Multiplier gearbox, sensors and couplings:
The gearboxes are made out of plastic double gears 48/12, 3.1mm bore, the outer gear having 48 teeth, the
inner gear 12 teeth.
The gearboxes are made out of plastic double gears 48/12, 3.1mm bore, the outer gear having 48 teeth, the
inner gear 12 teeth.
The gears mount on 3 mm shafts running between angled mounting plate. There are four gears, one driving the other, to produce an overall multiplier of 64:1. I experimented with more gears in the train, and also with 50/10 spur gears but found that the gearboxes are prone to jamming at higher ratios. Additionally, at higher gear ratios the force required to rotate the DEC or RA axis becomes considerable, as the gears act as a centrifugal limiter. The 64:1 ratio gives consistent and reliable performance. An interesting possibility would be to move to a higher gear ratio and drive the gearbox in the reverse direction with a small motor, as a reduction box, so as to make a complete GOTO system for the EQ2.
The gearboxes are coupled to the RA and DEC shafts by two Module 1 gears - a 40 tooth gear on the gearbox shaft
engaging with a 58 tooth gear on the mount axis shaft. These gears were chosen after some experimentation purely to match the physical size requirements, but conveniently give an extra 1.45:1 ratio on the overall gear train. More details about mounting gears on the mount axis shafts are given later on.
The set of parts for each gearbox is as follows:
Technobots Ltd (www.technobotsonline.com)
1. 4x MFA Plastic Double Gear 48/12 3.1mm Part No.: 4600-011
2. 1x MOD 1 40T Plastic Gear Brass Hub Bore 4mm Part No.: 4600-138
3. 1x MOD 1 58T Plastic Gear Brass Hub Bore 4mm Part No.: 4600-140
4. Spare Model Transmission Grub Screws Part No.: 4607-067 (a set of 5x M3 allen screws plus an allen key. These are the hardened ones)
Sciencestore (www.sciencestore.co.uk)
1. 1x Strips & Plates Part No.: CDT0115 (Pair of Angle Mounted Plates, for motor and gear box construction)
2. Distance Pieces Part No.: CDT0119 (For locating axle rods when mounting them between holed metal sheets. Tight fit on 3mm diameter metal axle rods. Pack of 10)
3. Reduction sleeves 4mm to 3mm, Pack of 10 Part No.: CDT0118
4. Pack of 10x 3mm diameter shafts Supplied in lengths of 120mm Part No.: CDT0033B
5. Tripad Board, SRBP board 100 x 160mm with rectangular pads covering three holes at a time. Hole spacing 2.54 x 2.54 (o.1"), 39 strips x 20 (3 hole pads) Part No.: CDT0139
Rapid Electronics (www.rapidonline.com)
1. 2x Plug strip - 22-1705 TruConnect Turned Pin SIL Header 32-Way
2. 2x Socket strip - 22-1751 32 Way Turned Pin Sil Socket
3. 1x 9W DSUB plug - 15-0200 9 Way Solder Lug D Connector Plug
4. 4x Sensor – 58-0936 Kingbright KTIR0821DS Photointerrupter
5. Spacer - 33-3560 25.4mm M3 Hex Threaded F-f Spacers(25)
6. Spacer - 33-3540 5mm M3 Hex Threaded F-f Spacers (25)
7. Screws - 33-2850 Slotted Countersunk Machine Screws BZP M3 6mm Pack Of 100
8. Washers -33-1760 Steel Washers BZP M3 Pack Of 100
9. Screws - 33-1500 Slotted Pan Head Machine Screws BZP M3 6mm Pack Of 100
10. Screws - 33-1525 Slotted Pan Head Machine Screws BZP M3 25mm Pack Of 100
Maplin (www.maplin.co.uk)
1. Tripad board, Product Code JP52G (if you can’t get it at Sciencestore)
2. Enclosure box LH14
B&Q DIY (www.diy.com)
1. Steel flat 14mm x 5mm - FFA Concept Varnished Hot Rolled Steel Flat (L)2000mm x (W)14mm x (T)5mm EAN: 3232630203750
The gearboxes are coupled to the RA and DEC shafts by two Module 1 gears - a 40 tooth gear on the gearbox shaft
engaging with a 58 tooth gear on the mount axis shaft. These gears were chosen after some experimentation purely to match the physical size requirements, but conveniently give an extra 1.45:1 ratio on the overall gear train. More details about mounting gears on the mount axis shafts are given later on.
