Do I need a 3:1 gear reduction setup?

[clurgy]

Hi again fellow members,

I am in the midst of designing my 1220 x 2440mm router and was wondering do I need to use a 3:1 reduction drive setup that I see Peter @cncnutz uses, and it does seem quite common on other router setups I've found. Is the only reasons for this 3:1 reduction to increase torque and resolution in the stepper steps? I'm using quite large Nema 34 8.5Nm steppers with maximum power supplied to them, but just not sure if I can drive the gantry back and forth directly off the stepper shaft (with a pinion gear onto the pinion rack) or will I need the 3:1 reduction setup. Thanks in advance, and I've attached a snap shot of the model so far for reference

Regards, Lucas...

 

[cncnutz]

Hi Lucas

To my mind the 3:1 reduction does 2 things. It increases the torque of the stepper motor and it increases the axis resolution.

Here I'm talking about my Rack and pinion drive which is geared. My Z axis lead screw is direct drive.
All of my motors have less that 1/4 the power of the ones that you are looking at. Mine are 270 ozin or 1.9Nm but that is about right for my setup. Because I am rack and pinion driving the 1" pinion is effectively gears up the drive again loosing the torque I gained through the reduction but it gives me speed in return. Without the gear reduction I would have 1/3rd the resolution on the axis and I don't think the axis would work half as well. The current resolution is 0.013mm per step or 0.0005" per step.

Since the dual drive Y axis is powerful enough to move with me sitting on it and can still outrun the cutting ability of a 2.2kw spindle the balance of motor power, gearing, resolution and speed seems to be right for my setup. Rapid speed 600ipm or 15M per minute

Ignoring resolution for the minute, since the motors you have are 4.4 times larger and I'm only gearing mine up by 3 times in theory you still have more power but you will need to take into account the torque curves of the motors as well. Large motors have large losses and usually loose power very quickly as they speed up. Sometimes almost exponentially in extreme cases. Smaller motors have smaller internal losses and tend to be more linear in the way they loose power as they increase speed. It is possible that a small geared motor eg 400 ozin geared 3:1 could potentially match or out perform a larger 1200 ozin motor direct driving the same load once it reaches a certain speed.

Next you have resolution to consider. The geared motor already has a 3:1 resolution advantage over the direct drive. There you have to rely on microstepping. Since I have a 10:1 microstepping set up on my drive you will need 30:1 to match the same resolution. I am assuming the motor you are using is 200 steps per revolution or 1.8 degrees. If dividing one rotation of the motor into 2000 divisions seems crazy think about dividing it into 6000 divisions. You would need to research the accuracy of micro stepping to see if higher resolution is position accurate or not. No gearing will eliminate backlash in the drive belt so there is an advantage to direct drive and is much simpler to implement.

To my simplistic way of thinking using gears allows the use of a motor 1/3rd the size, while getting the same torque as a larger motor with 3 times the resolution and potentially equal or better top end speed. All that said there are so many factors to take into account when designing a machine it is hard to say what the best option is. Each has it's advantages and disadvantages.

These are just my personal thoughts.

Cheers Peter

 

[matman]

Peter,

Do you know where to purchase Mod 1 , 3 to 1 gear reduction drives for a NEMA 23 motors in Australia?

Thanks

Mat

 

[cncnutz]

No idea Mat.
I imported mine from the USA and they are Pitch 20 racks not Mod1

Cheers Peter

 

[safeairone]

This topic is of particular interest to me as I m driving my ~150lb/70kg gantry assembly up and down the Y axis rails using two 1125oz/in (8Nm) servomotors directly turning 20 pitch pinions over long rack gears.

Using the smallest pinion I could find to fit on the 1/2 (12.7mm) servo shaft, the gantry moves about 3 inches (76mm) per rev way too much to have any meaningful resolution per pulse and easy to lose steps on acceleration/deceleration.

I intend to solve this problem by adding 15:1 planetary gear reducers to the Y axis servos. This will make the Y axis travel almost exactly equal to the other 2 axis, which use 5mm/rev balllscrews.

I plan to source the gearboxes from the People s Republic through eBay I just wish the output was 1/2 (12.7mm) so I didn t have to fiddle with my 1/2 bore pinion gears:

Planetary Gearbox Nema17/23/34 Speed Reducer Gear Head for CNC Stepper Motor | eBay

Nema17/23/34 Planetary Gearbox Speed Reducer 5/10/20/30/50/100:1 for CNC Stepper.

www.ebay.com