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Fun with a VFD - lathe stops and auto traverse

Canobi

Iron
Registered Member
Joined
Feb 28, 2017
Messages
14
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16
#1
Hi folks :)


I recently got a new motor for my 1940s Grayson lathe, the one that came with it was on it's last legs and was even struggling in backgear. Well not anymore, though I had to get a VFD as it's a lot bigger in the power and speed departments.


After mounting everything and setting up the VFDs perameters required to prevent 1.5kw of overkill from destroying my lovely machine, I cobbled together a scrap bin control panel for the manual controls and mounted the drive underneath:





While doing so, I got to thinking about how other types of switchs and buttons could be utilised for controlling things. At the moment I'm using a three position rotary switch for selecting the motor directions, with the centre pin being the off position, much the same as barrel and Dewhurst switches are configured.


I made the diagrams below to show how it's wired to the terminals:









My game plan is to add more switches to the system by taking them from the main control switch and extend them to the lathe bed for use with the saddle.


To do that, they need to be placed in series with any one of the three VFD terminals and the direction switch, though it should be noted that switches on the forward/reverse lines will operate the system slightly differently to the one on the common line.


The forward reverse lines offer the most flexibilty as they operate independently of each other. Also, depending on the type of switch or button used, they can be made to both change direction or act as stops:




Conversely, the common line effects both the forward and reverse lines simultaneously and therefore more suited to serve as stop/run or disconnect:




There's not much more to it really, most of the interesting stuff is in the creative ideas for how a particular button or switch could been integrated and actuated.


My first thought in that subject was to use two momentary pushbuttons and magnates to hold a sliding actuator in position but changed it to a spring latch and lever system with two opposing micro switches as I'm not so keen on the idea of the magnates collecting swarf around the lever mechanism (it's actually upside down, it will be mounted to the apron the other way up so swarf doesn't get trapped between the levers, it was just easier to draw it that way up for some reason)




As it stands, there won't be enough time to move the cross slide as the carriage will change direction the moment the button is pressed. What I want it to do goes like this:

Tool reaches end of cut, saddle hits actuator = button is pressed, saddle stops for a given time.


Given time passes = saddle resumes moving in the other direction, add infinitum.


The delay times don't need to be huge, a short one for end of cut and a slightly longer one for setting up the next one would be enough.


Unfortunately, my VFD can't do that, so I turned to electronics engineering (my second passion) to design a PCB that could.


I present to you the T.A.R.D.I.S (Timer Actuated tRaverse Delay Integration System)




The board has been designed to plug directly into the terminals via right angle header pins and measures just 26x23mm.


Power is taken from the drive's 10v and 0v reference terminals and stays active so it's ready to respond to triggering. As such I included a way to disengage both trigger ports when I don't need them by including ports for switch in series with the common line.


At the centre of the system are two 555s timer chips which are configured as one shots (ie, when triggered they put out a single pulse for a given duration and then stop until retriggered) to provide two independent channels with a delay period that can be set from as low as 1 nano second all the way up to 1 hour.


The output from the two 555s then actuate a series of mosfet transistors which control the connections from the forward/reverse terminals to the common terminal.


I'm still modifying the layout design as I've not accounted for the main controls in this version and there are one or two other things that I want to include as well such as status LEDs for active lines but I'll be cobbling together a prototype to test the timers and fet arrays function as intended soon, so while I wait for parts to arrive I'll be making the changes to add the missing bells and whistles and then wait for the results of my proto in case I need to make changes to the main circuits if the test results prove unfavourable.



Anyhoo, that's about as much as I've got for now but feel free to jump in with questions, ideas, designs or suggestions :)


Until next time, stay safe, stay happy and keep those chips flyin' ;)
 

Canobi

Iron
Registered Member
Joined
Feb 28, 2017
Messages
14
Likes
16
#3
I decided to take an exercise on simplifying the latching mechanism, handily it now makes for a decent beginners milling project too.
 
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