The set of parts for each gearbox is as follows:
Technobots Ltd (www.technobotsonline.com)
1. 4x MFA Plastic Double Gear 48/12 3.1mm Part No.: 4600-011
2. 1x MOD 1 40T Plastic Gear Brass Hub Bore 4mm Part No.: 4600-138
3. 1x MOD 1 58T Plastic Gear Brass Hub Bore 4mm Part No.: 4600-140
4. Spare Model Transmission Grub Screws Part No.: 4607-067 (a set of 5x M3 allen screws plus an allen key. These are the hardened ones)
Sciencestore (www.sciencestore.co.uk)
1. 1x Strips & Plates Part No.: CDT0115 (Pair of Angle Mounted Plates, for motor and gear box construction)
2. Distance Pieces Part No.: CDT0119 (For locating axle rods when mounting them between holed metal sheets. Tight fit on 3mm diameter metal axle rods. Pack of 10)
3. Reduction sleeves 4mm to 3mm, Pack of 10 Part No.: CDT0118
4. Pack of 10x 3mm diameter shafts Supplied in lengths of 120mm Part No.: CDT0033B
5. Tripad Board, SRBP board 100 x 160mm with rectangular pads covering three holes at a time. Hole spacing 2.54 x 2.54 (o.1"), 39 strips x 20 (3 hole pads) Part No.: CDT0139
Rapid Electronics (www.rapidonline.com)
1. 2x Plug strip - 22-1705 TruConnect Turned Pin SIL Header 32-Way
2. 2x Socket strip - 22-1751 32 Way Turned Pin Sil Socket
3. 1x 9W DSUB plug - 15-0200 9 Way Solder Lug D Connector Plug
4. 4x Sensor – 58-0936 Kingbright KTIR0821DS Photointerrupter
5. Spacer - 33-3560 25.4mm M3 Hex Threaded F-f Spacers(25)
6. Spacer - 33-3540 5mm M3 Hex Threaded F-f Spacers (25)
7. Screws - 33-2850 Slotted Countersunk Machine Screws BZP M3 6mm Pack Of 100
8. Washers -33-1760 Steel Washers BZP M3 Pack Of 100
9. Screws - 33-1500 Slotted Pan Head Machine Screws BZP M3 6mm Pack Of 100
10. Screws - 33-1525 Slotted Pan Head Machine Screws BZP M3 25mm Pack Of 100
Maplin (www.maplin.co.uk)
1. Tripad board, Product Code JP52G (if you can’t get it at Sciencestore)
2. Enclosure box LH14
B&Q DIY (www.diy.com)
1. Steel flat 14mm x 5mm - FFA Concept Varnished Hot Rolled Steel Flat (L)2000mm x (W)14mm x (T)5mm EAN: 3232630203750
Gearbox Construction:
Gearbox assembly should be pretty straight forward looking at the photos. The first step is to machine the plastic angle plates. They need an extra 3mm bore shaft hole in the top corner to bear one of the sensor assemblies. The 3mm holes on the front face which will be behind the Mod1 shaft coupling gear should be countersunk, so that the screws holding the spacers can be flush with the plate surface, and not foul the gear. Both gearboxes are identical, except they have the Mod1 gear on opposite faces. Watch out for this when countersinking the holes.
