Milli a new composite mill kit

[spumco]

A channel/plate across the top could do double-duty as a drag/cable chain trough for the X&Z axes as well as connect the two columns. Or a hollow cast bridge could pass servo/sensor cables and lube lines across the top, with a shallow flange to keep a drag chain in place.

[catahoula]

Pete I like where this is headed. Since casting is the goal, why not include a bridge between the columns and cast it as one piece, like this? The bridge can be as low as the tip of the spindle, so having that much depth will certainly help with the X deflection. This way the alignment between the columns is set, and you can align it to the base as only two pieces.

[peteeng]

Hi Cat - I like it too. Although the "bridge" can be cast as a unit it would be too heavy to manage. One design rule is to make parts <50kg. The current column is 74kg so needs to be lightweighted a bit. The two columns and bridge would be close to 200kg as is so unmanageable. Its meant to be a benchtop machine but its too big as is although its footprint is smaller then prior desings.

I intend to have aluminium pads cast in for the feet and the rails. Then these will be finish machined. So the fit should be very close to correct. The machine base will be machined to flat as well so all alignments should be easily achieved. I'm just drawing up Milli-LG Series two so stay tuned... Peter

[pippin88]

I agree about placing the bridge in the middle rather than at the top.

Just needs to be higher than the max workpiece height (so about level with bottom of spindle when fully up).

[peteeng]

Evening All - Here's Milli-LG series 2. I have started to give it an aesthetic. I also bolted the columns on with some M16 studs. I wanted to see what the stiffness impact this had. Under 1000N load it gaps 1um so that's OK I'd use loctite in the joint as well. I used Class 8.8 studs at 65kN preload. Could preload a bit more... The stats are good even with only 2 bolts per column. I shall add a big washer next time around as well. Milli-LG2 uses 25mm cars and rails and the spindle is a BT-30.

Stats X 21 Y 34 and Z 77N/um so on track. I think the main thing now is to look at the machine base and the tool plate. Then go see Jenny Craig and loss some weight... In prior models using a steel table vs an aluminium one improved the stiffness quite a lot. Took X to 24N/um in this case. The nose height is 400mm. So on track I think.. Peter

[ardenum]

Pete I like where this is headed. Since casting is the goal, why not include a bridge between the columns and cast it as one piece, like this? The bridge can be as low as the tip of the spindle, so having that much depth will certainly help with the X deflection. This way the alignment between the columns is set, and you can align it to the base as only two pieces.

are you also building a double column lifting gantry?

[catahoula]

pete, maybe a multipart mold and cast it with webs like cast iron would be? So the columns are solid, but the thick bridge has an insert from the pour side that hollows it out. Still one piece but lighter weight.

ardenum, no, i went with fixed gantry so as not to have to deal with using limit switches and double motors/ballscrews to square the gantry. I just felt like doing some sloppy speed CAD instead of using words last night

[peteeng]

Hi Cat - External webbing like in cast iron is not ideal (Ralph has just filled his cast VMC in with concrete for instance). It suits the process (keeping sections thin and consistent, shrinkage warping etc) but for structurally efficiency it needs to have an external skin to provide max geometric inertia. So maybe two halves with internal webbing that are bonded/bolted together. Or if laminated metal the internal pieces will be spider web like but the outers will be solid (like our bones). Maybe the outer skins are steel to take advantage of its stiffness yet the inside is aluminium or CSA etc etc. Plus can use generative design to see what that gives... Don't know until I boil down the config and the size to final form. Technically this part of the exercise is called "form finding". I have looked at so many configs and geometries to get here. Now I have to decide on the "real" machine envelope and market opportunity. Re Milli-LG2 I think the 25mm rails have made a difference locally so the take away is don't be shy on the bearing size when it comes to stiffness.... Peter

This morning I check the model and found two of the bolts weren't set up properly. The bolt preload creates the most stress in the model. The column feet do not gap which is good. The bolted area sees 15MPa stress which is well below concrete and aluminium failure levels. You can see how the bolt pulls in the material under preload. Interesting info. A washer or backer will distribute this local load better.... next time around

My main interest in this model was comparing the bolted column to the bonded column stiffness. The bonded was 26N/um and the bolted was 23N/um so not too big a delta. I could use two 12mm bolts to get the area stiffness up and the bolts further out from centre.

[mactec54]

Hi Cat - External webbing like in cast iron is not ideal (Ralph has just filled his cast VMC in with concrete for instance). It suits the process (keeping sections thin and consistent, shrinkage warping etc) but for structurally efficiency it needs to have an external skin to provide max geometric inertia. So maybe two halves with internal webbing that are bonded/bolted together. Or if laminated metal the internal pieces will be spider web like but the outers will be solid (like our bones). Maybe the outer skins are steel to take advantage of its stiffness yet the inside is aluminium or CSA etc etc. Plus can use generative design to see what that gives... Don't know until I boil down the config and the size to final form. Technically this part of the exercise is called "form finding". I have looked at so many configs and geometries to get here. Now I have to decide on the "real" machine envelope and market opportunity. Re Milli-LG2 I think the 25mm rails have made a difference locally so the take away is don't be shy on the bearing size when it comes to stiffness.... Peter

This morning I check the model and found two of the bolts weren't set up properly. The bolt preload creates the most stress in the model. The column feet do not gap which is good. The bolted area sees 15MPa stress which is well below concrete and aluminium failure levels. You can see how the bolt pulls in the material under preload. Interesting info. A washer or backer will distribute this local load better.... next time around

My main interest in this model was comparing the bolted column to the bonded column stiffness. The bonded was 26N/um and the bolted was 23N/um so not too big a delta. I could use two 12mm bolts to get the area stiffness up and the bolts further out from centre.

