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2018 July 9

Monoprice delta 3d printer glass clips

Filed under: Uncategorized — gasstationwithoutpumps @ 14:50
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In addition to the legs I added to my new Monoprice Delta Mini 3d printer, I’ve also added a 120mm borosilicate glass plate to the heated bed.  The glass plate needs to be immobilized, so I designed some clips loosely based on designs I saw on Thingiverse.

This is version 7 of the clip. The clip here is shown upside-down, which is how it is built on the printer. The rim holds the plate down, and the two cantilevered sections fit under the aluminum baseplate.  The image is exported directly from OpenSCAD.

Here is another view, with the clip the right way up, viewed from the pin side, rather than the glass side. The existing pin for holding down the aluminum plate rotates into place in the semicircular hole left for it.

Several of the dimensions of the clip are critical, and getting them even half a millimeter off made the clip non-functional. Because I’m not very good at measuring (nor very observant about all the dimensions I needed to measure, it took me seven tries to get a design that would work.

To make the design easier, I recalibrated my printer based on the Make magazine dimensional accuracy test object. The test object was printing a little smaller than it should, so I scaled the steps/mm calibration from the 113 steps/mm to 116 steps/mm. To make the change, I gave the “m503;” command to get the current settings, then “m92 x116 y116 z116;” to rescale the steps, and finally “m500;” to save the settings in non-volatile storage. I power-cycled the 3d printer and confirmed that the settings had been saved with “m503;”.

I designed the clips with OpenSCAD and sliced them with Cura, using 0.1mm/layer, 3 layers for the walls, and 4 layers top and bottom (I think—Cura does not save any of this metadata in the gcode file except the layer height).  The V7 clip supposedly uses 70cm of filament, which at 1.24 g/cm3 or 3g/m for 1.75mm diameter PLA is about 2.1g of PLA per clip. At 2¢/g, the clips cost under 20¢ for materials, even with the 6 failed designs, but I spent a lot of time on the design and a fair amount on the printing (each print run takes about 15–20 minutes, so there were about 3–4 hours of printing involved).

Some parts of the design were simple, like getting the 120mm diameter circular arc and the 2mm wide lip to hold down the glass plate. The harder parts were getting the pin diameter and distance from the glass plate right, leaving room on the bottom for the alignment pin, and making the clip under the plate thin enough not to interfere with the button movement for the automatic bed leveling.

This is the top view of all 7 clip designs showing the evolution from a pair of circular arcs to a more solid object.

The bottom views are more informative. V1 did not extend below the aluminum plate at all, and had too small a radius for the pin. V2 extended below the plate, but did not wrap around it and the cutout for the plate did not extend back far enough. V3 extended the cutout for late back far enough, but got the angle wrong. V4 tried wrapping around the edge of the plate, but got the depth of the slot wrong—I also had to cut some of the print with diagonal cutters to make room for the alignment pin. V5 fixed the angle of the slot and left room for the pin, but did not correct for the change in angle by increasing the slot depth. V6 got the slot depth and other XY dimensions right and reinforced the side of the slot by adding triangular panels to the clip, but was too thick below the plate, so that the buttons wouldn’t press. V7 extended the plates to reinforce the slot for the whole length of the slot and had only 1mm below the plate.

Here are the clips in place, holding down the glass plate after reprinting Make‘s dimensional test.

The prints pop off the glass plate very easily—perhaps too easily.  I used the last of our spray fixative on the plate before printing, as an attempt with nothing on the plate ended up with no adhesion—I’ll probably go buy some hair spray to use as fixative for future prints.

The glass plate does result in a shiny bottom layer for the prints—a much smoother bottom layer than the slightly textured surface of the provided bed.

Here are the dimensions of the dimensional test printed on the glass bed:

nominal 25mm 20mm 15mm 10mm
center X 24.95mm 19.95mm 15mm 10mm
center Y 24.85mm 19.9mm 15mm 10mm
outer X 25.05mm 19.95mm 14.9mm 10mm
outer Y 25.15mm 20mm 15.25mm 10mm

Center Z height: 20.4mm  Outer Z height: 20.5mm

The X measurements are using the edges with a notch, and the Y measurements are from the perpendicular pair of edges. The center measurements are from the copy closer to the center of the bed. All XY dimensional inaccuracy is less than 1%, so the step size is adjusted about as good as I can make it (maybe I could change the X to 116.1 steps/mm and Y to 115.9 steps/mm) , but the Z dimension is 2–2.5% too large, so I should set the Z steps/mm to 113.4.

