Just theoretic question. Why do not use carbon or kevlar monocoque for cnc frame?

Just theoretic question. Why do not use carbon or kevlar monocoque for cnc frame?

Why use?

Because of stiffness, i guess :slight_smile:


Price, durability, damping, and mass. Cast iron damps vibration. If you make it from carbon fiber and fill it with loose sand, then you might have something. There are also issues with red-hot chips of steel eroding epoxy-based shapes, and it’s a pain to embed junction points for screwing in metal parts.

Cost and manufacturability come to my mind. Depending on the size you may find cases where these choices can make sense. lathes can be made of concrete http://hackaday.com/2011/09/27/they-may-be-for-developing-countries-but-we-want-a-concrete-lathe/

I have no doubt that you could design a monocoque frame with some type of infill (most likely a fiber reinforced aggregate ) that would outperform cast iron in every way. Many of the high end machines are using epoxy with aggregates as the frame of very good machines. No real reason not to include fabric as well as it has higher yield strength (almost an order of magnitude over cast iron). The downside to cloth composites is they are more expensive to work with compared to just making a big sand mold and pouring in molten metal. Cast iron more common because it is pretty cheap and relatively easy to produce.

As to hot metal chips and such, that is why they make chip guards and chip trays.

I’ve seen some machines using turcite ways, and am curious how they seal them against chip damage.

@John_Bump The little I have seen, the turcite has been up under a piece of cast iron with wipers and such to keep clean.

I’ve been thinking about putting turcite/moglice on one of my older lathes, but man it seems like you have to design from the ground up to keep ways chip-free. (Maybe flood everything with coolant to wash stuff off?) I’ve added wipers and there’s still metal under the carriage sometimes.

If you intend to cut light materials you can use any type of these material to do your frame. Vibrations and constraints come to mind when you cut deep, and mechanical resistance will reach a point of saturation where your frame will simply brake or slowly start to split which could result into a hazard problem.

Try to picture a tool holder crossing at fast speed the room you are in and you can imagine the risk you are putting on the people around you.

Aluminium alloys could be used if the overall weight of the machine is a concern. I am thinking about Zamax for instance. In any case you need to think solid before cutting solid and light materials like kevlar or carbon do not present the proper mechanical characteristics to be chosen for a frame.

Those materials are usually chosen when weight is a concern. If you are building a CNC for eventual transport to the ISS then you might have a justification. Otherwise, what’s the point? Especially when factoring in all the cons mentioned above.

All materials all have their advantages and disadvantages. You can build something strong enough out of just about anything if the structure is designed properly.

If you are building a one-off machine, the expense of creating patterns for casting in metal are high. Building out of composites doesn’t need hundreds of pounds of molten metal.

During WWII, they built machines from concrete that worked just fine.

@John_Bump Steffan Gottswinter (I probably butchered the spelling of his name) on Youtube has a video where he re-did his lathe with some turcite (or something similar) Interesting but it was a major tear down and rebuild.

Do you have hardness and compression ratings for whoever is selling the carbon fiber or did you test it yourself if you made it (resin matters)?

You could also try a vibrations test with an FSR sensor sandwiched in between a length of the material and a solid surface. Look at the output of the FSR on the oscilloscope. I think that would determine rigidity but it would only matter for a single product. You don’t ever see hardness rating on that stuff (probably good).

That’s only if you are making it yourself which I assume you are being in the DIY CNC group.

@Andrew_Hodel It wouldn’t work that way very well. Composite fibers are wonderfully strong in tension but they are lousy in compression. You would need to add another material that was good in compression such as concrete fill. It is the same as working with concrete with rebar where the compression forces are handled by the concrete and the tension forces are handled by the steel reinforcement. Even better when you pre-load the steel tension.

When you buy composite fiber, you have some generic strength ratings for any of them but some are certified ratings required for use in things like airplanes. Also lay-up technique matters a lot when you want the most strength. The matrix (epoxy) isn’t that strong structurally but it does add some compression strength.

Can’t you just compress the fiber + plastic or whatever bonding agent you use then watch for vibrations with a scope?

It’s gonna vibrate at larger intervals until it breaks regardless of pressure, granted with higher pressure the break should come sooner.

Isn’t that the purpose of testing?

Once you test both materials you see what works for the dimensions of what you want for a cnc.

I don’t see what is so hard about openly publishing that data especially from huge manufacturers. It is fun to test but people don’t talk about that which is strange.