Improved MF70 CNC Underway
Posted by retromaster on December 24, 2009
Here are a couple of aluminum plates, some of the very first milled by my CNC-converted MF70. These are intended for the Y axis of the new, improved version of my MF70 CNC. A standard type 606 ball bearing fits in the circular recess in the middle, just like the bottom one in the photo. The outer holes are for mounting the axis stepper motor, and the inner holes are for attachment to the table. At the moment I am about to start the process of milling a couple of standoffs that will go in between the table and these plates. This will hopefully extend the woefully limited Y axis travel of the MF70.
Milling new parts is going rather slowly at the moment. The current mechanics of the machine are not very precise and rather inefficient. To ensure that the motors do not miss any steps, they need to be driven at a rather low speed. This is in contrast to the built-in spindle of the MF70, which seems to do best with relatively light cuts with a high feed rate, due to the rather high RPM (5k to 20k). The new parts should help alleviate this problem.
One other thing that does not help the situation is that the torque obtainable with the current stepper drivers is quite beneath the potential of the motors I use. To overcome this, I’ve started to work on a new controller board that implements bipolar chopper drive instead of the unipolar L/R drive of the current board. In addition, the new board will increase max current per coil from 0.5A to 0.6A. For the new board, I am also seriously considering to switch to the LPT port instead of USB, as this will enable me to use readily existing software such as EMC2 without a lot of work.