Screwing

[mwybo]

I am about to start assembling my Pooduck. My question is related to the size of hole to drill relative to the size of screw.

For example, when screwing the bottom to the transom using a #12 screw, for example :
- Do I drill a #12 hole in the bottom and a #10 in the transom

- Is a pilot hole the hole in the bottom or the transom ?

- How deep should the counterbores be in 3/8 plywood ?

- What is the general rule regarding screw sizes and relative hole sizes ?

Thanks

[maa. melee]

The pilot hole is the hole where the screw will grip and thread itself into the wood. The piece that needs to be drawn tight, in your case the bottom, needs to have a hole drilled in it such that the screw won't thread in the wood but small enough that you get maximum holding power. The idea here is to have the screw snugly thread into the transom and loosely hold the bottom until you draw the bottom on tight (by turning the screw). The counterbores are only as deep as you can plug or putty over. Are you using bungs or small caps? Probably not. Leave a few hundreths of an inch above the face of the flat head screw so the putty can be held in (try to avoid feathered putty plugs but have a shoulder for the putty to full). Now if you want the general rule for pilot holes, take out your calipers and measure the minor diameter of the screw (that is from trough to trough across the screw's threads). This should be your pilot hole diameter. Some might even take that and go one size smaller to really get a snug fit.

http://www.ruklic.com/images/specgraphics/particleboardscrews.gif

[Steve Miller]

Here is some info from the Glen-L site:

http://www.glen-l.com/weblettr/webletters-8/images8/bb-plate6c.jpg

"When driving screws through plywood, it is essential that compression wood be left underneath the head (see Plate 6-C). By this term, it is meant that the screw should not be countersunk so far that it would eliminate the top layer of plywood. The top layer of the plywood should be compressed under the screw head. This gives much better holding power. Screws in plywood that is under tension, such as in the application of curved planking panels, should never be counterbored. They should be driven just below the surface of the wood.

When driving any screw, it is imperative that the shank and pilot holes are of the proper diameter for the wood being used. Over-drilling or under-drilling will provide poor holding power for the screw or possibly cause the wood to split. Although tables are given recommending the proper sizes, these may have to be varied to suit the work. Obviously, the threads will not cut into oak as well as into a soft wood such as spruce. There are many types of tools available that will drill, countersink, and/or counterbore for screws in one operation, and are well worth the small amount that they cost."

[Nicholas Carey]

Originally posted by mwybo:
I am about to start assembling my Pooduck. My question is related to the size of hole to drill relative to the size of screw.

For example, when screwing the bottom to the transom using a #12 screw, for example :
- Do I drill a #12 hole in the bottom and a #10 in the transomNormally, you'd use a Fuller tapered countersink/drill bit of the appropriate size (see below for details on that).

- Is a pilot hole the hole in the bottom or the transom ?They're both looked at as one hole—the pilot hole.

If you want to get picayune about it, the piece nearest the screwhead would have the clearance hole and the piece furthest away the pilot hole, but only if the two holes are different diameters.

- How deep should the counterbores be in 3/8 plywood ?Depends…

Are you going to be plugging the screws, puttying them, or leaving them exposed?

If you're going to plug the screws, they need to be countersunk suffiently that you can put a plug reasonably deep into the countersink.

If you're going to putty it, it just needs to be countersunk enough to put a sufficiency of putty in.

And if the screw is going to be exposed, the countersink should essentially leave the screwhead flush.

- What is the general rule regarding screw sizes and relative hole sizes ?Depends on the type of screw.

If it's a "traditional" wood screw with cut threads, the pilot hole should be drilled [roughly] the root diameter (the "root diameter" is the diameter of the screw in the "valley" between the threads). How close you get to the root diameter depends [a lot, actually] on the wood you're driving the screw into: nothing like breaking a critical screw to liven up your day. Usually, in my experience, a little experimentation/experience is required to figure out the appropriate drill size. A hardwood like white oak or ironbark is going to be a lot less tolerant of a tight fit than a soft wood like, say, cedar. Getting it wrong can mean failure in two ways: the wood could split, or the screw head could torque off.

If you want to get pedantic, after you drill the pilot hole, you might want to get a counterbore (not a counter sink) and drill a clearance hole the shank diameter (the shank is the unthreaded portion of a traditional wood screw) in the topmost part to be fastened together.

If you use a tapered Fuller bit, it's not an issue. Nor is it an issue if you purchase a stepped bit of the appropiate size (tapered bits are a lot easier to find :D ).

However, in either case, you want to keep the clearance hold fairly tight so the screw doesn't leak.

BTW, the easiest way to pick up [accurately] the root or shank diameter is with a dial caliper calibrated in 1/1000ths of an inch (about $29 for one quite sufficient for woodworking).