Gearbox assembly should be pretty straight forward looking at the photos. The first step is to machine the plastic angle plates. They need an extra 3mm bore shaft hole in the top corner to bear one of the sensor assemblies. The 3mm holes on the front face which will be behind the Mod1 shaft coupling gear should be countersunk, so that the screws holding the spacers can be flush with the plate surface, and not foul the gear. Both gearboxes are identical, except they have the Mod1 gear on opposite faces. Watch out for this when countersinking the holes.
One of the MFA spur gears in each gearbox has to have a hub made for it so that it can be firmly attached to the 3mm shaft with a grub screw. Gears with a 2.9mm bore push fit for 3mm shaft are available, but will not withstand the torque without slipping. Use the 14mm by 5mm steel flat to construct the hub. Cut off a 20mm length of the flat. Drill and tap two M3 holes to match the existing holes in the MFA gear. Cut off the 12 tooth spur gear from the wheel and gently sand the surface flat. The hub will sit on this surface. On the other side of the gear, countersink the holes so the hub attachment screws will sit flush to the surface. Screw the hub to the gear, and use the gear as a pattern to drill an accurately centred 3mm hole for the shaft. Drill the hub to 3mm and see if the shaft fits – better a snug, but easy, fit rather than a loose one. The next step is to drill and tap an M3 screw hole through one of the long sides of the hub to intersect the shaft hole. This will take the grub screw to lock the gear to the shaft.
The two 3mm shafts have to be cut to size – note that one shaft extends to hold the external MOD1 gear. Use the 25mm spacers to assemble the angle plates and insert the shafts. Thread the gears, washers and distance pieces onto the shafts. The washers are needed to limit the forward and backward movement of the free running wheels. Follow the pattern and numbers of washers as in the photographs, remembering that space is needed later to mount the sensors. If the gearbox locks up when turning the shafts experiment with the numbers of washers and distance pieces so that it runs freely.
The MOD1 gear (4mm bore) is attached to the protruding 3mm shaft using the plastic reduction sleeves. Push reducer into the MOD1 gear hub, one from each side, so that the lips of the reducers are up against the gear. Then jam the whole assembly onto the 3mm shaft, pushing it up to the angle plate as close as possible.
Finally, when everything is settled, locking dimples need to be drilled into the shafts. These are needed so that the MOD1 gear and MFA gear with hub can be securely locked to the shaft. Screw the respective grub screws in the two gear hubs home so that the shaft is marked. The MOD1 grub screw has to penetrate the plastic reducers to do this. Disassemble the gearbox to remove the shaft and drill dimples into the shaft where it is marked by the screws. With the shaft in a vice, start with a centre drill and dimple the shaft at the mark. With a 2.0 mm drill, widen the dimple so the cone of the 2mm drill just fits in the dimple. This should provide just the right sized locking dimple for the 3mm grub screws. Re-assemble the gearbox. When turning the shafts there should be no slippage of the gear wheels.
The two 3mm shafts have to be cut to size – note that one shaft extends to hold the external MOD1 gear. Use the 25mm spacers to assemble the angle plates and insert the shafts. Thread the gears, washers and distance pieces onto the shafts. The washers are needed to limit the forward and backward movement of the free running wheels. Follow the pattern and numbers of washers as in the photographs, remembering that space is needed later to mount the sensors. If the gearbox locks up when turning the shafts experiment with the numbers of washers and distance pieces so that it runs freely.
The MOD1 gear (4mm bore) is attached to the protruding 3mm shaft using the plastic reduction sleeves. Push reducer into the MOD1 gear hub, one from each side, so that the lips of the reducers are up against the gear. Then jam the whole assembly onto the 3mm shaft, pushing it up to the angle plate as close as possible.
