/ HELP - last considerations before building 1750x1000x250 mm all-around milll

[Bart]

Hi,
I've started designing this machine around april and it developed from a small (400x400mm working area) fun project to a full scale 1750x1000 milling machine.


When I started thinking about Mill and designing it, I barely knew what is CNC and used very simple methods for my first design. However due to my innate urge for kaizen ;) and cost/benefit optimization and thx to many threads on couple of cnc forums I came of with current design.


However I still have some doubts about it and I'd like to ask you to help me with them.


Design spec


Price ~3100$ +/- unknowns :)
Steel Construcion:
Dimensions 2000x1460x400mm
Weight: Bed 230 kg; Gantry 70 kg; Gantry with all the equipment on it 150 kg
Working Area 1750x1000x250mm


Motor Drivers: 4x Toshiba TB6600 drivers
Stepper Motors: 4x (2 for X axis) Sanyou Denki Sanmotion 103H7823-1730 2Nm 4A 2,4mH
X,Y,(Z?)
1,5 MOD R&P
25 Linear Supported Rails


2,2 kW chinese Spindle + VFD Hitachi X200/NE-S1/ TOshiba VFnC3


Bed design...
according to Inventor it flexes 100 um when z axis is all the way down and there is 1130 N force applied to spindle collet and 20 Nm torque.





At first I wanted to have it all welded, stress relieved, milled and ground at one place but it turned out that the closest plant that could do it is 100 km away and it would cost 3,000$ - way to much for my budget. So I decided to get it welded by a friend of mine and stress relieved in nearby city and level the surface for the rails using epoxy resin - total cost 800$ - much better...




For this width, its rather impossible to drive it from only one side and driving it from underneath the table would require additional 30 kg of weight, to have the ball-nut stiffly coupled with gantry.
Using one stepper motor running two ballscrews is not practical as equivalent mass of whole system would be 255 kg to handle by only one stepper motor - acceleration would suffer greatly and the price would be quite high. So I decided on R&P due to the lenght, width of X axis, as well as cost od double ballscrews for X aixs.


Couple questions that I have


Mounting ball-screws is quite straight forward but I have some doubts about R&P mounting. Does the angel between line of mounting bolt-pinion meshing with rack and rack axis ('a' on the pic) has to be as close to 0* as possible? I haven't seen any discussion about this and it seems to me that the bigger the angel the less stiffness I get and less efficiency.



What are the benefits and drawbacks of making a reduction using pinions vs timing belts and if timing belts than which one should I use? Anyone knows informative article on timing belts?

For the Y axis - I wounder whether I should cram a rack between Y rails or place it on top of the gantry. It seams to me that best situation is when force is applied at the height of lower rail but how does it look in practice?

How does Z axis work when driven from rack and pinion?

Does it make sens to buy vector controlled VFD for 350$ instead of u/f controlled for 250$?


Any practical considerations and loose thoughts from those that have some experience in heavier machines would be greatly appreciated.


As to leveling and assembling machine.
I plan to level the machine by
1. making square leveled epoxy pads on the floor
2. placing bed support on this pads and leveling its tops with epoxy
3. placing bed on the support and leveling rail mounting surface with epoxy
4. flipping the bed upside down and leveling bottom of the bed with epoxy

Best Regards
Bart