[peteeng]

Afternoon all and sundry - I have been thinking about this truss idea but haven't really made a model that suits. My inertia around the truss is that to make them the usual way is labour intensive. But I thought how about I laser cut the sides as a truss? So I moded the last model and unfortunately its half the stiffness of the solid sided one. This has to do with membrane vs member action. In a truss the load has to travel down the members. And with torsional stiffness the load has to distribute itself over a relatively small area or volume in this case. So the volume of material in shear is low so the shear deflection is high. With a membrane such as solid sides the shear volume is large so the shear strain is smaller so the deflection is smaller. In this case about half the stiffness in the trussed version. So I think trusses are out because they are complex and don't do well in torsion. In bending however they can be efficient but its torsion that is our enemy at the moment. So the best way to do this is to make the column as big as possible (outer dims), then thin it out until it achieves the target stiffness. So onward to the machine base. Peter

[peteeng]

Evening all - Switching to steel suddenly I'm hitting the stiffness numbers. I'm still wondering about cast CSA so I did a quick calc to show that steel will be about the same weight but 4x stiffer then the solid CSA. So material stiffness really helps. The 200x200x20mm steel column weighs 112kg/m and the 200x200 CSA one weighs 108kg/m3 so nearly the same. The steel column is 15.7e9mm4GPa rigidity while the CSA has 4.0e9mm4GPa. So the case rests. Onward with steel... Peter

[joeavaerage]

Hi,
I have reached the same conclusion that steel and cast iron are still the best at stiffness per unit weight, stiffness per unit volume and stiffness per unit cost.
There may be contending technologies but I don't believe any of them are yet a match for all three metrics and at best can only approach steel/iron and seldom
exceed it.

I've come to think in recent times that cast aluminum might be a worthy contender, certainly stiffness per unit weight. There is a least one top end manufacturer (Kern) that
uses cast aluminum particularly for its thermal properties. Like any material you have to design according to its properties....

Craig

[peteeng]

Hi Craig - Yes alum will be better in transferring heat then steel. I'll develop the steel design a bit further then look at aluminium. I'd prefer alum vs steel. But I see that for any given fixed geometry steel will be stiffer and can be thinner because of the 3x modulus. So Aluminium may never get there in terms of max rigidity. Vibration mitigation will be interesting in steel. I think laminates maybe an answer. But need to get the whole machine concept to a given level before I think about that... thanks for chirping in occasionally. Maybe its time to rethink the scope of the machine, its suddenly gone down a rabbits black hole that I did not expect....Peter

stiffness per unit weight - using this metric ie the specific stiffness of steel and aluminium are the same. steel is 7800/200= 39 and AL 2700/69= 39 but specific stiffness in this case does not help. Usually in structures if you can make the aluminium section bigger then the steel section so its geometric inertia being to the 4th power takes over and the Al wins in rigidity and lighter weight. But if the dimensional envelope of the two is the same steel wins as its inertia is similar but its modulus is far greater. Now I have that clear in my head steel manufacturing has various ways to be done. Laser cut parts like I use at present but brazed (even soft/hard soldered) then finish machined seems to be worthwhile. Or thick plates and bolted is the other path... Conceptual thought clouds needed and some ZZZZZ's

[joeavaerage]

Hi peteeng,

stiffness per unit weight - using this metric ie the specific stiffness of steel and aluminium are the same. steel is 7800/200= 39 and AL 2700/69= 39 but specific stiffness in this case does not help. Usually in structures if you can make the aluminium section bigger then the steel section so its geometric inertia being to the 4th power takes over and the Al wins in rigidity and lighter weight. But if the dimensional envelope of the two is the same steel wins as its inertia is similar but its modulus is far greater.

That is a masterly summation of the properties of aluminum, very impressed. Basically if you are dimensionally unconstrained aluminum looks good stiffness to weight but
if constrained then steel still wins out.

It seems to me that this project has broadly the following goals:
1) It has to be stiff, say 20N/um or better on all axes, if its not stiff then no-one will want one no matter how attractive the price.
2) Cost, it has to be affordable
3) Weight and to a lesser degree size so that it be practical in the home workshop.

The first requirement, stiffness is material agnostic, so long as you design to the material of choice you can make it arbitrarily stiff, and it needs to meet a spec to be useful.
The second requirement, namely affordability is NOT material agnostic. In fact to make it affordable you will have to compete against many other manufacturers around the world
and thus I would suggest that the cheapest material be chosen that can achieve the required stiffness....and for that steel or maybe cast iron or even cast aluminum are probably the
likeliest candidates. The weight/size requirement is material dependent also, and steel, iron and aluminum will figure in a short list of contenders.

