RETROFITTING THE X3 MILLING MACHINE (3)

Transcription

1 RETROFITTING THE X3 MILLING MACHINE (3) Dick Stephen concludes the CNC conversion with a description of the wiring details and an Electronic handwheel 16. Stepper motor wiring (Bipolar parallel) Wiring up the Stepper Motors In part 1 of this series of articles, Table 1 (MEW Issue 102 page 33) gave the characteristics of the MAE 23 Frame HSX stepper motors. The Table indicates that the motors can be connected in either a uni-polar or a bi-polar configuration. The motors produce the greatest torque when connected in a bi-polar configuration. Fig 6 shows the lead colours of the 8 leads for the beginning and end of each of the 4 coil windings in the motor. To connect the motor in bi-polar parallel configuration each pair of coils is connected in parallel with the beginning of each coil and the end of each connected together. Thus for this arrangement, the following leads are connected : - 1. Red and Yellow-White. 2. Yellow and Red-White. 3. Brown and Orange-White. 4. Orange and Brown-White. This gives four leads 1 and 2 ; 3 and 4. The four conductors in the screened cable I used to connect the motors were coloured Red, Yellow, Blue and Green, the screen is used as the earth lead. These were paired Red & Yellow; Blue & Green. I connected Red to 1, Yellow to 2, Blue to 3 and Green to 4. Connect all three motors up in exactly the same way so that when you come to connecting up to the motor drivers there is little chance of an error. If subsequently when the motors are being run one of the motors rotates in the wrong direction all that is required to correct the rotation is for the connection one pair of the leads at the driver to be interchanged. When connecting the leads in the terminal block, professional panel builders would probably use the purpose made crimp on connectors. For my purposes, I believe in tinning the bare ends with solder to make the connection secure. Photo 16 illustrates the completed wiring of one of the motors. The cable screen, used as the earth lead, is connected to the mounting back plate. Use a solder tag attached to the back with a 2.5 mm screw. It is very important to ensure that the motors are securely earthed. The cable is secured to the back plate using a cable grip, and is further restrained by a rubber grommet fitted into the aluminium motor cover. Assembling the Stepper Motor Drive Unit In earlier articles in MEW (issues 88 & 89) I described a stepper motor drive unit incorporating Gecko drives and software and control card from DeskCNC. A number of readers found the article useful, however they did not feel there was adequate information for the electronics beginner to construct the unit. I am therefore offering a step by step description of the construction of the unit. Potential constructors are advised to follow the construction carefully as the unit operates with high voltages and substantial current. If any reader is at all concerned by any aspect of the construction and does not have access to qualified assistance, he should contact me and I will endeavour to help. My telephone number is given at the end of the article. Materials required for electronics 1. Four Gecko 201 micro stepping drives. Three would be fine for normal X, Y, Z working. Adding a fourth makes provision for driving a rotary table. I have used these drivers for some time and have found them excellent. Contact Showing enclosure with transformer, capacitor, rectifier, and 5v module. 2. All the following items are supplied by RS Components RSwww.com or Tel a. Enclosure b. 4off, 4-way X plugs c. 4off, 4-way Chassis X sockets Model Engineers Workshop

2 d. 500VA Toroidal Transformer 2 x 30V e. Bridge Rectifier 12A/200V f. Electrolytic Capacitor 63V 15,000 ìf g. Mains switch dpst Rocker on/off illuminated red h. 2off spst mini vis switches i. DC-DC converter to generate a 5V regulated supply j. Pack of terminal blocks and k. Pack of w 36 k Resistors l. Tube of heat sink compound The following are required if the reader intends to build the manual control unit. a. 2off Rotary switches break before make 4-pole 3-way b. Momentary open switch c. 1off, 6-way panel socket d. 1off, 6-way cable plug e inch pitch strip board 70 mm x 100 mm (tracks running length wise). f. 2off, 14 pin dil sockets, 1 8pin dil socket. g. 4off, 1k resistors (contains 10) h. 1off, 1.8 M resistor (contains 10). i. 4off, 2N2907A PNP transistors (contains 5). j. 22 swg tinned copper wire. k. 24 swg tinned copper wire. 18. DeskCNC and one Gecko drive added. 4. The following are available from US Digital a. 1, 500 line optical encoder S1-500 b. 1 4 inch coupler for the encoder c. Cable CA-434-1FT d. LS7184 Quadrature Clock converter 5. In addition to the above you will need a 3 m length of 5 amp 3-core mains cable, 3m of 6-core screened 5.9 mm diameter cable (hard to find in short lengths you will need to shop around), 3 mm 4 amp stranded cable red, an assortment of colours of stranded electronic wire, heat shrink in various sizes, cable ties, rubber grommets piece of aluminium 300 mm x 150 mm x 3 mm 19. Front panel viewed from rear showing component layout piece of aluminium 300 mm x 100 mm x 3 mm 8. 1 piece of pvc sheet 300 mm x 100 mm x6 mm 9. 1 piece of Vero strip board 80 mm x 80 mm 1 10 inch pitch off, double pole double throw switches The assembly The most difficult part of the assembly of the driver unit is mounting the components in the enclosure and cutting the holes in the front and rear panels for the switches and sockets. Photos 17, 18, & 19 illustrate the mounting of the components in the enclosure. Photo 17 shows the toroidal transformer, the bridge rectifier and the electrolytic smoothing 20. Wiring the SLR sockets. February/March

