My first heated bed worked OK but it was slow to warm up and hard to remove objects.
The second one was only ever intended for experimenting with vacuums and magnets but I ended up printing most of my Mendel on it. It worked well but limits the build area.
I have now replaced it with a full size version, using the lessons learned from the first two.
The first bed was the same size as HydraRaptor's table (200 × 200mm) but the build area is only about 150 × 150mm. The warm up time and power are both proportional to area, so I made this one just big enough, i.e. 150×150mm. Removing the 25mm border nearly halves the area!
The other innovations were to make it easier to build. I replaced nine AL clad resistors with four TO220 resistors. These are rated at 50W and 155°C, so are actually totally within spec when I run the table from 48V giving 188W. Instead of having to tap two M2 holes for each resistor these only need a single M3 hole. That is much easier to tap as the tap is a lot sturdier.
They are also lower profile of course. I just noticed that 47Ω ones are less than half the price, so I should have used four of those in parallel instead.
The thermocouple is mounted with a clamp made from PTFE.
Since this bed has a steel plate on top none of the holes need to be blind. That makes drilling and tapping easier as well.
On my previous magnetic bed I placed the magnets in blind holes that were almost all the way through. That required a milling machine to get flat bottomed holes. On this version I just drilled almost all the way though, leaving a lip to retain the magnet.
This is the top side. The magnet in the middle was done with an alternative technique. I drilled a through hole and then jammed the magnet in with a few strands of copper wire. That gets it flush with the surface, giving maximum magnetic force, but it pulled through on first use. I will have to glue it with high temperature epoxy I think.
After a suggestion by Enrique that wool was a good high temperature insulator my friend Steve gave me some carpet underlay made from wool. I used it to insulate the underside, thanks guys.
For the steel plate on the top I used the cover of an old CD ROM. It is only 145mm wide unfortunately. I think it is mild steel with nickel plating. Not as good as the stainless steel springy piece I got from inside a toaster.
So here is the finished article with the biggest bit of Mendel built on it. It was quite hard to remove. I had to remove the steel plate and bend it a little as intended. I had found that things I built recently on the small table could be removed without lifting the plate. I think the Kapton gets less grip as it ages. I tried cleaning it with alcohol and sanding it with very fine emery paper, but that seemed to make it worse if anything. It seems that shiny Kapton gives more grip than matt.