Take the deck off and check for bent blades and see how they align in the middle. Other than that a Big hammer, and patience, and take into consideration that it will never mow perfect again. Finding perfect alignment will be difficult if not impossible.
All the the time here. Takes using a BFH and knowing where to use it. The last deck I straighten took four hours to do but of course it bent in a way that both spindle were out alignment along two bent blade. Even after four hours beating it was only 99% right. Sometime I even bent spindle shafts or broken spindle housings.
Also having a knowledge of how the deck should be aligned helps along with a good straight edge. I use a four foot level and tape measure to do this.
Also take into account that sometimes heat is required to shrink the metal back into shape because it stretches when bent and that expanded metal needs to go someplace. Also the hammering back into shape can cause work hardening and stress cracks that then have to be repaired with welding.
Right. It just not something that most DIYers can do. I have been personally straightening decks for over 10 yrs now and still a PITA to do most times. I think the worst deck I had to straigthen in the last year was my own deck. Basically I park my mower on top a large stump while the deck was engaged.
That why I both MIG and Gas welders, grinders, and other tools to reform these decks. Sometimes it requires cutting and re-welding areas due to over stretched metal. I have gotten fairly good both types of welding but I have been doing it for 20 yrs too.
You got do all this while maintaining the alignment of all the moving parts. Just because you get the blades back to their level plain doesn't mean all the moving parts are working correctly.
I do get in decks that have been welded with stick welders that just looks plain horrible. Stick welders are not meant to be used on sheet metal, too much heat.
Basically it take a lot patience and a few headache pills for me.
Doing a 38" Murray right now and got a 42" time cutter waiting to go next
How some one managed to bend a 3/8" steel plate with a rolled up lip for strengthening beats me but it is 1/16" out at the bolt holes which is 3/4" at the blade tips.
And as the others have said gas welding, slow as grass growing in the winter but the only thing that works properly on pressed decks
Fabricated decks can be electric welded byt care must be taken not to leave the welds too brittle
GEneral rule is gas makes soft welds that can be bent and rarely crack ( but they can sag )
Electricity make stiffer welds that can become initation points for further cracking.