I recently watched another great Dr. D video showing his beautiful heavy duty mill enclosure. There was a quick mention of a switch used to activate the drawbar and software used to disable the drawbar when the spindle was active. My mill had a mechanical latch that needed to be moved each time I used the drawbar. This latch was wired to a microswitch and served as a safety mechanism to disable the spindle while the drawbar was active.
I decided to nix the mechanical latch on my drawbar and use the dry contacts on my VFD. I figured that when the spindle was turning, I would program the dry contact to open. I would use these open contacts to disable the drawbar (see below the dashed line on the attached schematic).
After a bit of experimenting, I noticed a potential problem. If the spindle is stopped, and the drawbar is active AND you press the button to turn on the spindle, nothing will happen at that moment, and all seems safe. However, when drawbar is released, the spindle will start spinning immediately. Typically, when I release the drawbar, I put one hand on the tool to stop it from falling out and crashing into the mill. I certainly would not want the spindle to start spinning while my hand was on the tool. It may seem unlikely that anyone would turn on the spindle while the drawbar was active, however if you use a manual edge finder, it can happen.
To solve this problem, I added a small $8.00 relay I found on Amazon and wired it with a holding circuit (see above the dashed line on the attached schematic). With this setup, each time you release the drawbar, you must hit the drawbar reset switch to enable the drawbar. Now the only way to have the spindle active while your hand is on the tool, is to hit the reset button while simultaneously having your hand on the tool. Not very likely.
I hope others may find this helpful.