I recently got an old post vice for cheap but part of the jaw is chipped off. I’ll post pictures here in a bit but I was wondering if I could just weld in the broken off part, build it up and grind it flat or not. Would normal welding rods be fine ir would it need to be a hard face? 

Maybe but it might be cheaper and easier to make new jaws. You can grind and plug weld a dovetail if you don't have access to a mill. 

Frosty The Lucky.

So just get a piece of tool steel( assuming that’s what the jaws are supposed to be) and then plug weld them in? Easy enough. I might have access to a mill through my college

What type of metal would I use for the jaws?

I would use 4130 or 4140 steel or for really tough steel a piece of old grader blade.

 

I can't control the wind, all I can do is adjust my sails. ~ Semper Paratus

Medium carbon is plenty hard and durable, tool steel is prone to work hardening and embrittlement.  You don't want to weld the jaws to the vise if you can avoid it. I was referring to plug welding the male dovetail to the jaw if you don't have access to a mill. There should be two screws through the jaws holding them in the dovetail. 

4130 or 4140 is a good choice. I do not recommend grader blade unless you can cut it with a file. If it dulls the file instead of being cut it's not suitable for much of anything other than a road grader cutting edge.

Frosty The Lucky.

I guess I don’t get what you’re referring to when you’re talking about plugging the male dovetail. I’m familiar with the joint but don’t get what you’re talking about.

That looks like it has some kind of layer on top of the jaws - the cross hatched section underneath looks like the original face (on all teh vices I've seen) 

 

I'd chip off the broken layer and see if the hatched layer is good.

Oh yeah you’re right, I didn’t look that closely but I think you’re right. I can see what looks like the original face of the jaws 

Mine, FYI. 

 

Personally, I would grind off the broken stuff and bring it down to the original. Then make sure the original is smooth so as not to put ding marks in your hot or cold iron. A post vice really doesn't need hardened jaws. I us 1/4" jaw protectors on my post vice for this reason.

 

First off, my apologies for the empty quote I couldn’t figure out how tor remove it, and second, that is likely what I’ll do. I’ll probably grind down the jaws to the original and if need be fill  any pockets with some weld to avoid large divots. And while I’m here, anyone have tips on how to remove the bolt that the jaw hinges on at the bottom? I cannot get it out. I have already tried heating it up.

Edited April 5, 20233 yr by Mod30
remove empty quote.

Have you soaked it in a good penetrating oil like Kroil or a 50/50 mixture of acetone and Dexron automatic transmission fluid? Is the bolt held in with a nut or a wedge? What I do, if the bolt is threaded with a nut, is run the nut on till almost flush with the bolt and wack it with a hammer, usually breaks free if it has been soaked in the above penetrating oils.

I’ve only used WD-40 which worked to get the nut off but I can’t get the bolt out. I tried your method and I still haven’t been able to knock it out. 

Well I haven’t tried any penetrating oils though.

It looks like a file was welded to the jaws. An old trick for better grip. I have a post vise that had the same modification. Removed it by carefully cutting the welds with an abrasive wheel on an angle grinder.

Take the screw out and do all the above whilst pivoting the jaw. Leave the nut on a few threads and tap the nut lightly with a hammer to loosen the bolt or use a lead or brass hammer so as to not mess up the threads. Use a punch and after heating the hole to a black heat drive it out. If this doesn't work,,, get a bigger hammer and be prepared to dress or remake the bolt, which shouldn't be a problem for a blacksmith if you have the proper threading dies. 

The punch should be a flat face punch just a bit smaller than the screws diameter.