MATE Building Instructions - Page 3

 

PREPARING THE "MATE" EXTRUSION

  1. Very carefully scribe a light vertical centre line on one end between the vee's at the top and bottom of the extrusion. Similarly, scribe a horizontal centre line across the same end, using the bottom faces of the mounting lugs as a reference.
  2. Using a compass, or dividers, ensure that the intersection of these two lines is at the exact centre of the 27. 5mm base. If not, re-check your marking out. When satisfied, lightly mark the with a sharp centre-punch. Repeat the marking-out on the other end of the extrusion.
  3. With a fine felt-tipped pen, draw a line across the top face of the extrusion 23mm from one end. Draw another line across that end 20mm down from the top face. Extend these lines, down and along the sides, until they meet. At the same time, mark the mounting lugs at 23mm from the same end and along their junction with the base.
  4. The enclosed 23mm x 20m area and the lugs on either side are now cut away with a hacksaw to minimise the extent of interrupted cuts on the crank-case nose section. Be careful not to mark the surfaces.

MACHINING THE CRANKCASE (1-1) PART ONE

  1. Centre the extrusion on your mark in a 4-jaw chuck, with the nose section pointing outwards. The extrusion should extend 26mm from the jaws and rest squarely between them. The original sawn end of the extrusion is an accurate reference and the edge of a steel ruler placed across it should be equidistant on either side from the chuck face.
  2. With a fine knife tool, take light facing cuts from the cylinder block to 25mm from the nose-end of the extrusion. Feed the tool inwards until it just starts to cut the base at the felt-pen line. Now, using the logitudinal feed, reduce the extrusion to the Ø27.5mm base. Before proceeding, confirm that it is still centred on your mark. Reduce further to Ø26mm, up to the cylinder block face. All traces of your earlier hacksaw cuts should now have been erased and your extrusion should start to look like a crankcase.
  3. Change to a tool with an approximate 2mm end radius. Set the topslide over to 7-1/2 °. Wind topslide right back and, using the saddle only, position the tool just to the right of the end of the extrusion. Lock the saddle (if possible) and wind the topslide across until the tool just touches the cylinder block face. Zero the topslide dial.
  4. Retract topslide fully and move cross-slide in towards the work. Take the first light cut, traversing topslide smoothly with a hand-over-hand movement. Zero the cross-slide dial and take another light cut. Measure the starting diameter of the nose and note final dial reading for Ø17mm.
  5. Continue traversing cuts with the the topslide, up to its the zero setting at cylinder block face. Approaching the Ø17mm, take a light finishing cut and the radius at the junction of the cylinder block and the crankcase nose should now blend smoothly.
  6. Re-align the topslide with the spindle axis, unlock the saddle, change to a knife tool and take light facing cuts across the end of the extrusion, reducing the crankcase nose length to 24.5mm from the cylinder block face.
  7. Centre-drill the nose and follow through with ascending drills to Ø10mm x 40mm deep. Bore to nominal Ø12.7mm to give a light interference fit to the poosphor-bronze bushing. Carefully break the edges of the bore.
  8. With the crankcase upright in the chuck, mark the uppermost jaw and the one towards you with a felt-tipped pen. Now slacken these two jaws sufficiently only to remove the crankcase.
  9. Wash the crankcase, the bushing and the venturi thoroughly in a solvent, or warm, soapy water, removing all traces of oil. Coat the bushing with Loctite #601 and press into the crankcase until the faced end is flush with the faced end of the nose. Allow the adhesive to cure.
  10. Mount the crankcase, across the lugs, in the milling machine vyce. Square-up against your marked centre-lines on the unmachined end. Centre the machine spindle between the vyce jaws and set it over at 7-1/2 ° from vertical.
  11. Centre-drill a pilot hole for the venturi at 6.5mm from the cylinder block face and follow through with ascending drills to Ø7.5mm, drilling right through crankcase and bushing into the centre of the Ø8.0mm bushing bore. Recess the 7.5mm hole with a 3/8" slot drill to a depth of 4mm, for the venturi seat. Check that the venturi is an easy, but shake free sliding fit before proceeding.
  12. Remove the case from the vyce and clean thoroughly in solvent. Assemble the venturi to the crankcase with Loctite #601 as before. Use a 3mm drill through the spraybar holes to align them at 90 ° to the crankshaft axis.