As you know I have been building and commissioning a new mill. I elected to have 115kg cast iron axis beds cast for me. They were not cheap, but worse was to come, namely
I had to pay and pay dearly to have them machined to a high standard. Not withstanding that I've got superb axis beds they were too expensive to reproduce. If I could find an
organisation that would machine them at a much sharper price then they would become price attractive.

By this stage my budget was near shot so I had two "L" shaped pieces of 32mm med-tensile steel cut for the frame. They were spaced apart by bolted 20mm plates.
I always intended that the frame be welded such that it forms a tube. Of course once you start welding you have to stress relieve. Unfortunately there is no-one in Christchurch
with a large enough oven....that I have found yet. This would require that I send it to Auckland, across Cook Strait and 1000km north, plus the $6.00/kg for the heat treatment itself.
I have work for this machine right now, mostly small parts and circuit boards that I've always done with my mini-mill and therefore don't require the extra stiffness all the welding
creates so I have pressed it into service just with the bolted frame with the cast iron beds bolted to that. Its working well, even if it is less stiff than I had hoped for. Perhaps in a few months
I can take it out of service, do the welding and get it stress relieved....but in the mean time I need to make parts.

While I can't see myself designing a new mill anytime soon, much less have the budget for it, I've already had some ideas. Particularly rather than casting axis beds which are attached
to a frame I would cast the Y and Z beds into the frame, the frame in two bolted pieces not unlike Hass VF2's. Its just too hard to beat the stiffness and damping properties of cast iron!

Craig

[StrawberryBoi]

I do wonder at a combination of bolting and adhesive bonding. Usually the biggest issue with an adhesive in stiffness situations is the flex of the adhesive, but it would only improve the stiffness by adding it to the bolted connection and would not require stress relieving with the right selection.

Thermal curing could be an issue, particularly since max properties almost always require a thermal aging process, however you won't require anywhere near the temps of stress relieving and so could use a controlled heat source and insulating material and do it essentially in situation.

If you needed vibration dampening you could also try a controlled layer height of EG along the sides of the plates. It wouldn't need to fill the entire cavity to provide some additional damping.

[ProtoTypeV2]

Hi Peteeng,

I have been following this thread for a while now and made a quick mockup of what I think you could do to manufacture your column. The idea is to make a hollow cast column that can later be filled with a fluid or fluid/sand mix to add mass and help with vibration dampening.

The simplest solution to this would be to suspend a 2L bottle inside a steel tube and fill the area in with a composite or expandable foam. After the machine is assembled the bottle can then be filled with fluid/sand to add mass.

I have attached a .stp file and a couple images to show how the structure would look. Basically there is one casting with two hollow sections. The outside hollow section gets filled with a composite or expandable foam to help strengthen the structure. The inside hollow section has a port at the top and bottom to add/remove a fluid for added mass. This is a complex casting, but could be simplified if the outer shape were just the casting and the inside chamber was a blow molded container. Maybe this could even be designed such that the column is an extruded aluminum section that is capped at one end and filled with a high durometer urethane.

Matt

[StrawberryBoi]

Interesting, ProtoTypeV2!

I could see an advantage to adding mass that can be just drained/pumped for moving, but I'm not sure that really will help with damping. Water and oil are incompressible fluids and air in the mix adds compressibility but not damping. You would need a sort of mesh feature and something viscous like oil, because ultimately mass may prevent a large movement by adding inertia, but mass alone isn't converting the energy from mechanical to heat, which is the passive damping.

A shock absorber typically works by forcing oil through restrictions which makes a lot of heat, this converting the mechanical motion to heat. A rubber damper does the same within the material. The mass that is the iron in big machines is partially for structural stiffness and majorly for damping, as cast iron also acts as a (poor, but better than steel) damper.

So the goal is to have a stiff, well damped assembly, not specifically a high mass assembly.

[peteeng]

Morning All -
Strawb - if the connection is bolted and its designed correctly then there is no purpose for adhesive in the joint, friction will do the job for you. If you want to use an adhesive then use a wicking loctite no need for curing. Anerobic adhesives are good things. If the machine is stiff enough then damping is not needed (broad statement), the primary goal is to have a machine that is stiffer then the tool deflection allows so it does not bounce or scrub. I'll have to think of how to express this clearly. Plus high mass does not necessarily help vibration mitigation. I use mitigation because the common term "absorption" is not an accurate expression of what is happening.

For those in the weight is great debate, if the machine was on a space station (so its weightless) would it perform differently to when being on earth? Its stiffness is the same.

