Clearance measurements - general advice?

[DrUsual]

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

[FrankV]

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

[mikenmel08]

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

[DrUsual]

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.

[DrUsual]

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

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.

[FrankV]

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.

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).

[DrUsual]

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).

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!

[soofle616]

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!

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.

[DrUsual]

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.

[Mjolinor]

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.