I've been a bit adventurous recently and experimented with cutting steel on my router (do a search for AVOR in this forum to find a description). The machine was originally intended for wood-cutting with maybe a bit of aluminium but I was in a "what the hell" mood and tried some bright mild steel. Basically, a project calls for a kind of 3D jigsaw in 3mm steel plate, silver-soldered together. The original designer called for tapped holes into the edges of the plates to take temporary bolts, which sounded a bit fraught and a lot of work. I could have bought laser-cut plates but they cost, and I wanted to keep the project in-house, so to speak. This is a hobby activity, not commercial, which moves the goalposts a bit - I don't cost my time, within reason. So I drew up the structure in Fusion 360 with a tab-and-slot design to make it all self-jigging, and set it all up on the router to see what happened.

Problem is cutter speed. After trying a standard endmill (2.5mm), which worked but looked fragile, I bought a couple of 2.5mm 3-flute carbide coated cutters. Only 5mm flute length and on a 6mm shank, so much stiffer. Recommended for dry cutting of steel (which suits me - keeps the machine cleaner). However, according to the data sheet these cutters should be run at around 4600RPM. I've been cutting fairly successfully like this, playing with depth of cut starting with "very conservative", but this is really below the speed at which my spindle should be run - I know from experience that torque is dropping off fairly quickly at these low speeds. Curiously, I seem to be suffering from small surges of speed/torque(?) from the spindle - at fairly regular intervals of a few seconds there is a slight "thump" from the spindle, a tiny bit of extra vibration from the cutter, and then it calms down until the next time. I'm wondering if I'm running below the speed at which the torque feedback or speed control or whatever mechanism is used is effective. I'm slightly increasing DoC each time, to put a touch more load on the spindle (although I suspect that I am really not needing much torque to turn these tiny cutters) and I have played a little with the cutter speed - increasing spindle speed seems to help although I don't want to go too far over the cutter manufacturer's recommendation.

Does anyone else have any experience of the typical HY VFD/spindle combination at lower than usual spindle speeds? Anyone played with any of the tuning parameters to aid performance at low speeds? I'm also looking through the HY manual to see if there's anything that might help my understanding, but decoding Chinglish at the same time doesn't make it easy!

BTW, it is a water-cooled spindle, so no overheating problems at low speed.