Hi Proto - thanks for contributing. Liquid damping like used in tall buildings has a few issues with machines. The tank has one frequency that its good at working with. A tall building has one natural freq, a machine has many. Filling columns with foam or composite does not contribute to the column stiffness, The prior calculation comparing steel to concrete shows that up very quickly (at this scale). One issue with Milli is that I have been driving the dimensions down which has forced the use of high modulus materials. In bigger machines the geometric inertia is dominant and then lower modulus materials do not matter. Machines of this small size that I can find are in the order of 1 to 5N/um static stiffness. Milli will be an order of magnitude stiffer then these and I feel vibration won't be a big issue. But until the machine basics are figured out, keep vibration as a second order concern. Many people try to solve the vibration issue before its a real issue.... Cast Iron is held up as damp but it is half the stiffness of steel and its primary property is its castability. Damping is a bonus not a strong reason to use it. If you are going to make many objects casting is a solution. Another CI property was lubricity for the ways but now we use rails this is not important anymore.

AM is taking over and we have new materials to play with....Depending on what literature you read you can find figures in which steel and CI have similar internal damping properties. Material & Machine damping is complex, much easier to design a cars shock absorber... again we say shock but its a motion damper, it does not "absorb" the "shock"

I think I'll re formulate Millis objectives as Craig has kicked it off. I'm lucky I have all of those things needed by Craig within 2 hrs drive with reasonable costs...Peter

[joeavaerage]

Hi,

I think I'll re formulate Millis objectives as Craig has kicked it off. I'm lucky I have all of those things needed by Craig within 2 hrs drive with reasonable costs...Peter

About the only time I've been envious of an Aussie too by crikey!

Craig

[peteeng]

Hi Craig - Well although they used to be 2hrs drive away, they are 2hrs drive plus a 2hr wait at the state border (they are checking every vehicle!) and as I am not essential I probably won't get across so may as well be 1000km away... Lockdowns been extended a week.. So on my router build I have decided to get access to the shed it is in and pull it apart and bring it home. It won't get finished where it is... should have been finished months ago... Interesting times. Peter

[joeavaerage]

Hi peteeng,
you may well have heard that NZ went into a very strict lockdown earlier this week as a response to one community case of the Delta variant. Its looking
like the governments abundance of caution may do the trick and suppress the outbreak, but it has caught me out as well. All my tools and equipment is at
work , of course as is my mill. There are dozens of small, and some not so small jobs, I could be doing on it.

As you say interesting times.

Craig

[peteeng]

Hi All - To round out the issue of the size of the column in concrete I did the math. If the steel one is 200x200x20mm thick as before, the concrete solid one (or an EG one) has to be 281mm square. This would weigh twice as much as the steel one but be of equal stiffness. To be lighter it would need to be bigger again and hollow. The solid 200x200 has an I=133,333,000mm4 and the steel section is 78,720,000mm4 so the steel is 78.7/133.3= 50% the inertia of the solid concrete yet its 4x stiffer mainly due to the material stiffness delta. OK so moving along to a more detailed design....

As Milli is small its driven by the packaging size of its std components more then the chosen geometry for the parts. This is why the geometry is constrained, I am sizing down not up. Peter

1) It has to be stiff, say 20N/um or better on all axes, if its not stiff then no-one will want one no matter how attractive the price.
2) Cost, it has to be affordable
3) Weight and to a lesser degree size so that it be practical in the home workshop. Benchtop/stand size minimum weight
4) For my purposes it will mill composites and aluminium so a HS spindle such as a GPenny metal maybe the answer
5) My parts tend to be long & high (moulds) so 600x300x400mm envelope maybe good
6) I prefer a fixed table vs a moving table
7) max part weight 50kg (prefer 40kg) for easier transport and manual management
8) may think of something else soon - all good to go for the next 1000 entries...

[ardenum]

Hi All - To round out the issue of the size of the column in concrete I did the math. If the steel one is 200x200x20mm thick as before, the concrete solid one (or an EG one) has to be 281mm square. This would weigh twice as much as the steel one but be of equal stiffness. To be lighter it would need to be bigger again and hollow. The solid 200x200 has an I=133,333,000mm4 and the steel section is 78,720,000mm4 so the steel is 78.7/133.3= 50% the inertia of the solid concrete yet its 4x stiffer mainly due to the material stiffness delta. OK so moving along to a more detailed design....

As Milli is small its driven by the packaging size of its std components more then the chosen geometry for the parts. This is why the geometry is constrained, I am sizing down not up. Peter

1) It has to be stiff, say 20N/um or better on all axes, if its not stiff then no-one will want one no matter how attractive the price.
2) Cost, it has to be affordable
3) Weight and to a lesser degree size so that it be practical in the home workshop. Benchtop/stand size minimum weight
4) For my purposes it will mill composites and aluminium so a HS spindle such as a GPenny metal maybe the answer
5) My parts tend to be long & high (moulds) so 600x300x400mm envelope maybe good
6) I prefer a fixed table vs a moving table
7) max part weight 50kg (prefer 40kg) for easier transport and manual management
8) may think of something else soon - all good to go for the next 1000 entries...