3 21. Working on the front panel. capacitor mounted on the 100 mm x 300 mm aluminium plate. To improve thermal conduction between the rectifier and the plate (which acts as a heat sink) coat the Fig. 6 23HSX stepper motor lead colours: Fig. 7 Gecko drive mounting plate underside of the rectifier with heat sink compound before screwing it to the plate. Also attached to the plate are 3-terminal 32Amp terminal block (block 1) and a 6- terminal block (block 2) and two solder tags for earth connections. Mounted on the PVC shelf are a 5-terminal block (block 3) and the DC-DC converter soldered on the piece of Vero strip board. The strip board is attached by 4, 3 mm screws supported on PVC spacers (note the wire solder tags attached to the strip board). Photo 18 illustrates the unit with the 300 mm x 150 mm aluminium shelf on which are mounted the 4 Gecko drive units and the DeskCNC control board. The Gecko drive units are attached to the aluminium plate with 3 mm screws. Heat sink compound is applied to the undersides of the Gecko units to improve the thermal contact between them and the plate, which also acts as a heat sink (not strictly necessary given the current drawn by the motors). The positions of the holes for the screws are given in Fig 7. The aluminium plate I used wasn t quite 150 mm wide so to install the control board I added a piece of PVC as shown. The shelf is supported on two pillars 40 mm long to give sufficient clearance between the Gecko drive units and the top of the enclosure. Also illustrated in Photo 18 is the rear panel with the 4 SLR sockets for connecting the stepper motors to the drivers. Photo 19 illustrates the layout of the front panel switches. The positions are not critical. Begin by fitting all the components as illustrated in Photos 17, 18, 19. The 6-way change over switch to select CNC or manual should be assembled and fitted. The switch consists of 3 dpdt (double pole double throw) switches mounted side by side. I tried to find a suitable rotary switch but was unsuccessful, so I had to resort to making something suitable. The plate for mounting the switches is shown in Fig 8. Attach a bar to the three levers to operate all three switches simultaneously. Do not fit the Gecko drives or the DeskCNC controller board at this stage. I have made extensive use of terminal blocks to minimise the number of solder joints having to be made in the confined space of the enclosure. All the connections to the Gecko units as well as the control board are with screw terminal blocks as well. With all the hardware for the unit fitted in the enclosure and on the front and rear panels work can begin on connecting up the drive unit. You will need two sizes of soldering iron, a small electronics one with a tip diameter about 1.5 mm and a larger one with a tip diameter of about 3 mm. You will also need some rosin cored electronics solder. Begin by soldering the leads to connect the SLR sockets to the Gecko drive units. Connect the four earth points together using 22 swg tinned copper wire covered with heat shrink (the yellow wire in Photo 20). Connect a length of green cable to the earth point of one socket and solder a tag on the other end. This will be connected to a convenient earth point in the enclosure. Now solder the connections to the Gecko drives to the socket pins. Choose the same four colours as in the motor cable. Cut off four equal lengths of each colour and remove the insulation for 6 mm from each end. Now tin both ends of all the wires with solder. Poke the end of the solder into hole in a socket pin and heat the pin until a small amount of solder melts into the hole. Poke the tinned end of a cable into the 32 Model Engineers Workshop

