Clearance measurements - general advice?

I think I made the disclaimer on another thread that I have zero mechanical engineering background...

I'm trying print a box on my Dreamer and want to have a pressure-style closure. I.e., the lid fits down over the box and is just enough bigger that it'll close, but will also stay on -- needs some pressure to open. I've been trying this with both round and square boxes.

Working on a round one now. The outside diameter of the bottom is 74mm. The sides of the lid are a 3mm wall, so, the inside diameter of the lid needs to be 77mm+, right? I've printed lids at 77.25mm, 77.5mm, 77.6...all the way up to 78mm, and the lid still doesn't fit over the box.

Am I missing something fundamental about the precision of the printer?

TIA

If wall sides are 3mm then 74 + 3 + 3=80mm (Assuming you mean the outside dia of the lid)

Did you measure the outside diameter yourself of just go on what the machine told you?

No, the inside diameter of the lid, because the lid fits over the edges of the box bottom. So, ID of the lid should be larger than the OD of the bottom, right? Tinkercad only measures (easily) the OD of a circle, though, so I did 74 OD of the bottom + 3 mm lid walls = 77 inside diameter of the lid, plus clearance. I was thinking that that clearance wouldn't need to be much, though, and even making the OD of the lid 78mm is leaving the ID too small to fit over the 74mm box bottom.

Quote:

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Originally Posted by mikenmel08

Did you measure the outside diameter yourself of just go on what the machine told you?

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I did break out the ruler and measure the box bottom myself -- it could be a hair over 74mm, but I don't think it's even 77.25mm.

Quote:

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Originally Posted by DrUsual

Tinkercad only measures (easily) the OD of a circle, though, so I did 74 OD of the bottom + 3 mm lid walls = 77 inside diameter of the lid, plus clearance.

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Ok, so this is the confusion. What do you mean by 3mm walls? If you mean the wall is 1.5mm then it would be 3mm total, but if the wall thickness is 3mm then the outside diameter must be the inside (74mm) plus the 2 x 3mm = 80mm outside dia for the lid (plus a bit of clearance of course).

Quote:

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Originally Posted by FrankV

Ok, so this is the confusion. What do you mean by 3mm walls? If you mean the wall is 1.5mm then it would be 3mm total, but if the wall thickness is 3mm then the outside diameter must be the inside (74mm) plus the 2 x 3mm = 80mm outside dia for the lid (plus a bit of clearance of course).

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GAH.

I had a funny feeling that the answer would be that I'm an idiot...

You're correct, I only accounted for 3mm of wall, not 6mm.

At least I know my printer's working correctly. :)

Thanks!

Quote:

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Originally Posted by DrUsual

GAH.

I had a funny feeling that the answer would be that I'm an idiot...

You're correct, I only accounted for 3mm of wall, not 6mm.

At least I know my printer's working correctly. :)

Thanks!

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Don't feel bad. I'm a mechanical engineer and I do stupid stuff like that from time to time. Actually, things like that are why I call my friend chris whenever I have a carpentry project to deal with because for some reason I can never keep it in my head that 2x4's are not actually 2x4" so I always end up cutting things wrong.

Another good lesson here...we're doing these in Tinkercad, and we started by creating the bottom of the box, then duplicating it, then scaling the duplicate/lid. My gut feel is that scaling the OD of the lid should scale proportionately the ID, so that it maintains the 2mm lip. Re-thinking, though, I'm guessing that scaling is increasing the thickness of the lip as well as the diameter of the hole and the exterior? Probably something very fundamental to an engineer, not quite as obvious to a liberal arts guy. :)

Most drawing packages are a series of adding and subtracting shapes so if I need a lid to fit a box then I use the same shape in the drawing package and they fit perfectly.

try it in openscad - made for boxes.

Also makes it easy to slope the inside of the lid, so it starts out easy to fit then get's tight as you push down.

Quote:

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Originally Posted by curious aardvark

try it in openscad - made for boxes.

Also makes it easy to slope the inside of the lid, so it starts out easy to fit then get's tight as you push down.

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That's exactly what I'm looking for, I'll try Openscad tonight. Thanks!

well I'll look at your specs again and knock a script up for you.
brb....

right openscad program to generate round boxes with lids attached.

The script contains both lid and box generator. Just remove the '//' from the bit you want.
Generate, f6 to render, export as stl and then. switch to the lid section (add // back in box and delete from lid sections)
and repeat.

Think I've covered all the measurements you might need.
Attachment 3593

Also here's the script for copying and pasting:

Quote:

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//Parametric Round box with tight fitting lid by Curious Aardvark

$fn=100;


cd=74; //box diameter
tw=3; //thickness of box wall and lid rim
tb=3; //thickness of box base
hb=50; //height of box



hr=10; //height of lid
blt=2; //thickness of lid top
si=1.5; //slope index. difference between the diameter of the lid rim from the bottom to the rim. Adjust slightly untill lid fits tightly enough.


