Advice sought on new Mill build

[embraced]

Hi all,

I've posted the same thread on CNCzone but I would appreciate feedback from here too.

Soon I'll be undertaking a new project to replace my CNC'D BF20 which I've outgrown.

Proposed specs:

• 520x, 280y, 550z travels
  
• X and Y motors will be 600W AC servos (Chinese)
• Z motor is yet to be decided on.
• Possibly a counterweight/gas spring to help lift the Z axis (the head alone will weigh over 80Kg)
• 2505 double-nut ballscrews on all axes
• HIWIN 25 rails on all axes
• 2.2kW (3.7kW peak) Servo-spindle (10.5N.m from 0-2000RPM, top speed of 8000rpm, Chinese product)
• BT30 spindle cartridge, 12000RPM max speed. (Chinese product)
• Pneumatic/hydraulic tool release mechanism (comes with the spindle).
• Table will be donated from an old RF45 - that will have the dovetails milled off for the linear rail bearing blocks.
• Machine will be controlled by a KFLOP/Kanalog combo with Mach3/Kmotion

Machine will primarily be constructed of thick-wall tubular steel, welded together and stress relieved before being machined.
I'm thinking about epoxy-sand filling all the hollow sections to avoid the dreaded ringing that's almost certain to occur on a steel structure. Will see what the community says...
Solidworks estimates the weight to be about 400Kg at this stage.


Anyway, half of the components listed are already in my possession, so it's not so much of a fantasy.


You will notice a lot of detail in the photos is absent - namely how the axes are slaved together and other detail like that.

I'm looking for any input that could help.

[Boyan Silyavski]

Hi,
i am no expert but looking at your drawing common sense tells me that your Z is definitely weak, the bed also.

Interesting that last night i was contemplating the way i could do something similar.


IMHO:

-Forget about that fancy ribs on the Z. Use Triple profile welded together and reinforce it with say 60x10mm or even better 100x10mm ribs without undercutting them for "looking good" purposes . Or make it all from 100x10mm plate welded together, with ribs and so,, or why not even 20mm thick where needed.

-fill the bed with profile, don't leave empty spaces there. Only empty space should be for the ball screw to pass. Fill the empty spaces with short profiles perpendicular to the other.

[embraced]

Thanks for the input silyavski.

I've changed the column to 250x250x9 hollow section for added strength. This will give me a LOT more volume for the epoxy sand formula to be poured into.
I still intend on keeping the bracing on either side of the column as I feel that it might help with the construction (and I also see all the VMC columns have flanged bases).

As for the bed, your suggestion is good. It will also allow me to put more of the epoxy sand mix in.

I've also added a way for my counterweight to go across the column. See the following pic:

[Boyan Silyavski]

I still intend on keeping the bracing on either side of the column as I feel that it might help with the construction (and I also see all the VMC columns have flanged bases).

What i meant was not to remove it but to make it even stronger extending it to the top, at the back and interfacing it together. I meant not thinning it for visual design purposes.


Thats a big chink of steel. Instead of epoxy, i would use aluminum scrap, melt it and pour it in the column. Money wise it would be even cheaper. But whatever you have at hand.
Or some special cement /+ sand+stones/, that does not crack or shrink, for industrial flooring. It has epoxy in it.

So your will drive it 1:1? What motors and screws? I have always thought it wiser such a machine to be driven 1:3 or 1:2 at least,servo or stepper. Personally i don't like directly driven screws. I prefer pulleys and short belts

[JAZZCNC]

Hi,
You need a miny version of my recent aqusition.!! . . . .


The column won't be the weak link thou it could be made much stronger.! It will be the narrow head and weak base. Would look at making the Head have wider foot print by either using wide section or placing wide think plate on front of Section.

The base is far too Flimsy IMO and will require much more substance and bracing to avoid twist and stress from every thing above. You shouldn't rely on fastening the base to a frame for strength because this will introduce error in the frame by pulling and twisting. The base is the foundation and your starting with crumbling bed rock.!!
Look at the base of the machine above it's one Solid cast Iron lump with coolant tank built into it so it's very very heavy and dense to resist the forces from above.

From the outside the column looks like hollow box section but it's actually made up of 5 Thick steel plates welded and braced. If you look and zoom in you can see thru the lifting holes just how thick the material is. You can also just see the Red counter balance weight which is a Big lump of steel on round linear guide. That head is perfectly ballanced and I can lift it with one finger.!!

Personally I would forget using section and Epoxy Etc for the strong parts and make a weldment machine using thick steel plate and lots of bracing. If you do go down the Hollow section route just fill the Section with Sand it's much easier and dampens resonace good which is what your wanting. It's also easy to remove when moving.!

[embraced]

Hi Jazz,

I can't see the attachment.

[Blackrat]

pouring aluminium into the tube will just make a loose slug, the ally wont bind to the column

[JAZZCNC]

pouring aluminium into the tube will just make a loose slug, the ally wont bind to the column

It could also distort the column with the heat coming from it.!!

[Jonathan]

You want the X-axis rails attached directly to the bed, not the bearing blocks. It'll save material and keep the bearings (i.e the support) in the same place in relation to the spindle regardless of the X-axis position. The same goes for Y - orient the rails like on the machine Jazz posted.

You've mentioned using 25mm ballscrews, but it's a bit early to be deciding that yet. On a router we'd say use 16mm screws, as the critical speed isn't a limiting factor for this size machine. However on a milling machine it can be worth using bigger ballscrews if the increased stiffness is beneficial - but you can only know that when the frame (etc) is designed.

pouring aluminium into the tube will just make a loose slug, the ally wont bind to the column

Yep, and it won't do a lot for damping either, which is the purpose of the epoxy...

[embraced]

Thanks for the advice everyone.

I will tackle the column design first.

So from the suggestions, I've moved from RHS (rolled hollow section) to a welded plate construction. the rear of the column has pockets to put the E/G mixture into, and to perhaps have another plate bolted on to seal the pockets and act as a torsion box.

Am I on the right track?