Thread: cnc for marble
The distance traveled per revolution with rack and pinion is the pitch circle of your pinion gear diameter * pi, for example 1inch 25.4mm pitch circle diameter pinion gear, 25.4*3.145 = 79.883mm distance traveled per revolution. Divide that by your stepper motor step count which is normally 200. 79.883/200=0.399415. So the resolution is 0.399415... so to get that 0.001 resolution I was talking about you'd need a 300:1 gear ratio, which is crazy. 0.001 might be a little optimistic... but the point being that the problem of smooth slow feeds will be much worse with rack and pinion.
Don't get me wrong rack and pinon is great for soft material routers that need high speeds but I don't think it's correct for your application. Of course you could probably make it work but it wouldn't be the ideal solution.
What specifically do you think is 'wrong' with it? As is do you think there is malfunction with the driver or are you wondering if another setting would be more appropriate?
Just to reiterate on earlier 0.001 resolution was a bit optimistic. 0.01 is more realistic and would be fine.
Last edited by Rufe0; 12-01-2016 at 11:54 PM.
i was wondering if another setting would be more appropriate
Maybe, try a few out. It depends on your steppers, drivers and how well they work together, there is probably a sweet spot. I would stay clear of the really high ones though, mircosteps aren't always as good a thing as you might think they would be for a number of reasons. Have a read about it http://www.micromo.com/microstepping...-and-realities . Obviously remember if you change your microsteps you will need to adjust the settings on your computer otherwise it will come out the wrong size.
Your also Wrong about the Resolution and Micro steps because no one who runs steppers would run at FULL step 200ppr. WHY.? . . . Because Resonance and the affects it has on the motors is crippling to motor performance when full stepping. Infact you'll be hard pressed to find decent quality drives that provide Full step mode. Half Step(400ppr) is usually minimum. In real world use then between 800 and 2000ppr are used to give best balance of performance/smoothness.
In this application cutting hard material and Steel framed machine the resonance will be high so using higher micro steppping would be used to help smooth motor performance. Micro stepping however shouldn't be thought of as way increasing resolution, though it does to some degree dependant on motor quality.
Resolution should be calcualted based on pitch. R&P will minimim ratio of 3:1 (typical 5:1) dependant on several factors like Pinion size, motor Rpm, desired Rapid speeds etc.
Lets look at it again with realistic figures using same pinion size. Lets start by applying a minimum Ratio 3:1 with minimal 400ms setting and seeing the speeds, resolution this would give.?
With an affective pitch of 79.833 / 3=26.611mm pitch which based on usable RPM of typical motor used being 700rpm this would give 18627mm/min or 18.6Mtr/min which is ridiculous speed for cutting stone that would never be used.
The resolution would be 26.611 / 800=0.066 which is still more than enough resolution.
Now lets go to more likely setup that would be used. 5:1 ratio still giving a speedy 15.966 Pitch and using 1600Ms which is often optimal for motor smoothing.
Rapid speed: 15.966 * 700= 11,176mm/min or 11.1Mtr/min still a nice healthy speed which you'll never use cutting stone other than for positional rapid moves.
Resolution: 15.966 / 1600= 0.009 far more than needed.
Now the Resoultion figure isn't exactly true figure but rather theoretical figure and in reality there will be some mechainical loses but the higher it is to start with the higher it will balance out to in the real world.
Also the higher ratios have increased torque which will be needed with R&P because it's quite inefficient compared to ballscrews so will need more power.
Helical R&P it's better than straight cut and provided you protect it from debris and keep pinion tensioned into rack properly then it's the perfect solution for long machine like this.
Last edited by JAZZCNC; 13-01-2016 at 09:16 AM.
"The real compromise is that as you increase the number of microsteps per full step the INCREMENTAL torque per microstep drops off drastically. Resolution increases but accuracy will actually suffer.
The consequence is that if the load torque plus the motorís friction and detent torque is greater than the incremental torque of a microstep successive microsteps will have to be realized until the accumulated torque exceeds the load torque plus the motorís friction and detent torque.
Simply stated, taking a microstep does not mean the motor will actually move!"
A microstep doesn't guarantee movement. For my point about wanting a high resolution for smooth slow speed to be valid and useful the resolution has to be real world accurate resolution.
So how many microsteps can you guarantee? Well ofcourse your motor could stall completely, but lets put that to a side for a moment. 1 microstep/full steps you can guarantee them. 2 yeah thats fine. 4 probably OK. 8 maybe I wouldn't be confident. More than that I would say you definitely couldn't call that guaranteed accurate movement.
From the point of view that over doing it is better than under doing it, using 200steps/rev for this calculation means you definitely will get that accuracy. However realistically 400/2microsteps or 800/4microsteps is probably the right thing to base the calculation on.
When the machine is built you can still run it at whatever microsteps you like, nothing stopping you doing that.
I'm suggesting something like 20mm pitch screw with 1/3 belt drive. 20mm/3 = 6.6mm / 400 = .016mm or / 800 = .0083. 0.016mm - 0.0083 real world accuracy. Or better yet a 10mm pitch screw with a belt drive.
There are so many variables that come into play that affect the performance so much more than these technical technical details. Quality and spec of drives and motors along with voltage will have much greater affect, as will Machine stiffness and Resonance.
Motor Ms theory is based on Fixed parameters for a particular motor/drive/voltage/ combination which mostly goes out the window in real world because of too many variables make the machine setup a moving target.
The simple fact is that what your quoting from the article regards Ms/torque etc doesn't have any noticble affect in real world use because of all the other variables come into play. But running the motors in MS range that lets them get affected by resonance certainly does. This is a 100% fact not theory with 95% chance of happening if you run at full step with poor spec components.!
The simple fact is that there is NO IDEAL setup and each combination of components and machine design/Stiffnes needs to be setup to suit that machine. . . .But there is a Limit.!!
We are not going to agree on this probably but I'm sure you'll come to realise what I'm saying when you have built a machine.!
Last edited by JAZZCNC; 13-01-2016 at 02:31 PM.
Your not saying anything that I haven't said. I said he should try out different settings to find what works best.
I'm saying on top of that he should design his new machine with high resolution in mind for smooth slow speed assuming that you won't get better than 2 or 4 microsteps accuracy. That way if you set your machine to 8 or 16 microsteps and it achieves that accuracy then great you've not lost anything. If it can only achieve half that accuracy then thats OK because you designed it with that in mind from the start.
By barrykavanagh365 in forum Gantry/Router Machines & BuildingReplies: 1Last Post: 18-11-2015, 08:14 PM