@thegasman2000 - you might be interested in this, that's my DIY heatbed for home PCB etching http://www.thingiverse.com/thing:1931575

kr15_uk,
Hi, I am seriously considering a heat bed mod, using a pcb heater like you have shown.  I am considering one from ebay, that is a 6x6 already made (never did have much luck etching my own).  Two questions,
1- how to you measure / control the heat to the bed.  Where / how is the thermistor or ??? connected and what does that attach to?
2- do you have any design files or gerber you used to create the print you would be willing to share?

I would love to see mode details on your mod. 

Thanks

Although I went with a 3D printed spacer, I think you could achieve the same just using simple washers from the hardware store.  If I remember correctly, there are some alignment holes/tabs in addition to the two screws that hold the sensor down.  But, just for a quick-and-dirty test, you could try using washers to offset the sensor housing.  Or, maybe even easier, stack up some pieces of paper/thin cardboard to get the sensor offset you need?

Thanks

I chose to print a 0.1 inch (2.5mm) spacer for the sensor assembly.  Sandwich it between the floor of the enclosure and the height sensor.  After installation, adjust the z-offset.

How does the optical sensor work? Do I need to add some foil so it stops where is should?

210degree C is on the low side but depends from the ABS you are using.
You can print ABS on cold bed if it has small footprint. Biggest concern with ABS and cold bed is warping and layer delamination.
Never tried on blue painters tape but on BuildTak it sticks quite well. You can get 165x165 BuildTak of Amazon - thats exact size of Jr bed. You can often find rebranded BuildTak (in UK Amazon store you can find Prima BuildTak - it's the same BuildTak but with extra Prima logo twice as cheap - some kind of a promotional stuff I'd presume)

But if you use Cura as your slicer you can set any temperature bypassing NFC chip set temperature.
Only problem in my case was getting temperature above 214degree C on my Jr with stock 25w heater block.
I've replaced my heater with eBay 40w cheapie - worked a treat.

I have been reading that its possible to print ABS without a heated bed, using painters tape sanded down and a large skirt. I have some samples of filament coming in the post and will give it a shot. 210 degrees is ok for ABS right?

Thanks

Do you have to cut into the build plate holder?

Yes. Because of the silicone heater I got wasn't completely flush in that end.

Update on my build:

I have added a relay that is connected to the back fan in the davinci.
When the back fan turns on, the relay gets a 5v signal so the 230v for the heater switches on.
The heater heats up to 60 degrees centigrade around 10 sec before the nozle temp is ready.
When the print is done, the fan stops and the 230v switches off.

This way it is automated, and the bed is not heating after the print is done.
So when I go get the print, it is completely loose from the build platform.

Very, very nice work!  I made similar changes to my printer.  But, I didn't take the time to wrap up all the details so nicely.  I have a big pile sitting in front of the printer including temperature controller, power supply, relay and wire! 

Did you use the same heating element I listed?  If so, I'm wondering if you're seeing any contact between the x-axis hardware and the table when the z-axis is zeroed.  I notice you're still using the original glass for the table?  I was concerned it might not like the heating cycles.  So, I milled a piece of aluminum to roughly the same thickness of the original glass.  After everything is assembled, the parts *just* touch when the z-axis zeros before a print.  I'm on the fence on the fix.  Either print a spacer for the z-axis homing sensor or mill the aluminum down further.  Curious if you ran into this?

Again, very nice work on the details!

Yeah, looks like the same type of silicone heater. I am using a 12v switched power supply, 150w and can handle up to 12A.
Yes, I am using the glass that came with the printer and I have heated it up to 90 degC maximum, but running it mostly on 60.
The X axis bumps just a little bit before printing, but I dont think it is a big deal.
I am thinking about going for an aluminium plate for the more even temp distribution and more exact messurement for a thermistor.