Substitute MDF for metal on a DIY router?

[posix]

What, if anything, can be gained if I start substituting bits of MDF with bits of aluminium or even steel? Ok, stiffness, that I understand but a rather stiff structure can be made with torsion boxes as well.

If i start putting bits of aluminium I will loose weight and I guess stiffness will be the same as with MDF as aluminium is not a particularly hard metal. So I end up with a router that's able to cut wood, MDF and light aluminium. So no point in switching to alu, really, right?

So if I go straight to steel, what happens? Does your ordinary jgro or joe-style router turn into a mill?

[paulC]

If you think about the aircraft industry they want max strength for weight. I am sure that a hugh amount of research has been put into these questions and they still use aluminium torsion boxes. So I suspect aluminium torsion boxes are the most efficient solution without switching to carbon fibre.
Paul

[fyffe555]

Its a properties of materials 'thing'. Al. is 20 times stronger than MDF. But MDF often appears stiffer because its thicker. Think about the weakest link - no point in building a steel jgro as the bearings wouldn't match the capacity of the frame...

[posix]

what would happen to them? they'd get all squishy?

[jeffs555]

If you think about the aircraft industry they want max strength for weight. I am sure that a hugh amount of research has been put into these questions and they still use aluminium torsion boxes. So I suspect aluminium torsion boxes are the most efficient solution without switching to carbon fibre.

The problem for most mills and routers is rigidity, not strength. The aircraft industry is mainly concerned with strength vs weight, so is not really a good example for cnc machine design. If you have ever been on a commercial airliner in turbulance and watched the wings, you would see that they are far from rigid.

[jdholbrook]

On that subject..
If I wanted to substitute AL for MDF what thickness of AL would you suggest for a piece of MDF that is 3/4x12x20 ??
Would 3/8 work?

Thanks
James

[paulC]

The problem for most mills and routers is rigidity, not strength. The aircraft industry is mainly concerned with strength vs weight, so is not really a good example for cnc machine design. If you have ever been on a commercial airliner in turbulance and watched the wings, you would see that they are far from rigid.

A wing flexing a few feet when you have a wing span of 200 feet supporting 240 ton of dead weight and this weight suddenly doubles when you add say a 1g of force when you hit a an air pocket is hardly saying that torsion boxes are not rigid. I would rather have a bit of flex than them snap off.
The forces inflicted on a wing are huge and even a solid mass would move conciderably under them.

Mills need mass to overcome vibration as much as for rigidity.

[posix]

so what, now? you're confusing me (and everybody else I presume)

do we need mass or strength or rigidity for cutting steel?

[ger21]

All of them.

[posix]

ok well that's cleared up a few things then. so make torsion boxes out of steel then?

[paulC]

so what, now? you're confusing me (and everybody else I presume)

do we need mass or strength or rigidity for cutting steel?

Sorry.
Your initial question was around replacing MDF with metal. You have two options, solid or torsion box construction. A torsion box can provide more strength/rigidity for the same weight of material if designed correctly. Of course it involves a lot more work.
Comprimises include sofisticated extrusions such as 8020 which steal a little from both worlds.

For machining steel or other metals you need mass. The more the better. This helps stop things bouncing around with the much larger forces required.
Machining aluminium on a router style machine is something you can do, but only slowly as its a bit of a comprimise. Slow speed equals less force.
Paul

[spalm]

Posix,

I have built and studied both a jgro and a torsion-pipe-rail-with-adjustment-plate-thingy machine. Aluminum or steel will not solve their problems. The jgro has free floating rails that flex like crazy. The torsion-pipe has much improvement but has the problem of having to tighten the bearings to support the gantry/Y-carriage at a 45 degree angle and that alone will cause a lot of stress on the structure. It also has the inherent problem of tilt.

Not to knock these designs down (they are inexpensive), and I am all for building rigid structures, but as mentioned, find the weakest link. In these two, it is the linear bearings, not the MDF.

Steve