[kostas1]

Hi all,

I am building the gantry of my 2x1.2m machine and I am wondering what material to use in gantry sides: aluminium 5083 or iron(steel)? Side is something like the attachment. According to its size, I calculate that 15mm thick aluminium will weigh ~4.5kg while 7mm thick steel will weigh ~6kg. However, the cost for the iron piece will be 1/3 of the aluminium cost and this seems good for my pocket!
What should the thickness of gantry side be for steel and for aluminium? What problems might occur if I do the gantry's sides with iron(steel)?
What do you suggest?

Thanks in advance,
Kostas.

[JerryBurks]

Given your dimensions, the aluminum parts will be a bit stiffer than the steel, or, to achieve same rigidity you could use thinner aluminum (like 10mm). But I don't know if either one would be good enough for your intended application. To stay with your design you could make the sides a laminated sandwich from 3mm aluminum sheets with maybe a 20mm plywood core. This would be super rigid and save you weight as well as money. Alternatively use gussets, tubes, box design or similar to stiffen up the sides.

Better remove the holes in the side. It does not save much weight and weakens the plates where it counts for forces in Y-direction.

[kostas1]

So you mean that 10mm aluminium equals 7mm of steel?

[kostas1]

Given your dimensions, the aluminum parts will be a bit stiffer than the steel, or, to achieve same rigidity you could use thinner aluminum (like 10mm). But I don't know if either one would be good enough for your intended application. To stay with your design you could make the sides a laminated sandwich from 3mm aluminum sheets with maybe a 20mm plywood core. This would be super rigid and save you weight as well as money. Alternatively use gussets, tubes, box design or similar to stiffen up the sides.

Better remove the holes in the side. It does not save much weight and weakens the plates where it counts for forces in Y-direction.

I am not sure also if laminated sandwich could do the job (rigidness etc)...it seems to me a fragile method to use wood in cnc...
I am not sure if this could make the diferrence...Did anyone do this approach you suggest with laminated sandwich?

Thanks.

[FandZ]

With your given measurements, 7mm vs 15mm thick, the 7mm steel gantry would actually be more rigid. Steel at the same thickness is about 3 times more rigid than aluminum but it also weighs 3 times more. So really weight vs rigidity shouldn't be one of your concerns. To get the same rigidity they would weigh about the same.

One things you should consider will be the flatness of the steel you receive vs aluminum. Hot rolled steel usually is a little rounded towards the center where as aluminum is usually flatter. If you go steel, cold rolled steel may be better option for you since it has tighter tolerances.

[PaulRowntree]

The best plan (IMHO) would be to replace flat plates with a 3D structure to add stiffness. Put 2 2"x0.5" sticks edge on, running the full height of each side plate, and it will be much, MUCH stiffer. So you can then make it lighter, which makes it cheaper and faster to accelerate, which makes it more fun to use. Use the material you can most easily work with. Everybody wins.

Box structures are much, much stronger than flat structures for the same weight.

Cheers!

[JerryBurks]

So you mean that 10mm aluminium equals 7mm of steel?

Yes, but only for a solid plate.... the bending stiffness of a beam or plate goes with the 3rd power of the thickness (i.e. a plate of double thickness is 8 times as rigid). That means, even if the aluminum modulus of elasticity is only a 1/3 of the steel modulus, you need about 44% thicker aluminum plate than steel to achieve the same rigidity.

JB

[JerryBurks]

I am not sure also if laminated sandwich could do the job (rigidness etc)...it seems to me a fragile method to use wood in cnc...
I am not sure if this could make the diferrence...Did anyone do this approach you suggest with laminated sandwich?

Thanks.

Aircraft builders have used sandwich structures for decades to achieve high rigidity. And they don't use massive plywood cores but Nomex paper or aluminum honeycombs or Balsa cores.

You can not build a heavy duty CNC mill with that but for a wood/aluminum router I would not hesitate. I just finished my entire machine in plywood (no aluminum skins but massive cross-section) and bet it is more rigid than many aluminum machines.

