06/04/2008 by peter.

For those who are interested in the wonders of CNC (probably none of you) I thought I would write a quick post to describe wht i have done so far.

AS mentioned here the control board has been replaced and I am controling everything with VB Express 2008. Here is a little more detail

The protocol for the machine

The reason the people who built this machine originally used both parallel and serial comms was because of the amount of data to be transmitted. so to do the same over a single serial link will require some packing. Unfortunately there is no standard way to do this, so therefore there is a requirement for me start by writing a protocol to handle all this info. The basic idea is to use the serial port like an 8 bit wide parallel port. Each byte is then an instruction. The prelim spec for the control protocol is here in OO format. if anyone actually wants to read this in MS Word .DOC format let me know and i will convert it.

After being able to communicate with the machine i needed to write some code to send commands to the machine and, more importantly, create lines and arcs from which everything else can be created. So i wrote some code to implement Bresenham’s line and arc algorithms and then wrote an XML based machine control language (since i am not a G-code fan, i think it is a bit out of date). Once i work out how i am going to do Post the code without it being mistaken for HTML i will post a sample programme in this post but until then you can see the programme that made this drawing here (simply open with notepad)

any more data wanted or people wanting to help develop this software, let me know

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06/04/2008 by peter.

I just noticed that there have been a spike of hits on my site. After a little snooping through the stats i see that slashdong gave me reference. It looks like someone has been reading what i write. Apologies for anyone else who was waiting for the next instalment. I will try and post some more stuff about the progress as things get done. Comments welcome (they let me know that people are in fact reading this stuff since most of my stats say that the visiting browsers are just search engine robots).

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05/04/2008 by peter.

Sorry for the lack of posts but my adventure into the world of CNC machines took me of at a slant so here is progress on the two projects. (Also no one is commenting so i don’t know if anyone is actually reading this or whether i am just wittering into the cyber darkness.)

Almost all written and just waiting for a PCB to be made and the last little bit (the electromyography bit) to get working. I’m attempting to build an EKG to start with since that’s easy for me to test and then i will just change the probes and the software for the final version (and possibly the gain of the input amp) The PCB will hopefully be made on my nice shiny new PCB router once i can get it working. It does a very good job of ploughing all the way through the board or just compliant at me at the moment. The breadboard prototype is now looking like this and there’s more to add (also just noticed that the serial port 9pin D-sub has fallen off in that pic but it shuould be hanging off the MAX232 there) . Most of the parts for the beta model have been bought including thisrather swanky box (the red one) which requires holes to be made in it for controls which will hopefully be done with my Boxford Duet now that i have finished the first attempt at the software.

Screenshot 1
Screenshot 2

Its all still moving along. It will be finished one day….

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16/12/2007 by peter.

Some time ago I bought an old Boxford duet which had a somewhat old interface, requiring a serial port for general communication and a parallel port to send commands to the stepper motors. The software ran in DOS and was somewhat out dated. I have therefore started designing new hardware that only uses the serial port and should eventually only use USB, new firmware and new control software. This has proven to be of interest to people so i will document my endeavours here.

The original controller was replaced with a new one

Until i have a diagram drawn up the circuit works like this:

20 pin connector going to the stepper board is in fact used as a 10pin connector with 0V,5V,12V coming in and the direction and clock bits for the 3 axis going out. The logic therefore gets its power from the 5V pin of this connector. The 25pin D connector supplies connections for the spindle motor relays (enable, clockwise, anticlockwise) input to the PWM board for spindle speed and, access the rx and tx pins of the serial port plug on the back of the control box and a line indicating the condition of the emergency stop button.

The heart of the new design is a PIC18F4550 with the serial port pins (PortC 6 and 7) connected to a MAX232 for level conversion which is then connected to the appropriate pins from the 25pin D-sub. This provides a link from the computer to the machine. A serial to USB adaptor is then connected to the port on the back of the control box by a null modem cable and this plugs into my Windows Vista laptop and is controlled by a programme written in VB express 2008. Back at the PIC, PortD 0,1,2,3,4,5 are connected to the 20pin header pins Xclk, Xdir, Yclk, Ydir, Zclk, Zdir respectively. PortD 6 and 7 control the direction of the spindle motor and have a 1k resistor connected to them and then that goes to the base of a BC547 to switch each of the direction relays by pulling low the appropriate pin of the 25pin d-sub. A similar setup is used on PortA0 to control the spindle enable relay. the PWM input comes from the PWM output of CCP1 and at the moment simply goes staight into the input of the PWM board in teh control unit by way of the appropriate pin of the 25pin d-sub (i know a PWM running off a ref voltage generated from a PWM is a bit silly but it will be modified at a later date.) PortB 6 and 7 and PortE 3 are used for the ICSP port. There is a 20MHz local osc there too but it’s not being used at the moment as I am using the internal osc, PortB 0 is connected to the emergency stop line but this has not been implemented in software yet, neither have the opto encoders on the spindle but they will connect to PortE 0 and 1 eventually. The rest is just LEDs for debugging, flyback diodes for the realys and caps for the Max232 charge pumps.

more to follow

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