Finally, when everything is settled, locking dimples need to be drilled into the shafts. These are needed so that the MOD1 gear and MFA gear with hub can be securely locked to the shaft. Screw the respective grub screws in the two gear hubs home so that the shaft is marked. The MOD1 grub screw has to penetrate the plastic reducers to do this. Disassemble the gearbox to remove the shaft and drill dimples into the shaft where it is marked by the screws. With the shaft in a vice, start with a centre drill and dimple the shaft at the mark. With a 2.0 mm drill, widen the dimple so the cone of the 2mm drill just fits in the dimple. This should provide just the right sized locking dimple for the 3mm grub screws. Re-assemble the gearbox. When turning the shafts there should be no slippage of the gear wheels.
Sensors:
Optical sensors are used to detect the rotation of the gear teeth of the final gear of the gearbox. There are two sensor assemblies, arranged so as to detect the passage of a gear tooth with a small phase difference between them. This phase difference is used to decide the direction of rotation – forwards or backwards. Each tooth thus generates four ticks – two rising edges and two falling edges. There are 48 teeth on the gear, giving 192 ticks per revolution of the final gear. The overall resolution is thus 192 * 64 * 1.45 = 17,818 ticks per axis revolution.
Optical sensors are used to detect the rotation of the gear teeth of the final gear of the gearbox. There are two sensor assemblies, arranged so as to detect the passage of a gear tooth with a small phase difference between them. This phase difference is used to decide the direction of rotation – forwards or backwards. Each tooth thus generates four ticks – two rising edges and two falling edges. There are 48 teeth on the gear, giving 192 ticks per revolution of the final gear. The overall resolution is thus 192 * 64 * 1.45 = 17,818 ticks per axis revolution.
The optical device in the sensor is the Kingbright KTIR0821DS. It is an IR emitter and a photo-darlington receiver in a single, compact and very thin package, intended to operate in reflectance mode. Unfortunately the size and pitch of the gear teeth is about the same as that of the emitter and receiver, so that when a tooth is illuminated the receiver is facing the gap between teeth, and when the receiver is facing a tooth the emitter is radiating into the gap. The end result is that no signal is ever received. The solution is to use two devices facing each other across the gear wheel, oriented so that the emitter of one faces the receiver of the other, interrupted by the passage of the gear tooth. The geometry is such that it is not possible to make use of the unused emitter/receiver pair as the second, phase delayed
detector. So we end up needing two separate sensor assemblies, and a total of four KTIR0821DS devices. The pictures illustrate the arrangement.
The KTIR0821DS devices are mounted on small pieces of tripad stripboard to make a sandwich that rotates about an M3 shaft and straddles the gear wheel. Each piece of stripboard has a 3mm hole to take the M3 shaft, a matching 4-pin plug and socket to clip them together, and a single KTIR0821DS device. The 4-pin plugs and sockets are made from the Turned Pin Header and Socket strips. Space is so tight that the header pins must be extracted from their plastic base and soldered directly into the stripboard – they can be made a tight fit into a 1.5mm hole.
Make sure that the KTIR0821DS devices are oriented on the stripboards so that when assembled emitters face receivers. The pitch of the leads on the KTIR0821DS devices does not quite match the hole pitch of stripboard, so the holes must be drilled out to 1.5mm. Once done, pass the leads through the holes, bend them over and solder into place so that the package lies perfectly flat on the other side. Solder wires to appropriate emitter and receiver leads and glue in place with small spots of glue from a glue gun. When plugged together the stripboards should make a neat sandwich and be spaced so that a gear wheel can move without fouling between them. Use a dremel grinder or file to grind flat the copper-side solder connections on the stripboard, consistent with robustness and conductivity.
To assemble the sensors to the gearbox, start by drilling out two of the 5mm M3 spacers to a 3.1 mm clear hole. Start with the lower sensor assembly first, feed a 25mm M3 screw through the angle plate, and thread washers, stripboard, bored out 5mm spacer, stripboard, 5mm spacer as locking nut and locate the end of the M3 screw into a final 5mm spacer screwed onto the facing angle plate. Tighten up, and make sure the sensor assembly can rotate about the M3 screw and intersect cleanly with the path of the gear teeth. Use a flat screwdriver to lever the 4-pin plug and socket to adjust spacing and orientation/tilt of the stripboards if necessary. The final rotation position of the sensor assemblies can wait until the head-end electronics are ready.