Here is the OpenSCAD code for the clip. It is not the most elegant way to describe the clip (because of the evolution of the code over the 7 versions), but it works. (Update 2018 July 15: I’ve made the clips available on Thingiverse: https://www.thingiverse.com/thing:3001881)

// Clip for holding down 120mm diam glass plate
// on Monoprice Delta printer
// Kevin Karplus
// 2018 July 6 

// xy plane is top of the clip.

thickness= 3;   // thickness of glass in mm
radius = 60;   // radius of glass in mm

overhang_thickness = 2;   // thickness of overhanging lip that holds glass
overhang_width = 2;     // how fare lip extends over the glass.
glass_angle = 42;   // arc in degrees of lip for glass

above_plate= thickness+overhang_thickness;

plate = 2;      // Existing plate is 2mm thick
below_plate=1;  // Max thickness below plate
                // (2mm space, but need 1mm travel for bed leveling)

height = above_plate + plate + below_plate;

pin_radius = 5.5;   // radius of circle cut away for pin
big_pin_radius = pin_radius+4; // size of ring around pin

pin_center = radius + 11;

plate_angle = 100;   // angle of plate edges in degrees.

intersection()
{
    difference()
    {
        union()
        {

            // Part that holds down glass
            intersection()
            {
                difference()
                {   // outer-edge
                    cylinder(r=radius+4, h=height, $fn=180);  // outer edge
                    translate([0,0,overhang_thickness]) cylinder(r=radius, h=height, $fn=180); //glass edge
                    cylinder(r=radius-2, h=3*height, center=true, $fn=180);// inner edge
                } // end difference

                // wedge to reduce arg
                linear_extrude(height=3*height, center=true)
                {   polygon(points=[ [0,0],
                        [100,100*tan(glass_angle/2)],
                        [100,-100*tan(glass_angle/2)]]);
                }

            } // end intersection

            // part that connects with existing clip pin
            translate([pin_center,0,0])
                difference()
                {   cylinder(r=big_pin_radius, h=height, $fs=0.2);  //outer edge
                    cylinder(r=pin_radius, h=3*height, center=true, $fs=0.2); // edge contacting pin

                    // cut to half circle
                    translate([3*pin_radius,0,0])
                        cube([6*pin_radius,6*pin_radius,3*height], center=true); 

                } // end difference

            // triangular fill to reinforce join between pin and glass
            translate([pin_center,0,0])
            {   outside_x= pin_center - cos(glass_angle/2)*(radius+4);
                linear_extrude(height=height)
                {    polygon(points=[ [-big_pin_radius, 0], [0, big_pin_radius],
                        [-outside_x, big_pin_radius+outside_x*tan(plate_angle/2)]]);
                };
                linear_extrude(height=height)
                {    polygon(points=[ [-big_pin_radius, 0], [0, -big_pin_radius],
                        [-outside_x, -big_pin_radius-outside_x*tan(plate_angle/2)]]);
                };
            } // end translate
        } // end union

        // cut away space for baseplate
        triangle_x = 40;  // large enough to cut everything
        triangle_y = triangle_x * tan(plate_angle/2);

        translate([pin_center+3,0,above_plate])
            {    linear_extrude(height=plate)
                    polygon(points=[[0,0],
                                [-triangle_x,-triangle_y],
                                [-triangle_x,triangle_y]]);
            }
    }

    // keep only those parts that don't interfere with the alignment pin
    // trimming the cantilevered below-plate part to a narrow rim
    union()
    {
        // everything above the plate is fine
        translate([0,0,above_plate/2]) cube([500,500,above_plate+0.001], center=true);
        // below the plate, make circular keep-regions
        keep_radius=22;
        translate([pin_center+8, keep_radius, above_plate])
            cylinder(r=keep_radius, h=height, $fn=180);
        translate([pin_center+8, -keep_radius, above_plate])
            cylinder(r=keep_radius, h=height, $fn=180);
    }
}
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2018 July 1

Adding feet to the Monoprice Delta Mini 3D printer

Filed under: Uncategorized — gasstationwithoutpumps @ 09:22
Tags: , , ,

The first functional prints I’ve created from the new Monoprice Delta Mini 3D printer that I bought earlier this week (see Three boxes this morning!) are small plates to attach rubber feet to the bottom of the printer, to raise it off the table and improve airflow.

