CNC Lathe
Back when I was in college I had loads of ideas of how to make myself some money, the main one was to make model airplane engines. I was thinking of all different ways of designing them so it would take minimal machining time and procedures, looking back on it now my design's weren't too bad, but I certainly wasn't going to make a living out of it.
At the same time we had to do ourselves a college project which had to be a solution to a problem, I thought that I could convert a manual lathe to a CNC and save time on making these engines. At the time money was pretty hard and my previous lathe was not worth much more than scrap. We had a college allowance of £10 which was barely enough to make the most simple project, but I didn't want to do the average college project. Luckily I was expecting an insurance payout (I was purposely knocked off a motorbike, as a passenger), which eventually got me the money I needed. So I bought my first proper motorbike (Honda hornet) and bought myself a lathe. The lathe was on an internet auction site labeled as a manual lathe, I could see that it was CNC. So I went and looked at the lathe, and gave the owner a really low offer, I couldn't afford anymore. Surprisingly he accepted the offer just on the basis that I was young and interested in machining.
I got the lathe home and couldn't machine a thing on it because it was three phase, I had no money left for a converter. So in the mean time I converted it from three to single phase. Slowly but surely I got the money together to buy an inverter for the motor. I got the lathe working, did all of the relevant coursework and hey presto I got a distinction. It was the biggest project that a college student had ever taken on, it led me to win a student of excellence award, and I won £250 which paid off the money for the inverter that I owed.
I started a job straight after college because I never had the financial backing to go to university, so I thought I would stick with it for a while and buy some of the things I never had. I updated my lathes stepper motor drives so it would work at a higher speed and resolution. I also spent quite a large amount of money on tools and parts for the lathe. Due to my long working hours and the amount of clutter that built up in my garage I had no time to do any of my projects. I didn't want to spend anymore money because I was considering quitting my job, I even sold one of my bikes - not the hornet.
At the time the only camera I owned was the one on my phone and it was terrible, hence the following pictures;
June - 2014
Now it has probably been about a year since I even turned on the CNC to my lathe, and unfortunately it blew the computers power supply when I tried it. So when I get back off holiday I'm going to spend a few hours just completely updating all of the electrical system, put some pictures on here and show a demonstration of it running. I need a new power supply, once I have completed my workshop I will then buy one.
September 27/09/2014
When I got back off holiday in June I had much more pressing matters to attend to, but now I have the time. I couldn't even get to my lathe properly to do any electrical work, and said that once I built some racks and bench's to clear up my garage I would start work on the lathe, well now I'm at that point. I removed the computer from the lathe and found that the power supply was no good, so I managed to come across a free ex-office computer, which weirdly does not have a keyboard socket. So I ordered myself a USB keyboard, plugged it all in and then found that the back lights in my LCD have stopped working, so this is another thing I need to purchase before I can get the lathe up and running again.
October 08/10/2014
I haven't really had the time to get a computer screen, although I should be picking one up tomorrow. In the mean time I thought it was about time that all of the electrical's in the back were cleaned up. First on the agenda was the power supply, the original was part of a rack system and not all that professional looking the way it was setup. The power supply is powered by a large transformer, it is then rectified to DC and then smoothed out by large electrolytic capacitors. The supply gives out 80Vdc and 24Vdc, one for the motors and one for the control boards, most of it is not necessary.
Basically I removed everything but the rectifier and the capacitors, the board was full of fuses and parts for the 24V supply. One disadvantage that I removed was a power dump, when the supply is cut it discharges all of the power from the capacitors, thankfully the stepper motor drivers will do it instead.
The stepper motor drivers are known as micro stepper's, this is due to them having the ability to turn a motor in smaller increments than what they were designed for. Usually a motor has a resolution of about 200 steps per revolution, using one of these controllers can get it down to 51,200 steps per revolution resulting in higher accuracy and fluidity. I chose 2000 steps which is an easy number to divide down, I also chose to run the motors at their maximum power (5.6Arms).
It must have taken me a good hour to tidy up all of the electrical's, most of it was just re-routing wires and using a couple of tie-wraps. I wired all of this from scratch when I first got the lathe, so I spent another half hour checking everything over. It did all work originally, the only additions are the CNC system, there should be no doubt that it will work again.
Just a picture of the wiring behind the panel switches.
