3D printer question

[the_digital_dentist]

I am testing the MegaMax wiring and have a few questions about endstops for any of you 3D printing people who can help.

I am using the RAMPS 1.4 board and mechanical endstops.

Do you use both endstops on each axis of your printer?

Does the Z-axis minimum stop serve any purpose?  

I would think the printbed is the minimum Z point- do you actually set up an endstop at the precise level of the printbed?  

Do you level the printbed then adjust the Z-minimum endstop switch position each time you readjust the printbed level?

Thanks!

[Brent Bublitz]

I have a z axis micro switch to set the base height for the extruder. Keeps things from slamming into things and allows the software to home properly. Is that the same thing you are talking about?

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[The_Digital_Dentist]

If the z-axis minimum endstop is set above the printbed to keep the extruder from slamming into the printbed, how do you print a first layer on the printbed?

If the z-axis minimum endstop is set below the printbed to allow the extruder to print the first layer very thin, how do you keep the extruder from slamming into the printbed?

Thanks!

[the_digital_dentist]

After some digging around the web I find that people are making special precision adjustments for their Z-axis minimum endstops, so I guess it has to be set to just barely prevent the extruder from crashing into the printbed.  I have added a screw adjustment for the z-axis minimum stop and set up the other five endstops as well.

The photo shows the X and Z axis minimum endstops.  Immediately above the Z-axis stop is the adjustment screw that allows the Z-axis minimum stop to be set precisely.  The Z axis adjustment screw goes through the X-axis stop support piece.   Right now the lego pieces are just glued to the machine frame but they'll eventually get screwed down or possibly replaced with metal parts.

I still have to wire the printbed heater/thermister/thermal cut-out and then modify the config files for the firmware to get everything working right, assuming I can get the firmware to compile again.

[the_digital_dentist]

New question: does anyone know how fast the printbed heater is switched on and off with Marlin firmware?  I believe it uses PID control.

I am using a 24V heater and power supply so I am thinking of using an SSR to switch AC into the power supply instead of burning up the traces on the controller board.

If it switches power on and off several times per second I'll look for a different way to do it, but if it switches only every few seconds then I think I will use the SSR.

Thanks again!

[Tom Gralewicz]

I don't suppose the power supply has any power or feedback control inputs?

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Tom Gralewicz
Miller Electronics Recycling
(414) 380-1716
www.deadcomputers.com

[the_digital_dentist]

Nope.  It's just a cheap switching power supply.  I've noticed the inrush current makes the lights flicker every time it gets turned on, so I think I'll make some sort of MOSFET relay to switch the DC through the printbed heater.

[the_digital_dentist]

Something like this ought to do the job:

https://www.sparkfun.com/products/10213

I have used one driven directly by a PIC uC to switch 15A through a 5W LED for a microscope illuminator.  Low on-resistance means I probably won't even have to heatsink it, but screwing it to my aluminum machine frame would definitely mitigate all thermal risk.

[Tom Gralewicz]

I have some DC solid state relays (just like the AC ones we are using on the lighting system)  that should do the trick.

I'm not sure how happy the power supply will be going from no load to full load, you might want to put a resistor across it to draw an amp or so even when the heater is off.

Tom

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[the_digital_dentist]

Actually, further study of the board shows that the power for the printbed heater is switched by a MOSFET that is good for 50A and that power input is separate from the 12V that powers the board and the steppers, so I can apply 24V directly to the board and let the on-board MOSFET do the switching.  There is an 11A self-resetting thermal-fuse-thingy on the board, too.

I will put a resistor on the supply to pull some current even when the printbed heater is off.