That pen is never going to cut through wood. No wonder you are skipping steps!
0.25mm DOC and 15mm/s should never skip steps (in wood). 4mm DOC and 15mm/s might, sometimes. I’m guessing the root problem is that the bit is grabbing a mouthful at the deeper layers and it’s too much.
I hate to immediately blame the dremel, but there are a few things which could potentially be causing this from the dremel:
- shaft stiffness. The bit is always trying to grab the material and pull away chips. If it’s allowed to move, then it will pull itself into the work and grab bigger pieces of wood. This will add a lot to the load.
- runout: If there is runout, even if it is still, then it will also tend to grab the wood, especially when it’s deeper.
- non-perpendicular: If the router bit isn’t perpendicular to the XY plane, then as it plunges, it won’t be plunging straight into the slot it just cut. As it’s leaning to one side, that side will grab the whole 4mm at a time.
- Power: Even set to 18krpm, if it’s hitting resistance, that speed will drop, causing bigger chips, and higher loads. A 600W DW660 is much more powerful than a dremel and can keep the speed even when it’s hitting some resistance. This helps it blow through problems.
My immediate suggestion is to look closely at the dremel. You can just enable the steppers and push the end of the bit with your finger. If the dremel is moving in it’s mount, or the bit is moving relative to the dremel, then that’s a problem.
You can put a piece of wood on the bit, and spin it around. If the far end of the wood touches the board only on one side, then it’s not perpendicular. If the far end of the wood moves easily, then it’s not very stiff.
I don’t think 0.25mm DOC in wood should cause the zip ties to flex. I think you’ve got something else wrong. FWIW, deeper is better than faster, so I would drop the speed in half and double the DOC. But you should still be able to do twice that DOC, no problems. Slower lets the steppers have their max torque and when/if you do get a full bite, the machine will be moving slower and might just be OK.