Anyway, I think a solid slender plate of any material is just poor mechanical design if stiffness/weight is the goal.

[cyclestart]

To stay with your design you could make the sides a laminated sandwich from 3mm aluminum sheets with maybe a 20mm plywood core.

Rigidity would depend very much on the strength of the aluminum-wood bond ?
Any thoughts on how to create that bond ?

[kostas1]

Rigidity would depend very much on the strength of the aluminum-wood bond ?
Any thoughts on how to create that bond ?

This is exactly what I wonder...should aluminum and wood be glued together or just screws throughout them would do the job?

[kostas1]

Here is the detailed plan in 2d.

[JerryBurks]

This is exactly what I wonder...should aluminum and wood be glued together or just screws throughout them would do the job?

You would need screws through to transfer the forces of other attached parts into the sandwich. But you need to glue the skins to the core, e.g. with epoxy since bending stress in the sandwich plate becomes shear stress between the skin and the core. You probably would want to glue the skins on, anyway since you can then machine and drill the whole thing more accurately as one piece.

But I don't want to push unusual build methods. I mentioned that sandwich option just as one way of achieving higher stability. I know that most machinists see wood as inferior to metal construction and many builders use metal sucessfully when some double wall design is employed like a box or attached tubes. I just like to do things differently....

[crane550]

I am going to be the black sheep here and encourage weight- I have never EVER regretted making something extra beefy. Weight is absolutely your friend, it add stiffness, reduces vibration, and lowers tool deflection. My gantry is pushing 400lbs, and would not have it any other way. I would avoid wood. My first machine was wood, and I replaced every single part with steel or aluminum one at a time as the wood pieces failed or flexed.

[LazyMan]

I would not use wood as a permanent solution. It may provide the necessary rigidity and strength, but it is not very predictable and creeps too much over time even after sealing. Many have been satisfied using it but I not. As JerryBurks said, don't put cut outs in the gantry sides. I cringe when I see people make cut outs on there gantry sides as you save a very insignificant amount of weight, lose rigidity, and create more work for yourself. Not to mention, the extra weight can be a positive attribute assuming your motion system can handle it. Plus I strongly suspect most are making cut outs in the name of aesthetics, which I must say I have been guilty of before.

[JerryBurks]

Maybe we should start a new thread sometimes with a rigidity shootout. It would be interesting to actually compare practical machine stiffness (and maybe weight) measured with a standardized method as a function of build concept (with picture), size, materials and price. That is easy enough to do with a $12 dial gage and a $6 fish scale.

I think it would be valuable orientation for new builders to pick a concept that fits application and budget or, at least to avoid major mistakes.

[kostas1]

My gantry is pushing 400lbs, and would not have it any other way. I would avoid wood. My first machine was wood, and I replaced every single part with steel or aluminum one at a time as the wood pieces failed or flexed.

Oh, ouaou...what motors do you have? What rapid speeds can you succeed?

[kostas1]

Maybe we should start a new thread sometimes with a rigidity shootout. It would be interesting to actually compare practical machine stiffness (and maybe weight) measured with a standardized method as a function of build concept (with picture), size, materials and price. That is easy enough to do with a $12 dial gage and a $6 fish scale.

I think it would be valuable orientation for new builders to pick a concept that fits application and budget or, at least to avoid major mistakes.

Yes this would be great...I am new builder...could anyone give examples of box and attached tube...It seems to me very good to replace plates of metals with aluminium profiles(is this the 3d boxing?).

[louieatienza]

I am going to be the black sheep here and encourage weight- I have never EVER regretted making something extra beefy. Weight is absolutely your friend, it add stiffness, reduces vibration, and lowers tool deflection. My gantry is pushing 400lbs, and would not have it any other way. I would avoid wood. My first machine was wood, and I replaced every single part with steel or aluminum one at a time as the wood pieces failed or flexed.

