Engraving Bit Isolation Milling
In version 0.27, Otherplan added support for engraving bit isolation milling. This guide provides background information on why you might want to use engraving bits to mill PCBs, tips on choosing an engraving bit, and step-by-step configuration details.
Flat End Mill vs. Engraving Bit
Traditionally, we’ve recommended using flat end mills when milling PCBs on the Othermill. Flat end mills have many advantages. Their straight profiles and consistent diameters ensure that milled traces are of a consistent width, even if the piece of FR-1 used isn’t perfectly flat. Flat end mills are also great at clearing large areas of material, and large flat end mills can remove material very quickly.
Flat end mills do have downsides, however. Smaller tools, particularly 1/64" and smaller, are fragile and can be easy to snap. To avoid breakage, they require slower feeds and speeds, multiple passes, and can result in long milling jobs.
Engraving bits are a fantastic alternative to flat end mills for some PCB milling tasks. Because engraving bits are tapered, they’re made of more metal and are stronger and less prone to breaking than their end mill counterparts. Even very narrow engraving bits can mill successfully at comparatively high speeds and in only one pass. This makes them a great solution for quickly milling precise circuit boards.
Engraving bits also have tradeoffs. Because of their taper, they’re not suitable for clearing large areas of material or for milling through-holes or board outlines. Because the width of the engraving tool varies based on depth, warped or inaccurately measured boards can result in inaccurate traces. You’ll want to account for the thickness of double-sided tape or other fixturing to ensure proper depth. Also, the profile of the resulting traces are tapered, not flat, which may have implications for boards that require precise trace geometries (i.e. high-frequency circuits and antennas).
|Flat End Mill||Engraving Bit|
|Milling Time, large areas||Fast||Slow|
|Milling time, small traces/spaces||Slow||Fast|
|PCB Features||Traces, Holes, Outlines||Traces|
Should I use a flat end mill or an engraving bit?
While every PCB design is different, here are some rules of thumb to use when choosing tools for a job:
- If the spacing on your PCB is large enough to be milled with a 1/32" or larger tool, use a flat end mill to mill traces, holes, and outlines without requiring a tool change.
- If the board requires 10-mil trace-and-space clearance, an engraving bit will be much faster than a flat end mill.
- If the board requires sub-10-mil trace-and-space, an engraving bit is required.
- If the board requires precise three-dimensional trace geometry, a flat end mill will provide the best results.
- If your board is an irregular shape, has vias, or requires both through-hole and surface-mount components, use both a flat end mill and an engraving bit.
What type of engraving bit should I use?
Engraving bits are classified by tip size and angle. Tip size refers to the diameter of the top of the tool, while the angle refers to the taper. A 80° bit is somewhat blunt, while a 30° bit comes to a very sharp point. Because they’re tapered, the effective milling diameter of an engraving tool varies with the milling depth.
We recommend two engraving bits: 80° and 30°.
The 30° bits are sharp and come to a narrow point. They’re a good balance of precision and strength, and are what we recommend for PCB milling.
Configuring Otherplan for Engraving Bit Isolation Milling
In Otherplan version 0.27 or higher, it’s easy to set up engraving bits for PCB isolation milling. Simply select an engraving bit in a PCB configuration panel and Otherplan will do the rest.
Here’s a step-by-step walkthrough:
1. Measure the thickness of your circuit board using digital calipers and enter the thickness into Otherplan’s Material Size panel.
2. If you’re using double-sided tape to attach your PCB to the Othermill, measure its thickness with calipers and add an additional z-offset to Otherplan’s Placement panel. This will ensure that the engraving bit is milling to the intended depth on your material.
3. Import a circuit board design file.
4. Select an engraving bit for traces.
5. (Optional) Select an end mill for holes and outlines.
Changing Trace Depth
When milling PCBs with engraving bits, Otherplan defaults to a 0.2 mm trace depth. If you wish to change the trace depth, click Advanced and change the value.
Note that changing the trace depth will not change the feeds and speeds used, so your results may vary. Additionally, the default pass depth for engraving bits is 0.25 mm, so if the trace depth is set larger than this value, Otherplan will configure multiple passes.