Screw together possible soft and hard wood types with pilot holes and finding the right drillbits

Question

This is my first DIY project with wood and I'm already over my expected/set budget and timeframe so I want to ensure I don't make as many more mistakes before finishing the project, but whatever happens happens at this point I guess.

TLDR (context and info to follow):

1- Are my materials hard or soft wood? PINE OSB and Sandeply (never heard of this but the site says its hardwood)

2- What drill bit do I use in regards to joining hard and soft wood (assuming that OSB is soft).

3- #4 screw pilot hole into the side of wood drill bit size for both hard and soft?

Context:

I'm making myself a modular/3 piece deadlift platform following several step by step videos and have already cut and glued the pieces and rubber together and am now looking to fix them tightly with screws. 1 lifting platform and 2 separate wood/rubber platforms for 3 total pieces mostly for storage.

My base layer is one 3/4" thick Pine Engineered OSB. Pine is listed as a softwood, so is the OSB also softwood? My platform layer is 3/4" thick "Sandeply" which the site is listed as hardwood.

I'll be using 1-1/4" #8 Philips head flat wood screws on the surface of the platform to drill down and fix them together. I'd like to avoid any splitting, so I want to make some pilot holes with a countersink so the screws are flush. What drill bit size would I use given that I will be going through 2 different types of wood. Online says to use 11/64" for hardwood and 7/64" for softwood for #8 screws so I'm a bit lost here.

I will also be adding toggle latch clamps to the sides of the pieced to keep the three separate joined when in use and its recommended to use #4 wood screws. One search said that #4s need 1/16 " but when I ran search at the hardware store, the 1/16 " was being listed for the #6 screw so I'm a bit turned around. The latching mechanism and bracket will have one screw hole in the platform and one in the base OSB layer if that makes sense.

I'd like to run to the store today so I can finish screwing everything together and move the platforms due to local car thefts so quick and timely advice would be appreciated. TIA.

Answer 1

Shortest possible version: use anything you have handy.

Longer version: ply and osb don’t split like wood, so drill bit sizing doesn’t have to be so precise. In a perfect world, you want the hole through the top piece to offer no resistance to the screw threads. (In other words, the screws should fall right through.) In reality, any old countersink/hole will work through the top layer as long as you push hard while screwing to keep the plies together. If they separate, back the screw out and do over. Don’t predrill the lower ply with anything too big or you’ll compromise the holding power of the screw. You could easily get away with no predrill. Same for the latch: 1/16 is good because it fulfills the requirement of giving your screw a straight guide into the hole. If you don’t have the tiny drill bit, just use something smaller than the root diameter of the screw (ie, the shaft, not thread, diameter).

The sandeply is hardwood if they say it is, but it’s probably not that different from pine, which is a softwood. Fwiw, balsa is technically a hardwood, so rules sometimes have to be discarded.