RETROFITTING THE X3 MILLING MACHINE (2)

Transcription

1 RETROFITTING THE X3 MILLING MACHINE (2) Dick Stephen continues with the mechanical modifications. Fitting the ball screws The 12mm 2mm pitch supplied by THK ball screws are delivered with the nut fitted onto the screw rod. The first task is to remove the nuts. If you simply remove the nuts the end result will be small ball bearings all over the floor and a major job finding them and then putting the balls back into the nut housing. To safely remove the three nuts, keepers have to be made (see Photo 6) that are inserted into the nut as the screw is withdrawn. The keepers are 60mm long mild steel rod turned to precisely 10.68mm diameter with a 6 mm reamed hole bored through the full length. The surface of the keepers needs to be nice and smooth so that the keeper will slide easily over the balls. The 6 mm hole will be used later for screwing the nut back onto the rod (and removing it again). The principle is illustrated in photo 6 for a different style of THK ball nut. If you are unfamiliar with ball nuts I strongly advise that you remove the nut over an old towel spread over the bench. If you are unfortunate enough to accidentally drop the balls out of the nut the balls will fall onto the towel and not roll all over the workshop! If such an accident does occur, I have described how to put them back in MEW issue 100. Wipe a little thin oil over the surface of the keeper. To remove the nut unscrew until the end of the rod just enters the white nylon wiper at the opening of the nut. Insert the end of the keeper into the wiper opening. Slowly unscrew the rod while pressing the keeper firmly against the rod end. The keeper should slide into the nut with little resistance. With the nut safely located on the keeper, wrap a rubber band around each end. This will prevent the keeper from falling out! If there is any resistance immediately reverse the process and screw the nut back onto the rod. If the diameter of the keeper is greater than the size specified it will not enter the nut. If it is smaller there is a danger of the balls getting jammed between the keeper and the internal ball track. The screw rods The X and Z screw rods are supplied 500 mm in length, the precise length required 6. Ballscrew assembly and keeper. for these axes. The rod for the Y axis needs to cut to 205mm in length. The rod has been surface hardened and is difficult to saw through. If you have or can borrow an angle grinder this is the best tool to cut the rod. Once cut the rod ends are easy to face using a carbide tipped tool and to drill the holes required in the ends with HSS drills. Start by drilling and reaming the holes for the shafts and pivots in the ends of the rods. If you are gripping the rods in a chuck use a 4-jaw and clock to check that the rod is running true before drilling the holes. It is a good idea to use a protective strip of brass or aluminium to avoid marking the screw surface. For the extension shafts drill the holes 6.8mm to a depth of 30 mm and finish the holes with a 7mm reamer. The holes for the pivots should be 20 mm deep. The X and Z rods will extend beyond the end of the spindle. To prevent any damage to the rod it will need supporting at the rear of the spindle. I find the best way to do this is to turn up a tapered wooden plug with a 12mm hole that fits into the spindle. The plug is slipped over the rod and pushed firmly into the spindle. This will hold the rod firmly as well as keeping it centred. The shaft extensions should be turned next. The dimensions of these are shown in Fig 1. The shafts can be made out of EN1A leaded mild steel. This steel machines very nicely and produces a super finish, it also thread cuts very well. The shafts are made out of 16mm diameter rod. Cut off a length of steel about 5mm longer than shown in the drawing. Turn down the end of each shaft to just over 7mm for a length of 30mm. Now carefully reduce the diameter until the shaft end just slides into the reamed hole in the rod. The end pivots are turned to 7mm for the full length of 45mm. The shafts and pivots are attached to the rods using Loctite 326 adhesive. I used Loctite to attach the shafts to the screws when I fitted ball screws to my Wabeco mill. The joints show no sign of coming apart after years of use. Grip the screw rod in the 4-jaw and check it is running true. Squeeze a small amount of Loctite onto a piece of Perspex (or other plastic surface). Using a toothpick smear Loctite thinly on the inside of the reamed hole. Smear Loctite thinly over the end of the shaft. Push the shaft into the reamed hole twisting the shaft to spread the adhesive. You have about 30 seconds to set the shaft in position. Use a centre in the tail stock to hold the shaft in place while the adhesive cures for at least one hour. The