[peteeng]

Hi Ard - Milli needs to be design stiffness without help from the "table" or "bench" I wouldn't use the tri strut drive but some sort of triangulated exoskeleton with a 3 axis inner. But its complex, so far no joy in resolving a good concept with this exoskeleton. Peter

[peteeng]

Evening all - I think I've found a candidate for Milli's spindle. In another project I'm building a router with a 1200x1200mm footprint. I'm thinking of the next build to make a mould and vacuum cast the machine base out of fibreglass. For the router it would be 12-20mm thick, for the Mill it would be 40mm thick. It could be backfilled with CSA grout for Milli so the FG casting could be the same for each. So Milli is sort of condensing out again. FinMakers saddle is to be made from recycled carbon fibre and its gantry will be laminated aluminium and MR-MDF, look for the Frankenrouter thread. . Peter

[ardenum]

Hey Peter,

That looks like it is just a servomotor with an ER attachment. the 3000RPM confirms that. You could slash the price in half if you mill the ER part on the router yourself. or get one of ER 4.5 kW g penny, same price

[peteeng]

Hi - yes it seems the equivalent servo is more powerful at similar or less cost then you can do tapping with it. Ideally I have been looking for 6000rpm motor... So an AC servo with 1:2 belt seems to be the answer at the moment. Peter

[ardenum]

Hi - yes it seems the equivalent servo is more powerful at similar or less cost then you can do tapping with it. Ideally I have been looking for 6000rpm motor... So an AC servo with 1:2 belt seems to be the answer at the moment. Peter

in the range 6000-9000RPM you can get a used rexroth servomotor on ebay like the ms2n, msk series others like mad or maf go up to 11000RPM. I'm sure other manufacturers have similar offerings too.

It really is annoying to find a motor that you like, either you don't find it or its overpriced for the project. This is the main reason I decided to dabble in and make my own PMSM motor. If I figure out the workings and get a hang of it there won't be a situation I can't build or upgrade the motor with my kind of specs.

[peteeng]

Hi Ard - The issue is that milling steel 5000rpm plus is usually too fast. Unless someone wants to correct me on that. Ideally a single unit is good (ie no gearboxes/pulleys) to keep it as simple as possible. Many newish benchtop mills have 100-3000rpm variable or 6 speeds between 100rpm and 2500rpm say. so the 3000rpm that seems to be reasonable. If I use a pulley this gives the user a chance to change to high speed if say only doing aluminium (6000rpm or 9000rpm). Or slow down if doing heavier cutting via changing a pulley to suit themselves or mount a high speed spindle to suit themselves... I won't be able to please everybody on this spec.... but need to supply something that covers a lot of territory for most Makers. Peter

I think Milli will be either a) a high rail gantry design or b) a moving column design. My thoughts are high rail gantry at the moment....

[peteeng]

Hi All and Sundry - I've spent a little time on Milli today checking the Z axis travel and adding some holes for wires and adding a face sheet for an apron. The spindle does not come fwd enough unless I use a spacer to "hang" over the front making the apron useful. I could move the motors to the back of the machine then the travel would come fwd of the apron. So tHe current footprint is 1200x1200mm and the travel is Z250mm y960mm (fwd/back) and x760mm (sideways). I shall do final checks and some drawings and hopefully start making parts end of this week. Peter

[ardenum]

Hey Peter,

Is this millie or franken? or did they fuse with one another?

on another note...do you think that once inserts are glued into a granite plate, do I consider them as one or as separate for FEA purposes? I usually fuse all parts into one big chunk to run FEA on a single piece, usually...I tried doing multiple material FEA but I'm messing something up because every time I run it I get a different result...

small inserts usually are negligible but I'm trying to figure out how big of an insert I can put in until the plate itself starts to be significantly affected...

[peteeng]

Hi Ard - Yes its a fusion. I'm building Frankie-S1 from surplus parts. The machine base is being built from formply. Once I get a feel for this I will cost it out as an aluminium plate build ie all the 16mm plywood parts would become solid 16mm aluminium plate parts. Or I commit to making a mould and vacuum cast it in fibreglass and CSA... So this thread should be Milli. I got a bit confused I think too many things happening... Frankies saddle will be carbon fibre so thats interesting.

