[catahoula]

But the bridge is then asymmetric in stiffness. ie its stiffer in one dirn vs the other. Ideally need a rear skin so the front and rear skin are the same.

The bridge is not the gantry; it's not under so much torsion. Your analysis shows the X direction is the weak point for deflection by quite a bit. To keep the two columns parallel and resist X deflection, the important part is spacing out the bridge vertically, as far down the columns as possible. A skin surface on the back face is pretty much irrelevant for the loads here. Even a bolted structure with a similar shape would help quite a bit.

Cheers

PS shop is looking great and envy-worthy

[peteeng]

Hi Cat - Without getting too technical the solid bridge weighs 17kgs which is easy to manage and easy to cast solid. Its stationary so weight is not critical. So it may as well be solid. Plus the FE work shows its a very minor structural part but I think it will help with the column vibration. My main concern at the moment is to bring down the base weight from 235kg to 100kg. Its currently a 120mm thick slab of E=70GPa material. Quite a chunk of stuff.

I have looked at aluminium sandwiches, foams and grids over the last couple of days. Your diagonal truss image is called a diagrid and this maybe an answer for the base. I have played with bolted up webs in Al plate to get an idea if it can be stiff enough. The image is stiff enough in Y&Z well over the 20N/um target but torsion is the usual hurdle. There is also the issue of a counter mould or the use of voids to get cast rear details, can get complicated.... Stay tuned.. Peter

[peteeng]

Hi All + Sundry - I'm waiting for my computer to solve a large problem so I looked at the machine base. My thinking is to make it in two pieces that bolt together from the bottom, then it has to be flipped Hmmm so should be bolted from the top...Each half weighs 63kg in this version. Its an orthogrid across the major hardpoints. Once the model solves I can get this into the FE. These would be straightforward to cast...Peter

addition - This has turned out very well X is 20.5 Y is 31 and Z is 66 N/um. So I can work on this 2 piece principle a bit more. Perhaps a bit deeper and do a generative design on it. Or jump to an isogrid or diagrid.... my main concern is the faces. I've designed at 20mm but will CSA and fibres be good at that thickness? don't drop something on the base!! Perhaps I use aluminium top and bottom with cast web in the middle... Time to do some casting and find out about this CSA stuff.... Peter

[peteeng]

Morning All+Sundry - This morning I built the clamshell base. It hit the target deflection at 61kg per half. I'll have to detail it out with perimeter bolting, just like a gearbox or engine part. Currently its an orthogrid and not optimal but that's the next step now I know it could work. The columns will go the same way. Then the other parts can be tuned up a little and we'll have a lighter machine.

Get some stuff moved from RHS to LHS to finish painting my workshop floor then I can get serious in there!! Peter

[peteeng]

Hi All - I have made an isogrid base and it weighs 60kg per half. The static stiffness stats are X 23 Y 38 and Z 66 N/um so very happy. Casting is 16mm thick so I'll have to see how that goes with test casts. I have called this the Dragon base after SpaceX machine. Now to do the columns and then it can be detailed.... Pippin will be happy with isogrid!! Peter

[ardenum]

Wait are you going to cast it in iron now?

[peteeng]

Hi Ard - No it will be CSA/ALOX iron has never been in the selection list. Peter

[ardenum]

Hi Ard - No it will be CSA/ALOX iron has never been in the selection list. Peter

16mm thick concrete? I think you're playing with fire here. How about making the bed modular and bolting it together instead?
A properly heavy piece that dampens vibrations but made from 50kg pieces.

[peteeng]

Hi Ard - The bed is two pieces and is modular and is bolted together! 16mm UHPC concrete no problem. If there is then its ALOX/epoxy, if there's issue with that its machined aluminium with UHPC webbing.... layers of plan A, plan B etc. Main thing is to get the geometry right as geometry is the king. Peter

[ihavenofish]

16mm thick concrete? I think you're playing with fire here. How about making the bed modular and bolting it together instead?
A properly heavy piece that dampens vibrations but made from 50kg pieces.

most of the mixes ive seen - schneeberger for example - say 40-50mm is bare minimum.

[mactec54]

16mm thick concrete? I think you're playing with fire here. How about making the bed modular and bolting it together instead?
A properly heavy piece that dampens vibrations but made from 50kg pieces.