shaft can now be turned to size. Before starting to machine remove the back centre and check that the centre hole is truly central. If the hole is not central the shaft will have to be machined back to remove the hole (the shaft was made 5mm too long to allow for this). Grip the shaft in the chuck to cut away the hole. Grip the rod again in the chuck check it runs true with a clock and drill a hole for the back centre. Turn the shaft to size and cut the 1mm pitch thread for the shaft locking rings. If you have been careful setting up the job the rod and shaft should be perfectly concentric. If there is any eccentricity it will be impossible to correctly line up the nut and the screw. There is no option but to begin again and remake the shaft. Finally make the six shaft locking rings. The outer size is not critical, 26 Model Engineers Workshop

2 and thickness might be about 6mm. The internal thread can either be cut on the lathe or using a 12mm x 1mm tap. These are available from Tracy tools for a few pounds. Follow the same procedure to turn the end pivots to size. The finished X screw shaft and rear pivot are shown in Photos 7 and 8. Fitting the ball nuts Begin fitting the Y-axis nut. The existing Acme nut fitted to the machine can be used as the housing for the ball nut. The nut is attached by two 5mm hex. head screws and positioned with tapered dowels. Slide the mill base forward about 200mm to overhang the cabinet. This will give access to the hex. head screws. All that is required is for the existing hole in the bronze block to be bored out to 25mm diameter. Before starting, the following need to be made. Firstly turn a piece of mild steel (1inch diameter is fine) down to exactly the same diameter as the cylindrical part of the nut. This serves as a useful test nut to check the diameter of the hole in the housing rather than using the ball nut. Secondly turn a second piece of mild steel (20mm diameter is fine) for centring the bronze block. Turn down the piece of steel to 18mm for a length of 30mm. Then turn down a 25mm length to fit tightly in the existing threaded hole in the block. Part off at a length of 30mm. Finish the 18mm diameter section with fine wet and dry paper. Fitting the Y-axis screw Set the bronze block up in a 4-jaw and fit the centring plug into the threaded hole. Use a clock applied to the plug the accurately centre the block as illustrated in Photo 9. The hole for the nut can now be bored out. Check the diameter using the 25mm test nut. The final fit must be good. Drill and tap the two holes for the 5mm hex head screws to secure the ball nut. The process of lining up the screw can now begin. First a second plug to fit into the 25mm hole in the nut housing needs to be made. The dimensions should be the same as those of the ball nut. Drill and ream 12mm through the plug. Rather than use the ball screw to line up the nut a length of 12mm diameter silver steel rod is used. As the rod will be used for the X-axis as well it needs to be at least 650 mm long. Set the rod up in the lathe and check it runs true with a clock. Turn down the end to 6mm for a length of about 12mm. Turn a sharp point on the end. Before commencing with the lining up a rear support for the screw needs to be fitted to the Y- axis slide. The support is made out of a piece of 8mm thick aluminium alloy. The dimensions for the support are given in Fig 2. Fix the Y nut mount to the base of the mill, fit the plug into the mount and fit the slide in place. Attach the front bearing. If the orientation of the nut mount is correct you should be able to pass the 12mm silver steel rod through the front bearing and the hole in the nut plug with little resistance. There should also be little resistance to moving the slide over its full travel. If there is resistance this may be 7. X axis shaft extension. 8. X axis rear pivot. 9. Centring the Y axis nut mounting using a DTI. 10. Centring the nut block using a microscope attachment. December 2004/January

3 11. Z axis motor mount and drive arrangement. 12. Z axis motor mounting. due to longitudinal or height misalignment. Before doing any modification try to determine which of these is the probable cause. The longitudinal alignment of the three screws on my machine was very good. If the resistance is due to incorrect height initially try slightly slackening off the screws securing the nut mount. If this helps then try shims of different thickness and see if this cures the problem. Next slacken off the screws holding the front bearing assembly. Slip a piece of thick paper (not card) under one side and Fig. 1 Shaft extensions retighten the screws. If this helps then the cause is either the two mating faces of the bearing assembly and the slide are not absolutely flat. Check this. If this does not help, repeat the procedure