So you boolean the parts into a single part? The issue with that is the granite will be 60GPa and the steel will be 200GPa or are the inserts aluminium? E=70GPa. The FE should be able to connect things together without the boolean. As a first round FE "bonding" the parts together is fine. But if you want a more detailed answer you have to refine the model. Depending on your FE they can include the bondline in the model or you can model the bondline so its included. The further away from reality you get with the model the further the answer is from correct. In simsolid or Strand7 the systems I use I would model the parts as seperate and when meshing them they will be connected correctly. Then the correct modulus can be appointed to the granite and the insert. In simsolid I can define a bondline spec. eg 0.5mm thick epoxy etc and simsolid will account for the extra compliance in the connection. Thin bond lines are quite stiff even though epoxy is only 3Gpa... To figure out if the effect is significant make a small simple model and give it a push or shove and see what it does. This is called a sensitivity analysis. Make the bondline thick and thin or stiff or compliant and see how much it affects the answer. Then you can followup the result in the bigger model. What FE are you using? Peter

[peteeng]

Hi Ard - I did a quick model. I bonded a 50mm wide steel bar 10mm thick with 100mm overlap. ie the bondline is 50x100mm and 0.5mm thick. I pulled this apart with 1000kgf. The rear coupon is analytically bonded and the front image is epoxy bonded E=3.5GPa.

you can see the epoxy connection is a bit compliant. 2.8% stretchier. If the connection is being pushed together then its stiffer. The analytical connecion being stiff and not distributing strain across its length well shows the stress spike at the lap.

The other part to this is that the strain in a CNC machine is tiny so the relative stretch will be tiny. This model is highly loaded but I think the adhesive is modelled as a linear element, I'll have a play and see...

So to answer your question model as close to reality as you can..... Peter

[ardenum]

Hi Ard - I did a quick model. I bonded a 50mm wide steel bar 10mm thick with 100mm overlap. ie the bondline is 50x100mm and 0.5mm thick. I pulled this apart with 1000kgf. The rear coupon is analytically bonded and the front image is epoxy bonded E=3.5GPa.

you can see the epoxy connection is a bit compliant. 2.8% stretchier. If the connection is being pushed together then its stiffer. The analytical connecion being stiff and not distributing strain across its length well shows the stress spike at the lap.

The other part to this is that the strain in a CNC machine is tiny so the relative stretch will be tiny. This model is highly loaded but I think the adhesive is modelled as a linear element, I'll have a play and see...

So to answer your question model as close to reality as you can..... Peter

The inserts are aluminum, yes. I'd assume I'm actually getting lower values in FEA because I fuse everything as a custom material, copied AL 6061 specs and only changed to 60GPa. This also means the steel carriages and rails are treated as such. That's the general way I do to simplify the simulation, I use pre/post in NX with nastran as solver so if I try to run anything on multiple parts, especially an assembly, my laptop hangs and hardresets...

I think I'm beginning to see what I have to analyze, you cleared my head a bit. thanks

[peteeng]

Hi Ard - so your doing a comparative model. vs an absolute or accurate model. So as long as you keep things apples to apples and one concept is clearly stiffer then the other that's what your looking for. Your looking for direction not exact stress/deflection data. Peter

nastran on a laptop should solve quite a big problem. Do you have control over mesh density? If so make a coarse mesh. A course mesh will deflect the same as a fine mesh, within certain rules. Fine mesh is used to ensure the stress results are correctly interpolated locally. But the deflection will be the same.

[peteeng]

Morning All - Due to some correspondence in another thread I may revisit the alox E test coupons in grout and epoxy. My first attempts tried to make 6mm thick coupons so I could compare them to a 6mm aluminium extrusion directly. My aim was to visibly show that the test sample was stiffer or more compliant then the aluminium extrusion under gravity using a long cantilever. But the samples turned out to be not robust enough. So a thicker sample is needed that I can load up. So I shall do some FE to establish what length and size is good and try to find an aluminium bar to use as a benchmark. I'm taking some parts to a machinist today (for Frankies build) I'll go through his offcut bin and see what shows up. Peter

[joeavaerage]

Hi,
does small spheroids of carbon in an iron matrix qualify as a composite? Spheroids in iron cast really well, stiff as hell and machine good-oh. One of the best composites for
machine building ever invented!

Craig

[peteeng]

Hmmm - An engineer or metallurgist would say its an alloy. But it does fit the general definition of a composite. They are even calling reinforced concrete "composite" now.

https://www.twi-global.com/technical...osite-material Peter

[peteeng]

Hi Craig - What alloy are you casting in? Grey cast iron is used for machine structures as its damp. ie it has a lot of slip in between its grain structure creating a lot of internal hysteresis. Its modulus depending on alloy can be from 60-130GPa. I think 110-120GPa would be about the value for most cnc machine parts?? Density is about 7150kg/m3. Now in my world if I had the $$$ I would make parts out of intermediate modulus carbon fibre/epoxy. This would result in a material with E=130GPa, density 1500kg/m3 and it would be 10x damper (at least) then CI. I think that maybe better then CI. I have designed and supervised manufacture of submarine components in this type of material. High modulus CF would be even better.... Peter

https://www.toraycma.com/products/ca...more%20rows%20

[joeavaerage]

Hi,
for my axis beds I used grey cast iron, about 40kpsi tensile, as a nice balance of stiffness and damping.

