I recently installed a cup screw kit from AoA in my HW90. They don't provide much direction. Here's how I did it.
The cups are made deep enough to house both the screw head and the lock washer. But I wanted to minimize the amount of wood I removed from the stock. It's already fairly thin. So I dispensed with the lock washers. The first job was to trim the brass cups down until the top was flush with the bolt head. I bought a 5mm nut at Home Depot and used that to secure the bolt in the cup. I chucked it in my drill motor and used a file and paper to trim and polish.
The cups are already the correct diameter for the existing forearm holes, But they stick out, even when trimmed. So the next job was to deepen the holes. The cups measured 0.408" or about 13/32" in diameter. You could use a 13/32" drill bit but there are problems. First, it's not a common size. Second, the drill tip is not flat so you won't get a flat bottom hole. Third, a drill will dig in and possibly drill too deep. What you really need is a 13/32" flat bottomed countersink. You can make your own with a 1/4-20 bolt, a drill motor, a file and a hack saw.
Chuck the 1/4-20 bolt in the drill and either file or grind to reduce head diameter to 13/32".
Now cut an X on the top with a hack saw.
Next step is to create four cutting edges by filing some relief on the back sides.
I used my old smooth-sided file. It's an extremely useful tool, made by grinding the teeth off one side. I think a regular file will work too, but you need to be more careful. The cutting edges don't need to be gorgeous, they're just scraping a small amount of wood.
Final product.
This cutter has several benefits. It's super accurate. It's controllable, taking a little bit at a time. It doesn't touch the side of the stock at the top of the hole so it won't scar the visible finish. It's dirt cheap.
Cut the hole just deep enough so no part of the cup sticks above the stock. The cutter will rapidly fill with wood, so you'll need to cut a little, then pull it out and clear the chips.
Here's the installed cup.
The cups are made deep enough to house both the screw head and the lock washer. But I wanted to minimize the amount of wood I removed from the stock. It's already fairly thin. So I dispensed with the lock washers. The first job was to trim the brass cups down until the top was flush with the bolt head. I bought a 5mm nut at Home Depot and used that to secure the bolt in the cup. I chucked it in my drill motor and used a file and paper to trim and polish.
The cups are already the correct diameter for the existing forearm holes, But they stick out, even when trimmed. So the next job was to deepen the holes. The cups measured 0.408" or about 13/32" in diameter. You could use a 13/32" drill bit but there are problems. First, it's not a common size. Second, the drill tip is not flat so you won't get a flat bottom hole. Third, a drill will dig in and possibly drill too deep. What you really need is a 13/32" flat bottomed countersink. You can make your own with a 1/4-20 bolt, a drill motor, a file and a hack saw.
Chuck the 1/4-20 bolt in the drill and either file or grind to reduce head diameter to 13/32".
Now cut an X on the top with a hack saw.
Next step is to create four cutting edges by filing some relief on the back sides.
I used my old smooth-sided file. It's an extremely useful tool, made by grinding the teeth off one side. I think a regular file will work too, but you need to be more careful. The cutting edges don't need to be gorgeous, they're just scraping a small amount of wood.
Final product.
This cutter has several benefits. It's super accurate. It's controllable, taking a little bit at a time. It doesn't touch the side of the stock at the top of the hole so it won't scar the visible finish. It's dirt cheap.
Cut the hole just deep enough so no part of the cup sticks above the stock. The cutter will rapidly fill with wood, so you'll need to cut a little, then pull it out and clear the chips.
Here's the installed cup.
