Starting a RepStrap - some basic questions!

Hi all,

Firstly, have to say how impressive the whole RepRap project is and how well the open sourcing works.

I have various linear guides, belt drive slides, and leadscrew type stuff and am going to put together my first RepStrap.

It will be mostly Aluminium profile construction, and maybe not too disimilar (though definitely not as tidy!) as [www.grassrootsengineering.com]

Will be using Gen 6 electronics and standard firmware, and NEMA 17 motors (assuming the Gen 6 will struggle with larger 23 motors...?)

Some questions:
(1) Are there dimensional limitations to the workspace (other than mechanical stiffness etc) - would 500mmx500mmx200mm work with the standard software.
(2) Is there an optimum range for the mm/min and/ the steps/mm - I have a fair bit of flexibility on this side of things (i.e. use gearing or not) once staying within torque range for the motors. What are these figures (mm/min and steps/mm) for the Mendle?? I am assuming the G-Code and extruder are most efficient at certain speeds etc - is it fair to say that slow drives mean more accuracy or does the extruder need to flow at a minimum rate...
(3) Are there and other major issues with using a different mechanical setup to the Mendel etc once the the NEMA 17's can dirve the axes...?

Many thanks! Pictures to follow...

Edited 1 time(s). Last edit at 02/07/2011 04:16PM by Axim.

I suggest asking those questions at the cnczone instead. They have more accumulated expertise in building custom linear motion projects.

I've read elsewhere that the software can handle machines much larger than a standard Mendel.

I don't have a printer up an running yet, but I'll answer the other questions from my own machine design experience.

For steps/mm, I'd start out near the default of the Mendel. Once you get that up and running and successfully making parts, you can start work on running faster, or more precisely than a Mendel. If you start out too different from Mendel, it could be that much harder to troubleshoot and tune.

Other issues? Just keep the axes square and minimize the backlash.

> Will be using Gen 6 electronics and standard
> firmware, and NEMA 17 motors (assuming the Gen 6
> will struggle with larger 23 motors...?)
A while back I saw a design for a nema23 stepper extruder. Couldn't tell you what he was using to drive it though. The amp load of the motor will be what matters more than anything. I'm assuming the gen6 drivers are bipolar only just like the other reprap electronics, so also make sure that your motor is either bipolar or can be wired as a bipolar stepper motor.

> (1) Are there dimensional limitations to the
> workspace (other than mechanical stiffness etc) -
> would 500mmx500mmx200mm work with the standard
> software.
I think the firmware uses 32bit integers to hold the number of steps to take. This is really the only limitation I know of - other than structural, precision, print time, and energy consumption issues - that limit the build space. If you use a high microstep setting or lead screws and a large build plate, you might hit the maximum limit of int32.

> (2) Is there an optimum range for the mm/min and/
> the steps/mm - I have a fair bit of flexibility on
> this side of things (i.e. use gearing or not) once
> staying within torque range for the motors. What
> are these figures (mm/min and steps/mm) for the
> Mendle?? I am assuming the G-Code and extruder are
> most efficient at certain speeds etc - is it fair
> to say that slow drives mean more accuracy or does
> the extruder need to flow at a minimum rate...
Yes and no. A flimsy machine will need to run slower in order to keep inertia from causing the axes to flex. A cheap belt will also stretch a bit during sudden deceleration and acceleration. That said, you can increase your top speed by making the axes more solid and using a belt with steel cable reinforcement instead of the stretchier, cheaper kind. A lead screw would probably be even better assuming you could turn your motor fast enough to compensate for the fine pitch. Reducing the mass of your extruder and build plate will also help.

There is a happy median between too fast and too slow, but I suspect that it will be different for each printer design.

> (3) Are there and other major issues with using a
> different mechanical setup to the Mendel etc once
> the the NEMA 17's can dirve the axes...?
>
If the axes travel easily, have the range of travel that you want, and are very solid with no potential to "creep" over time, sounds like a good framework for a repstrap to me. Just keep your build within reason (IE, don't make a super huge build plate thinking you can print a life-sized car on it). Your axes will be more sensitive to misalignment since they are scaled up, so it may take more patience to get everything squared up, but it should be doable.

Hi Axim,
Welcome to the forum! Sounds like you have quite the collection of bits 'n' pieces to make a good start with. Best of luck with it!
No.6

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[numbersixreprap.blogspot.com]

Brillant stuff, thanks for the replies.

Does anyone know off the top of their head the default steps/mm (or mm/step!) for each of the three axes on a standard Mendel - I could go working it out from the drive pulley pitch diameter and steps/rev (200 ?), but someone might save me the approximation...!

Diameter does not matter. It is just the pitch times the number of teeth divided by the number of steps. So with T5 belt and 8 tooth pulleys it is 40mm/400 giving 0.1mm per half step, so ten steps per mm, more if you use microstepping. I.e. x16 would be 80 steps per mm.

For z it is just thread pitch (1.25) times the gear ratio, which I can't remember atm.

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[www.hydraraptor.blogspot.com]

From the RepRap Calculator

Steps per mm - leadscrew

Motor step size: 1.8°

Driver microstepping: 1/2 (or 1, 1/4, 1/8/ 1/16 etcetera)

Leadscrew pitch: 1.25 mm (M8)

Gear ratio : 8 : 21
(Motor pulley teeth count : Leadscrew pulley teeth count) (1:1 for direct drive - Prusa)

Steps per mm = (((360/Motor step size) * (1/Driver microstepping))/Leadcrew pitch)*(Leadscrew pulley teeth count / Motor pulley teeth count)

Steps per mm = 360/1.8 * (1/0.5)/1.25 * 21/8 = 840

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
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