What is the best way to adhere an old printed part to the bed?
I have not done this, but I believe this is the best method and I want to test it at some point soon:
Model a negative for the footprint of the part you want to adhere to the bed, in the orientation you want to adhere it, with just a few mm of height, and clearance suitable for your printer (probably around 0.25 mm) in the XY plane, and print that with thick extrusions on the initial layer so that it adheres well to your bed. It should look exactly like a &quot;thickened brim&quot; for the part you want to re-adhere. Don't let the bed cool after printing it.
Now, you have a slot to insert your part into that will not only hold it to the bed, but holds it to particular coordinates you chose to print at, which you can align to the coordinates of the new model you want to print on top of it.
If you can't get the clearances right to hold without knocking off the negative footprint from the bed, you could try adding some threaded holes to it, so that you can put a couple set screws through it to hold the part in place. This technique can also be used if you want to avoid modeling a full negative (e.g. if you don't have the original in CAD form where you can easily negative it) - just print a few posts to hold set screws in approximately the right places on the bed.