4 hole and re-heat the pin. When the solder melts push the wire in as far as it will go and hold until the solder sets. Solder in the rest of the cables. Pair the cables connected to each of the motor coils and join them together with heat shrink sleeving (see photo 20). The front panel should be wired up next. In Photo 21 the mains switch is on the extreme left, the next two switches are the switch to disable the Gecko drive units and the switch for the Estop (emergency stop) on the control board. One side of these switches is connected to ground. The two terminals are connected and a single lead then connects the two to a convenient earth point. Make the two leads that connect to the Gecko drives and the control board over long as they will be cut to length later. The manual-cnc switch and the 6-pin socket are wired up next. Begin with the socket. Solder in the central earth lead first (green) this is made long enough to reach a convenient earth point and terminated with a tag. Next connect the manual X,Y and Z step leads to pins 1,2 and 3 the single direction lead is connected to pin 4 and a lead for +5 Volts to pin 5 (this lead needs to be long). Now connect the X,Y and Z step leads to the bottom left contact of each of the switches (see Photo 22). Connect the single direction lead (black) to the bottom right contact of the left switch and then connect this contact to the remaining two contacts (see photo 22). Now connect a pair of step and direction (orange and black) leads to the two centre terminals of each switch. These paired leads need to be long enough to reach the X,Y and Z Gecko units. Finally connect pairs of step and direction (orange and black) to the two upper contacts of each switch long enough to reach the step and direction terminals on the control board. The power supply is wired up next. There are two power supplies one high voltage at approximately 45 volts which powers the stepper motors and a second +5 volt regulated supply derived from the 45 volt supply which operates the optoisolators on the Gecko drives, the CNC control board and the Manual control unit. Together these three draw less than 1 2 an amp. To derive the regulated 5 volt supply a DC to DC converter is used as this is the simplest method of generating the 22. Detail of wiring for the six way switch. required 5 volts. Constructors with some knowledge of electronics may choose to save a few pounds by building an alternative arrangement based on a 7805 or similar chip. To make connections to the Fig. 8 Switch plate DC-DC converter, the device is soldered onto a piece of 1 10 inch pitch Vero strip board. Photo 18 shows the converter mounted on the piece of Vero board. Four pins connecting the leads to the device can Fig. 9 Manual control circuit Fig. 10 Rotary encoder February/March

5 23. Completion of wiring. Fig. 11 Frame also be seen.the device is fitted into the centre of the piece of board and the tracks between the pins cut using a sharp 3mm drill bit turned with the fingers. Making the connections to the converter is straight forward as the converter is clearly labelled. The mounted converter is attached to the PVC shelf with 3mm screws in the corners and insulated spacers between the board and the shelf (see photo 18). The transformer is connected up next. The transformer suggested has two secondary windings each producing 30 volts at 8.3 amps. To get the required current for the driver these two windings are connected in parallel. This would appear to give a total available current of 16.6 amps. As a full wave rectifier with capacitor smoothing is being used this reduces the available current to 62% or just over 10 amps. The transformer windings must be connected together correctly. Connect the start of each (VSecondary) together and the end of each (0 V) together. For the transformer suggested this means (Red + Yellow) and (Black + Orange). The two ends are soldered together and secured in the terminal block 1. Red cable with a current rating of over 10 amps is used to connect the secondary windings from the terminal block to the ac terminals of the bridge rectifier. The second ac terminal of the rectifier is connected to ground via the terminal block again using suitably heavy earth cable. The positive terminal of the rectifier is connected to the 6-way terminal block and the negative terminal to ground. Photo 23 shows how this terminal block is wired up. The four terminals from the left are used for the power to the drive units. A lead is taken from terminal 4 to the +ve terminal of the smoothing capacitor. Connect the - ve terminal of the capacitor to ground. The backs of these 4 terminals are linked. A length of 2 amp red cable is connected to terminal 4 and soldered to the + input terminal of the DC-DC converter. The input and the output terminals connected to ground. The + output (+5 volts) is connected to terminal 5 of the terminal block and then link to terminal 6. From the rear of terminal 6 two cables carry 5 volts to the control board and to pin 5 on the Manual socket in the front panel. Connect two lengths of 2 amp cable to the front of terminal 5 and a second two to terminal 6 of the terminal block. These cables carry 5 volts to the Gecko units to operate the opto-isolators on the units. This completes all the wiring, which can be seen in photo 23. It does look a bit of a jungle ; however readers will notice that wherever possible cables have been colour coded and paired together with heat shrink to minimise the possibility of making subsequent incorrect connections. The CNC control board and the four Gecko units can now be fitted. Smear a small amount of heat sink compound on the underside of each Gecko unit before screwing them in place. To fit the two front screws it is necessary to remove the blue terminal blocks. These terminal blocks lift off; they fit on pins attached to the circuit board and are quite a tight fit. The terminal blocks on both the Gecko units and the control board are clearly labelled. Never the less readers should consult the installation notes provided with both. When connecting up the units check at the 34 Model Engineers Workshop

