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Gear dimensions, please?

Posted by jtoombs 
Gear dimensions, please?
February 19, 2014 06:45PM
Hi guys. I'm trying to design my own type of extruder with the parts I currently have. I am trying to use some bearings and bolts and springs that I already have in an extruder that is loosely based on the extruders that are out there already. Now for the gears. I mostly see the same size gears on a lot of the extruders like Greg's Accessible design. They are sometimes herringbone, helical, or just spur. However, I can't seem to find the dimensions of them. I would like to know the distance from the center of the small to the center of the big gear. And also the hole diameters. Thanks.
Re: Gear dimensions, please?
February 20, 2014 04:11PM
Hi jtoombs,

Here's the TL;DR: gear separation = 39.7055mm, large gear bore diameter = 8mm, small gear bore diameter = 5.25mm. (These values are as defined in the OpenSCAD document. They may print out a bit different - for example the small gear mates with a NEMA 17 motor which usually has a 5mm shaft diameter.)

As you point out, there are about a million different variants of a "Greg's" extruder. For the purposes of this discussion, let's use this one: Greg's Hinged Accessible Extruder. Download all the "thing files" in a zip, and unzip it somewhere. Also go and install OpenSCAD.

If you open up "gregs-wade-v3.scad" using OpenSCAD (or a text editor if you don't feel like installing OpenSCAD) you can pretty quickly find a variable called "gear_separation" on line 68 which is defined as
Line 68: gear_separation=7.4444+32.0111+0.25;
that would be 39.7055 millimeters, for those of you who don't have a calculator handy.

Just for fun we can try and compile the scad file to see what it looks like (F5 or Design->Compile) and we see that it only partially compiles, complaining about metric nut diameters and not being able to find "wade-large.stl" and "wade-small.stl". OK, so let's make a wild guess and edit "gregs-wade-v3.scad" so it tries to import the stl files that it was downloaded with. Make these edits:
Line 289: import_stl("large_gear.stl");
Line 333: import_stl("small_gear.stl");
OK, cool. Now we can see gears when we compile, and they actually move when we change gear_separation! If we uncomment line 30 we can see where the idler goes:
Line 29: //Place for assembly.
Line 30: wadeidler();

Now, what about all those warnings about ignored variables? It turns out you need four additional files: configuration.scad, metric.scad, functions.scad, and inc/parametric_involute_gear_v5.0.scad. You can get them on Greg Frost's Github page (among other places). Try it again... OK cool. Now we have a few more warnings to take care of. Let's fix the "layer_height" bug, which was described in a comment by jkeegan on the Thingiverse page (see link above). Insert this new line:
Line 62: layer_height=0.4;

Now we just replace the deprecated "import_stl()" with "import()" and we can compile with no warnings! Hurray!

OK, now where do the gears get their hole diameters? Open up "gregs-wadebits.scad" in OpenSCAD or a text editor, and you will see that this module creates the large and small gears.

For the large gear:
gear (number_of_teeth=43,
	gear_thickness =gear_thickness,
	rim_thickness = 7,
	rim_width = 3,
	hub_thickness = nut_trap_thickness+nut_trap_depth,
	hub_diameter = 22,
	bore_diameter = 8,

And for the small gear:
gear (number_of_teeth=10,
	gear_thickness = 9,
	rim_thickness = 9,
	hub_thickness = 18,
	hub_diameter = 18,
	bore_diameter = 5.25,

The attached zip file contains the collected bits of openscad code with minor edits, as described above.

Edited 1 time(s). Last edit at 02/20/2014 04:16PM by MattMoses.
open | download - Gregs_Hinged_Accessible_Extruder_minor_edits.zip (174.3 KB)
Re: Gear dimensions, please?
February 20, 2014 08:02PM
Wow! That was a lot more than I expected. That's just what I was looking for though. Thanks a lot for the extensive and in depth response MattMoses.
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