On the other hand…"new-style" screws (like drywall screws, among others) are made a bit differently: the threads aren't cut, they're upset. That means the root diameter is the same as the shank diameter. Also a lot of "new-style" screws are threaded full-length. The threads are typically much thinner as well.

One potential drawback of "new-style" screws [due to being threaded full-length] is that they have a tendency to "bridge". A good way to avoid that problem is to drill a clearance hole that matches the thread diameter.

There you have it. Hope it's useful :D [/QB][/QUOTE]

[Paulyboy]

Save yourself time and worries. Go to Mcfeelys screw site and get cut thread screws and a proper set of tapered drills with counterbores to properly seat the screws so no water can get by. Look at their illustration in the site and it'll make everything cleart as the water off the shores of Bermuda. (or gilligans isle, depending on your economic outlook)

[Smacksman]

I use a combined drill/counterbore/countersink bit when I have a lot of screwing with the same gauge/length screws. Expensive but they save hours.

Made by Stanley as I recall.

The drawings above are a bit misleading for normal wood screws. They have a shank that is visably thicker than the screw part.

And the top screws are dead easy to shear off in hardwood and are usually just plated.

[Bruce Hooke]

IMOOP:

3/8" plywood is not thick enough to allow for plugging the holes (with a wood bung). So, puttying (or nothing) is the usual solution. As others have said, to putty you want the screw to be just a touch below the surface of the wood. Going deeper is asking for the screw to pull through the wood.

I don't like the tapered bits because they drill the wrong shape hole. You want a stepped hole, not a tapered hole. So, I use a Makita "Quad-Driver" rig that drills a stepped hole. The down side of this and similar rigs is that you don't have a lot of choice on the size of the pilot hole drill bit -- it is pretty much one set size for each screw number.

In issue #54 WoodenBoat published a table of pilot and clearance hole sizes for different sizes of wood screw. I keep a copy of this table in my workshop and refer to it regularly. Here is what it says for the most common screw sizes:

#6
Shank Diameter: 0.138"
Shank Drill: 9/64" (#28)
Root Diameter: 0.095"
Pilot hole in dry hardwood: #44 drill
Pilot hole in wet oak frames: #48
Pilot hole in softwood: #51
Pilot hole in end grain: #40

#8
Shank Diameter: 0.164"
Shank Drill: 5/32" (#23)
Root Diameter: 0.113"
Pilot hole in dry hardwood: #38 drill
Pilot hole in wet oak frames: #43
Pilot hole in softwood: #47
Pilot hole in end grain: #33

#10
Shank Diameter: 0.190"
Shank Drill: 3/16" (#12)
Root Diameter: 0.132"
Pilot hole in dry hardwood: #31 drill
Pilot hole in wet oak frames: #36
Pilot hole in softwood: #42
Pilot hole in end grain: #30

#12
Shank Diameter: 0.216"
Shank Drill: 7/32"
Root Diameter: 0.147"
Pilot hole in dry hardwood: #29 drill
Pilot hole in wet oak frames: #32
Pilot hole in softwood: #37
Pilot hole in end grain: #26

#14
Shank Diameter: 0.242"
Shank Drill: 15/64" (softwood) 1/4" (hardwood)
Root Diameter: 0.166"
Pilot hole in dry hardwood: #25 drill
Pilot hole in wet oak frames: #29
Pilot hole in softwood: #32
Pilot hole in end grain: #19

Note: Shank drill means the drill used to go through the plywood (or whatever wood is just under the screw's head). Pilot hole means the hole in the wood that is underneath the plywood (the wood the screw's threads should be biting into).

If you are drilling the holes with standard drill bits drill the pilot hole first and then use that hole to guide the shank hole drill bit and then, finally, countersink the hole.

[carioca1232001]

I have just finished refastening and replanking the undersides of my 32 ft power boat.

Purchased the 3000-odd #10 screws, tapered drill bits, countersinks and stop-collars from Jamestown Distributors (JD)in RI.

Drop an email to Dave Francazio at JD:

davef@jamestowndistributors.com

You will be pleasantly surprised at how helpful the JD crew are.

[Billy Bones]

Screws? In a Pooduck?

Hmmm.

[gary porter]

Go to www.bitsnbores.com (http://www.bitsnbores.com)
look under the yellow menu bar for "Anatomy of Wood Screws" or something like that. There you will find about three good printable charts that will give you all the info you'll need.
good luck
Gary

[ 03-09-2005, 05:59 PM: Message edited by: gary porter ]

[gary porter]

Originally posted by Billy Bones:
Screws? In a Pooduck?

Hmmm.Nutshells, Shellbacks, all have screws in the plank to transom and stem connections. One could use nails or nothing at all except temporary fastners but srews are what Joel White called for. Sil Bronze is the best.
Gary