The computer uses the parallel printer port to control the stepper drives, as can be seen by the ribbon cable. A little red wire from the controllers goes through the back of the computer, it is supplying them with 5V for the logic circuit.
All that is required is for me to get a display, install the software on the computer, configure the software and then it should be ready to run. One other problem is that when I switch off the power to the lathe it causes the mains RCD to trip, which is a real problem at night.
October 11/10/2014
Today I managed to get myself a computer screen, connected it all up and switched it on. The computer came up with a login screen, luckily I managed to unlock it as the administrator as no password had been set. I installed the software and configured the software as below. It allows me to choose each pin on the parallel printer port to either step a motor, or control the direction. It also allows me to change the motors steps per millimetre, its speed and acceleration. I'm also able to control other things like coolant pumps, spindle or inputs such as those from limit switches. I chose only the motors as I could do everything else manually.
The next step was to see what kind of speeds the motor's were happy running at, it was a case of guessing these values, running a program to rapid back and forth to see if they lost their places. I remembered these values that I chose about 3 years ago, so not a great amount of time was spent on tuning. I thought that it was now time to make something in the lathe, so I created a program to machine a valve stem for my air rifle. In each part of my program it stopped one pass before the finish so that I could measure and then alter the tool. Below shows a pass that was supposed to be 6.5mm (measured at 6.61mm), the tool got altered for the next finish pass of 5.97mm (measured at 5.97mm). The great thing is that this is keeping its accuracy, the major bonus is that depths can be machined accurately instead of guessing.
I wanted to finish off the valve stem using the CNC, but unfortunately the lathe keeps tripping the mains due to an earth leakage, although its very random. I have a feeling that the earth leakage is coming from the inverter, otherwise it could be a transformer. I would say that for now the lathe project is complete, I may update this page when I resolve the earth problem, other than that there should be no reason to bring up this project again.
One useful procedures the lathe is good for is machining domes on the heads of bolts, so this is what I made.
October 08/10/2015
I quick update on the lathe. I decided that I needed to use the CNC again and I keep having problems with the house RCD tripping due to an earth leakage. I tested the insulation resistance on all of the transformers and they were all way within what they should be, so it came down to the inverter. Inverter's do have some earth leakage through the large electrolytic filter capacitors inside of them. The AC in is rectified to DC, the negative terminal is connected to ground which is why earth leakage can occur. The manufacturer states that the inverter must have excellent grounding, I personally cannot see why as the earth shouldn't carry current, the motor is earthed, the supply is earthed and the inverter is completely plastic. I made the choice to remove the earthing wires to the inverter and now the lathe works perfectly, so good that it hasn't tripped once.
The longest program I dared to run would have been about a minute, but below is a 25 minute constant program cycle;
January 06/01/2016
I chose to make another couple of modifications to the lathe today, one of these being some homing switches. There are some limit switches already installed on the lathe so I thought that these could be use to home the lathe and allow me to setup quicker, however this was not the case. I connected up the switches to the computer and the software registered that they were working, unfortunately the software kept randomly coming up with alarms even when the switches were not in use, I chose to disconnect them. The main modification I made was to stop the large power transformer for the stepper motor controllers from tripping out the mains, it probably does it 50% of the time. There is a component called a negative coefficient thermister (NTC), basically a resistor that looses resistance when it get's hot, it can be used to limit the inrush current to the large transformer. I installed one as shown in the pictures below and the lathe has not tripped once, something I wish I had done earlier.
March 20/03/2016
This is the final entry on the CNC lathe as I have sold the contents of workshop, in under a week from writing this the lathe will be gone. Before selling the lathe I chose to install two cooling fans for the stepper motor drives instead of the one, they were also fixed in place with a bracket to ensure they would not move around. The picture to the right shows the amount of tooling and inserts that I built up.
Then there are all of the work holding tools;
I advertised all of this for £3300, after a week of advertisement it had sold. I had about 8 weeks until I left the UK and thought I would struggle selling it, I guess I was wrong and probably could have finished more projects. The only CNC's on the market are either the hobby ones or the industrial 3 phase ones, so I think it sold quick because it filled this gap in the market, a capable CNC for the home garage. Considering the money I spent on the lathe including the tooling I was quite happy with the money, that certainly couldn't be said for the miller. Thankfully the buyer wanted the lathe for their own satisfaction and not to break it down or sell it on.
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