Milli a new composite mill kit

[peteeng]

Hi Pippin - I flicked through Bambachs thesis looking for clues for dampness and weight loss. He spent a huge amount of time testing and analysing various methods. He did use Portland with an Aluminium expanding agent. CSA will be much better then Portland. Probably not known much 20 years ago when he did his thesis. In the damping tests the concrete did much better than expected and he comments that this was probably because they didn't use any damping factor for it in their calculations. So concrete has considerable damping within itself, which is expected as it is not a homogenous material. I looked at the constrained layer stuff and he comments that the thinner the interleave the better, The commercial damping materials where around 1mm thick and he calculates that 0.15mm would be ideal. I have discussed this elsewhere I think but at this thickness laminated metal with epoxy resin fits his commentary perfectly. So you can layer up metals then mill or cut metals to shape leaving internal holes for light weighting then glue up and finish machine. I would use a glass tissue between layers to control bondline thickness. Typical tissue is 0.2mm thick... you could use paper tissue as well.

Aircraft use laminated metals to counter fatigue cracking in some applications and to get damp structures. natural and acoustic vibrations drive airframe fatigue so damp crack tolerant materials are used. . Airbus use tonnages of aluminium laminated metal in the airframes like the A380. look up a material called GLARE, https://www.compositesworld.com/arti...-the-spotlight and other fibre metal laminates FML's

It also gives the possibility of using steel outers with aluminium inners for light weighting taking advantage of steels stiffness on the outside like a sandwich structure.

A note on some concrete expanding agents. They use fine aluminium powder. Concrete and water being alkaline attack the aluminum resulting in outgassing of hydrogen countering the initial shrinkage. But H2 also attacks high strength steels used for reinforcing or inserts and can result in steel embrittlement. I'd like to use aluminium inserts in the CSA but again its alkaline so would get attacked by the concrete. So I will epoxy coat the aluminium if I do this.... Time to do some yard work.. Peter

[peteeng]

Hi Pippin - Re laminates - I built a 50x500mm cantilever. One is solid aluminium 2.5mm thick the other is 2mm al/0.5mm epoxy and 2mm al... I then did a modal analysis and they both free vibrate at similar frequencies, 10.32 vs 10.25 64.14 vs 64.63 etc. I then did a dynamic analysis using 3% damping ratio with a 0.5sec load spike. The laminated beam stopped vibrating in 4 cycles the solid beam in 7 cycles. Can't show this easily I had to run the output in slow mo and count cycles. This is based purely on the elastic ratios of the materials as damping in this case is same for all materials. (I have sent a note to the software people to change the damping to a material property vs a structures property) . Hysterisis and interfaces would improve this I'd expect. So laminates work. Peter

[peteeng]

Hi Pippin - I dug deeper into the damping analysis and its disappointing. Due to the damping being applied at a structural level vs a material level the two examples are actually the same result. I'll have to model it in my meshing FEA to investigate this further. I figured out how to plot the decrement and the two samples have the same decrement using this software. I used 10% damping to slow it down to plot it clearly. Manually counting cycles in the video was a poor method!! Now what I'm worried about with using concrete is its microcracking behavior. I hope CSA does not do this! Will ask the chemist. This was triggered in my backyard. I have had some earth moved for the new shed and it has cracked exactly like concrete does. So once cracked its fine in compression as the cells get pushed together but in tension they get pulled apart. This is a loss of modulus/stiffness which we don't want in machines. So I think fibres are mandatory and a plasticiser which is in the CSA mix (usually PVA glue). cheers Peter

[pippin88]

As an amateur, I would assume computational vibration analysis is more difficult and fraught than bending / stress analysis. Certainly I've come across of a lot of papers proposing and validating methods for computational damping prediction.

Re: CSA
I haven't found anything about CSA and machine tools, or vibration damping.

The main question for me is: how long does it move for?

Early movement is not so much a problem. E.g. if casting shrinks or expands for the first 7 days, just 'age' the casting for a long enough before machining.

The problem with portland based concrete is that it keeps moving.

CSA won't be much better for castings if it keeps moving over time.

It may be a great choice for filling rigid forms - filling steel tubes or lasercut skin? This has been done with portland concrete and epoxy/polymer concretes.

[peteeng]

Hi Pippin - CSA is much more stable then portland. Its chemistry is quite different to Portland. But its a punt... I'll ask the chemist a few more Q's. Vibration analysis is getting easier but I'll have to hammer simsolid to change their damping processing to a material level . In Strand I can do this but takes hours to set up and tune model vs minutes in simsolid... Reading up on CSA says that in the initial cure the reactions are complete and stable so there are no long term reactions going on as does in Portland. So as said its a punt... I think I'll make Milli smaller as a test unit vs a big machine. cheers Peter Pippin have you a thread on your machine?

[pippin88]

No thread yet. Not much time of late. Many other projects to tick off the list before chewing on one this large.

I'm still going around in circles.