MACHINING THE CRANKCASE (4) PART TWO

  1. Measure and note the amount to be removed to reduce to 23.2mm the distance from the front to the rear face of the cylinder block. Remove any burr which may have been raised against the top face.
  2. Return the crankcase to the 4-jaw chuck, with the unmachined end extending 3mm from the jaws. Centre on your punched mark and square end against chuck face, as before. Tighten only the two previously marked jaws and alignment should be spot-on. Do not proceed until you are happy with this aspect.
  3. Again, using the fine knife tool, take light facing cuts to finish the cylinder block at 23.2mm from its front face. This finished rear face is now the primary reference plane and the crankcase must not be moved in the chuck until all subsequent operations parallel to the crankshaft axis are completed.
  4. Centre-drill the rear face and follow through with an Ø8mm drill for a depth of about 22mm. You may wish to assess your prowess by noting the concentricity of the two 8mm bores, but at this stage it's now only of academic interest. Don't attempt to adjust!
  5. Bore the crankcase cavity Ø23.5mm x 18.4mm deep. Chamfer the rear entry 45° x 1mm wide.
  6. Prepare the lathe gearing to cut the Ø23.5mm x 1 mm pitch thread for the backplate. Set the 60° internal threading tool accurately. Use this tool to turn the 24.6mm undercut, which serves both as thread run-out and for conrod clearance.
  7. Cut the thread, removing 0.05mm per pass, reducing to 0.01mm approaching 0.5mm depth. Clean thread with a toothbrush and test fit the backplate. A smooth, shake free fit is required, so continue removing 0.01mm at each pass until it appears the backplate is about to enter. Take a couple of passes at this setting and test again. If it will not fit, continue to cut, testing at each 0.01mm reduction. The nominal thread depth is 0.55mm, but should be used as a guide only. Although it is common practice to lap threads together, the introduction of a lapping compound between two close fitting aluminium parts will result in disaster. The parts will irretrievably lock together as one.
  8. Bore the crankshaft bushing to Ø9.7mm, excercising care whilst cutting the remnant of the Ø7.5mm venturi spigot. Concentrate on obtaining a smooth and parallel bore, rather than aiming for a dead accurate size. The crankshaft itself will be sized to fit the bearing. To clear the crankshaft radius, chamfer the bushing face inside the crankcase 45° x 1.5mm wide.
  9. Remove the crankcase from the chuck, wash it and very carefully break all exposed edges with fine needle files. Don't go mad here, aim for a 45° chamfer with minimal width. Leave the edges at the widened part at the top of the cylinder block until later, as these are used as a reference in the next operation.

MACHINING THE CRANKCASE (4) PART THREE

  1. Re-establish the vertical centre-line very lightly on the rear face, extending it downwards from the top vee. Stop 3mm short of the 23.5 bore, leaving unmarked the backplate sealing face. Place a steel rule across the bottom of the lugs, its edge representing the horizontal centre line. Measure up from this edge and at 27.5mm scribe a small mark on the vertical centre-line.
  2. Mount the crankcase to a 90° angle plate with an 8mm bolt. Use a thick washer under the bolt head and protect both ends of the crankcase with a piece of notepaper. Place the angle plate on a flat surface (plate) and align the vertical cylinder axis at 90° to that surface. Your mark at 27.5mm must clear the edge of the angle plate by at least 1mm. Tighten the bolt very securely. Measure halfway along the top vee and mark a dot with your felt pen.
  3. Mount the lathe face plate and bolt the angle plate to this, centreing your pen mark on the spindle axis. Face the top of cylinder block down to the 27.5mm mark.
  4. Transfer the angle plate with crankcase still mounted, from the lathe face plate to a bench vyce (with protected jaws). Scribe accurately lines from corner to corner on the machined top face of the crankcase. Carefully punch a light centre at their intersection. Set a pair of dividers accurately at 7.75mm and mark out the PCD for the transfer passages, scribing lightly at each diagonal. Centre-punch these marks. You should now have five punch marks, the central one indicating the precise cylinder axis.
  5. Return the angle plate to the face-plate (sounds more like instructions on kitchen management!), centreing accurately on one of the transfer port marks. Centre-drill, following with ascending drills to finish at Ø5mm x 18mm deep. Drill the remaining three passages, centreing on each in turn. Take care not to chamfer the edge of the holes.
  6. Centre carefully now on the cylinder axis mark. Centre-drill and follow with drills up to, say, Ø10mm x 21mm deep. Then bore the crankcase for the lower cylinder, finishing at Ø16.5mm x 19mm deep. Take fine cuts approaching the final bore, so as to eliminate burring at the edge of the transfer passages.
  7. Reduce the cylinder block (deck) height to 27.2mm, by facing the remaining 0.3mm from the top face. Chamfer the bored entry 45° x .25mm wide. Remove from chuck and wash. Carefully break remaining edges at top of cylinder block.
  8. Mount the crankcase inverted in the machine vyce and drill the (4) four Ø3mm mounting holes in the centre of the vee's in the lugs, at 3.5mm from the end faces. Break the edges of these holes with a #3 centre-drill, twisting it between the fingertips.
  9. Scribe fine lines on the rear face of the crankcase at an included angle of 20° to the centre-line and from its intersection with the top face. Extend these lines to 3mm short of the 23.5mm bore. Do this neatly, as there is no way of removing these lines. Mount the crankcase in the machine vyce, using a steel rule or square to align the crankcase at 20° to the vertical. Fix to milling machine table.
  10. Mount a 1/4" slot drill and set the depth stop so that the cutter is arrested at the intersection of the top face of the now canted mounting lug and the side of the crankcase cavity. Centre the cutter on the cylinder axis, moving the machine table sideways now until the side of the (lowered) cutter just touches the lower edge of the cylinder bore where it breaks into the crankcase cavity. Raise the cutter and traverse the table a further 2mm from the cylinder axis. Plunge cut the con-rod relief down to the stop. Repeat the whole setting-up procedure for the other side. Remove now, wash and carefully de-burr the crankcase. With the exception of the cylinder screw holes, it is finished.

 

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