Steel frame cnc router design/build

**CraftyGeek** · 02-09-2013

Right given the lack of negative comments on the last revision of my design, i'm starting to move on to the next stage which brings me onto another set of questions...

Gear ratios & microstepping.

After reading around, it seems that the general consensus is that less microsteps are better ie, either use full steps or at most, half steps.
My understanding is that the accuracy of the stepper motor gets lower when the number of microsteps is increased...I think this also has an effect on the torque?

Couple this with the gear ratio from the stepper to the leadscrew makes the choices a little open ended as far as I can tell.
The general flat answer for the ratio that I seem to see is to use 2:1.

I'm only thinking about the X & Y axis at the moment which are both going to use 1610 screws. Belts/pulleys will be 5mm HTD 15mm wide.
Full step geared 2:1 gives a resolution of 0.1mm
Half step geared 2:1 gives 0.05mm
Full step geared 1:1 also gives 0.05mm

0.1mm sounds quite a coarse resolution to me - 0.05mm sounds more like it. So why gear 2:1 & use half step when the same resolution can be had with full steps & 1:1?

or perhaps go for a somewhere inbetween like 1.4:1 (eg 28/20) full stepped = 0.07mm ??

Am I missing something here?

**JAZZCNC** · 02-09-2013

Originally Posted by CraftyGeek

After reading around, it seems that the general consensus is that less microsteps are better ie, either use full steps or at most, half steps.
My understanding is that the accuracy of the stepper motor gets lower when the number of microsteps is increased...I think this also has an effect on the torque?

Am I missing something here?

Erm. . . !! You've been the wrong places then.? First you'll struggle to find drives that allow Full step and most start at half.

This is for a good reason. Don't think of MS in terms of increasing resolution but more in-terms of smoothing the motors action.
Full step gives rough action in comparison it's also common for resonance to be a issue if run at full step.
Main reason why you'll see folks run at full step is because they are chasing speed and have slow parallel port so struggle with pulse's needed for higher MS and higher speeds. They are often running low voltage has well.!!

Your PC parallel port will most likely be bottle neck for higher speeds with higher MS. So again you'll probably select the best MS to suit your PP motor/drive combo.! . . . . . If your using Mach3 and you want very Smooth motors and Higher speeds the invest in a Motion control card Like the SS it's well worth the money.

Personally I wouldn't run under 1/4 (800ms) and prefer to run nearer 2000MS which gives much smoother motor performance.
Often your PC and drives coupled with how much machine resonates will mostly determine the best MS to suit for machine.
Every machine tends to have it's own resonance which can affect how the motors/drives perform so you'll probably end up playing with differant settings that suit your machine best.? . . . BUT. . . I can tell you with great confidence you won't run them at Full step or very likely 1/2 has the motor performance will be rough in comparison to higher MS with possible resonance issues to boot with 1/4 or 1/8 being more likely your best option.

Regards the ratio then 2:1 will double torque and resolution of 10mm pitch screw so you won't have any issues here if you do this. It will half the speed thou.!!
If your mainly cutting woods.plastics etc then I suggest running 1:1 for the speed. If your cutting mainly Ali then I'd suggest 5mm pitch and 1:1.

If you want a balance of both then 2:1 would be ok but really not required with 10mm pitch screws if using 3Nm motors with enough voltage (60-70Vdc).

**CraftyGeek** · 03-09-2013

The drives I have at the moment have full steps on them...so I don't know any different in that regard. They have full, 1/2, 1/4 & 1/8 - so it looks like I should just use 1/8 step & forget about it.

I hadn't considered the torque side of things.
If my intention is to mainly cut wood/mdf & then aluminium plate/sheet occasionally....given that i'll be stuck on a low voltage (24v) for a while before being able to upgrade the driver/psu combo do you think that a ratio in between might be a better option? So I don't sacrifice too much speed, but still get a torque boost.

**JAZZCNC** · 03-09-2013

Originally Posted by CraftyGeek

I hadn't considered the torque side of things.
If my intention is to mainly cut wood/mdf & then aluminium plate/sheet occasionally....given that i'll be stuck on a low voltage (24v) for a while before being able to upgrade the driver/psu combo do you think that a ratio in between might be a better option? So I don't sacrifice too much speed, but still get a torque boost.

No I'd stick with 10mm pitch @ 1:1 has you'll be speed limited with 24V so the extra lead of the screw will help here. With 3Nm motors torque won't be an issue really unless really heavy gantry or running 2 x screws from one motor.!!

**CraftyGeek** · 03-09-2013

Originally Posted by JAZZCNC

With 3Nm motors torque won't be an issue really unless really heavy gantry or running 2 x screws from one motor.!!

I am going to be running the X-axis with 2 screws & one motor....also the motors I have at the moment are 1.8Nm. Not ideal I know - but its what I have to start with.

**JAZZCNC** · 03-09-2013

Originally Posted by CraftyGeek

I am going to be running the X-axis with 2 screws & one motor....also the motors I have at the moment are 1.8Nm. Not ideal I know - but its what I have to start with.

Oh dear your stuffed.!!!. . . Lol

Well in this case then your really bating uphill and may struggle bad With those small motors.? . . . So with only 24V then I'd suggest running 3:1 to increase torque and suffer the speed loss.
When you have larger every thing then drop back to 1:1. . !!

Make sure the motors are wired in parallel not series has well else your double stuffed when speed rises. . .Lol

What drives are you using.?

**CraftyGeek** · 03-09-2013

I bought the drivers years ago when there wasn't anywhere as much choice at the cheaper end of the market:
Single Driver

Also have the LPT quick connector from there as well.

I plan on replacing it all when the machine is running....but that will be a way off.