The final step is to terminate the sensor leads into plugs and sockets that mate with the DB9 plugs as shown. The gearbox cases are made from a Maplin box (product code LH14) cut in half, and held in place by M3
studs.
Shaft couplings – DEC axis:
The basic idea for the DEC axis is to remove the M12 shaft that holds the counterweights and remove the cast aluminium finned nut that the counterweight shaft screws into. The finned nut is replaced with an M12 threaded hub holding a MOD1 58 toothed gear, which is itself locked into place with an M12 studding connector nut. The connector nut can then take the counterweight shaft. Connector nuts are basically long nuts used to connect lengths of studding (see www.shop4fasteners.co.uk). Use the zinc plated BZP ones rather than the stainless steel versions – much cheaper
and easier to machine.
I replaced the counterweight shaft with a beefier 35cm length of M16 studding. The M16 studding mates with the M12 connector nut through a second M12 connector drilled and tapped internally to M16 along half its length. A short M12 stud joins the two connectors together.
The cast finned nut is locked to the DEC axis by three grub screws around the perimeter. Loosen the grub screws and undo the finned nut – I used a mole wrench wrapped in a rag on it since it does not have proper flat faces. Removal of the cast finned nut should be considered a one time operation: the grub screws score the M12 thread on the DEC shaft, which in turn damages the internal thread of the aluminium nut on removal. The ends of the grub screws are themselves deformed by tightening, and damage the internal grub screw threads of the aluminium nut when unscrewed. Once the nut is removed, dress the M12 threads of the DEC axis shaft with a needle file and thread chaser. The nut can go into the junk box. Note the number and order of the plastic and fibre washers between nut and bearing surface. Remove the DEC axis from the top, again noting the number and order of washers. If you wish to keep the finned nut, then undo the DEC axis from the other end – beneath the OTA bed is a recessed nut that can be undone with a box spanner. Undo this and remove the DEC axis from the bottom.
Make a hub for the Mod1 gear from a 48mm length of 30mm x 4mm steel flat. Start with drilling a 4mm bore hole at
the centre of the flat. The flat will be attached to the gear by two M3 screws. Mark the position of these two holes symmetrically about the 4mm hole and drill small diameter (1mm) pilot through holes at these positions. Note that the screw heads on the reverse side must not foul the bearing surface of the DEC axis, so position the holes at the extreme edges of the flat. With a 4mm shaft threaded through the gear and flat, drill through the pilot holes of the flat and through the plastic gear. Drill and tap the pilot holes in the flat to M3 and the gear wheel to 3mm clear. On the reverse side of the MOD1 gear to the brass hub remove the raised plastic lip around the bore and screw the flat and gear together firmly, making sure the 4mm shaft easily passes through the assembly. Punch out the existing brass
hub of the MOD1 gear with a drift, and drill and tap the new hub and gear bore to M12.
With the DEC axis installed in the casting, screw the MOD1 gear onto the projecting M12 thread, with the plastic side of the gear facing toward the bearing surface. The thick fibre washer sits to the bearing surface and a thin plastic washer to gear surface. Make sure the magic EQ2 grease is smeared over the surfaces. Holding the top end of the DEC axis where the OTA sits tighten the MOD1 gear by its new hub, if necessary with an adjustable wrench. It should be firm but not tight. Check the DEC axis sits firmly and does not wobble, but will rotate easily. Finally screw an M12
connector nut onto the protruding M12 stud, and with the adjustable wrench on the hub use a spanner to tighten the connector nut against the hub. This locks the MOD1 gear to the DEC shaft, so it needs to be tight – a three grunt job.