6 same time all the wiring very carefully. If you are unfamiliar with electronics have a friend who is check it over for you. When it is all connected connect the stepper motors to the unit. Do not turn on the power. If you have a universal test meter measure the resistance between each motor terminal on the Gecko units (phases A D) and ground. The resistance should be about 20k. If by chance there is a short to ground at any of the connections the Gecko unit will be damaged if the power is turned on. Manual control unit I have been using a CNC mill for about 5 years. I possibly have used the mill as a manually operated machine almost as much as in the CNC mode. The problem using stepper motor drives manually is the back e.m.f generated by the motors when the armatures are rotated. This back e.m.f. generates a resistance, which I found unacceptable. It also seemed a bit daft to have a motor drive, which wasn t being usefully employed! This prompted the design of the manual operating unit. In essence the device is a box having a handwheel and switch arrangement. Turning the handwheel rotates an encoder generating signals which then cause the specified axis to move in sympathy. Having now made the unit and tried it I cannot understand why I didn t make one years ago! Similar devices have become available over the last few years for professional machines, and will frequently be described as Electronic handwheels. Photo 24 illustrates the unit. Sorting out the design, and sourcing some of the parts was the most difficult aspect of the project. US Digital is by far the cheapest source of encoders that I could find, as well as the only source I could find of Quadrature clock converters. Encoders are available from RS-Components costing considerably more. The construction of the electronic control board is very straight forward. The circuit diagram for the control board is shown in Fig 9. The layout of the board illustrated in Photo 25 follows the diagram given in Fig 9. Readers should have little difficulty constructing the board if they follow the circuit diagram. Connect the Reset line to one terminal of the momentary open switch and the other terminal to ground. Pressing the Reset switch will set both dividers to zero. A problem with strip board is making neat secure cable connections. RS-Components supply strip board pins for this purpose. I have never found these pins satisfactory. Photo 26 illustrates the method I employ, using 24 swg tinned copper wire. The hole in the board needs to be slightly enlarged using a tapered broach. A length of wire is bent to form a loop and the ends pushed through the enlarged hole. A 1.5 mm panel pin is passed through the loop and the ends of the wire are pulled tight, bent parallel to the copper track, cut off 1mm long and then soldered in. The loop is twisted through 90 degrees, the cable passed through and soldered in. The result is a neat secure connection. A test meter is required for wiring up the rotary switches as it isn t obvious how the tags on the switches are connected. 24. Exterior view of manual control unit. 25. Components of manual control unit. 26. Technique for making stripboard connections. February/March

7 27. Encoder and detent assembly. The range switch selects the basic encoder clock rate (500 x 4) of 2000 per revolution of the encoder giving a movement of 2 mm on X and Y and 1 mm on Z. The second switch position selects the 1 10 clock rate giving a 0.20 mm movement on X and Y and 0.1mm on Z. On my original version, a final position selects the rate. These clock rates are sent to the second rotary switch, which selects the axis. With benefit of hindsight, the divide by 100 feature has been found to be an unnecessary luxury. Thus the circuit diagram does not include this feature. The encoder connections are illustrated in Fig 10 as well as being clearly shown on the unit. The frame for mounting the encoder is illustrated in Photo 27 and Fig 11. Fig 12 shows the other small parts associated with the encoder. The index disc is made out of 2 mm engraving brass and has 20 positions. I have not shown a design for the detent I made as I am sure readers will be able to make a better one than mine! The length of the 1 4 inch shaft for connecting the encoder to the knob is also not shown as this is determined when fitting the encoder in the frame. It is important the line up the encoder shaft and the connecting shaft accurately. I was unable to source a suitable knob for the unit so I resorted to making one out of a piece of Paxolin I had. The knobs for the axis and range selection I made as well, as RS-Components didn t have what I wanted. Final comments Photo 28 illustrates the completed machine. I transferred the Newall digital readout from my Wabeco to the new mill. A digital readout is, I think, essential in order to get the best out of your mill. I had to resort to the Newall as I cannot read the Vernier type easily. A digital readout also makes calibration of the axes extremely easy. I was amazed when I calibrated the three axes to find that the backlash on all three was zero. The entire conversion has taken 28. Project completed. me about 2 months to complete. A significant amount of this time involved head scratching, trying to work out how best to carry out some of the jobs. Now it is done I am very pleased with the end result and consider that I now have a mill Fig. 12 MCU components that would have cost me many times the amount it finally cost, which was less than 2000 (including the cost of the X3). As noted above, should prospective constructors require further guidance, I can be contacted on Model Engineers Workshop