on the other side of the bearing assembly. If this still does not help then it is almost certain that the nut mounting is too high. Remove the nut mount and skim off the underside of the nut mount (taking off no more than 0.02 mm). Replace the nut mount and see if the resistance has been reduced. If this helps keep reducing the height until the slide moves freely over the full length of its travel. If you only want to take off a very small amount rather use 280 grit wet and dry paper on a flat surface. It is worth taking a lot of trouble getting the alignment right as the amount of backlash in the ball screws depends on the alignment. Screw the ball nut onto the screw rod. To do this, remove the retaining rubber band from the keeper and slip the keeper onto the 6mm pivot. Screw the rod into the nut while maintaining pressure on the keeper until the keeper is forced out. Fit the nut into the mount and secure with the two 5mm screws. Fit the slide in place and slip the front bearing over the shaft. Try turning the shaft with your fingers. If you have got the alignment correct the shaft should be easy to turn. The THK ball screws suggested are very sensitive to the alignment. The slightest degree of misalignment will cause them to lock up. Getting the screws to work really well isn t easy and does take a lot of time and fiddling. It took me about two days to get the Y screw to work perfectly. With the screw aligned the bearing for the back support can now be fitted. Remove the screw and the nut and insert the nut plug. Fix the back bearing support to the slide. Reassemble and insert the 12mm silver steel rod until the sharp end touches the back support. Give the rod a gentle tap with a hammer to mark the position of the bearing. Drill a 6.5mm hole in the support at the place marked. Bore a 15mm recess 5mm deep for the bearing. Secure the bearing with a small amount of Loctite. Replace the support and clamp in place. Remove the slideway and using the 4mm holes in the support as a guide, drill and tap the holes for the screws to secure the support. Fitting the X-axis screw Before commencing the fitting of the X ball screw the 6mm i.d. ball race should be in the X screw back support. Pick up the centre of the existing bearing hole in the support. Bore out the hole to take the 6 mm ball race. Fit the ball race and secure it with a small amount of Loctite. The X-axis bronze nut is not quite high enough to accommodate the THK ball nut. 28 Model Engineers Workshop

4 To make a new nut mounting a piece of brass bar 60mm x 40mm x 30mm is required. Begin by facing all six sides square. This is easiest done on the lathe using a 4-jaw chuck. The position of the centre of the 25mm hole for the ball nut needs to be determined. To do this fit the steel centring plug used for the Y-axis into the threaded hole in the bronze X nut. Measure the height of the centre of the plug from the bottom of the nut. Scribe a line the measured height on the face of the brass block and a second centre line at right angles. The hole needs to be bored where these two lines intersect. Photo 10 illustrates how I centred my block using my home made centring microscope fitted into the lathe tail stock. If you don t already have one of these microscopes you should make one. Details for making a similar device appeared in Model Engineer Vol. 183 No.4103, and another design was published in MEW issue 98. If you make one don t use a front silvered mirror use a prism. With the block centred the 25 mm hole for the ball nut can now be bored. The holes for the 5mm hex. head screws and the locating tapered dowels need to be drilled in the base of the nut mount. This poses a really difficult problem. If these holes are not precisely placed the screw will be impossible to line up accurately. I used my centring microscope to position the holes for the dowels. I clamped the X-axis bronze nut in a vice on the Wabeco mill picked up the centre of the existing threaded Acme screw hole. I zeroed the digital readout and refocused to microscope on to the base of the nut and picked up the co-ordinates of the dowel holes relative to the set zero. I then replaced the bronze nut with the Fig. 2 Fig. 3 Fig. 4 December 2004/January