The cast finned nut is locked to the DEC axis by three grub screws around the perimeter. Loosen the grub screws and undo the finned nut – I used a mole wrench wrapped in a rag on it since it does not have proper flat faces. Removal of the cast finned nut should be considered a one time operation: the grub screws score the M12 thread on the DEC shaft, which in turn damages the internal thread of the aluminium nut on removal. The ends of the grub screws are themselves deformed by tightening, and damage the internal grub screw threads of the aluminium nut when unscrewed. Once the nut is removed, dress the M12 threads of the DEC axis shaft with a needle file and thread chaser. The nut can go into the junk box. Note the number and order of the plastic and fibre washers between nut and bearing surface. Remove the DEC axis from the top, again noting the number and order of washers. If you wish to keep the finned nut, then undo the DEC axis from the other end – beneath the OTA bed is a recessed nut that can be undone with a box spanner. Undo this and remove the DEC axis from the bottom.
Make a hub for the Mod1 gear from a 48mm length of 30mm x 4mm steel flat. Start with drilling a 4mm bore hole at
the centre of the flat. The flat will be attached to the gear by two M3 screws. Mark the position of these two holes symmetrically about the 4mm hole and drill small diameter (1mm) pilot through holes at these positions. Note that the screw heads on the reverse side must not foul the bearing surface of the DEC axis, so position the holes at the extreme edges of the flat. With a 4mm shaft threaded through the gear and flat, drill through the pilot holes of the flat and through the plastic gear. Drill and tap the pilot holes in the flat to M3 and the gear wheel to 3mm clear. On the reverse side of the MOD1 gear to the brass hub remove the raised plastic lip around the bore and screw the flat and gear together firmly, making sure the 4mm shaft easily passes through the assembly. Punch out the existing brass
hub of the MOD1 gear with a drift, and drill and tap the new hub and gear bore to M12.
With the DEC axis installed in the casting, screw the MOD1 gear onto the projecting M12 thread, with the plastic side of the gear facing toward the bearing surface. The thick fibre washer sits to the bearing surface and a thin plastic washer to gear surface. Make sure the magic EQ2 grease is smeared over the surfaces. Holding the top end of the DEC axis where the OTA sits tighten the MOD1 gear by its new hub, if necessary with an adjustable wrench. It should be firm but not tight. Check the DEC axis sits firmly and does not wobble, but will rotate easily. Finally screw an M12
connector nut onto the protruding M12 stud, and with the adjustable wrench on the hub use a spanner to tighten the connector nut against the hub. This locks the MOD1 gear to the DEC shaft, so it needs to be tight – a three grunt job.
Shaft couplings – RA axis:
The gear coupling to the RA axis is similar to that of the DEC axis, but more difficult as there is so little protruding RA
shaft to work with. First make a hub for a 58 tooth MOD1 gear as for the DEC axis, but drill and tap the new hub and gear bore to M10 rather than M12.
The gear coupling to the RA axis is similar to that of the DEC axis, but more difficult as there is so little protruding RA
shaft to work with. First make a hub for a 58 tooth MOD1 gear as for the DEC axis, but drill and tap the new hub and gear bore to M10 rather than M12.
Now remove the DEC axis shaft from the EQ2 casting by following the DEC axis instructions above. Put a plastic bag over the casting to prevent the sticky grease travelling. Adjust the altitude adjustment of the EQ2 so that the RA axis is horizontal. Prise off the plastic cap at the bottom of the RA casting. This will expose the top of an M10 nyloc nut that holds the RA shaft in the casting. Undo the M10 nut. This will require a box spanner because there is very little space between the nut and the casting walls, most sockets will be too fat to fit. Pull the RA shaft out of the casting and be ready to catch a couple of plastic and steel washers (as before, note the order).
While the mount is disassembled it is worth making sure the RA shaft is securely locked to the DEC casting. Pull the
RA slow motion gear off the shaft (note any washers), loosen the small M3 RA setting circle lock screw and remove the RA setting circle. Unscrew the RA clutch from the casting.