I agree totally, and have also touted the benefits that mass plays in machine design.

Of course, one has to design his/her machine within the constraints of their available material, tooling used or needed, cost, and power of the drive system used. In that sense I guess many of us build our machines backwards; that is build around whatever electronics, linear rails, and leadscrews we could find, as opposed to designing a machine frame, picking out the correct linear components, THEN figuring out how much power we will need and buy accordingly.

The other issue I see deals with what exactly would you be machining? For sure, if it's mainly metal and hardwoods you want to cut in one pass, you'd want as much mass as you can afford. For mose everyone else, you'd have to consider what that extra mass and ridgidity is going to cost you. And it's not just metal, you're talking maybe moving into high-powered servo territory, profile rails, etc. Also consider that if you don't have high grade motion components (hardened and ground ballscrews or gear racks) and have a lubrication system for them, they're going to life out much quicker. Heavy commercial machines have self-lubricating drive components. One could argue that the won't see as much use as a commercial machine, but then that would just prove that you don't need to go through the expense. Also, oddly, the more expensive the drive and linear components, the more unforgiving it is of any slight misalignment. One cannot just take some 35 series rails and slap it on a piece of steel tubing, since tubing is not perfect. The profile rails will telegraph any imperfections, which can cause binding and premature wear.

Anyone know why many of the best sounding handmade acoustic guitars are such? Because their light weight allows the wood to vibrate and contribute to the sound. A lot of factory made acoustic guitars, where they may be made overseas, are built a lot heavier, don't have as much volume, and feel a bit 'dead' to play. It's somewhat analogous that the heavier the machine, the higher it's resonant frequency, and the more vibration-resistant it becomes. A lot of this may not be as apparant to most of us since we use lower-powered spindles and take shallower cuts than on a commercial machine. More weight also leads to less tool deflection since the system has a higher 'inertia' so to speak. The spindles alone on some commercial machines can weigh in the hundreds of pounds, with double digit horsepower ratings.

I thnk that if I had to choose between a wood product and an aluminum one of equal stiffness, I probably would lean toward aluminum, even if it meant (and maybe in paricular because it means) a thicker cross-section and more weight. Plus I'm lazy and I'd have to paint the steel. It also might be more cost/benefit effective (and easier) to put the weight in the base, where you'd see a difference, as opposed to having it all in the moving parts, with a flimsy base that will shake with every move. I would also use the highest powered spindle you can, so you don't have to make the machine push through as much through what you're cutting, and you can run it at a lower speed and still have enough power.

Steel frame CNC's seem to be the high water mark of the DIY crowd, but really, steel is used more in entry-level commercial CNCs. Most of the top end stuff is cast iron, which is stiffer and way more vibration-damping than steel.

My first machine was also wood. I won't say I regretted doing it that way; that was what I could afford, it was easy to cut with hand tools (and even by hand on some parts), and I leared a lot from the experience. Heck, before I upgraded the leadscrews, I had about $500-600 into the machine, complete sans software. And I didn't mind running bits into the table, or crashing the carriage into the gantry or bed. I did make my second machine out of phenolic, but mainly because it was what I had. And while the increase of weight I feel helped the performance of my machine, I think upgrading from 1HP to 2-1/4HP made a more substantial gain.

[JerryBurks]

I agree, too. At the end, you can build a great or a crappy machine from most materials. It just depends if the design accommodates the properties of the material. I am confident my new all-plywood 32x36" machine can match the rigidity and position accuracy of most aluminum or steel frame gantry routers of the same weight class, but then it was not much cheaper to build and at over 500 pounds it is not exactly very light weight. It just made sense to me because it was easier to fabricate the parts with my equipment.

[kostas1]

Thanks for your useful comments on this subject...my view is that profile aluminium (ie. 80x40) could make the gantry rigid and stable. Here is a sketch up of what I mean...this would be more rigid than a single plate,right?