rebuilding a transformer -looking for tips

They're normally stuck together with wax so if you heat them to melt the wax and apply a bit of pressure they should squeeze together.

ok -i'll try that and let you know

andyadler is on the money.

Just out of curiosity...
What were the specs of the original transformer and how did it die?
Are you looking to improve the performance or just get some more life out of it.

It is hard (but not impossible) to match the performance of a machine wound transformer by hand.

Andy, can i remove the wax any other way?  i fear if i heat the metal in the trafo plastic housing of the cores; it will melt

d- glitch i don't know exact specs, but it would be 220 V =>20 V, about 40 W (or less)
it was the PS for a battery charger of a Drill, and i forgot to monitor the charge for a couple of hours - and it was dead.
after disassembling, i found a thermal fuse ( 2A) open.  all i had to do is replace it, and put the blades back
it is only a hobby project to repair it ( with lots of time -die to the sancted corona thing)

I was thinking more of removing it from the plastic before heating it up.

that seems to be the way to go - and then clean them with towels separately -right?

I was thinking more of heating the whole stack in the oven and then putting a weight on top. Like you I've taken them apart but I've never put one together again. The wax sticks them together to stop them buzzing.

I think you want to keep the wax, not remove it.
Definitely remove the wire bobbin before heating core pieces.
Build the whole stack and heat it under pressure.

I would use a few binder clips to hold it together and keep it aligned.
https://www.amazon.com/Coofficer-Binder-Clamps-Assorted-Different/dp/B07DXSBT5J/ref=asc_df_B07DXSBT5J/?tag=hyprod-20&linkCode=df0&hvadid=241898301275&hvpos=&hvnetw=g&hvrand=17803826033690609731&hvpone=&hvptwo=&hvqmt=&hvdev=t&hvdvcmdl=&hvlocint=&hvlocphy=9001863&hvtargid=pla-472025243243&psc=1

I am a bid fan of passive safety devices.  Can you replace the thermal switch?  It may have saved you from a fire.  And one a fire starts, you never know where it will go.

>>  I was thinking more of heating the whole stack in the oven and then putting a weight on top.   <<  ok i can do that, but then what?  i have to place the blades one way, then the other each time, so i must place each blade separately

d-glitch  -  build the whole stack - then heat it??  see above
 
i have several heating options a paint stripper, and 2 ovens  - one has a top grill, the other warm air
what would you think is the best method?  put them all in a ceramic pot, and heat them all - then clean each with a paper towel ? just lloking for options, and the best, or easier way

Oh, you're using the alternate-lap method of laying out the laminates, I was assuming the butt-joint method. In that case then you will either have to wipe the wax off when hot or reassemble the core using butt-joint method. Both methods are used, see figure 2 in https://www.degruyter.com/document/doi/10.1515/phys-2017-0124/html

thanks, but i do not understand the conclusion; nor what method is best, or how it influences the power outpit, temperature, or such

It was just to demonstrate that either stacking method works.

Suppose you want to make a transformer from "scratch."  You have to define scratch.

Option 1

• You can buy laminated E-I core pieces and a matching bobbin.  
• The laminates will be epoxied together.
• Wind the wire on the bobbin, clamp the pieces together, and you are done.
• There will be three gaps between the E and I pieces.

Option 2

• You can punch or cut E-I laminates out of sheet stock.
• You wind wire on an appropriate bobbin.
• You build up the core alternating the direction of the E-laminates.
• The gap area will still be the same, but the 3N gaps are distributed over a larger volume.  
• Gross electrical performance is probably close to Opt 1.
• I can imagine some improvement in fringing fields and localized heating.
• The final assembly is more complicated but probably more rugged as well.  

Even if your XMFR started out as Opt 2, you can rebuild it as Opt 1.

dglitch - that is not really for a  small DIY project like mine
i can as well buy a 20 V transfo new then

i tried with the paint stripper,  but got nothing off the plates -
how best remove the wax ?

i found i could remove it 2 ways :
-rubbing it off with isopropylalcohol
rubbing it off with steel wool ( seems a bit better for thicker residuals )

right now, i'm scraping it off the plates with a small stanley knife with a 45° blade, and cleaning it with steelwool.

can you  suggest what i should use as "WAX" when the reassembly is complete?