Epoxy granite casting
or
Laser cut steel skin with some sort of damping infill (?epoxy granite ??CSA)
- progressed a bit on a design for this but it looked like cost / weight / stiffness would end up similar to full epoxy granite for the non-moving parts (easy to just make the fixed base and columns larger section out of epoxy granite and not worry about stress relief etc). Good strategy for the moving parts / saddle.
or
Steel tube weldment with some sort of damping infill (?epoxy granite ??CSA)
or
Some combination of above strategies

[peteeng]

Hi All- This article studies long term stability of various concretes . says CSA is very stable, no long term shrinkage, cracking or warping. Other types do have these. Encouraging. Peter his thesis is too big to upload, can find it in a search.. Peter

shadravan - dimensional stability of concrete slabs on grade

[peteeng]

So many circles, rabbit holes and black holes....Peter

[CL_MotoTech]

I work in concrete in and have done so for a decade. I've never met a concrete that doesn't crack.

We use a lot of Sika products, including that horse hair in the video from above. It will still crack. I see it all the time.

That said, cool project! I've enjoyed following along!

[peteeng]

Hi Moto - Have you used CSA? or all Portland cement? Peter

[peteeng]

Hi All - Milli has grown from a benchtop machine to quite big. I think I have a good philosophical stance on it now so I'm going to rebuild [Milli No14] from the BT30 power head backwards and make things as small as possible.... Philosophy is cast CSA main parts with machined aluminium motion subplates. The rough subplates will be bonded onto the cast parts then finish machined by a machinist I use on his 5 axis Mazak. That should be accurate... it will be a test case for the CSA. I think I'll use the steel fibres at a good loading. Will need to figure that out. Looks like I go to Sydney in 2 weeks and get it from their main office (900km drive). Heading south to visit the family and will drive by Blueys warehouse. See attached machine comparisons. If you know a good little mill tell me so I can add it to the data base.... That will be a good project for 2021.....Peter

I like the idea of a long bed, so maybe 250x500x400Z will be a good beginners envelope.... I finished YaGs machine base two days ago so now I wait for the motion parts....

[CL_MotoTech]

Hi Moto - Have you used CSA? or all Portland cement? Peter

We use both. Portland is far more commonly used for large pours where a multi-yard truck will be on site. CSA is used for small patches.

I am not a concrete finisher, but the guys I talk to find tell me that some CSA is very hard to work with. We use a Sika product, I can't recall the number, but they often have to use a retarder in the mix otherwise the mix sets off too quickly and becomes unworkable. I've seen a number of poorly finished CSA patches as a result. Some of the CSA mixes we use will literally be plastic like in 20 minutes. So out of a 50 or 80 lbs bag, you are trying move all that material in like 40 minutes. Just getting it thoroughly mixed can take 20.

Anyways, good luck!

[peteeng]

Thanks Moto - My casts will be straight forward I think. Will cast some paperweights first to figure out a few things. The mix I'm going to use says 30mins at 20degC pot life so that's enough to get it into the mould.... could be famous last words. I used to do polyurethane casts and it had a 6min pot life and it was hot cast. Got to be organised with that stuff!! Peter

[spumco]

Peter,

I'm at the point of planning/purchasing materials to fill the column, base, and (possibly) head of my mill. I don't want to hijack your Milli thread with off-topic questions... PM, or is there an existing EG/CSA thread you think appropriate?

Thanks,
Ralph

[peteeng]

Hi Ralph - The EG thread is enormous, takes a year to read. Either start your own thread or ask here I don't consider it a hijack. I'll summarise my thoughts. This is for filling metal parts:

1) Epoxy is stable, reliable but is the most costly material
2) CSA as far as I can determine has not been used yet in machine tools but technically looks very good. Low cost
3) expanding concrete is portland with CSA or aluminium derivatives. Low cost
4) concremate is plaster of paris/portland and is slightly expanding. Moderate cost
5) portland cement - cheap but shrinks and is unstable over time

All need to be used with a filler to minimise cost or improve stiffness or control shrinkage or cracking

Fillers
1) Minerals such as sand and crushed aggregate
2) man made fillers such as glass beads, metal fibres, plastic fibres, glass and carbon fibres

All work except pure Portland cement as it shrinks and cracks eventually providing no coupling to the structure. It does provide mass. This is similar to polyester and vinyl ester resins they shrink and can decouple from the structure. But if its steel or iron or aluminium PE&VE can be considered if a high filler ratio is used as this shrinkage will be minimised. I would not use PE or VE for cast parts. They definitely will change shape over time from experience of using these things for 30 years.

Epoxy - is very stable but at E=3.5GPa is very low modulus. CSA however starts at E=25GPa so is already a good stiffness. Add basalt or hard 10mm aggregate and you'll get to 40GPa easily. people can pick the wrong epoxy. Pick one suitable for infusion so its very thin with no thixotropes. Laminating resin has thickeners which are designed to get thicker when mixed this is counter productive. Technically with thin epoxy or CSA you can pack a container with 10mm aggregate then pour in the epoxy or CSA until filled, walk away and let set. This makes mixing, placing and proportioning much easier than batch mixing.

