Bolts

Bolted fasteners (bolts) are available in a wide variety of head styles, thread pitches, diameters, and lengths. A quick walk down the hardware store aisle offers a huge variety of options.

US Bolts

Unified National bolts are offered in different outside diameters. For a given size diameter, US bolts are available in course threads (UNC) and fine threads (UNF). Larger bolts have an Extra Fine thread option (UNEF). Bolts below 1/4” are specified by wire gauge size and threads per inch. Larger bolts are by fraction of an inch diameter, and threads per inch. These threads are cut at a 60 degree angle, so coarser threads will have a smaller minor diameter due to the deeper thread cut. In general; there is a roughly 15% increase in strength for each step of thread pitch (i.e. UNC to UNF).

Standard fit and drill size following are marked for 75% thread engagement, for aluminum, brass, and plastics, and for a “standard fit” clearance hole. Sizes include both drill size and decimal equivalent.

Metric Bolts

Metric bolts are specified by the Outside Diameter (in mm) and the Distance between Threads (in mm). There are a number of different national standards for metric fasteners; which introduces sometimes considerable differences in what standard thread sizes are sold and used in a region. Some of the more common fastners are listed below.

Standard fit and drill size following are marked for 75% thread engagement, for aluminum, brass, and plastics, and for a “standard fit” clearance hole. Sizes include both drill size and decimal equivalent.

Standardization

For consistency and more easy maintainence, teams should standardize as much as possible on a small amount of fastener sizes (preferably one). Though you will need a small amount of other fastener sizes, such as for fastening motors or other COTS parts, use of a single fastener size/pitch angle (in multiple lengths) simplifies the design and construction process as well as making it easier to find tools and parts during competition.

Many teams recommend 10-32 (UNF) cap head or hex head bolts, generally used with a 3/8” wrench and a 5/32” hex key. This size is also standard for many COTS parts, including the CIM motor and the VexPro VersaPlanetary gearbox.

For international teams, consider M5x1.0 as your standard size. This has similar characteristics as #10-32, but is much more common internationally. You will still need a quantity of standard (imperial) bolts for COTS parts included in your KoP.

NOTE: Steer clear of 10-24 fasteners! Unless you’re very careful, these are easily confused with 10-32, and the different thread count per inch can cause damage to parts when used incorrectly. 10-32 also is advantageous when tapping holes in thinner sheet or tubing wall, since you come much closer to recommended thread count.

Addendum

Shoulder Bolts

Shoulder bolts are similar to normal bolts, with the same sizes and thread counts, but rather than being completely threaded have an unthreaded (constant OD) section, followed by space for a bolted connection at the end. For example, a 3/8” shoulder bolt can be used as a fixed axle for 3/8” bearings (FR6ZZ) or bushings to provide a rotary point for an robot arm or an extension.

Countersinking

When bolts are used in an application where the head cannot be exposed, they can be countersunk into a surface with a countersinking tool. This will require access to a drill press (or a hand drill, with less accuracy).