Depth setting rings are plastic collars that are securely pressed onto the 1/8 in. shank of a drill bit, end-mill, router bit, or similar rotary cutter. They are 0.20 in. (5.1 mm) thick by 0.25 in. (6.35 mm) dia.. The shank side of the ring (top side in the picture to the right) is accurately positioned (d ±0.004 in.) relative to the tip of the tool. If the shank-side of the ring is pressed against the collet when the tool is loaded, the tip of the bit will be a known distance (d) from the face of the collet. Knowing this distance, the programmer can plunge the tool, confident that the depth of cut will be as expected and will remain the same no matter how many times the tool is changed. If all of the tools in the programmer's magazine have been fitted with depth setting rings, an automatic tool changer can load and unload tools with the tip of each one the same distance from the face of the collet.
This is especially helpful with micro-tools where "touching-off" on some reference surface to calibrate the tool height can damage or break the tip of the tool. Rings also find widespread use in inlay pocket cutting where relatively large diameter cutters are used to "hog out" basic shapes, followed by much smaller diameter finishing bits that clean up the edges, sharpen the corners and add fine detail to the finished part.
Most PreciseBits 1/8 in. shank bits are available with or without depth setting rings. There is no charge to add the rings, but the user is cautioned to make sure that their equipment is compatible with their use.
The most common values for d are 0.570 in. (small diameter micro-tools) and 0.800 in. (industry standard). However, with the advent of mechanically etching printed circuit boards in the mid 1970s, a new standard was implemented for the special purpose tools developed for this application. To wit:
Today, only machines made by LPKF and T-Tech adhere to this special standard. Systems from AccurateCNC, MITS, Sun and EverPrecision are compatible with the original industry standard depth of 0.800 in.