Looking for some advice

[DeusXmachinist]

I'm an engineering student working on a team for an engineering competition. We've run into a design issue: the chassis we are building has to be lightweight (this is the key design constraint) but we have fairly limited resources in terms of machining (just a fairly basic student shop with a couple lathes and mills that are meant for cutting fairly mundane materials like aluminum and stuff). We have looked into making this thing out of carbon fiber but I have heard that it is really hard to work with. The chassis doesn't require anything really fancy, but we would need to drill into it to mount bearings and stuff to it. My questions are:

1)How easy is it to machine carbon fiber? Do we need special bits/safety measures (ie will the dust cause us to choke and die?). Also, by putting holes into it do we ruin the integrity of the piece?

2)If carbon fiber won't work, what other lightweight materials will? I have no idea what sorts of composites are out there or how easy they are to machine.

3)What means are there for putting threaded holes into carbon fiber? Obviously you can't thread a laminate material, so how would you bolt onto the stuff?

The chassis is basically just a frame that we are bolting stuff to so drilling/threading holes is the extent of the machining we would have to do. I am by no means an expert machinist in any way and would appreciate any advice I can get.

[swiss-1]

Haven't worked with carbon fiber but .....
High rpm & Low pecking feed rates should help tearing.
Feel tools for heat - lower rpms if hot to touch or any tool discoloring especially!

Drill, ream for light pressfit bushings as guides to protect and strenghthen walls - countersink for bolts heads - allen wrench style. Mist for dust/ spray bottle keep switching fingers (Hee he).
Just some ideas!

[sendkeys]

What size is this project? I'm guessing it's small because of the great cost of carbon fiber .What about going with titanium? No easy to machine but you can do it. http://www.machinedesign.com/BDE/FAS.../bdefj1_6.html



If it was me i would just use 7075 aluminum. Super easy to work with, very strong and lower cost.
Weight could be the same or less then the carbon if you have to over design because the skill/experience isn't there to make it functional. It would give you more time on the design too because it's easy to work with.

Most the time with this type of work you see a mix of carbon/titanium. When cost isn't a problem.

[ImanCarrot]

Here's an excellent thread on this site about laminate stuff...

http://www.cnczone.com/forums/showthread.php?t=18483

[mxtras]

What about using a rigid foam?

Or possibly an Aluminum Composite panel - like Alpolic?

HERE'S AN OFFER - If it is a small component, I think I can hook you up if you can provide me the data. I machined a smallish package handling robot for a group of VMI students last year. It was a gift - material, machining and shipping was donated. I can probably do the same for you. They asked for one set of parts, I made them six. They used four.

Rather than threading the base, you can use through bolting with a sandwich plate (like a lap patch) -or- Alpolic will hold a thread if the forces aren't rediculous.

Scott

[mxtras]

Oh - on threading carbon fiber - - use threaded inserts or sandwich construction, not threads in the material. I don't think you will have much success threading a properly layed up panel as there is not really a whole lot of resin there to support the thread shape and the fibers aren't going to do it. If you have a resin rich panel then maybe but then you are pretty much defeating the purpose of using an exotic material and woud be just as well served using glass.

My opinion on another question you asked - placement of the holes relative to the fiber direction can destroy the integrity of the panel, yes. I doubt you will be designing on the edge of the limits of the material/laminate and will likely not have issues but in the real world, the position of holes and joints are carefully evaluated with respect to the fiber orientation and panel configuration. Local areas may be tailored to accomodate the addition of voids if their position is known in advance. In your situation, I don't think it would be a huge concern but you can't just turn it into a piece if swiss cheese and expect it to remain just a rigid as the solid panel was - I am sure you know this.

Other assembly options for such a project -

Use tabs on intersecting panels that go through slots and are then constrained by adhesive or other means such as pins, notches, keyholes, dovetails, rivets.... Snapping things together can work - bolting is the easiest, less design-intensive method. Working to assemble something that basically snaps or otherwise mechanically locked together takes a bit more thought or iteration than threaded fasteners but can be very effective.

Scott

Scott