*Disclaimer - this is my first post here and if I've put it in the wrong thread I apologise. This may seem more like a question but I plan on updating this page with budget information and pictures and documentation as the build progresses*
Hello CNC zone members and viewers. I've fallen in the later category right up until recently today. I've been lurking in the darkness collecting the information I needed from this forum and other corners of the internet to create my own CNC router. Today I would like to present my machine expectations, model, and some of my parts. I'm going to proceed with the build with a plan of completing the build before April 20, 2021.
So lets get into it:
I've attached some screen captures of what my machine should look like, that should help add context to what I'm planning to build.
This will be my first machine, I've never done anything with a scope quite like this in the past. Here the boxes I would like to tick. Some I know are a little beyond my components, but I'm aiming for upgradability.
>250"/min or 9.8m/min Rapid (and ideally when cutting light material like soft wood)
High accelerations as to be proficient with more complex and precise CAM tool paths (I'm under the impression that this will decease cycle times of complex parts far more than faster cutting speeds)
Accuracy to be limited by ball screws. Hoping for a resolution of less <0.02" or 0.5mm but I haven't calculated my resolution yet. (I'm working on a budget, but should I fall further in love I want to know I can upgrade my components for greater precision)
Cutting hard wood, aluminum, and brass is the current goal. Limping the cutter through steel may also happen on occasion.
THE FRAME
I plan on using the 2.5" x 2.5" x 0.125" steel for the frame of my machine. I settled on a larger cross-section and thinner wall, after doing some FEA in solidworks and reducing the wall thickness until the deformation on the frame rails was within allowable tolerances. I started with .250" steel and that increased the cost and weight of the machine significantly. I plan on filling the frame tubes with sand to help dampen vibration and add mass.
I've attached a screen capture of my simulation deformation results. This test was done by fixing all 4 legs of the machine and putting a 500N force on the face of the gantry. I felt as though this would give rough, but generous estimation of the strength of the machine as the table feeds into the spindle. I've not yet taken any classes on Simulation so the results may not be worth much, but I tried.
THE GANTRY
This machine has taken inspiration from ThisOldTony's router build, and Wade'O Design's Mill. I Really like the idea of have the ball screw within the structure of the machine, but I didn't like the added complexity of using a C-Channel for the gantry. I've decided on a 6" x 4" x 0.25" A500 Steel Section for Gantry Beam. I'd like to bolt it in place to make it removable should I ever need a larger distance between my Y axis rails should I upgrade it in the future.
The Gantry's total length from end to end will be ~55". My model is still set to change as I actually procure parts for to start building.
THE TABLE
The table of my machine will be mounted to the 2 Square tubes spanning under the gantry. I'm planning to use 0.25" steel tubes for those rail supports, and they will be mounted much as ThisOldTony's are, bolted in place, trued up, then tacked in place. I've been considering adding additional supports perpendicularly between those supports as I need cross members to allow the leveling epoxy to flow, and it would allow be to support the rails from the bottom with an additional "middle Leg".
LINEAR MOTION
These decisions are entirely budget driven. I feel like I've managed to source the best deal in my area of linear motion supplies.
All linear rails will be mounted to an area flattened with Precision Epoxy Product, except maybe the Z axis which I may machine myself on my grandpa's mill.
I plan on purchasing the 4' x 4' CNC kit from Bearings Canada. All parts included in the kit are in the link below.
Link: https://www.bearingscanada.com/4-X4-...-guide-kit.htm
I am worried that 1605 ball screws will have potential for whip. But I've calculated 1605 ball screws over a 53" span with a safety factor of 1.25 at 1070RPM.
Linear Speed in m/min = RPM x Screw lead in meters/rotation
= 1070 x 0.005m
= 5.35m/min
I'm aware that this number is below my goal of 9.8m/min, I'm not sure whether or not I'm using the correct value for the lead of the screw, or if I can try to get a 10mm screw from the supplier with the kit insead of the 5mm as shown online.
This has been my major dilemma because I need to know my lead before I can appropriately size motors.
MOTORS
I've bought into the marketing behind closed loop stepper motors. The main draws for me are the quieter operation, lower power consumption, and the impossibility of losing steps ruining an entire part. My gripe is that purchasing good closed loop steppers is beyond my budget, and I'm not aware if cheap chinese closed loop steppers are any better than decent quality open loop steppers that are sized appropriately.
I haven't even gotten into sizing motors for the moving table, but I've used Rexroth Indrasize and have attached screen captures of how I setup that calculation. I used 9.81 m/s^2 for my acceleration. I'm completely unaware is that number is even remotely reasonable. I feel like if the axis moved as fast sideways as it would if i dropped it that would be quite fast, but I'm not sure. This calculation implies that I need to have a stepper with at least 2.2Nm of holding assuming the running torque is 70% of the holding torque.
I've been on ebay and have found 3 Axis nema 34 closed loop stepper kits with drivers and power supplies for less than $800 dollars. This is attractive to me, but seems to good to be true.
Link: https://www.ebay.ca/itm/3Axis-US-Wan...EAAOSwCnpcf142
Any assistance on sizing motors appropriately, and insights about chinese closed loop steppers would be appreciated.
SPINDLE
I plan on using a chinese 2.2kw spindle from ebay as well. I explicitly want water cooling for reduced noise, and I feel like this is the most power/money I'll be able to come by.
Link: https://www.ebay.ca/itm/CA-2-2KW-ER2...fpPei#shpCntId
I believe I read somewhere that lower rigidity can sometimes be remedied with more power, and I feel like the chinese spindle is the best I can afford.
CONCLUSION
This ended up becoming quite a lengthy post, but I'm really hoping to get some insights from others, and I would hate for someone to see this thread and decided they don't have enough context to help me.
I will try to be as active as possible and have set notifications to instant email. As I've said above, any help is appreciated. I'm even willing to send someone money for there time if they bring enough insight and experience.
Ultimately this is a learning experience and the goal is to learn as much as I can. I'm planning on documenting this project and using it as a resume skill for my upcoming co-op term December 2021. I also just want my own machine but refuse to spend $3k for an 12" by 12" aluminum extrusion machine from china.