When to use or not to use the drop hole allowance for precision metal parts

A dovetail tool of choice when working with precision hardware is a cutting tool that creates a trapezoidal shape or dovetail groove in a part. Due to the form of these tools, special consideration is required to achieve long tool life and excellent results. This is especially true when machining O-ring grooves, as this operation requires the tool to drop into the part to begin cutting. Using proper tool entry methods, especially knowing when drop hole allowances are required (and not required), is important to prevent common dovetail accidents.

What is a drop hole?

When designing a part with O-ring grooves, taking dropout allowances into consideration is a critical first step. Drop holes for precision hardware parts are eccentric holes that are milled during roughing/grooving operations. This feature allows the use of larger, more rigid tools. This is because the cutter no longer needs to fit into the slot, but into a hole with a diameter larger than the diameter of its cutter.

Why consider adding drop holes?

A tool with a hole allowance has a larger neck diameter to tool diameter ratio than a tool without a hole allowance. This makes the drop-hole tool stronger, allowing the tool to take heavy radial depths of cut and fewer steps. Using drop holes will allow stronger tools to be used, which will increase productivity and extend tool life.

Drop holes are sometimes not allowed in the design of precision metal parts because it leaves points of increased stress concentration. Common examples of disallowed vulnerabilities include:

1. In high pressure applications;

2. In seals that require high reliability;

3. Places where dangerous or dangerous liquids are used.

The problem with drilling allowances is that the extra clearance for tool entry can create weak spots in the seal, which can then be compromised under certain conditions. Ultimately, downhole limits require customer approval to ensure that the application allows.

When adding drip holes to precision metal parts, it is important to ensure proper feature placement to maximize seal integrity. According to the diagram below, the recessed hole should be placed outside the center of the groove, making sure that only one side of the groove is affected.

You must also ensure that the recessed hole feature is placed on the correct side of the groove. Since the O-ring acts as a seal between the pressures, it is important to have the drop hole bordering the high pressure area. As the pressure moves from high to low, the O-ring will be forced into the fully supported side for a proper seal (see image below).

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