Minimill/2 restoration

This Web page describes some work on two Minitech Minimill/2 3-axis tabletop mills. This page last updated June 6 2011. To email @ me for details, check my ordering page.

Intro

A few years back, I acquired two MiniTech brand Minimill/2 systems and an old PC which ran them. The MiniMill /2 was an early generation (1980's) of desktop milling machines, primarily for CAD/CAM training and for small part work. These ran under an MS-DOS based program for simple CAD design, and for control of stepper motors which advanced a small milling or drilling tool over an X-Y table driven with Acme screws. I've cleaned up one of them and it's operating under its old CAD program.

Progress

In early 2009 I cleaned up one of these. The rails were all rusted, so I used "Bar Keeper's Friend", an abrasive metal cleaner with some rust-reversing properties, to clean the rails. The Acme threads looked OK and I cleaned and relubricated them. I pulled the old DOS program and copied it to a deskop computer running Windos 98 and MS-DOS. The program appeared to operate but I needed to confirm how to power the mill and connect it to the PC.

I contacted MiniTech and spoke at length with Dave Cummings and Jack White. The company was very cooperative and sent me descriptions of the cabling to the mill from the PC (parallel port), and assured me that Mach 3 (a readily available CAD/CAM software product) could run this mill. They said it pleased them that there was still interest in their early product and they enjoyed how people upgraded or restored these machines. They confirmed that it really DID take a 20A 12V power supply to run these low-voltage steppers.

The a/c cord powers two ac outlets and runs the spindle motor. An external power supply is connected by a two-wire cable. Their original supply was an Astron RS-20 which supplies 12v 20A - jack white positive other negative. I've confirmed these 1.5V steppers DO draw 15-20 A when stepping! Only one axis at a time. PC control is from a parallel port, internal optical isolated 4N33's to UNC5804's (from MiniTech docs). No encoders, no return signals to PC or "stops" that I see. Range of motion, 5" X 10" X 6" height approximately. The steppers are 200 step per revolution, and that advances the table 1/10 inch; that suggests 2000 steps/inch.

I acquired an old high-current power supply and wired it up to the mill. I ran the program and used its manual "jog" features to move the mill around. Sure enough, I could get the mill in motion! So I ran the sample project file, and watch the mill go through its paces. Sure enough, the ammeter showed 12A current draw and surges above that as each motor operated. In Oct 2009 I got a switching supply which I'll try that has more current.

I put the project aside until late in 2009, when a few people discussed small computer milling or drilling machines with me. I've brought up this Web page at the time.

2011

In 2011 I looked them over in more detail, and got both mills cleaned up and running.

Here's a general sketch of the mill to show travel of the table and head. You can look at the front of the mill, and here is the side view of the mill.. Here's a view of the head.. The rails are 1" diameter, the Acme threaded shaft is 1/2 inch, 8 TPI. The steppers are 200 steps per rotation, so each full step is 1/1600 inch, or 8 steps per 5 mills travel.

This photo looks at the 1-inch diameter rail of the X-axis and the bearings. There's a similar shot of the 1-inch z-axis rail and bearing. The 1/2 inch 8 TPI Acme threaded drive shafts ride through some kind of plastic Acme nut. The Acme shaft couples to the stepper with a torque dampener of aluminum.

Herb Johnson
New Jersey, USA
here is how to email @ me