CNC Lathe


A while back I bought a Colchester Triumph CNC lathe with a faulty control. The owner of the lathe was quoted a huge sum to repair the control so I purchased it from him fairly cheaply. Once I got the 2.5 tonne beast home I found it wasn't in as good mechanical condition as I thought. The bed was more worn than I thought and the Z axis screw was a bit rusty due to having been sitting for a long time. On further investigation I found that the central lubrication pump did not work. In fact there was no way it had ever worked as it was missing a valve! The lack of oil explains the wear and rust.

I completely stripped the saddle and checked it over. The X axis (cross slide) was in pretty good condition and the screw had obviously been replaced in the not too distant past. The sliding ways on the bottom of the saddle were pretty bad but they are made of Turcite, an epoxy + PTFE compound. The saddle is simply held in place a few millimetres off the bed then Turcite is injected into the gap and allowed to harden. You get an instant perfect fit with low friction. Instead of rescraping you can simply sand it back to shape. It is still a slow process though. Sand a bit off, drop the saddle on and slide it back and forth a few times, sand off the shiny bits and repeat.

As this is a fairly complicated machine and I wanted to do some clever stuff with the electronics, EMC was an obvious choice for the control software. It is open source so if I need any special functions I can write them myself. For instance the buttins on the control panel are all interfaced through a custom module for EMC.

I couldn't find any motor drives to suit the original Fanuc AC servo motors so I fitted two large DC servos instead. of course shortly after I did this someone brought out a drive that could run the original motors! Servos need an encoder so the computer can keep track of the motor's position. The Fanuc motors had these built in but the DC servos didn't. There was very little room on the X axis and I ended up fitting the encoder on the end of the screw in a waterproof housing. To get the cables to the encoder and limit switches I used copper pipe. Copper pipe works very well as it is water tight and tough while remaining fairly compact. 

Originally this machine was fully enclosed with a panel that projected through the housing for manual control. I tend to do quite a lot of manual work so I got rid of most of the enclosure and the large panel. Instead I fitted electronic handwheels and a few useful buttons to control the machine. On the top panel there are buttons to turn the spindle forwards and reverse plus a handwheel sensitivity control. This allows me to quickly switch between 1mm/turn for accurate work, 10mm/turn or 100mm/turn for quickly traversing.

This shows the whole bed. Originally you couldn't see most of this as it was behind a big sliding door. The rail along the bottom is the last remnant of the old enclosure. It used to be quite a bit higher and the door slid along the top of it. There used to be a big casting where the bottom panel is fitted on the saddle. This projected through a slot in the cabinet and the original hand wheels and control panel were then bolted to this casting.

This is inside the control cabinet.  Towards the top right is the computer. Below that are the contactors, fuses and overload trips. On the left at the top is the 7.5kw spindle inverter. Below it is the mains switching. In the middle at the top is the LCD monitor and below that the two control panels. The object at the bottom with a blue lid is a dehumidifier. As I live near the sea, damp is always an issue and electronics don't like damp.

Here is the wiring on the back of the front panels. The left hand panel has power switches and assorted knobs. The right hand panel has a lot of illuminated buttons. It was originally controlled by the Fanuc computer but I added an adapter  so I could control it using Modbus over a USB connection.

These two boards are make up the adapter. The right hand board is an Arduino which is a small computer on a board and includes a USB connection. The  left hand board is  my own design and interfaces the Arduino to the panel. The tracks on this board were routed out on my Bridgeport CNC mill. I used a clever piece of software called Visolate which takes the output of a PCB design package and works out how to cut it with the shortest cut paths possible. The tracks look a bit weird but they all provide the correct connectivity.

This is the stack of boards assembled on the panel. 

Here is a closer look at the right hand side. As you can see there are a lot of contactors, fuses and current trips in this machine. Towards the top are the interface boards that handle the hand wheel encoders, buttons and other inputs. The bunch of orange boxes towards the top left are relays for various switching tasks. All of these boards are wired back to a Mesa Electronics PCI interface board in the computer.

Moving round to the front, here are the main controls.

This panel controls power on and various overrides such as feed rate and spindle speed. The knobs are read by the Arduino on the left hand panel.

This panel controls a lot of the lathe's functions. The label is just paper with a plastic cover laminated over it. once I have finalised the layout I'll print on self adhesive vinyl then laminate that to make a completely water and oil proof label. A lot of the upper buttons will be fairly familiar to users of CNC machines. The gear buttons control the electronically operated gearbox. Teach doesn't do anything yet. I hope to write  a module to handle this at some point. Below we have tool selection. These buttons aren't really relevant for CNC use but they are really handy for manual use. First I enter the dimensions of each tool. Each tool has a unique number. This only has to be done once unless I move the tool in it's holder. Now when I change a tool I just press it's button and the computer automatically applies offsets for that tool. I can read the position of the cutting edge directly from the screen.

Here is a video that I took a couple of years ago. I didn't have coolant plumbed in at that point.

Another video, showing one way of cutting a keyway on a CNC lathe.

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