Add option for magnets on bottom of baseplate

[kbob]

Hi. Here comes a new feature out of the blue.

A great way to use a thin style baseplate and use the magnets in Gridfinity bins is to line the drawer with a thin steel sheet, then have magnets in both the bins and the baseplate to stick to the sheet. There is just enough room in the corners of a thin baseplate for a 6x2 magnet.

You can see bottom magnets in use in this Mastodon thread.
https://chaos.social/@kbob/113634630860874471

So this patch adds a bottom magnet option.

Some design choices I made (changeable, of course).

• There's a configuration boolean in the Magnet Hole section, "enable bottom magnet".
• Bottom magnets use the same settings for crush ribs and chamfers as top magnets.
• Bottom magnet holes are always made printable.
• Bottom magnets always have a 2.5mm hole up to the top of the baseplate for pushing the magnet out.
• If there's room, bottom magnets are added in the padding area as well as between squares. Enough room is defined as one millimeter of padding past the magnet edge. (hardcoded)

Bottom magnets are most useful with thin baseplates. Chamfers don't work with thin plates; the walls are too thin at the corners. So it might be better to always disable bottom magnet chamfers or disable them for thin plates.

[Rahix]

Very nice, I like this feature a lot!

Chamfers don't work with thin plates; the walls are too thin at the corners. So it might be better to always disable bottom magnet chamfers or disable them for thin plates.

Yeah, I think they should be disabled for thin plates automatically then.

I also noticed that 1x1 plates get cut-off bottom magnet holes in each corner right now.

And nit-pick: Your ø2.5mm hole for magnet removal kind of cuts into the supportless hole bridges because the bridges are generated for a ø2mm hole here (the 1mm radius):

gridfinity-rebuilt-openscad/src/core/gridfinity-rebuilt-holes.scad

Line 304 in 0db70cf

screw_hole ? screw_radius : 1, magnet_radius, magnet_depth, supportless_additional_layers);

Not sure how that could be fixed without adding yet another parameter to block_base_hole()... On the other hand, it prints mostly fine the way it is right now.

[kbob]

Thanks, I did not notice that. There's nothing magic about the 2.5mm diameter. I will change it to 2mm and update the PR.

I don't have a good answer for the 1x1 plates yet.

[Rahix]

I don't have a good answer for the 1x1 plates yet.

I think the best option is to either drop the magnet holes for 1x1 baseplates because there is just no space for them or to throw an error in this situation.

I do not think anyone will need 1x1 baseplates with bottom magnets.

[EmperorArthur]

Remind me to look at this in a week or two. My printer is down, but with #305 merged, baseplates are now on my todo list to overhaul.

@Rahix 1x1 baseplates are useful for test prints and some "This tool goes here" things. Though, I don't know enough about your PR to make a judgement call one way or another.

[Rahix]

ping @EmperorArthur :) (also for #301)

1x1 baseplates are useful for test prints and some "This tool goes here" things. Though, I don't know enough about your PR to make a judgement call one way or another.

Fair! I guess an option here could be to special case the 1x1 baseplate to add 4 magnets "external" to the basic grid square. Although I am really not sure if people who need a 1x1 baseplate are interested in mounting it using magnets... @kbob, thoughts?