31 Mar. 1979|
Wood to metal fasteners,pool furniture made pvc pipe,online wood carving magazines - Review
We have self-feeding fastening tools available, and fasteners and tools for wood & composite decking for dock builders. 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). 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.
This fastener guide gives a great pictorial overview of just about every fastener type, and here is a good overview of possible head types..
Machine screws are generally stronger than wood screws, have finer threads and are made more precisely. The metric coarse thread is actually between UN coarse and fine thread, and the metric fine thread is finer than the UNF threads.
Metric threads are specified in ANSI B1.13M-1982 (R1995), which is very nearly equivalent to the original ISO 68 specification. UNJ or MJ: These threads are used in situations where fasteners must withstand high fatigue stress, predominantely the aerospace industry.
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. There are numerous standards that fasteners are manufactured to, and those standards describe everything from material chemistry to surface finish to heat treatment. These washers are probably most effective in joints where the recommended tightness cannot be achieved, such as soft metal, plastic or wood joints. Galvanic Corrosion: When placed in contact along with an electrolite (like humid air), certain metals form little batteries that corrode each other. The threadless shank allows the top piece of wood to be pulled flush against the under piece without getting caught on the threads. The head-to-shaft junction is more curved than in a wood screw to prevent tearing of the dry-wall. This may be surprising given the almost universal recommendation that fine threads be used in sheet metal and other thin-walled materials. Blake's "What Every Engineer Should Know About Threaded Fasteners: Materials and Design," recommends not using fine metric threads. Because of this, efficiency is a concern, and the 60 degree thread profile in standard fasteners is unsuited. According to this Unbrako whitepaper on the Fastener Act, over 85% of failures are due to fatigue and not a simple overloading situation. 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).
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. The Unified standard doesn't demand that the roots (vallies) of external threads be rounded, but just about all fasteners less than 1" come that way because their threads are rolled (see Blake book ref.
Proof stress is similar to Yield stress except that it is slightly less (about 90%), and only applies to fasteners. The thread geometry causes them to yield slightly before the Yield stress level of the material, so Proof Stress can be thought of the true yield--in other words, the fastener will behave like a spring below that stress level. The referenced formulas may at least provide a rough estimate for sheet metal, where thread engagement is limited. There can be enough metal transfer from a file to your stainless piece to cause rust to star. Galvanized hardware is a good alternative to stainless but be aware that the process of making something galvanized requires that it is dipped in a molten metal vat (hot dipping).