For filling metal machine structures I would use a class C CSA (expanding). If it micro cracks it will not be a problem. Its readily available and 10x cheaper then epoxy. I'd maybe use a plastic fibre as used in concretes or I plan to use steel fibres to improve stiffness.

I'm sure whatever you do will work in making the machine heavier (damper) but if you are trying to make it considerably stiffer then you need think about a few things... stuff you add to the middle of members does not help global stiffness, it can help local stiffness...Filling things with these substances is a one way ticket, once done it can't be undone...regards Peter

resins are about 1100kg/m3 and CSA is 2200kg/m3 so your ahead with mass as well

[peteeng]

Hi all you Lurkers, Swarfers and dust makers - keep at it... I've shrunk Milli a bit to Milli No14. I started at the power head and made everything driven by package sizes. This means the size of mounts, bearings, motors etc. I'll resize after a simulation. Its a much better benchtop size... I have also added a load rod. I have found that holding the base feet and loading the machine makes for the wrong load path and maybe its stiffness is bigger or smaller, not sure. Now the load will always be through the table, bed and column so is more accurate to reality. The machine feet will sit on a "bench" with friction so can move slightly even lift its feet if it needs to. Now its smaller the second table motor may not be needed. So the tool can reach all the table and its 600x300 so still big. Th spindle nose to table is 410mm and currently the nose can touch the table but I'll lift that a bit. The column will need to be sturdier I think but we start here. Out to move some tools into the workshop. Peter

[catahoula]

pete, are you sure two Y screws is the best way? I've noticed in at least one software manual (centroid Acorn i believe) that Screw Mapping is not allowed with paired screws. especially in the context of building affordable & accurate benchtop mills, screw mapping seems to be an essential feature.

edit to add: i understand your rationale for three rails, but seems like using a thicker table and offsetting the two rails a bit toward the middle (maybe 15% from each side so as not to create excess cantilever on the edges) would be better overall

[peteeng]

Hi All - This new full model approach has opened up a can of worms. By pushing on the tool and pulling on the table I can see that each structural loop (or loop direction) has a different stiffness. The tool moves 3x more then the job (or table side of the loop). BUT this cannot be! they occupy the same space in reality so now I connected the tool and the job and its stiffer but that's not the correct point of application of the load. The load is applied by the ballscrew. The load we have been applying to the "tool" is actually a reaction. Its now complicated, I need a Bex.... But to keep it simple and in line with how a machines static stiffness is measured I'll go back to a load applied to the tool. After a little lie down I think... Peter

[peteeng]

Hi Cat - Screw mapping is not on my radar for this mill. Now Millis been shrunk it may go back to a single screw. I don't get serious with drives until I finalise the structure. I've only just started to include the table in the structural model so the table design is probably one of the next hurdles to jump... Peter

[spumco]

Peter,

Thank you for the summary. Yes, the intention is to add mass for damping and (hopefully) stiffen it. The amount I'm doing is not what I'd call significant, so the cost difference between CSA and epoxy is negligible compared to the frustration of getting it wrong. I have the mill completely torn down and I don't think I'm going to make it worse by filling the various cavities.

Matrix:

I found an extremely low viscosity epoxy resin:

https://www.theepoxyresinstore.com/p...ve-epoxy-resin

For CSA, I can get straight CSA cement somewhat locally:

https://www.ctscement.com/datasheet/...=Professionals

or I can get a blended product (no Portland in it) that is essentially a mortar mix:

https://www.ctscement.com/datasheet/...0&t=Homeowners

Either is fairly available, the cement-all mix is very local to me.


Fillers:

I can get glass, PE, or carbon fiber of various strand lengths. Steel fibers are not easily available.

Just about any aggregate is available, including (fine to very coarse) aluminum oxide and silicon carbide blast media. I also have a significant quantity of aluminum and steel machining swarf - more than enough to fill everything 3x over.


Process:

I will not be doing a vacuum infusion process - don't have the equipment or experience. So it's either mix and place/pour, or fill the cavities with aggregate/fibers and do a pour-over.

Maximum depth is 4", and all components are cast iron. The mill base is open on the bottom to the coolant tank so I expect a high humidity environment (if that matters for the CSA long-term). As I mentioned earlier in the thread the column will be boxed-in with a 1/4" steel plate bolted & epoxied to the back after filling.

It's winter here so I'm not concerned with CSA lighting off too fast, and I can warm the aggregate/fibers & components if I do an epoxy pour.

Questions:
How much fiber to add, if any, per x-volume unit?
If/when the CSA micro-cracks, won't it de-couple from the 'molds' and be mostly useless?

So... armed with those details do you have any further suggestions?

Thanks,
Ralph