First...Try using a phillips head slightly larger than normal for that size screw. Not too larage. The tip of the phillips head must still fit in what remains of the screw hole.
Second thing if that does not work.......Can you get hold of the sides of the screw head with a pair of pliers? If so, use a pair of locking pliers (vise-grips, etc); lock the pliers tightly around the screw head; and twist the pliers (and the screws) off. Needle nose vise-grips for tight places best.
A tool called screw extractor was designed for people in exactly your situation.
Inserting it into the busted Phillips and applying unscrew action will back it out. Available from large hardware and automotive stores.
Take along one of the good screws for size comparison.
Sometimes you can get a straight screw driver to bite what is left of the screw head. Try different sizes, with a sharper point.
Sears has been advertising the "Screw Out" (no kidding!) with Bob Villa up here in Canada on TV-seems like just the job for it!
Both good thoughts, will probably try them both and dump the drill idea...One of my handyman friends called , his suggestion was to take a phillips screw driver of exactly the same size , insert it into the hole and tap lightly to try and indent a new phillips insert into the existing screws. Sounds like a good idea, however I know when I first tried to take the screws out with a normal screw driver , they did seem to be pretty firmly in place, so not wanting to kill myself getting them out, I really did not try too hard, because I knew my neighbor had a power screw driver. You live......You learn....will try all three suggestions if necessary. Amazing how you miss coming home from work and playing music, even if its only for a few minutes. thanks.
get a Screw Out at Sears, you'll need it again someday for something around the house or garage. only $20
next time, try tightening a bit before attempting to loosen. I learned this trick from an old pro machinist many years ago; still works the last time I needed to uncover an amp that just wouldn't cooperate.
There are also products called "Easy Outs" that are used to remove bolts whose heads have snapped off or been rounded. (They look like funky drill bits.) I don't know if these would work for your application, though. At hardware store or a good automotive supply store. Keep us posted if you use that Craftsman device. I'm sure that there are many others, besides myself, here on Agon who are curious if this "goober" really works! Keep you woofers a woofin'`and your tweeters a tweetin'!
As a pro, drill it carefully with a drill slighty larger than the threaded part of the screw, you just want to take the heads off, do it to the rest of the stripped heads......... then you will be able to remove the plate, when you remove the plate the threads of the now drilled screws will be pertruding. use a vise grip to remove the screws. walla! damage free! get new screws and your done, and forget the power screw driver, when you install the new ones!
Like Fatparrot said, they are called Easy Outs. First you have to drill a hole into the screw head so you can fit the easy out in there. The easy out is nothing more than a coarse, reverse threaded screw. They do work well if you know how to use them. If you are unfamiliar with them, I suggest you practice on a screw first, and then attempt the project at hand.
Be careful if you do decide to drill. The metal shavings may be attracted to the speaker magnet. If they get in around the voice coil you could have big trouble.
If the screws are countersunk then I think the best method is the one mentioned by Chichiuno. Most of the other methods involve some type of impact onto the speaker cabinet, which is not good. A high quality easy out still needs to be given a good tap in the drilled hole to set the edge in order to grip the damaged screw. If the cabinets are pretty sturdy this should be a minor concern.
All the above mentioned methods will work but drilling off the screw heads is the least stressful to the cabinet. If the screw heads are accessible, a pair of high quality diagonal cutters can grip the sides of the screw by actually digging into the sides & you can back them out while squeezing HARD.
If you're unable to drill I would take the approach which has the least amount of impact, so the first approach is the easiest. I would start with another screwdriver & if at all possible, lay the speaker face down so you can exert more pressure on the screwdriver. If you try this first use a high quality screwdriver & pay attention to the tip to screw line up.
Herman, good point! If I ever have to drill metal on ANY audio gear, I will put a small circle of Blue Tack around the drilling site, so that any metal shaving will be held by this material. Also, one of those mini hand held computer vacuums used while drilling comes in handy!
take a small file or dremel and cut a straight cut into the screw then use a straight blade screw driver. go slow with the extraction. the power driver isn't always a good tool
Great ideas guys. Another tried and true method on breaking the grip of a "frozen" fastener is to tap the end of the screwdriver firmly but not too hard, when sitting in the slotted head.
A friend suggested a vice grip wrench . Has a turnable nut to tighten and then grips down. A little sweat, however got them off, refastened the post and the music is back ! THX
Screwing around again, eh Darryl?