I am putting up a sheet of 3/4" plywood over a drywalled wall with steel studs on 24" center. It needs to be anchored very well and I'm not sure what type of fastener to use. I was thinking maybe drywall screws pre-drilled with washers but I'm looking for suggestions.
Thanks for any help on this.

 

What are you attaching to the plywood, how heavy is it and would someone be hurt or something damaged if it failed???

Screws have very little holding power in steel studs.
In commercial applications the normal way to fasten to walls with steel studs is to open the wall and brace between the studs with a backing board, normally 2x6 dimensional lumber then put the drywall back up.
If you span three studs then they will all share the weight.

I have for moderate strength spaced flat head metal screws every four inches and spanned three studs on a surface mounted piece of plywood.

Maybe someone else has a different idea.

 

The plywood is being used as a stiffener for braces that will attach to our shapers fence in our shop. The guy that built the braces before used some 3" long sheet metal screws shot into the drywall not even hitting the studs. So the design was pointless. I have been charged with redoing this design and what I came up with is to run a full sheet of 3/4 along the wall and screw it into 4 stud spans and the build 28" wide boxes of 2x6 to screw into the 2 sets of studs as well as into the plywood and the attach to the guide fence for the shaper. Its a rented shop unit so tearing into the walls to brace behind the dry wall isnt an option.

 

Can't really picture what a " stiffener for braces that will attach to our shapers fence" does and what sort of forces are on it.

Bottom line is that metal studs are very thin and screws into them have very little grip.
Best you can do is put LOTS of screws into the studs and possibly several butterfly type anchors into the drywall for good measure.

 

Glue the plywood to the drywall with construction adhesive and use fine thread drywall screws. You might want to locate the studs first and countersink the holes in the plywood so that the screws don't want to strip as you pull them down tight.

 

Use the self-tapping bright screws with machine thread that are made for attaching drywall or other materials to steel studs. Go about every 6" up each stud. Use screws long enough to penetrate about 3/4" to 1" beyond the face of the stud - 2" or 2-1/4" should do it. You can get the proper screws from a drywall supply house, or you may be able to find them at a big box center.

Half-way between each stud, along the top, install a Molly fastener. This may be the butterfly fastener that GregH was talking about. Or install two in each bay, about 5-1/2" off the center of each stud.

 

We use a shaper / planer to fine edge acrylic sheets. There wont be much force but it needs to be as stable as possible so that the alignment fence on the shaper doesnt move. The last design would allow the fence on the entrance side of the bit to move as much as 1/8" if pushed with easy to moderate force. The braces are simply 2x4 boxes attached to the back of the fence and then into the wall. The fence is made of 2x4 x 1/8" wall aluminum stock. I knew that my options were seriously limited by the steel stud framing and being a rented facility I dont have much room for adding what I really need to beef it up. Thanks again for the ideas..greatly appreciated.