When you are machining a hole for a fastener, you must select an correct hole size because the size of the hole determine how well the fastener will function. A hole that is too small will cause the fastener to struggle, and a hole that is too large will cause the fastener to spin uselessly without gripping the material. You must understand the difference between a tap hole and a clearance hole.
A tap hole is a smaller hole designed so that you can cut threads into the hole, and a tap hole allow a fastener to grip the material. A clearance hole is a larger hole that allows a bolt to slide through the hole without the threads catching on the material. You must use a chart to determine the correct dimensions for these holes because the dimension for a tap hole and the dimensions for a clearance hole is different.
How to Make the Right Hole for Fasteners
The type of metal you are using affect the size of the hole you should drill. If you are working with steel, you should aim for a thread engagement of approximately 75 percent. A thread engagement of 75 percent provide enough metal for the threads to grip the steel, but a 75 percent thread engagement also prevents the tap from requiring too much torque.
If you drill a hole in steel that is too small, the torque required to use the tap will increase, and the tap may break inside the hole. If you drill a hole in steel that is too large, the screw will strip out of the steel. Aluminum and brass require different hole sizes because aluminum and brass are more softer metals than steel.
Aluminum and brass are prone to galling, and galling is a process where the metal seize and tears during the machining process. For aluminum and brass, you should aim for a thread engagement of approximately 60 percent because a 60 percent thread engagement make the tapping process smoother and reduces the risk of the tool getting stuck. You must also consider how the head of the fastener sit in the surface of the material.
If you use flat head screws, you must use a countersink to create a conical recess so that the head of the screw sits flush with the surface. You must match the angle of the countersink to the angle of the screw because a mismatch in angles will cause problems. For example, if you use a 90-degree countersink with an imperial fastener that require an 82-degree angle, the screw will not sit flush.
If the screw does not sit flush, the screw will either sit above the surface or the screw will rock inside the hole. If you want to hide the head of a fastener completely, you must use a counterbore. A counterbore is a flat bottomed recess that is designed for socket head cap screws.
You must ensure that the depth of the counterbore is at least as deep as the height of the bolt head because if the counterbore is too shallow, the bolt head will protrude from the surface. The method you use to drill the hole is important for the success of the machining process. When you are drilling a deep hole, you should use a technique call peck drilling.
Peck drilling is a technique where you retract the drill bit frequent so that you can clear the metal chips out of the hole. If you do not clear the metal chips, the metal chips will pack together in the hole and create heat. Heat can cause the drill bit to jam or the drill bit to snap.
Additionally, you must use a lubricant when you are drilling in metal. Using a cutting fluid is a requirement because the cutting fluid keeps the heat low and keeps the threads clean. Cast iron is an exception to this rule because cast iron is a self lubricating metal, so you can drill cast iron without using a cutting fluid.
If you follow the correct dimensions and use the correct technique, the hole will serve the fastener correctly.
