Mendel90 Build - Parts input required pls.

Hello all!
First post in the 3D realm.

I've been thinking of 3D printers for awhile now, and made it out to the Victoria Maker faire.
I ended up meeting with Dan and John there (Hi Guys!)

So now that I've seen the printers up close and personal, I HAVE to have one.
I'm going to be building one based on the Mendel90 machine and I would like some
input on parts that would be better for this build.

What I have so far:
Ramps 1.4 ULTIMATE KIT incl SD Ramps and wiring Mendel RepRap
Reprap Ramps 1.4 smart LCD controller inclunding smart adapter
12pcs LM8UU 8mm Linear Ball Bearing Bush Bushing and the rails

Other parts I've been looking at:
Nema 17 78 oz-in steppers (1.8A)
Switching Power Supply for LED Strip Light AC to DC 12V

I'm also considering my build with some 1/4" dual start acme threads Link Insted of using belts.
There isn't too many builds based on the acme rods... and from what I've been hearing, is that there would be greater
accuacy with the rods. (I've built a CNC Router based on acme rods and the accuracy is really good)

Now if the rods wont offer superior performance over the belts(t2.5 timing belts) then maybe I wont bother.
But I'm willing to give it a shot if the steppers listed above would push the machine without issue.
I prefer accuacy over speed, but I think the dual start acmes would be ok?

How are the A4988 stepper drivers at pushing 1.8A? (with heatsink)
And will the 350Watt PSU be enough to drive everything with a heatbed too?

Thanks for any input, and I will post some pictures later with my ideas...

Sorry for the long post!
Chris

1.8 amps is really pushing the drivers. Generally you can run them at closer to 1A and still have enough torque. You usually don't want to run motors at their max rated current.

A 350W supply will be more than enough. A standard RepRap with heated bed will run off of approximately 200W at 12V. Better to have overhead though, especially with cheap Chinese power supplies.

ACME threaded rods would be more accurate, but much slower to accelerate which will actually reduce print quality at all but the slowest speeds, because of hysteresis in the plastic extrusion. It is possible to compensate for mathematically, but AFAIK most firmware does not support this.

Mogal Wrote:
> I'm also considering my build with some 1/4" dual
> start acme threads Link Insted of using belts.
> There isn't too many builds based on the acme
> rods... and from what I've been hearing, is that
> there would be greater
> accuacy with the rods. (I've built a CNC Router
> based on acme rods and the accuracy is really
> good)

I think it is probably true to say that in the CNC area screw drives are usually the appropriate solution for the requirements of CNC, but that does not mean they are the best solution for all problems. Belts can be as accurate as screws, but there are other reasons for choosing screws in a CNC machine, where speed is less important.

The accuracy of 3D printing is much more to do with extruding perfromance than movement, unlike CNC. Most people who try 3D printing with screw drives end up migrating to belts for X and Y, there is a practical advantage to using screw drive on the Z axis.

If you put 1.8A through 1.8A motors they will probably get hot enough to melt plastic brackets. We normally use 1A with 1.7A motors.


The lead screws move 2.54mm / turn whereas a belt moves 40mm, so yes you get more accuracy but also 15 times slower all things being equal.

---

[www.hydraraptor.blogspot.com]

Ok, thanks guys! I will stick with the belt drives.
what threaded rod are you using for the Z axs?
(I presume no backlash nut is required?)

As for the steppers, I'm having a hard time finding appropriate steppers.
Could you point me in the right direction pls?

These are what I was looking at:
5pcs NEMA17 78 oz-in CNC stepper motor/1.8A

Thanks again!
Chris

Mogal Wrote:
-------------------------------------------------------
> Ok, thanks guys! I will stick with the belt
> drives.
> what threaded rod are you using for the Z axs?
> (I presume no backlash nut is required?)
>
> As for the steppers, I'm having a hard time
> finding appropriate steppers.
> Could you point me in the right direction pls?
>
> These are what I was looking at:
> 5pcs NEMA17 78 oz-in CNC stepper motor/1.8A
>
> Thanks again!
> Chris

Those steppers should be fine. You just want ones with at least 60 oz-in torque and are rated at less than 4V or greater than 1.5A.

For the Z axis, most people use plain threaded rod. 8mm or smaller. Some people use acme lead screws. Either works, but stainless steel is recommended as it tends to have more accurate threads. Backlash nuts are generally not needed as the weight of the x-axis mostly eliminates backlash in Z.

As mentioned before. Screw offers much lower speed. Only advantage is that they are less "elastic". Theoretisaly u can get better acuracy in corners but belt is accurate enough. For example im using t2,5 belt and 14z Al wheel. 14x2,5=35mm on 16x200=3200steppes => 0.01mm per step (more then enough for me).
Stepper motors have less torque on higher frequency. Mine motors goes significantly down on 20000hz.
If i use threaded rod "Trapezoidal" Tr 12x3. I get 3mm on 3200steppes. 20000/3200=6,25x3=18.75mm/s. Normal printing speeds are 30mm/s many peaople use 60mm/s (especially with PLA). 18mm/s can work, but u will print realy sloooow with same result as on 30mm/s.
U can use gears 4:1, but it lead backslash, and u are back as with belt.

Just by the way, I saw those exact steppers here for a lot less:

[www.aliexpress.com]