self tapping screw into pre-existing hole (need help with sizing)

[pharmvet]

I am attaching some 3/4 plywood to an angle iron frame (deer stand floor) As an afterthought, I'm planning to use self tapping screws that will counter-sink and be flush with the plywood. I already have holes drilled into the angle iron. I was planning to use drywall screws screwed in from the bottom (just long enough that they would not completely penetrate the plywood. Anyway, the small threads did not really catch the wood as good as I wanted, so I decided to remove the drywall screws and drill up through the plywood so that I could see where to go with self tapping screws that will go in from the wood side and catch the angle iron. Here is my delima. The holes in the angle iron (3/8 " angle iron) are 5/32. Most of the self tapping screws with heads that will counter-sink are listed as #4 or #6 etc. Here is my question:

What size "self tapping" screw will I need to get to fully utilize an already existing 5/32 hole in the angle iron?

 

[whistlepig]

I am attaching some 3/4 plywood to an angle iron frame (deer stand floor) As an afterthought, I'm planning to use self tapping screws that will counter-sink and be flush with the plywood. I already have holes drilled into the angle iron. I was planning to use drywall screws screwed in from the bottom (just long enough that they would not completely penetrate the plywood. Anyway, the small threads did not really catch the wood as good as I wanted, so I decided to remove the drywall screws and drill up through the plywood so that I could see where to go with self tapping screws that will go in from the wood side and catch the angle iron. Here is my delima. The holes in the angle iron (3/8 " angle iron) are 5/32. Most of the self tapping screws with heads that will counter-sink are listed as #4 or #6 etc. Here is my question:

What size "self tapping" screw will I need to get to fully utilize an already existing 5/32 hole in the angle iron?

If you are drilling through the plywood and trying to hit an existing hole in the angle iron underneath that simply isn't going to happen. Good way to waste a day. Use self tapping screws and drill a new hole through the wood and metal at the same time.

 

[chopped]

just a thought but how about carriage bolts?

 

[pharmvet]

There will be a small pilote hole coming up through the plywood so that I know where the holes in the angle iron are.

Regarding carriage bolt - That is plan B if I cannot get self tapping screws to work.

 

[whistlepig]

I have done something similar on the floors and ramps of horse trailers that have rotted out from horse pee. I use dial calipers on the screw. I measure on the body of the screw between the threads and this is the size drill I use.

 

[CharlieS]

they make a self tapping screw with a drill point. so it drills the proper size hole and threads itself. you just need to figure the proper length and size you want.

 

[whistlepig]

they make a self tapping screw with a drill point. so it drills the proper size hole and threads itself. you just need to figure the proper length and size you want.

You are right. These are the kind of screws that I use. I still pilot drill for them. Makes it easier.

 

[BeezFun]

What size "self tapping" screw will I need to get to fully utilize an already existing 5/32 hole in the angle iron?

I have a tap drill chart in the shop, here's one I found online. You will either need a #10 or #12 to fully utilize the existing 5/32 hole, depending on whether you choose a coarse or fine thread. I'd choose fine for that application, so #10 would do the trick.

 

[mrmikey]

Another option is elevator bolts Elevator Bolts - Tycoons Group Enterprise Co., Ltd. > I prefer them over carriage as you don't have to bury them as deep in the plywood to get them flush and they have a bigger head so they're lest apt to pull thru....Mike

 

[studor]

+1 on the fine # 10 or an M5 for those of metric hardware. You could also use a 10-32 bolt if you wanted to tap the holes. I recently made a garden cart and tapped the angle to attach the plywood to with 1/4 -20 bolts countersunk to be flush -- easier to take apart than with carriage bolts and no nuts to lose.