No spindles that are of the same type you are using are suitable for milling steel, the spindle bearing arrangement that is designed for these high speed spindle is where the problem is, apart from a cartridge type spindle and a independent drive motor I don't think there are any of this type of spindle that is made for low speed use just for steel or any type of metal cutting
If you take light cuts with what you have and have plenty of time for the job to be done you can use what you have, engraving should not be a problem, it's when using a Endmill it pulls down on the spindle releasing the preload on the bearings once this happens then you have a mess and it does not take much for this to happen
With the sand mixture unless it can bond to the steel or aluminum it is not going to make it anymore rigid, rigidity starts with the design of the machine adding pieces can help sometimes but it is always best designed for your needs at the start before you build your machine
Mactec54
Thanks Sir! I have not seen much for low rpm spindles either. I see some interesting 5 bearing power heads with reduction gearing. ( might be interesting) would bumping up to 2.2kv+ 4 bearing with an ER20 collett help?
I never intended to be cutting steel with this machine but i do have the time and its seemed to work quite well with small bites. Thats really all i need for now as i am just milling holes and slots for now.
Now that this machine works. I can build the next machine differently.
I did find this Spindle which would be a 4 pole, don't know how they have the bearing configuration though and it is a high price
https://www.aliexpress.com/item/32996693858.html
Mactec54
This spindle unit looks like it could work? . 8k rpms and mentions milling steel.
I would probably need to switch to the square linear rails instead of my round rails for my z to get any tight tolerances happening.
Im also thinking about embedding my vise into my frame/base when i do my epoxy fill. Its dovetailed on the bottom side so it would be permanently locked in. It would drop my vice jaws down 2 inches lower. Hmm
I filled my gantry void with epoxy, sand and several sizes of gravel/pebbles. The gantry took less than a cubic foot. My base fame will need about 4 cubic feet.
I am also replacing my round bar linear rails on my Z axis with 20mm square linear rails. Ive got that about half done now
Still looking at different spindles for low rpm milling.
I took a chance on that bt30 ebay spindle.
Looks like it should work fine..
After further review, i was wrong. Its totally 3 phase