Place a couple of M10 nuts onto the end of the RA shaft and tighten them together securely. With a spanner on the end nut, hold the casting and firmly try to turn the RA shaft left and right. If it does not turn with reasonable force (do not overdo it), then all is well. If the shaft rotates, locate and undo the grub screw in the DEC casting that locks the RA shaft. Using the spanner tighten the RA shaft home, reinstall the grub screw and tighten firmly. Return the RA clutch
into position, and the RA setting circle and slow motion gear with plastic washers onto the RA shaft.
Re-insert the RA shaft into the RA housing. Arrange a sandwich of three thin M10 steel washers and the original two plastic washers and place over the end of the RA shaft protruding from the base of the RA casting. Re-install the nyloc nut but reversed, so the nylon locking ring is into the casing. If you have a new nyloc nut available, use it. Hold the RA shaft and tighten the nyloc nut so that the axis rotates but does not rock in its housing or have free play or end float. My preference is to adjust it on the tight side. With the extra washers in place several free threads of the nyloc nut should protrude (if not, add or remove packing washers so that they do).
Cut a 27mm length of M10 studding and install a couple of M10 nuts on one end. Screw the stud into the exposed threads of the nyloc nut. With a spanner on the M10 nuts and holding the RA shaft tighten the stud into the nyloc nut as firmly as possible. Loosen the M10 nuts without disturbing the stud, and tighten the inner nut home onto the nyloc nut with the box spanner, again as firmly as possible. Swing the RA axis to and fro and make sure nothing comes
undone. So far I have found this arrangement to be perfectly reliable, but it may be best to have a tube of Loctite 243 threadlocker in reserve.
Finally, thread an M10 washer and a larger M10 penny washer over the stud and screw the MOD1 gear onto the stud, plastic side to the RA casing. Make sure the M10 washers and MOD1 gear do not rub against the RA casing. Fabricate an M10 half nut (cut a full size nut in half) and tighten it up against the MOD1 gear hub. This locks the MOD1 gear to the RA shaft, so it needs to be tight – another three grunt job.
It is important that the stud, MOD1 gear and half nut project as little as possible beyond the base of the RA casing – they will interfere with the base of the mount at large altitude settings. The arrangement described here will not permit altitude settings beyond 70 degrees before the MOD1 gear collides with the mount base.
While the mount is disassembled it is worth making sure the RA shaft is securely locked to the DEC casting. Pull the
RA slow motion gear off the shaft (note any washers), loosen the small M3 RA setting circle lock screw and remove the RA setting circle. Unscrew the RA clutch from the casting.
Place a couple of M10 nuts onto the end of the RA shaft and tighten them together securely. With a spanner on the end nut, hold the casting and firmly try to turn the RA shaft left and right. If it does not turn with reasonable force (do not overdo it), then all is well. If the shaft rotates, locate and undo the grub screw in the DEC casting that locks the RA shaft. Using the spanner tighten the RA shaft home, reinstall the grub screw and tighten firmly. Return the RA clutch
into position, and the RA setting circle and slow motion gear with plastic washers onto the RA shaft.
Re-insert the RA shaft into the RA housing. Arrange a sandwich of three thin M10 steel washers and the original two plastic washers and place over the end of the RA shaft protruding from the base of the RA casting. Re-install the nyloc nut but reversed, so the nylon locking ring is into the casing. If you have a new nyloc nut available, use it. Hold the RA shaft and tighten the nyloc nut so that the axis rotates but does not rock in its housing or have free play or end float. My preference is to adjust it on the tight side. With the extra washers in place several free threads of the nyloc nut should protrude (if not, add or remove packing washers so that they do).
Cut a 27mm length of M10 studding and install a couple of M10 nuts on one end. Screw the stud into the exposed threads of the nyloc nut. With a spanner on the M10 nuts and holding the RA shaft tighten the stud into the nyloc nut as firmly as possible. Loosen the M10 nuts without disturbing the stud, and tighten the inner nut home onto the nyloc nut with the box spanner, again as firmly as possible. Swing the RA axis to and fro and make sure nothing comes
undone. So far I have found this arrangement to be perfectly reliable, but it may be best to have a tube of Loctite 243 threadlocker in reserve.
