A while ago, I milled out a set of large letters for Ingrid Solvik. They would spell out BANK over a large doorway in a commercial she was working on. Since they’d need some handling and mounting in a somewhat difficult location, she wanted them done in foam. More specifically: insulation foam, as this was easy to get on short notice and carried a reasonable price tag.
In total, we milled twelve letters, which would later be sandwiched together in layers of three to form the final letters. This meant we had to do a fair bit of milling, and since this was a bit of a rush job, I set the feed rate rather high for our first try.
The pros of a high feed rate is not only that the job gets done quicker, but also that heat buildup is lessened along the way. Partly because the milling bit spends less time in any given spot, but mostly because the milled chips of material is what actually transports most of the heat away as they fly off. Higher feed rate = more material getting removed = more heat removed.
The cons of a high feed rate is that the milling bit might break more easily if it’s moved through the material faster than it’s able to cut its way through, that you might get less precision if the milling bit is slightly bent (but doesn’t break) and that the cut surface gets a rougher finish.
The latter con became rather apparent during the first run. The insulation foam wasn’t exactly ideal milling material; the cut surface had lots of almost-but-not-quite severed “strings” of foam left. These had to be meticulously rubbed off by hand and left a terrible mess. So, in an attempt to reduce this, I lowered the feed rate.
Which worked, to an extent.
However, as I got more into the swing of things and worked quicker, heat started building up in the milling bit without me noticing. When I got to the second-to-last letter, the temperature quite clearly reached the melting temperature of the insulation foam, as I heard the milling sound change ominously and a foul smell appeared.
I stopped the Shopbot within a couple of seconds – but a respectable layer of melted foam still managed to accumulate on the milling bit (see the photo above). Luckily, it was easy enough to remove with pincers after a bit of a cooldown.
Lessons learned: When milling an unfamiliar material, regularly check for heat buildup. And prepare for a major cleanup operation afterwards if the material is insulation foam, those little wisps of foam get everywhere. I even had to vacuum the walls up to shoulder height!