12 Oct. 1989|
Wood machine screw,hgtv kitchen designs islands,making wood furniture at home - Reviews
Wood Screws: These have a coarser pitch (few threads per inch) than sheet metal or machine screws, and often have an unthreaded shank. Sheet Metal Screws: Usually threaded all the way to their head, these will work in wood, but wood screws shouldn't be used in metal (this is based on hardware store employee advice, not experimental evidence).
Drywall Screws: The coarse thread version is meant to secure drywall to wood while the fine thread version is for attachment to metal studs (commonly used in office construction). Slotted, Phillips and Square drives: The main drawback of slotted heads is that power driven screw drivers easily cam out. Carriage Bolts: These have a square shank that sinks into and grips wood when a nut is tightened. Flat and Oval Head: The most common type of head for wood, these heads end up flush or below the surface of the wood when installed.
Security Heads (tamper proof): These screws have heads that are either impossible to reverse or require a special driver to operate. Hex Washer Head and Truss head: These screws have a built in washer to help distribute load to a wider area. This talks about different screw types and their uses, also a bit about washers and rivets. Machine screws are generally stronger than wood screws, have finer threads and are made more precisely. Socket Screws: While many hex cap screws may be found in vehicles, socket head screws are becoming more popular and have some space saving advantages over hex cap screws.
Shoulder Screws: These have precision ground shanks that remain above the head of a hole and provide a simple way to make an axel for a wheel.
Set Screws: These are threaded along their entire length and are typically used to secure a shaft from rotating. Here's a great doc from tessco that talks about different screw types and their applications, grade and strength information, and screw material guidance. Coarse threads are less likely to cross-thread, or jam because the screw is inserted at an angle. In 1949, Canada, the United Kingdom, and the US agreed to a Unified thread that is largely the same as the American National thread that came before it, and screws from both systems are interchangeable.
Extra fine threads and minature screws: The Machinery's Handbook has listings of dimensions for really small screws.
Power Screws and ACME Threads: Power screws' job is to translate rotational motion into linear motion. According to the Machinery's handbook, tightening by feel is only +-35% accurate, and using a torque wrench only improves the accuracy to about +-25%. Another method recommended by the Machinery's Handbook is to measure the torque necessary to break a test bolt, and then use 50-60% of this value. Washers help distribute load and prevent the screw head from digging into the joint material. These washers are probably most effective in joints where the recommended tightness cannot be achieved, such as soft metal, plastic or wood joints. Sems: These are screws with freely spinning captured washers--washers that are permanently attached. A massive guide about material selection with links at the bottom to just about everything else you'd want to know about screws, including dimensions, installation torques, heat treatment, thread creation, etc.
USDA --- The United States Department of Agriculture, one portion of which, the Forestry Service, has numerous publications on wood and trees. V block --- A piece of wood with a V-shaped groove cut into one face to securely hold dowels or other rounded objects in position while performing operations such as drilling. VS --- Variable Speed; just refers to any machine that has a variable speed control, particularly for machines, such as a drill press or a wood lathe, that may come with either fixed speed or variable speed. Morse taper --- The standard for the machine taper on the shanks of drill chucks, drill bits and lathe centers.
My bowl site has a more extensive writeup about the effects of movement in service on small craft projects (such as my bowls) that use different woods glued next to each other. Most sources (like the Machinery's Handbook) define screws and bolts in terms of how they are installed: if you turn the head it's a screw, if you turn a nut it's a bolt. The threadless shank allows the top piece of wood to be pulled flush against the under piece without getting caught on the threads. Most of these screws are self-tapping in that they only require a pre-drilled hole (pre-drill sizes), but some come with self-drilling (shown in above pic) or self-tapping tips. The head-to-shaft junction is more curved than in a wood screw to prevent tearing of the dry-wall.
Consider the following picture and imagine that the screw (or spring) is clamping the red box against the ground with 10 lbs of force. Now we need to select a screw with enough strength so that it can withstand the combined external load and pre-load from tightening. The Machinery's handbook recommends 50-80% of ultimate tensile strength, or 75-90% of yield or proof strength (75% for reusable joints, 90% for permanent).
While there are many arguments for tightening a screw past its yield point (for instance), from this author's viewpoint, if an external load yields a screw, and if that load is ever removed, the screw will now be permanently stretched and loose.
The Machinery's handbook has a different formula for bolts with tensile strengths over 100ksi, but due to some doubt about its origins, we don't use it. Any unmarked machine screws smaller than that are probably Grade 2; we show the higher Grades for reference only on those sizes.
According to "Fundamentals of Machine Component Design", 3rd addition, by Juvinall and Marshek, p. This offers some formula (also found in machinery's handbook) for calculating the shear area of threads, but it's uncertain how one would apply that formula given the imbalanced thread load. If the surface of the joint isn't smooth, it's more likely that the screw will compress higher spots over time and come loose. In these cases, the washer would likely not be entirely flat and would indeed dig into the screw surfaces. That’s interesting about steel shavings enabling rust to start from filing a SS screw.
