As with any woodworking project, if you remove one step from the manufacturing process the results could be disastrous.
Screws can cause a variety of problems, including bending, twisting and breaking. Choose better not necessarily more expensive screws to reduce those occurrences.
It usually happens as you hang a door or install a shelf. Youve got a balancing act going as you hold one piece, steady a second piece and attempt to drive a screw all at the same time. As you tighten the screw the last few threads, the connection pulls tightly and the screw snaps.
I was interested to discover why certain screws broke and others didn’t. It will be as amazing to you as it was for me.
Because they are easy to acquire, drywall screws are included in this test. However, I wouldn’t consider using them when building fine furniture.
I bought four different boxes of screws to test a common drywall screw (because most woodworkers use this screw as they start woodworking), a zinc-plated screw off the shelf from a home center, as well as a premium square-drive screw from McFeelys (mcfeelys.com) and a box of Spax screws (also considered premium) purchased online (highlandwoodworking.com).
First, I was surprised to discover that premium screws were more affordable per piece () than those purchased at the home center ().
Screw Fabrication
How a screw is created is interesting and influences a screws strength. Some manufacturers bring in steel stock that is ready to be made into screws while others bring in stock that needs to be pulled and thinned to the appropriate size; bulky material is passed through a series of shaping stations that stretch the stock to arrive at a needed diameter.
Once the material has been sized, it is fed into a machine to shape the head. A second or third time, the machine will pound the stock to create the recess for the head of the screwdriver. Ive been told this process shakes the entire facility floor with each pound.
Head Designs
The ease of driving screws was made easier by the inventions of square-drive recesses and Phillips. Each decreased the foot-pounds required to keep the bit in contact with the head of the screw.
There are many types of screwheads, but the most popular for woodworking is the flat head. This is where the top of a head meets the surface when the screw has been properly seated. Other head styles include:
Trim head: These are like the flat head but with a smaller diameter flat surface.
Oval head: This screw also has a countersink design but its surface is slightly elliptical.
Pan head: A head with a dome-shaped shape that is elevated above the woods, while the flat under the head acts as the bearing surface.
Truss head: Similar to a pan head screw, but with a lower head profile and a slightly larger bearing surface.
Washer head: The washer has a larger head
bearing surface, used in cabinetry for mounting wall cabinets.
Counterbores and pilot holes help to reduce the tiny amount of shredded steel collected on the tip of the driver, which is a clear indication of cam-out.
You may also find other designs from time to time.
The countersink is located just below the head. The countersink on wood screws is generally an 80 to 82 angle on American screws. This angle is compatible with most commercially available countersink bits. Again, you occasionally find screws being fabricated with other angles.
Recess Design
There are many recess types for screws, but I have found that woodworkers tend to use only three designs.
The slot head was the first recess used in screws. Slot-head screws are what I use for the majority of my projects, if any. This is a decision that I make to build reproduction furniture. Because it is difficult to maintain, this oldest recess has seen its use decline over the years.
As you apply significant torque to drive the screws, the screwdriver will engage with the recess.
Henry Phillips first patented the Phillips recess in 1936. This recess, sometimes referred to as cross-drive, is still popular in the U.S. woodworking industry. The ability to maintain contact between the screwdriver and the Phillips screw recess is greatly enhanced versus the slot screw. People exert approximately 3.7 m-pounds of pressure to keep the driver engaged into a Phillips recess. If you don’t want to achieve an antique, authentic look, this is the recess that most woodworkers use.
The slot head allowed the driver slip, causing damage to the screws heads. However, the Phillips recess introduced a new problem called cam-out. Cam-out is caused by excessive torque being applied to a screwdriver’s recess.
The square drive is the third type of screw recess that is used in woodworking shops. This design reduces cam-out by reducing or eliminating it. To keep the square-drive screw in contact with the recess, it takes only 1.2 m-pounds pressure. You are less likely to let the driver slip.
I use square-drive screws for assembling jigs and shop fixtures. Why? I was experiencing problems with screws breaking so I decided to give a deck screw a try to see if it made any difference. The screws I selected were a square-drive design. There was a difference, but I later found out it wasnt due to the recess. It was due to a difference between the screws. Read on.
Thread Styles
A countersink on a wood screw (right) is tapered at 80-82 in the U.S. (90 in the U.K.), while the countersink on a typical drywall screw (left) is slightly concave or of a bugle design.
Once the recess and head are formed, screws can be cut to length. Then, threads can be added. The shank was cut in the shape of threads. This allowed the threads to be the same diameter as the shank. This is a great way to date screws from older furniture. Today, threads are pressed into the shanks. The thread profile is therefore larger than the shank’s diameter.
To show how threads can be added to screws, hold your hands in a praying position and then slide your righthand back until the tips of the fingers touch the palm of the left hand. In this example your right hand is the thread-cutting tool while the left hand is the screw. Now, apply pressure with your right hand and slide it forward across your left hand.
Thread patterns for woodscrews have traditionally been a single lead, or one parallel thread, wrapped around the length of the screw. This design is still the most-used thread style in woodworking screws.