Here is a video of a portable machine design similar to what you started with, there are a few videos of it doing jobs, it is a simple build which could quite easy turn into a hobby bench top machine

[catahoula]

I meant internal webbing, something like this. If the mold is built so the front rail face is on the bottom, which you would want for the flat reference, and the mold box/walls prescribes the shape of the columns+bridge, then it wouldn't be too hard to add an insert from the top (a lid with hanging protrusions) that would prevent the concrete material from filling the triangle shaped areas. Still leaving plenty of area to pour into on the columns. Of course, wall thickness and draft angles are going to be key with a low tensile strength material. Maybe not tough enough for shipping etc. Just a thought. Might be a better concept for cast iron or steel fabrication.

[peteeng]

Hi Martec - Good find - yes that has good possibilities... Peter

Hi Cat - Yes I understood. 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. Now it will bend in two directions the same and the webbing is not good in torsion...I can make outer skins and fill with foamed concrete or PU foam....probably stiffer then the webbing... Optimisation of the structural parts is a little bit down the track... Peter

[peteeng]

Evening all - I have been concentrating on the static stiffness when the tool is in the worst position. But most of the time the nose will be 200m from the table not 420mm. So I ran a bonded version with the nose at 200mm. So the stats here are X 58 Y 83 and Z 83 N/um very happy with that. I also started setting up the bolts but forgot to connect them to the base. So in Opps you can see the gantry etc. come off!! I'll have to fix that. Milli-LG2-No2 has been tweaked a little. I worked through the parts and made them a little bigger when possible. I'll also update the studs to cap screws these will look better. I used 2xM16 bolts and a thick washer... I'll also have to figure out how the cars connect to the gantry then I can proceed further... Peter

I did the topcoat on the LHS of the shed this morning, another step to a working shop!!

Heres the geniSYS mill- Peter
https://www.hydratight.com/en/produc...table-cnc-mill

[peteeng]

Morning All & Sundry - This mornings puzzle. What's wrong in this image? Peter

[peteeng]

Morning All - Its tweaking time. I adjusted a few dims and sorted some details. Last to do before we move to motion parts is figure out how to mount the cars to the gantry. Current description:

Milli-LG2 - No4
1) Intended to be benchtop machine. footprint 650mm x1200mm fits buts its heavy 420kg. Next is to lightweight Milli.
2) X 550mm Y 350mm Z 350mm spindle reaches all of the table
3) Nose height up 425mm and down 75mm
4) table size 550mm x350mm
5) Material is cast aluminium, machined billet aluminium, cast CSA/ALOX with Al inserts, laminated aluminium, E80 UHPC if your in Europe. carbon fibre if you have the $$$ or Beryllium if you need to send it to Mars.

Any comments appreciated. Maybe 450mm Z?

So after I figure the gantry cars I'll fit the motion parts;
then lightweight Milli. May cut her down a little to suit the ER16 power head but the 25mm flanged cars dictates the tool plate size which happily takes the BT-30 head. Its looking and feeling robust. Onto sorting the cars. Peter

[ardenum]

The bed is relatively thin, with bolting the columns all the way through it, you could get rid of that hole. Mounting would also not be a problem.

[peteeng]

Hi Ard - One other informal design rule is that everything can be accessed from above and easily. I presume you mean bolt upward thru the base? The hole is not a big deal and its much easier working from the top then the bottom. If the machine is placed on a bench access below is a problem. (the column may have more holes in its lightening phase anyway). I have decided to use a sub-plate for the the gantry connection. This will be steel so it takes threads and screw torque well. It will be bolted to the cars first and this will allow the sub-plate to be used for setting up the column rails. Then the gantry will be bolted to the sub-plate using M16 bolts. The cars use 4xM6 screws and an M12 bolt has the same area ie stiffness. So an M16 is stiffer. 4xM6 provide 31kN preload and an M16 provides 60kN preload so all in all its a stiff connection. There will be at least 6 or 8 screws here so an M12 or M10 is good enough... will depend on space and access. Have to make a CAD wrench to check spanner clearances. Peter

addition star log 2021012411-35 I have spanner clearance yippee

[ardenum]

Initial setup is no problem, place the bed on the table so the bolting area sticks out, bolt one column, then rotate 180 degrees, bolt the other one. Do you need access to that once it's all bolted together afterwards?

[peteeng]

Hi Ard - Currently the bed weighs 235kg (519lbs) so (rotate 180degs is not an easy step. Would be easier to drill holes thru the bench plus its a 6 person lift to get it on the bench (ie a crane is needed)...The columns weigh 69kg each so once bolted and "rotated" its heavy. Think of this as a 120mm thick slab of aluminium 1200x650mm. I and others don't have cranes. I'm aiming at halving current part weight but that may not happen easily.... Ideally this is a bottom up assembly with max part weight 50kg or a 2 person lift... Peter

and yes may need access to adjust column vertical or horizontal. Depends on degree of registration precision I decide on once this is presented to the machinist vs cost. Ideally its fully machined to spec so its place and bolt up...

[peteeng]

Hi All - I have completed the Round 1 form finding of Milli. Designed 115 parts but only 60 parts in the model. So nearly half the parts built have been scraped. Typical of this sort of thing. I have added the bolts to the gantry sub-plate and all seems good. I'm now going to A) optimise structure or B) add the motion bits. My thought is to optimise because this may change the form details again. I also have to get back to YaG my router build so I can make Millis parts and moulds!! I'm going to use coupling nuts as thread inserts with a washer, nut and bolt on the back for extra grip, will epoxy the rear bolt in. No6 Still very stiff although its been made a bit wider and taller. About X=22N/um so over target. So I'll do a bit on YaG get the metal bits on order then I can back to this. My thoughts would have settled by then.... Plus I have to do some test casts of the CSA and ALOX...
Keep Making - Peter

[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