Finally, thread an M10 washer and a larger M10 penny washer over the stud and screw the MOD1 gear onto the stud, plastic side to the RA casing. Make sure the M10 washers and MOD1 gear do not rub against the RA casing. Fabricate an M10 half nut (cut a full size nut in half) and tighten it up against the MOD1 gear hub. This locks the MOD1 gear to the RA shaft, so it needs to be tight – another three grunt job.
It is important that the stud, MOD1 gear and half nut project as little as possible beyond the base of the RA casing – they will interfere with the base of the mount at large altitude settings. The arrangement described here will not permit altitude settings beyond 70 degrees before the MOD1 gear collides with the mount base.
Attaching the gearbox – DEC axis:
The DEC gearbox is attached to the EQ2 casting by first attaching a steel flat to the DEC casting, and then a piece of
angle iron to the flat. The flat is attached by making use of the M3 hole used by the RA setting circle locking screw and making an additional M3 tapped hole in the DEC casting. Start by removing the DEC axis shaft from the EQ2 casting by following the DEC axis instructions. Then remove the RA shaft from the RA housing, and remove the RA slow motion gear, RA setting circle and clutch from the shaft, also as described in the RA axis instructions.
The DEC gearbox is attached to the EQ2 casting by first attaching a steel flat to the DEC casting, and then a piece of
angle iron to the flat. The flat is attached by making use of the M3 hole used by the RA setting circle locking screw and making an additional M3 tapped hole in the DEC casting. Start by removing the DEC axis shaft from the EQ2 casting by following the DEC axis instructions. Then remove the RA shaft from the RA housing, and remove the RA slow motion gear, RA setting circle and clutch from the shaft, also as described in the RA axis instructions.
Drill and tap an M3 hole in the DEC casting as shown in the pics. Stuff rag into the casting to catch any swarf. The steel flat is 30mm x 4.5mm by 71mm. An iron angle piece 30mm x 30mm x3mm and 58mm long is used to make a T piece to the flat. The angle iron is screwed into place so the top surface sits on DEC casting, its long edge parallel to the DEC axis. The gearbox is attached to the angle iron by four M3 screws so that the MOD1 40 tooth gear on the gearbox engages smoothly and at the right height with the MOD1 58 tooth gear on the DEC shaft. Try various positions of the gearbox relative to the 58 tooth gear before drilling and tapping the holes in the angle iron and gearbox base for the final position. The final position should arrange for the teeth of the two MOD1 gears to fully mesh without lifting the gearbox. There is almost certain to be some eccentricity on the 58 tooth gear, so this has to be accommodated for in the final position. Make sure that the M3 screws can be installed and removed without having to dismantle the
gearbox. When attached the axis should be able to swing freely to and fro, and with only a rather satisfying rustle and whir from the gears turning in the gearbox.
Attaching the gearbox – RA axis:
gearbox. When attached the axis should be able to swing freely to and fro, and with only a rather satisfying rustle and whir from the gears turning in the gearbox.
Attaching the gearbox – RA axis:
The RA gearbox is attached to the EQ2 casting by first attaching a steel flat to the RA casting, and then a piece of angle iron to the flat. The flat is attached by making use of the two M2.5 screws that attach the altitude scale pointer, and the M5 cap screw which appears to have no purpose. The steel flat is 30mm x 3.5mm and 65mm long. An iron angle piece 30mm x 30mm x3mm and 60mm long is screwed longitudinally to the flat so the top surface sits on the RA casing, its long edge parallel to the RA axis. The gearbox is attached to the angle iron by four M3 screws so that the MOD1 40 tooth gear on the gearbox engages smoothly and at the right height with the MOD1 58 tooth gear on the RA shaft. Follow the same care as with the DEC axis before deciding the final position.