Without such a blocker, UV rays will cause some woods (padauk and redheart are excellent examples) to change color, usually in an unattractive way. Used with pully wheels to transmit power from a motor shaft to the shaft of a power tool such as a wood lathe.
In woodworking this is used as regards the flow of air in a roof (see soffit ventilation, the flow in and out of a workshop, and the flow of air around hazardous chemicals. In woodworking, this is a term that you'll see in discussions about finishes and glues in addition to wood sap and resins. When this happens to lumber that is floating to a processing mill, the logs sometimes sink, and in any event are referred to as wetwood, not to be confused with green wood (unseasoned wood) which is not fully saturated with water, although in some species, the green wood can be close to water logged. After application to wood and drying, this compound makes the wood resistance to attack by fungi or insects and also makes the wood less likely to absorb moisture from the atmosphere, thus reducing the likelihood of both decay and movement in service. Weathering does not include decay in wood, but is sometimes used also to refer to changes in surface finishes due to the effects of weather. The condition can occur in both softwoods and hardwoods and the wood is usually exceptionally difficult to dry without defects.
Often used to give a desired color to wood and then covered by a more permanent finishing agent. It includes both full object carving, representational or otherwise, and also the decoration of larger wooden objects such as religious alters and furniture by using gouges and chisels to shape the wood surface. High quality wood flour is made from hardwoods for chip size consistency but low grade wood flour is sometimes made from softwoods.
The primary source of mechanical defects is drying the wood (see drying defects), but there are others.
There are two types of meter; one type measures the electrical resistance of the wood, the other measures the dielectric property of the wood. When the wood absorbs moisture, it expands slightly and when moisture leaves the wood, the wood contracts slightly. A method called "turn-of-nut" can supposedly get within +-10%, but this relies heavily on a reliable starting point from which to start counting turns (see Machinery's handbook). Alloy steel socket head cap screws will most likely have a greater strength than SAE Grade 8 unless their manufacturer says otherwise. Because of this, fasteners are designed to fail in the bolt, not the threads, so most nuts are more than adequate--just make sure you use a similar grade of nut compared to the screw. Also, if the surface is damaged by the screw or nut, it can lead to problems with future re-installation.
Waxes are used as (among many other things) wood finishing agents and they share the common characteristics of (1) being plastic (malleable), (2) having a higher viscosity than most wood finishing agents (some are in the form of a semi-solid paste), and (3) being totally waterproof.
Mineral spirits are often used to moisten planks for photography when they are to be sold on the internet or represented in catalogs because mineral spirits make the grain pop and enhance the color in a way similar to what is likely to happen to the wood when a permanent finishing agent is applied, and then the mineral spirits evaporate off the plank although I'm not clear on how long that takes. To restate this in another way for clarity, it is the weight of the water in the wood divided by the weight of the wood without any water. No, your 5 lbs of upwards force would only reduce the pressure between the red box and the ground; it would not stretch the screw at all. Wax is usually applied and maintained in thin layers which are buffed for a shine (much like waxing a car) and while a wax finish is easy to apply and maintain, generally looks great, and has the advantage of being waterproof, it is not a hard, durable surface and does not do much to protect wood from wear. You can get the same results for pictures by spraying wood with a light coat of water, but there are some woods, especially dye woods that will suffer water stain so mineral spirits are a better idea. Also, wood changes size across the grain proportionally more than it does along the grain (and it's actually a little more complicated than that) so even wood cut from the same plank will move differently based on orientation. None of its force goes into pushing outward, whereas a 60 degree thread has a substantial force component away from the axial direction of the screw.
In fact, the screw would not feel any change in force whatsoever until the upward force exceeded 10 lbs and the reb box lifts.
For instance, be sure to use a hardened washer for high-strength screws and bolts (Grade 8 and socket head cap screws).
Compounds that include complex urethanes are sold as polyurethane finishes and they tend to be quite hard and durable and waterproof and they can be had with UV blockers to protect wood from color changes due to sunlight. If this seems small, keep in mind that the ultimate strength (breaking strength) of a Grade 2 bolt is 74 ksi, so a #6 screw could theoretically hold 672 lbs in pure tension. Here's a good explanation of these effects along with a lot of other great screw information.
There are some formulations that protect against ultraviolet rays that could otherwise cause color changes in wood. If you're wondering why bolts you see in cars and weight machines are so large, it's partly to gaurd against loosening and fatigue failure in addition to safety factors. Oven dry basis is used in wood science because one wants to always use the dry wood as the basis for all calculations.
The degree to which movement in service is a problem varies greatly among wood species and how an object is constructed from wood. Composite materials do not suffer this problem, or suffer it to a much lesser degree than pure wood, which is one of their advantages.