I looked at what Thingiverse had for accessories for the printer, and several people had designs for feet, but I didn’t really like any of the designs, so I decided to design my own.

Because 3D printing is so slow, I decided not to try to print tall feet, but to use some rubber feet that I bought back in 2012 from Parts Express. (If you follow that link, you’d find that Parts Express no longer has these feet, but has slightly smaller black rubber feet for about 66¢ each.)  This meant that I only had to design and print adapter plates that could be screwed on over the existing  feet, with a central hole for attaching the rubber foot.

The plate attaches with M3 screws. The design calls for replacing the existing M3×8 Phillips head screws with M3×12 socket-head screws, and I decided to recess the sockets into the plate, both for looks and to see whether the printer could bridge over the recess.  (If the printer had not been able to bridge, then I would have made that surface of the adapter plate be flat, and used M3×16 screws instead.)

I spent a fair amount of time measuring the irregular hexagonal end of the uprights of the printer, so that I could match it and the get the screw holes in the right places.  The measurements were not perfectly consistent, so I had to decide which measurements to take as “correct” and which to compute based on the chosen parameters.  I decided that all the angles should be treated as canonical (multiples of 30°), since that seemed like a likely choice for the designers of the printers.  I decided that the two parallel edges and the distance between them would be my other defining parameters, since that allowed easy definition in Cartesian coordinates, which also seemed like a likely choice for the original designers.

I did the design in OpenSCAD, which does not have anywhere near the feature richness of a professional tool like SolidWorks, but which is (for a programmer) much easier to learn to use, and easier to get precise results with.

Unfortunately, OpenSCAD does not produce the pretty renderings that SolidWorks does, so I can’t show you pretty design pictures. I can, however, share the source code for the design, which you can modify to produce different designs, or just compile and print. The code is at the end of this post.

View of the adapter plate from the outside, rendered by Finder’s “Quick Look” on a Mac.

View of the adapter plate from the printer side, showing the countersunk hole in the center, rendered by Finder’s “Quick Look” on the Mac.

My first print was to test whether I had the holes in the right places, and whether the printer was printing things at the specified size.  (I was pretty sure it was not, as I had printed the Make magazine test piece for dimensional accuracy, and had seen that the printer was printing about 3% small.)  The test print was just a 3mm slab, printed with 10% infill and 0.2mm layers for speed.  OpenSCAD made it easy to create this slab, by intersecting the design with a rectangular prism of the appropriate thickness and location.

The two test pieces I printed. Test piece 1 is a little small and has a few holes misplaced.  On test piece 2, you can see a little “stringing” where the unsupported bridging filaments drooped, but the overall integrity of the bridge seemed adequate.

The slab showed me that the printer was indeed printing a little small, and that I had misplaced the hole for the existing printer foot by 1mm and the screw holes further from the outside edge by about 0.5mm.  I moved the holes, figured out how to do scaling in Cura to scale the part by 102.9% when slicing, and did another test print—a 4mm slab that included the end of the recesses for the socket-head screws, so that I could test the overhang capability.  I printed this one with 20% infill.

The final design is a slab 7mm thick, which 5 screw holes: 4 for the M3 socket-head screws, and one for a 10-24 flat-head screw for attaching the rubber foot. The 10-24 hole is countersunk and is at the base of a cylindrical recess deep enough that the head of the screw has clearance from the foot that is already on the base of the printer. Because the existing feet are just stuck on with double-stick tape, it would probably have been easier to remove them rather than make clearance for them.

Outside view of a disassembled leg.

Printer-side view of a disassembled leg.

Because the 10-24 screw will be hard to retighten once the leg is on the printer, I squirted a little low-temperature hot-melt glue onto the nut after tightening it, so that it would not work loose from vibration.

The three printed legs, with the rubber feet attached. If you look closely, you can see a little of the clear hot-melt glue in the right-hand foot, to keep the nut from loosening.

Bottom view of the printer with the legs attached.

Side view of the printer with the legs attached.

Closeup of the printer with the legs attached, showing the greater space now available for airflow.

I printed the legs one at a time, so that in the event of printer failure, I would only have to redo one leg, rather than all three.

To print the legs, you need to open the .scad file with OpenSCAD, render it, and output a .stl file. Then use Cura to slide the model. I chose a layer height of 0.1mm, a wall count of 4 layers (for strength and stiffness), 4 top layers and 3 bottom layers, a concentric top and bottom pattern, 20% infill (a compromise between strength and speed of printing), and no special build-plate adhesion. The top layers are excessive, as that face is buried against the bottom of the printer and does not need to be pretty. The first leg I printed had some sort of printer/communication failure and froze after only one top layer had printed, and it is still a perfectly usable leg.