//generates box - remove '//' to make box


//difference() { //remove smaller cylinder to make box
//cylinder(d=cd,h=hb);
//translate([0,0,tb]) cylinder (d=cd-tw,h=hb);


//} //end of remove smaller cylinder to make box




//generates pressure fitting lid - remove or add '//' to make lid


//difference(){ //removes sloping cylinder to make tight fit
//cylinder(d=cd+(2*tw),h=hr);
//translate([0,0,blt]) cylinder(d1=cd-si,d2=cd,h=hr);


//}
//end of lid generation

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Just copy and paste into openscad.


So to make the box it would look like this:

Quote:

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//Parametric Round box with tight fitting lid by Curious Aardvark

$fn=100;


cd=74; //box diameter
tw=3; //thickness of box wall and lid rim
tb=3; //thickness of box base
hb=50; //height of box



hr=10; //height of lid
blt=2; //thickness of lid top
si=1.5; //slope index. difference between the diameter of the lid rim from the bottom to the rim. Adjust slightly untill lid fits tightly enough.


//generates box - remove '//' to make box


difference() { //remove smaller cylinder to make box
cylinder(d=cd,h=hb);
translate([0,0,tb]) cylinder (d=cd-tw,h=hb);


} //end of remove smaller cylinder to make box




//generates pressure fitting lid - remove or add '//' to make lid


//difference(){ //removes sloping cylinder to make tight fit
//cylinder(d=cd+(2*tw),h=hr);
//translate([0,0,blt]) cylinder(d1=cd-si,d2=cd,h=hr);


//}
//end of lid generation

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And to make corresponding lid:

Quote:

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//Parametric Round box with tight fitting lid by Curious Aardvark

$fn=100;


cd=74; //box diameter
tw=3; //thickness of box wall and lid rim
tb=3; //thickness of box base
hb=50; //height of box



hr=10; //height of lid
blt=2; //thickness of lid top
si=1.5; //slope index. difference between the diameter of the lid rim from the bottom to the rim. Adjust slightly untill lid fits tightly enough.


//generates box - remove '//' to make box


//difference() { //remove smaller cylinder to make box
//cylinder(d=cd,h=hb);
//translate([0,0,tb]) cylinder (d=cd-tw,h=hb);


//} //end of remove smaller cylinder to make box




//generates pressure fitting lid - remove or add '//' to make lid


difference(){ //removes sloping cylinder to make tight fit
cylinder(d=cd+(2*tw),h=hr);
translate([0,0,blt]) cylinder(d1=cd-si,d2=cd,h=hr);


}
//end of lid generation

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Just change variables to change size of box and lid.

Oh yeah - for abs you need to make everything 1% larger in the slicing software. For pla - no adjustment is needed.

Any issues or confusion - let me know :-)

Quote:

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Originally Posted by curious aardvark

well I'll look at your specs again and knock a script up for you.
brb....

right openscad program to generate round boxes with lids attached.

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This is incredibly helpful; I always find I learn stuff more quickly with a good example to work from. Thanks!

it's a pretty simple script.

The only thing that might need adding is a variable to seperately adjust the thickness of the lid's edge. Also maybe enlarge the lid's minimum size to make it slightly larger than the box diameter.
It does depend on what you're actually printing with. Some filaments shrink, some don't.

I find that pla is actually less brittle and more durable than abs. Which is weird. I think pla has improved massively in the last couple of years and most of the negative things just aren't there any more.
The sweeper handles I make will crack if slightly too large and made of abs. But the pla ones just need more force to get them to click in - and just don't break.

If you need a polygonal box I've written a polygon generating script that's pretty useful.
Haven't got into modules yet - but when i do that'll be the first thing I turn into a module.

Quote:

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//curious aardvarks polygon generator

$fn=100;


//parametric polygon


ns=5; //number of sides
hs=65; //height shape
ds=50; // overall width of shape - measured across the diagonal




hull() {
for (r=[0:ns]){


rotate([0,0,(360/ns)*r]) {


translate([0,0,0]) cube([0.001,0.001,hs]);
translate([0,0,hs]) cube([ds/2,0.001,0.001]);
translate([0,0,0]) cube([ds/2,0.001,0.001]);
}}}

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The hull command makes what look like tricky shapes real easy.
Basically all it does is draw a series of line a 1000th of a mm thick and then wraps a hull round them.
I used it to make a feather plucker that fits on a drill :-)

What i really like about openscad is that the more I use it, the more things I find it will do.
Great learning curve.

Very cool...have you tackled doing this with irregular shapes at all? Not that I need to jump into something that complicated yet, I'm just eager to learn as much as possible. In fact, I'm about to sit down with OpenSCAD and do that this afternoon...