torqe and support the weight of blades and I dont think the pilots would like sloppy controls.

[harryn]

Honeycomb panels are widely used for very stiff structures. It is useful to note that the military helicopter parts are not simply honeycombs, but actually a mixture of cell sizes, carefully selected by detailed 3 D structural analysis. (including vibrations)

The fact that MDF and plywood can work is a good indication that a lot of materials will provide adequate stiffness. Almost any honeycomb will blow away MDF and plywood for stiffness. Cost is a different question.

A surfboard takes a real beating and actually is pretty good at damping vibration, partly because of the mixed material properties. (foam and fiberglass / carbon fiber mat) The R/C airplane guys (ok, women too) routinely put in CF and Al pipe stiffening ribs.

While you are designing in the composite material, there are a whole bunch of interesting variations of the gantry that are potentially stiffer designs. It is hard to look at a typical moving gantry system and not see that they contain some design concepts which actually amplify the effective torque loads and vibrations.

For me, the hardest part is to "hand make" straight, flat parts. Stiffness has not been that tough (Mostly using commercial Al angle)

I am starting to think that no matter how you build up a machine, they can benefit from some leveling epoxy in the right place. If you have a composite machine already, you are 1/2 way there.

[High Seas]

Originally Posted by Karlf I'm talking about cutting the part out of foam and then appliying multiple layers of bi directional fiberglass.

As I mentioned before with a bit of engineering. For example your most likely not get the rigidity you seek if your beam replicates the "typical" design of a Y axis. Most are using fairly conventional materials and methods. And over-built but more than satisfactory.

Effective composite construction has a bit more technique and process than laying up a bit of glass and foam.

Originally Posted by Karlf I'm planning on building a composite airplane this way.

I have similar plans too. (and a car and another boat..and and) But, you'll want to do a bit of review with a few books. Have you checked out:
HOW TO BUILD COMPOSITE AIRCRAFT by Martin Hollmann.
COMPOSITE AIRCRAFT DESIGN. By Dr. Hal Loken and Martin Hollmann.
Understanding Aircraft Composite Construction by Zeke Smith
Advanced Composite Construction by Zeke Smith
and Jack Lambie has one out there too..

These have some good examples you may want to consider.

Originally Posted by Karlf I'm hoping to use the cnc router to help speed the build up.

There are some advantages to using a cnc machine in the construction of repetitive part making - Get with JerryFlyGuy here on the
zone -- I bet he'll have some insights.

For fuselage buklheads, ribs, and if you want to build a plug for the hull - sorry fuse, the cnc could speed the process a lot!

Honeycombs are useful - but again expensive and you'll need some thought on placement in your beam -- and some technique to laying up. There are some interesting solutions that have come out using non crimped fabrics as reinforcement - vice typical bi directional that is crimped in the weaving process.

Flat and straight parts are easy-peasy. You just gotta start with a flat surface and keep it

All in all - an interesting approach, but there will be some time and $$ to get there - - But you'd have a one of a kind!
Until you make more (thats the real advantage of the composite methods - once your engineering, process, molds, jigs are set - pound them out. For example, how many shelby cobra/Lambo/whatever kits are on eBay?)

[Karlf]

Great stuff High Seas, but I'm a pilot not an engineer. I am very handy with building things and I have built a few race cars, so I know what a long project I'm getting into. The airplane I want to build is called a Cozy. It's a modified version of Burt Rutan's Long Eze. Burt licensed the design to another guy who sells the plans. It uses moldless construction. You cut a part out of foam, then cover it with fiberglass. Then you glue the parts together. Many of the parts are flat and straight. Others are cut flat then curved, then fiberglassed. The wings are fairly complex and I originally wanted to make a cnc hot wire cutter for them. Fortunately, there is a guy who has built the airplane and already has the cutter. He sells the precut wing foam for a few hundred dollars more than the cost of the foam. So I'll just probably just buy the wing foam precut.

Where I think I can gain a significant advantage is with the smaller flat parts. There are alot of them. The plans call for hand cutting the foam part, then fiberglassing it, then trimming the excess fiberglass. If I can machine cut the part, I'll save alot of time. If I can cut a rectangular piece of foam slightly larger than the part and fiberglass it. Then cut the part out, I'll have automated two tedious steps and I'll save huge amounts of time.

[High Seas]

Karlf - sorry for the delay in getting back to this one. Holidays I guess!
I am quite familiar with moldless construction and its approach and limitations. When we look at a gantry think of the loads and forces in the wing spar. You'll find some interesting approaches in unpowered aircraft (read sailplane) that use composite spars, fuse, etc. Check out the build of the Pioneer 3. (Jim Marske has spent a lot of enginnering thought and effort in using carbon rods as the spar caps -- top and bottom of the spar.) Such is the types of "engineered approach" to get a well structured gantry. Remember the loads on the gantry are going to be resulting from the router cutting forces and the router weight and torque from its position as well.

Re the Cozy - A great plan to do all the bulkheads etc with a cnc and the bits and pieces. DIy Boats are moving in that direction - especially class racing - like the mini-transats.

You might like to check out Vectorply (google) they have a bit of software the Laminator you can work layup schedules loads etc.

If I'd ever get my pen to work - I'd finish that book and share the hows and whys with you on a neat approach to moldless construction that takes it one step beyond.
Jim