BuildingAfloat

[Robin Hewitt]

I forgot to explain

A heavy pulley is a flywheel, flywheels have long been used to add inertia to systems, and if you look at the maths you will find they do it well. It takes surprisingly little effort to accelerate a gantry, if it weighs 20 lbs then a 20 pound force will accelerate it at 1G, same as if you dropped it. If you add unnecesary inertia to the system you have to watch out for that moment when you hurl the gantry in to reverse. That is the moment you risk losing steps and blowing your position. Unnecessary inertia is to be avoided for that reason.

As you like explanations I should also warn you about choosing stepper motors by holding torque alone. For some obscure reason the hobby market has settled on stepper motors with an arbitrary 200 full steps per rev, so the field coils have to turn the rotor 1.8 degrees. If you use a large rotor you get more leverage on the shaft but the pull in distance for 1.8 degrees is larger than for a small motor. Unfortunately as the distance increases the magnetic field falls away by the inverse square law so you tend to lose more than you gain.

Next problem is that the suppliers have found that nobody seems to care a fig for the actual motor performance, they only look at the holding/detente torque for the motor. Obviously the holding torque is measured with the minimum gap so big numbers are easy achieve so that is exactly what they do. What you are really interested in is the pull in torque and the inductance of the field coils.

You can "fix" a large 200 step motor by increasing the volts but that is expensive in the driver/PSU department. When choosing a motor you want to view holding torque as a simple indicator so you gett in the right ball park, what you really want is a graph of torque against speed and see how it goes with what you are trying to achive in accuracy while still giving a good rapid which gets more important as the size of the machine bed increases.

Edit: There is also a tendency to sell motors in sets of 3 for XYZ, why anyone would want to use the same motor for 3 completely different tasks eludes me.

[BikerAfloat]

Thanks for your reply Robin,

I forgot to explain

A heavy pulley is a flywheel, ...

... Unnecessary inertia is to be avoided for that reason.

Should have thought a bit more about my reply, I was finishing it before having to go out!

I was heading down the right track, I just didn't get to the end in time!

I have a real love for the old Mini and the A-Series engine (Yes this is still on track) sadly can no longer justify the expense of owning one along side my pickup truck :-( In the tuning manuals I had there was the formula that proved a few pounds off the weight of the flywheel equated to a significantly larger weight reduction in the vehicle. Obviously this effective weight loss was reduced with each gear, as the rate of acceleration decreased. Had I given myself a bit more time to think, it would have been clear to me that this holds true with the mass of the pulleys vs the mass of the gantry Doh!!!!

As you like explanations I should also warn you about choosing stepper motors by holding torque alone...

...Edit: There is also a tendency to sell motors in sets of 3 for XYZ, why anyone would want to use the same motor for 3 completely different tasks eludes me.

Oh yes, another can of CNC worms! I have done a bit of reading about this and was aware of the difference between holding torque performance and dynamic performance.

There are so many variables in each component, there comes a time when you have to say enough - any more information your brain will fry - and base your choices on what seems to be the best given what you know and what the rest of the world is doing in a similar situation.

I agree with your final statement, I had already thought that in an ideal world the motors need to be sized lower performance for Z, rising through Y to X as each axis in that order, has relatively more to do. However since this is not a racing machine where everything has to be maximum performance and minimum safe construction, I couldn't see the harm if Z&Y were a little over specified, when compared to X. As you will have seen, I have asked above if a single 3.1 Nm is enough for my X Axis.

As you can imagine I have done lots and lots of reading and seen large machines working with quite small motors where budgets are tight and small machines apparently grossly over powered when it has been clear money was little object!

My choice of motors was down to several parameters:

1) Most machines I have found in the size range I am after (and some at 8'x4') use Nema 23s, so this seemed to be a the most sensible choice for frame size. Incidentally it also fits in nicely with my choice of extrusions.

2) This page (Drives and Steppers~Solsylva CNC Plans) had the following statement: "Steppers in the ~300 oz.in. range can push a full sized router, but they lack the power to push the router to its limit. The steppers will most certainly stall before the router does." That ruled out the lower torque range of the Nema 23s. Also I have a copy of the TAB CNC Robotics book by Geoff Williams, his steel construction machine gets away with motors that are at best 220oz.in. So I decided on 300oz.in minimum.

Since this project does not have an unlimited budget, the best "Bang per Buck" method was going to have to be used. Also I chose to make the assumption that for a given frame size and therefore presumably similar construction a Higher holding torque should lead to a correspondingly higher dynamic torque.

My search led me to the 439oz.in (3.1Nm) motors from CNC4You at around £100 for four. At roughly 30% over the torque suggested by what I could find at the time, I guessed these would probably be man enough. My limited knowledge and obviously finite research makes these the best cost/performance ratio I can find. Given my resignation to the fact that finite budget machines are always a compromise, I am happy with my choice, but I am willing to be proved wrong!

Q: Does anyone know of 4 better motors out there for the same money, give or take a Tenner?

Now I'm going to be using belts to drive the twin screws the 4th motor is going to rest in a draw until I get round to my 4th Axis.

Cheers,
Geoff.