Don't worry about vibrations, if you have a steel shell it's easy to make it damp the vibrations. Just pouring concrete inside is enough or if you want to go the next step sandwitch an elastomer between the shell and a concrete core.

[peteeng]

Evening All - I made a new 12mm thick column with legs for the cars. I made one without webs and one with webs on the car tabs. Very little difference. The intent is to laser cut parts, braze together (Tobin Bronze) then finish machine. Zinc plate. Or maybe do it in stainless like my routers, have to cost both ways.

Now to look at the base as this is the big weighty object. The column is 47kg so in spec for weight. Peter

[joeavaerage]

Hi peteeng,
have you considered something like plates welded together with a sub-arc process?

I used to service a sub-arc machine that that could run at 600A continuously and lay down perfect welds 20 plus mm deep in one pass.
Another customer had two Lincoln DC600's in parallel for 1200A cont and welds 30 and 40mm thick!!

My thought for a column was to get two short lengths of very heavy duty Universal Column, some of which has flange thicknesses of 25mm and web thicknesses of
17mm. If they were machined or otherwise cut down into two 'C' sections and welded together using full penetration sub-arc welding...... Naturally they would have to be
heat-treated and so on, but could be done very economically in small to medium runs.

The machine I am thinking of held long lengths of steel plate, up to 15m long, in a jig that used hydraulic and pnuematic clamps to hold them together and the weld machine would quietly
advance under motor traction along the weld. They use it for making bespoke beams and columns for buildings and has seen very extensive use since Christchurch buildings are still
being built after the earthquake of ten years ago.


Quirky thing about sub-arc is its remarkably quiet and virtually no flash. When operating you can see the weld current at 400A plus and the 2.4mm weld wire disappearing into the
weld zone but is of course totally submerged in flux so you cant see the weld arc. In fact you can hardly hear it either, you can certainly hear the hum of the welder but the
weld itself sounds like a panful of bacon frying....thats it!

Really good, uniform, full penetration welds in 20mm plates easy as can be.

Craig

[peteeng]

Hi Craig - sub arc is used for very deep high amp welds as you explained. Unfortunately any process that melts the metal, then distorts the metal. So a stress relief would be needed. I've been looking at high strength solder. 200degs melt, metal does not even get to red and its 100MPa strength tin solder. I've done a few tests here and its strong enough for machine parts. My oxy will do that easy or it can be soaked in a 200C oven then touched up with soft flame as you introduce solder. I've done a lot of silver soldering in the past and its really good too. It runs at 600-700degs just red but silver solder is $$$ compared to tin. Tobin bronze is about 900C quite red. One future is laser welding have a look at that for thin metal at the moment. They are getting stronger and can do thick stuff as well. But it still is a molten process... Steel shrinks 2% when it solidifies this creates very large stresses...

But sub arc is out for this little bunny. Prefer not to do any process with heat but edge bolting has limits and costs as well. Brazing allows me to use laser cut parts as is, edge bolting requires laser cut parts to be machined and CNC edge work is expensive work. I'm also considering having bent flanges then epoxying these. This would be damp and easy to align and join. But I'm not sure about local stiffness, have to do some modelling or testing on that.

A company I do structural analysis for does lots of robotic sub arc on mining equipment, great for deep long welds... here's one of their trailers. Oh and they stress relive all of these large objects with vibrational stress relief as there aint any ovens big enough for them....Peter

https://en.wikipedia.org/wiki/Vibratory_stress_relief

I do vibrational analysis to assist with node identification points and areas that may be over stressed during vibration. We have had things fall apart in the process....

[joeavaerage]

Hi,
the real advantage of sub-arc is cost. If you weld two C sections together, you could make them say 5m long, then cut it in a power hacksaw to the length of the column.
A days set up and welding might cost $500-$1000 or so but then you'd have enough for ten columns. Bung them in an oven for stress relief and your in business...
There is a reason that sub-arc, despite its complexities, is still a valued technique for thick sections.

Craig

[joeavaerage]

Hi,

Oh and they stress relive all of these large objects with vibrational stress relief as there aint any ovens big enough for them....Peter

Yes there is s company around the corner from me that do vibrational stress relief, in fact they did it as an intermediate step when they machined my cast iron beds. Vibrational
stress relief is a hell of a lot cheaper than genuine heat treatment!

Craig

[StrawberryBoi]

Why not make a crenellation/tooth pattern on the edges as if the were going to interlock like a wood piece, but leave enough clearance for the laser tolerances such that they will fit non-interference. Continue the tabs out until they form an X in the corners and you can bolt to a threaded square stock. Like a square with four smaller (bolted) squares in the corners?

That would eliminate the need for edge machining the larger bits and you can just tap square stock for the corner anchors.