My intention was to cast the frame in two parts in SG iron, its considerably tougher and stiffer while having a modest amount of damping
and really costs little more than grey iron. In the event my budget ran out and I made the frame out of steel, plasma cut 32mm med tensile sides
and 20mm med tensile plates in between.

One of these days I will cast the frame as I intended.....

Overall I still contend that cast iron is a cost competitive material for machine building, its so versatile casting as well as it does, its properties are well understood and there is still
a great deal of experience and skill out there to have it happen. What's the bet within a hundred miles from your home there are several foundries capable of pours of over 500kg,
and dozens of engineering shops that could do the machining.

How many companies within your state, or even the whole of Australia stock all the epoxies, reinforcements, fillers, release agents for a full on composite build?
I have no doubt those companies exist, they do here in New Zealand as well, but they are few and far between by comparison to metal processing industry.

I have used carbon fiber for model areoplanes and in boat building on and off for years and have an enthusiastic appreciation about what these materials offer, particularly
strength to weight. When it comes to CNC however you need stiffness, weight be damned. Steels and cast iron can at best be matched by carbon for stiffness but the COST!!!!
Who cares about damping....its way down the list....stiffness trumps any and every possible metric, if the machine is not stiff...its junk, even well damped, its still junk.

Craig

[peteeng]

Hi Craig - Yes there is a foundry that can do up to 4.5 tonne pours within 6 hrs drive of me. I have used it for large mining vehicle components. They can also cast high strength Q&T steel which has been very interesting. There are at least three composite companies that can provide the materials for a cnc build within half a days drive from me. CI and CF have their place in the industry. CF is already used where high speed/accelerations are required due to the lower inertia, particularly with robotics. With machine bases mass is needed to overcome the inertial forces of the moving parts vs nailing the machine to the floor which some companies don't like doing. So its picking the "best" material for the job at hand. There is also a lot of industrial inertia in adopting new materials as supply lines and service risk has to be managed on expensive machines. Takes years to gain confidence in "new" processes and materials. Taken at least 20-30 years for the major aircraft makers to do primary structures in commercial aircraft. I have always been interested and involved in projects that need to reduce weight and I transfer that to my cnc machines. I'm looking forward to my first proper CF cnc part in Frankie. The interested client accepted the budget today so Frankie is full steam ahead. Then I'm back to Milli... BTW I've spent a few years in NZ on and off in the superyacht industry doing mast and structural analysis for a couple of companies... I like NZ many years ago I cycled around south island and hitched around for a year or so....Lived in Hawea/Wanaka for a year paid $38 per week rent. Now its a multi million dollar row...Peter

[joeavaerage]

Hi,
I rather suspect that many participants of this forum, not all, but many, find epoxy granite and other composite technologies appealing because
they believe that the traditional cast iron/steel are expensive, especially the machining. I have been down that path myself, trying to find some
magic self-leveling something or rather that meant I could build a machine without any, or any extensive machining.

I realised eventually that all the granite/epoxy granite/high tech concrete ideas were going to cost a bomb and/or had significant compromises
that I finally overcame my aversion to having something cast.

Those whom have used epoxy granite successfully have had a major build using costly materials and still, in most cases, have some machining done.
All this to avoid getting something cast??? Casting is just not that bloody hard!

Craig

[ardenum]

Hi,
I rather suspect that many participants of this forum, not all, but many, find epoxy granite and other composite technologies appealing because
they believe that the traditional cast iron/steel are expensive, especially the machining. I have been down that path myself, trying to find some
magic self-leveling something or rather that meant I could build a machine without any, or any extensive machining.

I realised eventually that all the granite/epoxy granite/high tech concrete ideas were going to cost a bomb and/or had significant compromises
that I finally overcame my aversion to having something cast.

Those whom have used epoxy granite successfully have had a major build using costly materials and still, in most cases, have some machining done.
All this to avoid getting something cast??? Casting is just not that bloody hard!

Craig

You mean casting in iron/steel? you have a contact to some cheap chinese forge maybe?

Machining costs depend on where you live eg. I wanted to have a custom alu heatsink machined 160x200x40mm, quotes varied from 800 to 3000 EUR. Machining a cold cast part sized 720x780x320 the prices were up to 8000.