round rails

[TrickyCNC]

Also, dont under estimate the 'bendyness' of steel

Just think about the title of this thread. What the round rails are made of, and what you need to do to stop them bending.

It's exactly the same for steel box section. It will need supporting and bracing too.

not knocking it, just reminding you and anyone else reading this, to think about the design, as much as your chosen material.

As a very extreme example, pick up that piece of 50 x 50 x 7m long box section Jazz mentioned, in the middle, and it will bend and wobble like a bendy wobbly thing !

[JAZZCNC]

As a very extreme example, pick up that piece of 50 x 50 x 7m long box section Jazz mentioned, in the middle, and it will bend and wobble like a bendy wobbly thing !

Come on.!! . . . Do the same with an 8x4 sheet.!! Give it to Rolf Harris and he'll play you a tune.!!!

Get a grip man it's all about design and bracing.! With MDF/WOOD it means more bracing, much more to get near steel.

I have worked both materials for years and have equipment to use both and if I could build just has strong, accurate, reliable and cheaper machine from wood then I would. I can't simple has that.!!

[TrickyCNC]

Come on.!! . . . Do the same with an 8x4 sheet.!! Give it to Rolf Harris and he'll play you a tune.!!!

Get a grip man it's all about design and bracing.! With MDF/WOOD it means more bracing, much more to get near steel.

Sorry, I missed this one.

I completely agree :)

I know ... and you know ... , but some people might be inclined to think steel is just rigid.

[Ger21]

Gerry in all honesty now.!! If you had both options available IE full Access to machinery and resources would you build from Wood.?

It would probably depend on the actual machine. If I needed a 4x8 (or larger) production router, then it would really need to be steel. For anything say 4x4 or smaller, I'd still go with wood. Mainly because I like working with wood, steel not so much. And I know how to use wood and make it work well.
As you alluded to, wood is much cheaper for me than aluminum.

The gantry beam in the pics has $30 in MDF, and $35 in phenolic, and about $15 in glue. Say $100 complete for a 68" (1700mm) long beam. I couldn't get an aluminum structure that would be as strong for anywhere near that cost. The Hiwins that mount to it were $350 (20mmx1700)

Calculations are showing a deflection of about .02mm with a 200lb load. I actually believe it will be stiffer due to may laminate construction, which appears to be about 2-3x stiffer than regular MDF.

Equally if you didn't have access to either equipment and had little to no experience using either wood or metal/Alu profile.! . . Say an office worker just looking for a DIY hobby in the back shed which of the two options would you say gave the best chance of success FIRST TIME of an accurate, reliable machine.?

I'd definitely recommend the aluminum extrusion. Or is it aluminium for you guys? It costs more, but is by far the easiest for a novice imo.

I'm with you on the CNCRP carriages, btw. Don't care for them at all.

[TrickyCNC]

I'd definitely recommend the aluminum extrusion. Or is it aluminium for you guys? It costs more, but is by far the easiest for a novice imo.

for ease yes, but remember it's not even just a case of buying an 80x20 extrusion 1700mm long ! a BIG chunk of ally is needed to be rigid at this length

[Jonathan]

for ease yes, but remember it's not even just a case of buying an 80x20 extrusion 1700mm long ! a BIG chunk of ally is needed to be rigid at this length

If you made a torsion box type construction out of aluminium or steel sheet that can be very strong compared to the volume of material it uses.
It's interesting to compare the young's modulus of aluminium and MDF:

1. We can loosely say aluminium is 14 to 28 (hard to find accurate figures for MDF) times stronger than aluminium based on the ratio of their young's moduli and similarly steel is 40.6 to 81 times 'stronger'. That implies that you can use 14 to 28 times less volume in aluminium than MDF for an optimally designed structure with each material.
2. Aluminium is currently roughly £5.50 per kg which equates to £14850 per cubic meter.
3. MDF, based on buying 18mm 8x4' sheets is £320 per cubic meter.
4. Jazz says £26 for a 7.5m length of steel, which is 4.4kg/m making steel £6627 per kg.

Therefore aluminium is 46.4 times more expensive than MDF per unit volume and steel is 20.7 times more expensive.

Taking into account the 14-28 ratio for the strength, we can conclude that the same strength structure should cost 1.7 to 3.3 times as much in aluminium (much more if you price up aluminium extrusion per kg), or 0.24 to 0.5 times the cost of MDF if you use steel.
So in conclusion... if just purely go by comparing the, for want of a better word, strength of the materials then steel costs less than MDF(!) and unsurprisingly aluminium is expensive. Clearly there's more to it than this simple analysis - as has already been alluded to there are many other factors in using each material that can override differences in cost.