5 13. View showing Z axis terminal block. brass one and repeated the above and drilled the dowel holes. The ball screw subsequently lined up perfectly with no further adjustment. If you don t have a centring microscope or a digital readout you will have to adopt the following rather unusual approach to drill the dowel holes. Firstly remove the Y- axis screw and ball nut and put to one side well out of harms way. Replace the Y-axis slideway positioning centrally in the dovetail. Lock the slide by tightening up the gib screws. Fit the front screw bearing to the table and secure with the two hex head screws. Fit the 25mm plug with the 12mm hole into the brass nut. Fit the table in the slideway. Pass the 12mm silver steel rod through the front bearing and slide on the nut. The brass nut as it stands is too high to pass under the underside of the table. Reduce the height until the nut just clears the underside of the table. The flange of the ball nut, when it is fitted will be just below the top of the nut when the nut clears the underside by about 1mm. Slide the nut on to the 12mm rod and fit the rear support to the back of the table. If you have bored the 25mm ball nut hole correctly the nut will pass under the table making contact on its underside with the slideway where it is attached. So far so good. Remove the rear support, withdraw the 12mm rod and the nut. Remove the table from the slide way. Clean the area where the nut is to be fixed and remove all traces of oil and dirt. Scribe a line connecting the centres of the two dowel holes and a second line connecting the centres of the two screw holes. These lines will help in correctly positioning the nut. Position the nut so that these lines are equidistant from the edges of the nut and parallel to the edges. Now take two pieces of scrap sheet metal 1.5mm thick about 80mm long and 25mm wide. One long edge needs to be straight. Clean the surface free of oil and dirt and stick some double sided adhesive tape to the surface. Stick these pieces to the slideway surface about 0.50mm from either edge of the nut. Check that they are firmly stuck down. Now apply some slow set epoxy resin glue to the underside of the nut and to the seating. Roughly position the nut. Refit the table into the slideway, pass the 12mm rod through the front bearing, through the hole in the nut plug, fit the 6mm end of the rod into the bearing in the back support and secure the back support to the table. If you have been careful the nut will be in precisely the correct position. Leave for several hours for the epoxy to cure before removing the 12mm rod and the table. The nut should be sufficiently well attached to the seating to allow you to drill and ream the tapered holes for the dowels. If you don t have a suitable taper reamer you could drill the dowel holes with a suitable drill and fit parallel dowels. The dowels can be secured in the casting with Loctite. If the adhesive holds well the holes for the 5mm hex head screws can also be drilled and tapped at this time. Dismantle the table and remove the nut. Clean off all the adhesive and remove the pieces of scrap. Fix the nut mount in place on the slide assembly. Fit the ball nut onto the screw, as described above, and then fix the nut in the mount with the two 5mm hex head screws. Fit the table into the slideway and the front bearing on the shaft. If all the lining up is correct the two dowels should slip easily in to the respective holes in the table. The same should apply to the rear support. Check that the screw is easily turned with your fingers. Fitting the Z-axis screw The Z-axis screw is the easiest of the three to fit. The original Acme nut is cylindrical 30mm in diameter. To fit the ball nut all that is required is to turn up a sleeve 30mm diameter and 30mm long bored to the diameter of the ball nut, 25mm. This sleeve is fixed in the cast iron housing with Loctite 326. The ball nut is secured with 4mm hex head screws rather than 5mm. This is to avoid the thread breaking through the side of the casting. To prevent the nut rotating, the clearance holes in the ball nut flange are sleeved for the 4mm screws. The upper bearing support will also need to be sleeved to accommodate the 6 mm i.d. ball race. The sleeve is secured with Loctite 326. Before the screw can be installed the 30-tooth 5mm pitch timing belt pulley has to be fitted to the shaft. The pulleys supplied by HPC have to be bored to fit the screw shaft as well as the shaft on the stepper motor. Drill and ream the 30 tooth pulley 12mm and the 15 tooth pulley 8 mm. The position of the pulley on the screw shaft is illustrated in Photo 11. Position the pulley on the shaft and tighten the grub screw to mark the position of the screw on the shaft. Remove the pulley and drill a 3mm radial hole in the shaft to a depth of 4mm. Replace the grub screw with one 12mm long. Turn down the end of the screw to 3mm for a length of 3.5mm. Fit the pulley onto the shaft and install the screw. A 16mm diameter spacer has to be inserted between the pulley and the upper thrust race in the lower bearing housing. With the screw installed measure the thickness required for the spacer. Leave a clearance of about 1mm between the top end of the screw and the upper 6mm ball race. Before finally fitting the screw in place clean the inside of the vertical column casting with a wire brush to keep contamination to a minimum. Clean the threads of the ball screw carefully and wipe light oil over the thread. Install the screw not forgetting the belt. Fig Model Engineers Workshop