(Update 2018 July 15: I’ve made the clips available on Thingiverse: https://www.thingiverse.com/thing:3001872)

Here is the code for the adapter plate for extending the legs of the printer:

// Kevin Karplus
// 2018 June 30
// Leg for extending height of Monoprice Delta Printer, to improve airflow under printer.


// create a screwhole for metric screw of given length,
// with head extending in -z direction and screw in +z direction
// Loose=0.10 for loose fit, 0.05 for tight fit, about -0.15 for threaded hole
module screw_hole(metric_size=3, length=10, loose=0.10)
{   union()
    {   cylinder(d=metric_size*(1+loose), h=length*1.1, $fs=0.3);  // body of screw
    translate([0,0,-metric_size]) 
        cylinder(d=metric_size*2.2, h=metric_size*1.1, $fs=0.3);  // counterbore for head of screw
    }
}

// make countersunk hole for 10-24 flathead screw
// surface on xy plane at (0,0), screw extends in +z direction.
// length is the length of the hole for the threads (past the base of the head)
// diam is the diameter of the hole for the screw threads
// depth is the depth of the countersink
module countersunk_10_24(length=35, diam=5, depth=3)
{
    // top_diam is the diameter at the surface (the xy plane)
    // assumes that cone has an apeture of 90°
    top_diam = diam + 2*depth;
    union()
    {    cylinder (d=diam, h=length+depth, $fs=0.3);   // hole for screw
        // countersink (adding 0.1 overshoot to avoid coincident faces)
        translate([0,0,-0.1]) 
cylinder (d1=top_diam+0.2, d2=diam, h=depth+0.1, $fs=0.3); 
    }
}


// polygon that matches the foot of the Monoprice Delta printer,
// with outside edge on the x-axis, with center of edge at origin.
// "base" is the length of the outside edge in mm.
// Long edge of plate parallel to x-axis at y=height, of length long.
//
// The default parameter values were measured from the printer.  
// The angles are taken as canonical, and the parameters were chosen as
// those that could be easily defined on a Catersian coordinate system,
// with the other side lengths calculated from the geometry.
module foot_poly(base=50,long=65, height=32)
{   
    // how wide trapezoid would be without trimmed-off corner
    extra_long= base+ 2* height/tan(60);
    echo("extra_long=",extra_long);
    
    short = (extra_long-long)/2 * cos(30);  // shortest edge
    
    // Width at widest point (right-angle vertex)
    width =long + 2*short*cos(30);
    
    // remaining edge
    side = (width-base)/(2*cos(60));
    
    // How high up is the widest point
    height_at_width= side*sin(60);
    
    echo("edges=", base, side,short, long);
    echo("width=",width, "height=", height);
    echo("height_at_width=",height_at_width);
    polygon(points=[ [base/2,0], 
            [width/2, height_at_width],
            [long/2, height],
            [-long/2, height],
            [-width/2, height_at_width],
            [-base/2,0]]);
}



module foot(screw_length=12)
{
    // For the 4 M3 screws that hold foot plate to delta printer
    hole_length=screw_length-8;  // how much of screw thread is left?
    counterbore= 3;  // depth of counterbore
    thickness = hole_length+counterbore;
    
    old_foot = 3.7;  // height of bore to avoid old foot
    
    difference()
    {
        color ("red")  linear_extrude(height=thickness) foot_poly();

        color("green") translate([22.5,5,counterbore])  
            screw_hole(metric_size=3,length=hole_length);
        color("green") translate([-22.5,5,counterbore])
            screw_hole(metric_size=3,length=hole_length);
        
        color("blue") translate([32.5,25,counterbore])
            screw_hole(metric_size=3,length=hole_length);
        color("blue") translate([-32.5,25,counterbore]) 
            screw_hole(metric_size=3,length=hole_length);
 
        // make hole for old foot
        center=18;
        color("brown") translate([0,center,thickness-old_foot]) 
            cylinder(d=14,h=old_foot +0.1, $fs=0.6);
        
        // make countersunk hole for 10-24 screw for new foot
        color("magenta") translate([0,center,thickness-old_foot])
            rotate([180,0,0]) countersunk_10_24();
        
    }
}


{  foot();  
}

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