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.
Price ~3100$ +/- unknowns :)
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
1,5 MOD R&P
25 Linear Supported Rails
2,2 kW chinese Spindle + VFD Hitachi X200/NE-S1/ TOshiba VFnC3
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$?
Any practical considerations and loose thoughts from those that have some experience in heavier machines would be greatly appreciated.
As to leveling and assembling machine.
I plan to level the machine by
1. making square leveled epoxy pads on the floor
2. placing bed support on this pads and leveling its tops with epoxy
3. placing bed on the support and leveling rail mounting surface with epoxy
4. flipping the bed upside down and leveling bottom of the bed with epoxy
26-12-2013 #2Does it make sens to buy vector controlled VFD for 350$ instead of u/f controlled for 250$?
I'm going to buy the Chinese 2,2 kW spindle. Do they perform better at low speeds with vector VFD?
Yes, the VFD will be used in sensorless vector mode which is not as good as the closed loop type but is still better than a standard scalar frequency mode but like I say make sure the VFD can work in vector mode with your spindle.
Last edited by EddyCurrent; 26-12-2013 at 08:47 PM.
I want to cut from Foam to stone, using a few different spindles if required. So
1. Does using sensor less vector mode give better toque at lower revs, on a standard setup.
2. Is this enough of a difference to be usfull ?
I clearly dont know what i need to know with this, so fire away with any info
Bart - i am designing/building a similar machine, hop you dont mind me jumping in and asking a question on your post
Why would VFD not work with a spindle? Vector drive is just a algorithm of controlling PWM - nothing more - so how wouldn't it work?
And as george uk said is that enough of a difference or in other words what is the difference in practice.
And how does a scalar VFD with encoder on a spindle behave.
not at all
Last edited by Bart; 27-12-2013 at 12:43 PM.
If the limiting factor is the difference in torque between the control algorithms, you should probably look into getting a bigger (e.g. 3kW) spindle and stick with the cheap VFD.
Nice design Bart!
I'm not an expert but I can give a few suggestions.
Have you considered using aluminium for the gantry in order to cut out weight? Here you have a very useful calculator to see what speed and acceleration will you have with your heavy gantry and 2Nm motors. I think that for a 150kg gantry and decent speeds you will need at least 4Nm motors and 70V drives.
Here you can find some useful information about rack and pinion. They build similar sized routers.
I've seen this web page but they don't have any equations or information on timing belt stretch, load capability etc.
Yes I did consider aluminium but only briefly as 1. it doesn't make sens too use it 2. it's too expensive .
ad. 1. If I use aluminium as a material for this gantry I end up with 60% more deflection then with steel, so to make it up I'd have to use 50%-100% more material and I would end up saving 30%-50% of a gantry mass and thus reducing:
gantry weight from 70 kg to 40-50 kg
total X axis moving mass from 150 kg to 120 kg - 20% reduction
total equivalent moving mass from 250 to 220 kg (that is including inertia of motors and gears) 12% of weight reduction for $$$ - see below...
And I would get into aluminum welding that is more expensive and problematic, although I probably wouldn't have to pay for stress relieving.
ad. 2. Prices in Poland are 7$ per 1 kg of Alu and 0,9$ for 1 kg of Steel.
I would save 30 kg so instead of paying 63$ for steel I'd pay 300$ for aluminium. For this 250$ I could buy motors and get much more acceleration or buy Leadshine digital drivers and get much more speed and solve inevitable resonance problems.
As you see although it's very tempting and everyone is using it, it doesn't make sens to use it. I think that aluminium is so popular because most people don't take under consideration Inertia of motors and screws that are much "heavier" then bare gantry.
Thx for the link but since there was no good calculator online I've created my own. It's free for non commercial use :) hehehe
According to it I should get good acc and Vmax but I'll see soon how does it work in real life :)
Please keep pointing out better solutions - if I can't defend mine it means that I can make it better.
With my other machine that does work in vector mode I found quite a difference but that's only because I tried it in both modes so could see the difference first hand. It does seem to be more responsive to varying load such that it does not slow down when load is applied in vector mode, also because I use it at very low speed for bobbin sanding it works much better in vector mode.
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