Comments sought on new build: A CNC Router for RC Gliders and Planes

[irving2008]

I reckon they're both right :) Jonathan's calcs are ok if you are not microstepping cos then you have to derate the torque significantly not to lose steps, so Dean's answer is pragmatic in the likelihood you will be microstepping.

[Jonathan]

I reckon they're both right :) Jonathan's calcs are ok if you are not microstepping cos then you have to derate the torque significantly not to lose steps, so Dean's answer is pragmatic in the likelihood you will be microstepping.

Microstepping reduces the so called 'incremental torque' - i.e. the torque available to move from one microstep to the next and hold a microstepped position - the torque when the motor is moving continuously (i.e. the operating point I calculated) is the same regardless of microstepping. Also above low speeds stepper drivers switch to full stepping.

Personally I would still use two motors, but for other practical reasons - not due to the torque rating.

[Neale]

I've just about reached the stage on my machine where I have to decide one motor/twin belts or two motors. Sounds like the received wisdom is to go two motors. So, that means buying one more motor, another driver - but the old analogue ones I have at present are obsolete, so that will mean two new digital drivers, and, what's worse, ditching LinuxCNC and having to use Windows/Mach3 to be able to home two motors, and, probably, an external motion controller. Ho hum...

[njhussey]

I've just about reached the stage on my machine where I have to decide one motor/twin belts or two motors. Sounds like the received wisdom is to go two motors. So, that means buying one more motor, another driver - but the old analogue ones I have at present are obsolete, so that will mean two new digital drivers, and, what's worse, ditching LinuxCNC and having to use Windows/Mach3 to be able to home two motors, and, probably, an external motion controller. Ho hum...

I'm in a similar situation regarding the single vs two steppers, I can either buy another Nema 23 3Nm stepper and driver (but mine are the AM882 so I'd have to source from China or ebay) or a Nema 34 and 240V driver...decided on the Nema 34 for a bit of safety margin...

[JAZZCNC]

I calculated that I only needed one Nema 24 3Nm stepper for the twin X axis ballscrews but on thinking about it and listening to Dean I'll be going with a single Nema 34 on the X axis.....just to err on the safe side...after all it's not my money now so I'll be getting a 240V driver to go with the Nema 34 as well...

Ok well feel I've got to comment here about theses calculations or calculators. While I'm not saying the calculations are wrong what I am saying is that they are miss leading and not giving the full picture so sending people wrong. I've helped several people who have gone off them and been dissapointed with performance.!!

They don't and can't take into account all the factors that make up a working machine. They don't account for miss alignment or poor materials and how machine is designed and built or the affect this as on friction etc.
They don't account for the affects of resonance. They don't account for Cheap drives running on PC with poor pulses from a ropey parallel port.
They don't account for the DIY affect and the numerous things that can affect how machine performs.

All these things come into play and have a big affect on the overall performance and the calculators can never know these variables so IMO they are only to be used as a very rough guide.

This particular case is a prime example because while the Calculators says 1 x 3Nm Nema23 turning 2 x 1300mm screws connected with belts should all be hunkydory I can tell you with confidence you won't get anything like the performance that as been suggested. Yes it will move them but at much slower feed rate and acceleration than the calculator suggests to give a stable working machine.
So Neil believe me your Not Erring on the side of caution your doing what's is needed to get the level of performance your looking for.!!

Neale:

The single or Twin motor argumant falls down to ONE real differance.! . . How stable and accurate do you want your machine.?
If you wan't very stable with virtually no chance of screws losing sync even with cheap electronics then you need single motor/belts.

If you want to use twin screws then while they do work well they only do so provided certain things are correct.?
They are much more sensitive to motor tuning and need careful attention not to push too hard.
IME They work best and get best performance from them with Modern digital drives and External motion control cards which can provide nice clean pulses compared to ropey parallel port.
IMO it's crucial they have some form of stall detect or Error signal on the drives so if one motor stalls or drive fails the system shuts down. I won't build a twin screw machine without this option and Those that have built using twins motors and had this happen at speed will tell you how scary the big potential for damage.

I build more machines using Twin motors than single motors because it's easier but will ONLY do so provided I can build like the above. I Only build using Motion control cards and Digital drives and still leave a comfortable margin on tuning for best reliabilty and safety.
I never have any issues with twin screws building this way but it comes with a cost. Single motor and belts while more messy and involved is cheaper and more forgiving with lesser electronics.
I never truely trust twin screws not to stall or twist up the machine in some way if pushing hard for exteneded periods and I'm always mindfull while using and setting up. Where has I have complete trust in single motor setup and it never enters my head in use no matter how hard I push, no matter how long I push it for.!!

Both work.!!

[EddyCurrent]

M = Misalignment
Q = Materials quality
D = Design and build
F = Friction etc.
R = Effects of resonance.
C = Cheap drives running on PC
P = Ropey parallel port.
Y = DIY effect
O = other miscelaneous effects

For each of the above assign a value between 0 and 1; 1 = 'perfect' or negligible, 0 = 'total shite'

JazzFactor = M*Q*D*F*R*C*P*Y*O

(Note: if any value=0 then JazzFactor=0 hence idea=total shite so don't even bother.


Actual calculation = theoretical calculation * JazzFactor

Example:

theoretical torque = 0.75Nm

JazzFactor = (0.9 * 0.95 * 0.9 * 0.99 * 0.9 * 0.95 * 0.9 * 0.85 * 0.85)

actual torque = 0.75 * 0.42

actual torque = 0.32 Nm

[njhussey]

Well I dusted off the frame this afternoon and took the last of the epoxy off. It's actually set really hard so I could have used it after all....but better safe than sorry I'm going to re - do it from scratch. 10 mins with a hammer and chisel (screwdriver) and and angle grinder with flapper disc and it's back to bare metal. Going to cut mdf sides this time and seal the gaps/edges/joins with silicone sealant. Got to do a bit more proper work now this afternoon on the lathe but hope to get the moat done tomorrow.





Video showing the hardness of the set Epoxy...

[njhussey]

Only managed 1/2 hour on this today but cut the MDF for the moat. Tomorrow I'll glue it together and seal it, thinking of sticking it to the steel with silicone sealant which will also act as a sealant to stop (hopefully) any leaks as I don't want a repeat of last time where despite my best efforts (obviously not good enough) I had leaks at some joins.

If I can get it together and sealed then I might go for a pour so it starts to set over the weekend. Whatever happens I'll not be rushing this....lesson has been learnt from last time :)

[EddyCurrent]

Neil,
I trust you read silyavski's thread about his epoxy method ?

[njhussey]

Neil,
I trust you read silyavski's thread about his epoxy method ?

With the metal strips?