6 The replacement gas spring can be fitted next. The gas spring suggested has the same stroke as the original but is 35mm shorter in overall length. To fit the spring an extension piece has to be attached to the end of the cylinder. The dimensions of this are given in Fig 3. I feel that the gas spring as supplied is more powerful than is needed, although those who use heavier cutter assemblies, tapping heads etc, may differ. As I tend to use lightweight cutters, for my purposes the compliance of the spring should be sufficient to support the weight of the head including the motor. The head of machine plus motor is just less than 35 K gm. The chosen replacement gas spring is adjustable (downwards). To achieve the correct compliance some of the gas needs to be let out of the cylinder. To measure the compliance I used the bathroom scales. I placed a board on the scales that just covered the top with a 12mm hole in the centre into which the eye attached to the rod was placed. I leant on the other end and noted the force required to compress the spring. As supplied the new spring was so strong that I was unable to compress it at all. The gas pressure can be reduced, by opening a valve at the end of the cylinder with an Allen key. Open the valve for a second and then close it. You will need to do this several times measuring the force each time until the spring compresses with a force of 35 K gm. To install the spring the nut housing will have to be raised as high as it will go. This can be done by turning the screw with your fingers. Adjust the height until the spring can be slipped on to the two 8 mm pegs. The spring is normally under compression and the force generated by the spring will prevent it from slipping off the pegs. Fitting the Z-axis stepper motor The attachment of the Z-axis stepper motor is illustrated in Fig 4, and Photo 12. The attachment comprises a 200mm x 12mm x 25mm aluminium alloy bar attached to the base of the column with two 5mm hex. head screws. The column already has a suitable hole tapped 5mm on the right side. Measure the position of the existing tapped hole from the edge of the column. Drill a 5mm clearance hole in the bar the measured distance from the end and a second hole 4.3mm (tapping size) in the equivalent position at the other end. Attach the alum. bar using the existing tapped hole and a clamp to position it and use the hole in the bar as a guide to drill in the column with an electric hand drill for the remaining 5mm screw. The pillars are best made using free machining aluminium rod. Make the pillars too long initially and reduce the length until the two pulleys line up exactly. When you have made all the parts assemble the mount, fit the pillars to the motor and attach the 15 tooth pulley to the motor shaft. Fit the pillars into the holes in the base and check that the two pulleys line up. Take it all apart and fix the 5-way section of screw terminal block to the underside of the base as illustrated in Photo 13. Connecting the motor to the drive unit will be dealt within a separate 14. Motor mounting assembly for X and Y axes. section. The pillars are attached to the base using Loctite 326. Attach the pillars to the motor, apply Loctite to the ends and fit the pillars into the base. Attaching the X and Y axis motors The mounting assembly for the X and Y axes is shown in Photo 14. The dimensions of the components of the assembly are given in Fig 5. The construction is straight forward and needs little further explanation. The front screw shaft bearings have a fitted collar that has to be removed. This collar is secured by some quite powerful adhesive. They do come off eventually, I got mine off without resorting to using heat. The clamp pieces do need to be a very good fit on the front 15. Setup for bending the motor covers. bearings, if they are to hold securely. The motor shaft is attached to the screw shaft using an Oldham coupling. These couplings will accommodate a small amount of misalignment of the two shafts, but the pillar holes in the clamp do need to be concentric with the hole that fits on the front of the shaft bearing. The motor and electrical wiring is protected by a sheet aluminium cover. If you have access to a sheet metal bender this is the easiest way to form the cover. Photo 15 illustrates how I bent the aluminium to form the cover which is held in place by six 3mm hex. head screws. A 5-way screw terminal block is attached to the inside of the back plate and can be seen in photo 14. The concluding part in the next issue will deal with wiring up the electrical and electronic elements, and the construction of the manual control unit. December 2004/January