3 Common Questions About Screw Pockets

1. Why Screw Pockets?
Traditional wood joinery evolved over hundreds of years into the twentieth century when electrically powered machinery became common place. Nearly all forms of traditional wood joinery involved the use of large, expensive, complex, and frankly dangerous machines. The most wide spread forms of joinery, such as mortise & tenon, dovetail, finger joint, and dowel construction had several things in common. They required precise time consuming setups. The required that both members to be joined be precisely machined for the joint to work. As previously stated, the machines involved were large, expensive, and required some skill to operate. In all the cases above the success of the joint depended on two conditions; the accuracy of the machining, and the use of glue to bind the parts together. To insure a successful joint the members had to be physically clamped together until the glue set. So, the economics of these methods of joinery were determined by the amount of labor required, cost of machines, low through put capacity, and time consuming clamping.

Woodworkers discovered that they could toe screw parts together successfully if they were to bore a pocket at an angle with a step bit in only one of the members to be joined. If they clamped the two members together they could then drive a screw into the pocket and thus join the parts. If they glued the parts the screw then acted as a clamp and eliminated the need for clamping equipment. This method initially became popular with hobbyists and smaller shops who did not want to invest in the equipment needed to make the other joinery methods work. A number of small jigs became commercially available and eventually several machinery manufactures began to offer powered equipment to produce these screw pockets, often called ‘pocket holes’. By the 70’s and 80’s screw pocket joinery for face frames and furniture were becoming widely accepted as an economical alternative to traditional methods.

2. Why Low Angle Screw Pockets?
As screw pocket joinery became more acceptable and popular, woodworkers began to look for more diverse ways to use them. They tried using them in panel joinery for store fixtures, case goods, frameless cabinets, etc. They discovered two major obstacles to using pocket holes for these applications. Because the pocket was bored with a step bit it was necessary to drill at a sufficiently steep angle for the bit to successfully create the pocket, usually between 14 and 16 degrees. Because driving a drill bit at such an angle into the surface of hard, slick materials like melamine created a high lateral load on the bit, they experienced a high amount of bit breakage. The other problem was that due to the shifting created when driving a screw into two parts at such an angle, the parts had to be clamped securely. This presented quite a problem for large panels, case parts, etc.

In the late 70’s and early 80’s Max Durney and his brother Sy were building cabinets, high end interiors, and store fixtures in Petaluma California. They took on a job that required that they make a large order of store fixtures to be shipped unassembled to Saudi Arabia and assembled there. Max felt that screw pockets would be the best method to accomplish this but he was dissatisfied with pocket holes for the reasons discussed. Max, being highly inventive, set about solving this dilemma. He discovered that by making a low angle pocket (6 degrees) by machining with a router he reduced the shifting force significantly and eliminated the problem of bit breakage. Furthermore, by drilling a true pilot hole into the pocket from the edge he further reduced shifting forces and caused the members to draw together rather than try to separate when the screw was driven. This greatly reduced the need for clamping and in fact many times two parts could be held in place by hand as the parts were joined. The method was so successful that Max patented the method and the pocket. This led to the development of the Castle line of screw pocket machines that were subsequently successfully marketed under the Castle brand. The Castle low angle screw pocket began to supplant pocket holes throughout the market, especially when the larger cabinet manufacturers began to adopt this joinery method.

The efficiency of this method is based on several factors. The machinery is not terribly expensive and almost anyone can use it. Only one member need be machined so accuracy is not an issue. Even if clamping is used to augment assembly, such as for face frames, the parts once assembled can be set aside while the glue dries, thus through put capacity is highly increased. Lastly, low angle screw pockets are fast, easy, and labor efficient.

_DSC3253

3. Why Mechanical Drive Motion?
After the Castle patent on the low angle screw pocket and method expired in 2003 the market opened up for other brands to adopt the method. Surprisingly none of the other major players were interested in switching from pocket holes to the low angle pocket. This created an opportunity for the designers at ARC Machinery and the technicians at Safety Speed Manufacturing to develop and build their own machines. The very first decision that was made was not to copy the methods of the Castle machines, which rely on pneumatics for operation and electronics to control the pneumatics. The downsides of this approach are numerous, most especially the affect that dirty air has on pneumatic components. Using double acting air cylinders and physical stops with electric sensors means that the operating cycle could be highly variable due to fluctuating air pressure and flow. In addition as components aged the cycle time gradually increased resulted in burned bits and other malfunctions. Because these systems were complex troubleshooting could be problematic, especially over the phone.
Arc’s designers decided to adopt an old traditional mechanical method based on the cam. This design created a cycle that was precise, fast, and extremely reliable. The cam allowed the SPM301 to engage both the drill and the router at the same time, slightly off set, which created a very fast and flawless cycle. Pneumatics are used only for the hold down, which is the simplest application available. The SPM301 designed cam action means that the cutting tools are always under precise control. The speed of the cycle can be easily adjusted. ARC adopted a rounded router bit which produces a round bottom pocket which more closely conforms to the head of the screw. This along with the shorter router stroke makes the SPM301 screw pocket shorter that the flat bottomed castle version, enabling pockets to be put in narrower parts. With the SPM301 design essential adjustments are more easily done, such as pocket depth, web offset, and cycle speed. In effect, ARC and Safety Speed Manufacturing has taken the superior screw pocket technology pioneered by Max Durney and delivered it in a unique and better manner.

Leave a Reply

Your email address will not be published. Required fields are marked *