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Bart
26-12-2013, 08:44 PM
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.

110831108211084
11081110791107811080


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.

11077

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

EddyCurrent
26-12-2013, 09:15 PM
Does it make sens to buy vector controlled VFD for 350$ instead of u/f controlled for 250$?

I found this was a problem only after I bought the gear, it seems the ABB ACS355 can not control my ELTE spindle (2.2kw 24000 rpm 400hz) in vector mode. I use the same inverter on another machine that has a standard 2.2kw induction motor and it works in vector mode for that without problem. You therefore need to read the manual for the VFD to see if it can handle your spindle.

Bart
26-12-2013, 09:29 PM
I'm going to buy the Chinese 2,2 kW spindle. Do they perform better at low speeds with vector VFD?

EddyCurrent
26-12-2013, 09:46 PM
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.

george uk
27-12-2013, 11:55 AM
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

Hi, this is new to me, have had a look on wiki, i get the basics. but not enough to understand.

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

Bart
27-12-2013, 12:14 PM
EddyCurrent
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.

george uk
not at all

Jonathan
27-12-2013, 01:31 PM
1. Does using sensor less vector mode give better toque at lower revs, on a standard setup.

Yes.


2. Is this enough of a difference to be usfull ?

That's a much better question! It depends what you're trying to cut. If you're trying to drill aluminium with HSS drill bits, or mill steel with standard (i.e. no special coatings) carbide cutters (i.e. anything that needs higher torque at low speed0, then the difference will be tangible, but that doesn't mean you can't get away with a cheap V/f control drive. If you're just milling aluminium and cutting wood/plastic etc, then you wont really gain anything.


Yes, the VFD will be used in sensorless vector mode which is not as good as the closed loop type

Sensorless vector control is still closed loop - you're just inferring the rotor position electronically (i.e. from the back-emf) instead of from using an encoder. That indicates the situation where sensorless control isn't as good - it relies on the back-emf which is hard to sense at low speeds, so it's necessary to have field oriented control with an encoder if you need full torque down to zero speed.

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.

paulus.v
27-12-2013, 04:37 PM
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 (http://www.mycncuk.com/forums/faqs-problems-solutions/1524-what-size-stepper-motor-do-i-need.html) 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 (http://www.cncrouterparts.com/pro-rack-and-pinion-parts-c-47_49.html) you can find some useful information about rack and pinion. They build similar sized routers.

Bart
27-12-2013, 07:17 PM
Thx paulus.v

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
11085

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.

Best reagards

EddyCurrent
27-12-2013, 07:55 PM
EddyCurrent
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.

When I try to run the spindle in vector mode it immediately puts the VFD into fault, the fault code (which I can't remember for certain) says something about various parameters having to meet certain conditions, which clearly they do not at the moment. I did not follow up on this and unfortunately I am not in a position to try the spindle just now. Because it worked fine in v/f mode I decided to leave further investigation till later.
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.

EddyCurrent
29-12-2013, 09:41 PM
I managed to get it all connected up again today, the drive trips on fault F1009 when I switch to vector mode, remember it works great in v/f mode.

Check parameter settings. Following
must apply for induction motor:
• 1 < (60 9907 MOTOR NOM FREQ / 9908 MOTOR NOM SPEED) < 16
• 0.8 < 9908 MOTOR NOM SPEED / (120 9907 MOTOR NOM FREQ / Motor poles) < 0.992

So point 1.
motor freq = 400
motor speed=24000
60 x 400 / 24000 = 1
well it says it should be greater than 1 and less than 16

point 2.
motor poles = 2
24000 / ((120 x 400)/num poles)) = 1
it says it should be greater than 0.8 and less than 0.992 so that's why it's going into fault.

I changed the nominal speed to 23760 and that solved the problem.

Now though it trips on over-current but I'd had enough today, and yes I do have the correct figures in for nominal current it's 9.5 Amps and max current is set to 17.6 Amps