Deep-thread screws were created due to the widespread use of particleboard in furniture manufacturing. This thread design has a thinner shank and deeper threads set at a sharper angle. Deep-thread screws are better suited for composite woods because they can withstand more splitting, burping, and displacement.
As the process of creating threads on screws evolved, the relationship of thread size to screw shaft changed. The threads on the right screw were cut into the shaft leaving, the thread diameter equal with the shaft. Threads on new screws (the screw on the left) are pressed into the shaft causing the thread diameter to be larger that that of the shaft.
Double lead is another thread design that uses parallel threads. These threads circle the shank and originate at the point where the screw is. The double-lead thread design produces more threads per inch. Screws drive quicker because of this. Double-lead threads can only be used for screws with a length of 3-15 cm. Double-lead threads provide stability for the screw during heat treatment, which reduces the tendency of the screws to bend.
Heat Treatment: Next Step
With the threads created, screws travel through a heat-treating process. Heat-treating is the most important step in making screws that are strong or weak. (The raw materials that are used to make the screws can also increase their strength.
Heat-treating is accomplished using a variety of methods. It is best to break the screws into smaller pieces, such as hoppers, before running the screws through heat treatment. This method allows the screws to travel more evenly, ensuring each screw is heat-treated properly. Manufacturers shake the conveyor belt while the screws move along. This helps increase efficiency. The screws will heat and mix more evenly when shaken.
Heat-treating temperatures vary depending on the length of the screw, but in general the furnace temperature should be close to 1,600 degrees Fahrenheit. The screws become more fragile if they are exposed to temperatures higher than 1,600F. They can snap if they become too fragile.
Heres the Shocker!
All screws should not be subject to heat treatment. One of the single-most important steps used to build strength in screws is left out of the process by some manufacturers. This is the main reason some screws break and others do not.
Drywall Screws
The properties weve discussed so far relate to wood screws. Contrary to what you may have heard, drywall screws should not be used for woodworking projects.
Besides being heat-treated at a higher temperature and being brittle, drywall screws have a smooth bugle head a curved transition between the shank and head. This design is to keep the screw head from breaking the paper of drywall wallboard (the only true use for drywall screws.
) The countersink of a drywall screw is not compatible with the recess created when using a standard countersink bit.
Also, drywall screws generally have thin shanks equal to the size of a #6 wood screw and oversized heads that are the size of a #8 wood screw. The oversized head helps to minimize tearing through the drywalls paper face as well.
Its easy to understand that if you tighten a screw with those characteristics, its more likely to break when driven into wood. The bugle-head design doesnt fit the countersink and the thin shank reaches a point of stress prior to the larger head being properly seated.
About our Testing The test consisted of driving a number each screw and keeping track of which screws broke or came out during driving. Always drill a countersink when installing screws in your work. For the test however, I simply drove the screws into maple. No countersink. There is no starter hole. This keeps the test on a level playing field.
Proper heat-treating is extremely important in wood screws. This manufacturing step is clearly demonstrated by the results of the test. Premium screws, which are the two rows to the right, were not only fully seated into maple but also seated with sufficient force to cause splits in the wood.
Surprise, Surprise
I was a long-time screw worker and a bit skeptical so I knew what to expect. Let me say, I wasnt surprised with the results of the test. But I was aghast at the severity of difference.
I began the test by driving five home-center screws. All five screws broke. They did not just break when the maple’s countersink came into contact with them; they broke at the mid-point of their length. Not one screw seated properly. Wow.
I didn’t want to come to a quick conclusion, even though it was difficult considering the results. So I drove home-center screw after screw. In the end, all 20 screws fell victim to the same type of destruction.
Next came the drywall screws. I was expecting the same results. Surprised again. The drywall screws performed better than the home center wood screws, but not much. I tested 20 drywall screws and found that only four were up to the task. Interestingly, most of the drywall screws reached the countersink portion of the screw, then broke. Clearly, this reinforces earlier statements.
Better Screws Beget Better Results
Down to the final two contenders. I wondered if these screws could withstand the test. After all, maple is a hard wood. Perhaps all the screws would fail and I would need to start over and rethink the test.
After driving the first Spax screw, it was clear that the test would not be repeated. This premium screw drove easily. As the countersink portion of the screw reached the maple, the screw properly seated as a screw should. The head was flush with the hardwood. All 20 screws were tested and seated perfectly without any recess or breakage.
The Spax did not fare as well with the McFeelys premium screw, which were the least expensive. You bet they did. I used 20 screws to drive them; they all fit perfectly and held their shape. The McFeelys screws, like the Spax, showed no signs of breakage or recess damage.
Premium screws were expected to perform better than others. However, I was expecting at least a few of them to fail during the test. Neither screw, the Spax nor the McFeelys, flinched at the job. This surprised me.
I will make sure that I have premium screws from Spax and McFeelys available. You can save money and also save time while you are building your project. And thats a savings of which Im happy to take advantage.
– by Glen D. Huey
From Woodworking Magazine, Summer 2008, which also includes A Better Design for a Blanket Chests, Better Finger Joints, Trim End Grain Proud, A Simple Sea Chest, The Best Crackle Finish plus Letters, Shortcuts and more.
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