I bought a set of 12" screws to drive through two 6x6 landscape timbers.
With my M18 Milwaukee Fuel impact driver I can start driving the screw, but it reaches to a point that I can't either screw further or unscrew it.
Why would this happen? FastenMaster screws claim to not need a pre-drilled hole.
Attached picture to help visualize the situation
If the pieces of wood you are drilling are not tightly pinned/clamped to each other, and are able to move around relative to each other while drilling, that is going to cause the screw to bind up and overwhelm your driver.
Myself, especially in treated I would use lag screws (basically bolts with wood threads). Lag screws of this length are 1/2" diameter. These must be pre-drilled to full size through the first piece, and to the specified undersize (3/8"?) into the second piece. They have a large hex head (probably 3/4" hex) and are run down with a normal socket wrench, or impact wrench for the hand-tool-impaired.
To drill them I would start with a tiny 1/8" pilot hole, then use a wide 1", 1-1/8" or 1-1/4" drill (whatever will leave clearance for your drive socket) down, to allow the head and a washer to be recessed into the wood and the head to be flush. After that countersink is done, then drill the main 1/2" hole.
1/2" lag bolts will do a much better job holding than those dainty little deck screws. I don't know what your service loads are.
You are working with pressure-treated wood.
That is a whole different story than dry wood.
Pre-drill the first 6 inches to make it easier for the driver to do its job.
I also do not see the self-drilling notch on the screws, thus would question the statement of no pre-drilling required.
When drilling you cut the wood fibers irreparably.
When screwing in dry wood, you deform the wood fibers, and they stay that way.
When screwing in pressure treated wood (it is moist inside), The fibers are forced out of the way, but they flex back into the original position thus tightening the grip around the screw - i.e. chocking the screw.
Pre-drilling is, more or less, for normal screws. A normal screw cannot cut into the wood, it merely pushes it out of the way. But you're buying the nice, pricey screws. Drilling? Where we're going we don't need drills! Just grab your impact driver and go to town! That's why you paid $20 for 12 screws, right?
I rather like Harper's point about lag screws. Lags do far better in this situation, but it's worth talking about something else important (screw or lag): technique. Most likely you put the screw in your driver and started driving. There's two things to keep in mind on screws of this type:
Your impact driver is an amazing tool. I remember buying a power screwdriver about 20 years ago and it wasn't anywhere near as versatile. The "impact" part vibrates the screw as you're driving it, which helps lower resistance. And for short self-driving screws, they pairs perfectly with an impact driver. Those 3" screws go in with ease! But the longer the screw, the more you need to "work" the screw, especially if you hit something tougher in the wood (like a knot).
At this point, the old Tim "The tool man" Taylor instinct comes out (that's why the character is so funny, because we all can relate to that) and you start to think "We need more power! Lean harder! Squeeze that trigger like your life depends on it!" Don't do that. You do need consistent pressure, but pushing too hard gets you where you are now: the screw is stuck and bent. You might also have rounded the head some, if you weren't applying enough pressure. Getting that out will be fun.
When a screw stops turning, it's a good idea to stop. The longer the screw, the more likely this is to happen. What you might want to consider is backing up and trying to slowly work it back down. The nice things about impact drivers is they can provide amazing torque. That usually translates into speed, but for long screws we don't want speed, we want it in. Backing up and going forward some can help work the tip (which is doing the job of a drill) further in.
If it binds up on a slow retry, I would suggest backing it all the way out. This will help clear some debris from the hole. You can then let your screw cool some. Then try again slowly (remember, you have a shaft now so the tip won't get as hot). Pre-drilling just saves you whatever heat and debris the length of your drill bit can take out first (which probably isn't a lot on a narrow-shaft bit). Some of your screws may go in without complaint, but some won't (especially on a 4x4, which is a lot more likely to have knots). Working the screws in a slow but deliberate manner can get you where you want to go: screwing without drilling. Trying to force it seldom goes well.
TimberLok bolts of the type that you linked to on the Lowes web site are an excellent product and I have used many boxes of them over many years. I've used the 6 inch ones to anchor 4x4 timbers to the side of other timbers of similar size. I always drill a pre-drilled hole through the first timber. The pre-drilled hole really does not need to be any larger than the long smooth unthreaded part of the shank. The threaded part can easily walk its way through the pre-drilled portion before engaging into the second timber.
I have used the 6 or 7 inch lengths when making raised garden bed frames out of treated 2x12 planks. Screwing into the end grain of one plank on the corners with the pre-drill hole extending through the 1.5" thickness of the first plank at the corner and about 3 to 4 inches into the mating end grain makes an almost unbelievably strong joint. For 2x12 frames I use 4 or 5 bolts on each corner. If using 2x10s for similar frames I use 3 or 4 bolts.
I've used the 8" lengths when making gate frames out of 2x4s. I lay the 2x4s flat to minimize the thickness of the gate. Pre-drilled holes through the edge of one 2x4 on the corner of the frame allows the long TimberLok screws to enter the end grain of the mating flat 2x4 frame piece. Even when the frames are made of a relatively soft material like red cedar the power driver can literally drive the head of the screw way down into the side of the first 2x4 without ever stripping out in the end grain of the other 2x4 with just 4 1/2 inches of embedment of the screws.
Lastly I've built some structures where I needed to butt joint a treated 4x4 up against the side of a 4x4 vertical post. Four of the 8 or 9 inch TimberLok screws draw the joint together so tight it is almost amazing.
That's just too much. I would predrill through the first 6x6 at the full width of the screw. Then predrill partway into the second with a narrower bit, so that the screw can fully grip into the second 6x6.
Just because you don't have to predrill doesn't mean you can't predrill.
Looking at the linked product, only the last few inches have threads. Since the upper part that will be inside the first 6x6 has no threads, it won't grip whether your predrill or not, so predrilling has no downside except for a little extra time.
I think the problem is that as it goes through the first piece of wood, and reaches the second, it tries to pull the second up to the first, thus binding it all. Were it just one piece of wood, it would probably carry on until it was buried - try it on a similar piece.
That's the reason. The solution is to pilot a hole through the first piece. Let's face it - you're trying to pull the two together, so as the screw tightens into the second piece, it needs to be free to slide inside the first, which it will do inside a hole just wide enough to allow it to slide. If it can't, then it binds, and the two pieces will never be squeezed together, which is the object of what you're doing.
And with 2 off 6"x6" timbers, there's a good chance when it's all tight, the sharp points of 12" screws will be sticking through the other side. Not a sensible idea, I think...
Three suggestions:
1: Get bigger gauge screws, your screws look way undersized for the task at hand.
2: Pre-drill the hole to about 1mm less than the width of the shank of your screws.
I think one of the reason could simply be that the length of the screw together with the nature of the impact driver could cause a very high amount of torque.
Like the torque limiter extension bars you can use when changing tires.
Maybe using a powerful "normal" screw driver might work better in this case.
You cannot use your impact driver for that. Get your 18 hammer drill. Put the handle on it and run it in hammer drill setting on number two. It will run them all day long. It will run 15 inches all day long. Your impact will not work. You will just burn it up.