It will work very well once he gets the mixture correct

[peteeng]

Hi All - The Sneeburger (and other suppliers) mix uses larger aggregate then I shall be using. A 16-20mm thick piece of terrazzo/synthetic granite or real granite is still a strong element. This is not low strength Portland cement. Ideally it will also be reinforced with 2-3mm steel fibres. Have to sort the recipe out... Peter

[peteeng]

Hi Al - Now we have Milli-LG3 geometry and material I thought I'd have a quick look at what's possible in steel and aluminium. So I built a monocoque version of Milli-LG3 and figured out its thickness so its 100kg. In aluminium it turned out to be 12mm thick. In steel it would be 4mm thick for 98kg... I don't think any of these would be locally stiff enough for the rails so it would have to be filled with something that would add lots of weight. Or I'd have to use hard to do internal features to support the shear requirements. This exercise made me realise maybe I'm using too big an envelope to solve the problem so use a crux form and minimise the geometry. This weighed 153kg and used RHS tubes 9mm thick. The column section is 200x100x9 and the base is 250x150x9. So I decided to go with the crux to see what it does. It needs to be 3x stiffer to get to the target static stiffness. But I noticed that the base seems to bend less then my big slab base... But the other issue with the crux design is that I don't think I can get the X and Y rails machined as the mill can't reach under the bridge? So the columns/bridge would have to be a bolt on... So more thought bubbles needed. Peter

[peteeng]

Hi All - Moved back to casting and simplified the base. Still can't get lower then 160kg for the base based on 2700kg/m3 material E70 in a single part. So I'll have to leave this for a while to get YaG moving again. Spent some time today lapping the tops flat and level. Need to finalise metal bits!! I think what I shall do is shrink Milli a bit to make it more manageable. So make the base 100kg and see what that looks like and then match the rest. 2 people can move 100kg OK 3 people easy.... Slip bars into the holes and lift, bend those knees!! Peter

Stats X 22N/um
Y 40 N/um
Z 61 N/um

[peteeng]

Morning All - Have completed a whole new series of designs called the Mini. The Crux was that every time I changed the geometry I seemed to go backward with the X stiffness. I decided to scale down the machine till the base got to 100kg, a 2 person lift or I can block and tackle from the roof. This means a table size of 250x500mm. Then the X dropped to 14N/um which drives me mad. Plus the Y&Z get better at 50N/um... Then it struck me the columns are not wide enough! The penny dropped so the Mini then got "wings" I just copied and flipped the columns around and booleaned them together adjusted the base and now I get X 28 Y29 and Z69 N/um... very good. So make the columns square and keep adjusting until they work.... The current wing design is a bit complex to make. Plus smooth out the base and maybe make a bit thinner. Holy Grail Batman!! Peter

[klaas123]

Good thinking Peter, evolution is a slow process but you are getting there!

[peteeng]

Thanks Klaas - The Milli simulation file has just clicked over 20Gb. I remember when you couldn't get a drive that big! Lucky its on a terabyte drive... Peter

[klaas123]

20GB, my first computer had a 20MB HDD

[lukahr]

Nice thinking with isogrid for base, I had same idea on my machine for saddle couple days ago, but haven't got to testing to see if it would be strong enough. It would be lighter for sure
Making isogrid mold would be easy for me, I would just cut negative with existing cnc machine and cast. What is the minimum wall thicknes with UHDC like nanodur E45, any idea ?

[peteeng]

Hi Luka- You will have to ask the Duracrete people but on their site its about 40mm. My isogrid is complex because its a two sided part. The "open" side which is poor dimensionally would need to be finished somehow so as to marry correctly to its counterpart. So I have flicked that idea. Being the first of its type for me I have to be careful not to make it too complex. Once I make a few parts and understand how it goes then I can get more complex with countermoulds. So Milli-Mini-3 is here. The column being an RHS section I can make it wider until it works in both directions,. The prior columns where a bit biased towards fwd/aft stiffness. This design is 22/26/50 N/um so it WORKS! I don't like straight cantilevers as they vibrate but once the geometry is known I can warp/taper it somehow. Another day closer... Peter

The base weighs 102kg the columns 56kg so managable...