http://blog.cnccookbook.com/2012/03/...tting-success/
I am providing this link because it gives what I consider good and well explained advice on milling aluminum in a gantry mill.
It considers what if the spindle is much faster than the spindle of a bridge or turret mill.
It considers the ideal feed rates which are *much* higher than what a stepper driven gantry mill is capable of in many cases.
It shows the interrelated factors that influence the designs.
Most importantly:
Quote:
8. Don’t slow down the feedrate too much!
If you go too slow on your feedrate, you run the risk of making your tool rub rather than cutting. This is a much bigger risk for CNC Router users than mill users simply because the spindle is going so fast. In order to maintain recommended chiploads with rpms that high you’ll have to keep the cutting moving smartly. Our 3/16″ cutter at 21K rpm wants to feed at 91 IPM, for example. If you slow down too much, say to 1/4 of that, many will think they’re babying the machine and tool. Nothing could be further from the truth. If you wind up going slow enough that the cutter starts rubbing at 20K rpm, you’re going to heat up the whole works and drastically shorten your tool life. For more on this rubbing phenomenon, see our article on chiploads and surface speeds.
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It would be very easy to gear down a CIM from a peak power output at 2,500RPM to 900RPM turning a 10tpi lead screw. You'd be close with a 2.5:1 ratio. Versus the 100:1 ratio I used above while targeting a replication of 1.8 degree stepper resolution.
I can with certainty write that my Oriental Motor PK268M-02A bipolar, 4 wire, 0.9 degree stepper motors will loose steps and bring operations to a dead halt at anywhere near these feeds.
Therefore the 'sweet spot' is somewhere between a gear ratio of 2.5:1 and 100:1. The lower one makes the ratio the more the precision will suffer. The higher one makes the ratio the lower the feed rate will be limited and depending on what you are carving your way through that could be a problem. With steppers the only way you could get near these feed rates would be a high power stepper geared up. Gearing it up reduces the resolution by that ratio.
Once we accept that we must loose stepper resolution by gearing up to get to the feeds to reduce the tooling issues it makes the CIM look much more reasonable. Plus in reality the Acme lead screws on most machines will limit the tangible resolution much more than it would appear. Most small gantry mill kits list a repeatable resolution of 0.005" or so for this reason. They might move into smaller increments but it can be unreliable to repeat it over and over.
Now we can slow the feed rate if we can turn the spindle slower.
So how about if we can get closer to a bridge or turret mill speed of 5,000RPM?
We could buy a cheap Chinese motor, motor control, power supply and mount off E-Bay for $150+.
400W Chinese spindle motor
However we have 2,500RPM+ CIM motors and motor controls.
Plus we are already providing batteries to drive the other CIM.
So we could gear the CIM 1:2 for speed and get 5,000RPM.
At more torque than the Chinese set as well (interesting the torque ratings on the Chinese motors are 0.35 or 0.5Nm).
Most of those are 0.5Nm or about 70oz-in a CIM at 2,500 RPM is closer to 170 oz-in (do consider the gear ratio).
You'd need to provide a spindle with the ER11 collet fixture to do this.