A clearance hole allow the shank of the screw to pass through the material without engaging the threads of the screw with the threads of the material. When a clearance hole is use, the screw wont grip the material in that area. Instead, the screw will use a nut or another component with a threaded hole to grip the material.
If the clearance hole is too small for the screw, it will bind in the hole. If the clearance hole is too large for the screw, it will not remain secure in the material. There are three type of fits for clearance holes.
How to Choose and Drill Clearance Holes
Close fits allow the screw to remain in place; the close fit is best for applications where precision is necesary. Normal fits are standard fits for the majority of assembly tasks. A loose fit allow the screw to move within the material and is used for situations where adjustments after the screw is assembled are needed or if the material may expand due to heat.
A clearance hole chart will allow people to determine the correct drill bit size for a screw. Clearance hole charts list drill bit sizes for screws of different diameters. For instance, a clearance hole chart will specify what drill bit size will provide a close fit, normal fit, or loose fit for an #8 screw.
The same is true for metric screws. If a person is using an M3 screw, they will use a 3.2 mm drill bit. Clearance hole charts uses standards from the ASME or ISO to ensure the correct size of the clearance hole for the screw.
The type of fit will depend upon the requirements of the assembly. Close fits is use for precision assemblies. For general applications, a normal fit is used.
A loose fit can be used for situations where adjustments are made after the screw is assembled; however, if screws are subject to vibration, a loose fit can lead to the screws coming loose over time. A person must decide whether they would like the screws to have a tight or loose fit. A tight fit can cause the screw to bind when inserted into the material, while a loose fit can lead to screws moving within the material.
If a person is to use flat-head screws, a countersink will be necesary to allow the head of the screw to sit flush with the material. The countersink will have the same angle as the head of the screw. An 82-degree countersink is the standard for flat-head screws.
It is important to drill the clearance hole before cutting the countersink. If socket head screws are used, a counterbore will be used instead of a countersink to allow the screw head to remain below the material. The type of material will affect the drill bit size and speed at which the drill bit should be drilled.
For steel materials, high-speed steel drill bits should be used along with oil to lubricate the drill bit. Additionally, peck cycles should be used when drilling into steel to prevent the drill bit from overheating. For aluminum materials, drill bits should be run at high speeds.
Additionally, the drill bit should be retracted from the aluminum while drilling to prevent the aluminum from welding to the drill bit. For wood materials, brad-point drill bits should be used to prevent the wood from splintering. Additionally, a drill bit that is stable when drilling into the wood can provide better results.
For plastic materials, drill bits should be run at slow speeds and the bits should be sharp to prevent the plastic from melting when drilled. Finally, for titanium or composite materials, carbide drill bits should be used along with coolant for these type of materials to prevent them from working hardened by the drill bit. In order to drill the correct size clearance holes, the correct drill bit must be selected.
Numbered and lettered drill bits are used for small machine screws. Fractional drill bits are used for larger bolts. Step drills are used for thin sheets of metal.
Drill bits must be sharp to provide the best results. If drill bits are dull, the screws will have to be manually force into the material. Additionally, a dull drill bit may create an oversized clearance hole due to the heat created by the dull bit.
If a person does not use a clearance hole chart to determine the correct size of the clearance hole, the screw may bind in the material. If the clearance hole is undersized relative to the screw, it is possible that the screw will bind and may even cross-thread when being inserted into the material. If the clearance hole is too large for the screw, there will be a reduction of the clamping power of the screw, and the screw head may even pull through the material.
It is important to deburr the clearance holes to prevent the screw head from not seating properly within the material. Finally, it is important to test the fit of the screw in nonessential materials to ensure that the clearance hole size is correct.
