Ian_C's workbench - P4 and S7 allsorts

Discussion in 'Workbenches, including workshop techniques.' started by Ian_C, 21 May 2017.

  1. Ian_C

    Ian_C Western Thunderer

    Seemed like a good idea to start a thread here rather than scatter stuff all over the place.

    First some observations on Slaters driving wheels in S7 that I bought for an 8F project. When I received them I put a couple of wheels on an axle in the motor/gearbox and ran them out of pure curiosity. There was quite visible runout on the first set, and subsequently on all the others too. Put all the wheels and axles in a collet chuck in the lathe and clocked the back of the rim to see by how much they were out of true.
    wheel clocking 1.jpg wheel clocking 2.jpg
    They varied from 2 to 15 thou TIR. I was surprised how small a runout is noticeable. The eyeball and the brain are rascals! 15 thou looks terrible, and even 2 thou is noticeable without looking too hard. Here's a question for S7 folk - what is the allowable variation in the B-to-B dimension? Apart from looking goofy, at what point does run out start to have an effect on clearance at crossings and check rails?

    Some advice elsewhere on WT suggested flattening the back of the wheels on fine emery until rims, spokes and axle inserts all end up on the same plane. Also checked and removed tiny machining burrs on the axle ends. It helps as well to have the axle ends and the back of the axle inserts absolutely free of dirt and debris before assembly. All seemed like sensible advice and it did improve matters a bit, but still noticeable runout on all the wheels.

    Thinking about how the wheels are manufactured gives us a clue. The tyre and the inserts are put in an injection moulding tool. The tool is closed and plastic is injected to form the boss, spokes and rim. After a cooling period the tool is opened and the moulded wheel is ejected. The geometrical relationship between axle insert and tyre depends on the accuracy of the tool and the dimensional stability of the injected plastic. I'm fortunate to have access at work to a guy who has spent his whole professional life in the injection moulding business. He helps us design moulded parts and helps our suppliers design and commission tooling to make those parts. Fair to say he knows his stuff. I took a wheel and axle to work and explained the issue. His opinion was that it would not be possible to end up with perfectly true wheels made this way. The reason is that the plastic (a glass filled nylon in this case?) contracts on cooling. On these 8F driving wheels the crank boss is quite large relative to the diameter on the wheel and the distribution of plastic around the axle insert is not symmetrical. Consequently when it cools it shrinks unevenly and pulls the tyre slightly out of true relative to the axle insert. If you look closely it is possible to see some sink marks in the boss between the inserts where the larger mass of plastic has cooled. There must also be some clearance in the tool to accept the inserts. So...a perfectly true Slaters driving wheel and axle is improbable. Spoked wheels without crankpin bosses ought to shrink more evenly, so should run truer, but I've not measured any yet. Not knocking Slaters though. There are limitations the process, there's a price we're prepared to pay for wheels and there's a need to see a return on tooling investment. I guess they suit most of us most of the time and in 7mm we'd be much the poorer without Slaters wheels. Oh, and quartering's done for us as well! Much to be grateful for.

    Can they be further improved? The answer is 'yes, a bit'. Some more experimenting and measuring in the lathe showed that:
    1. Taking a wheel off an axle and replacing it in the same position can cause the TIR to change slightly by about 2-3 thou, so there's a bit of unrepeatability in the assembly process.
    2. Taking a wheel off an axle and clocking it round one flat causes the runout to stay in the same position relative the the wheel, so most of the runout is in the wheel and not any inaccuracy in the axle end (caveat, 1 above).
    3. Changing wheels onto different axle ends causes some variation of runout but not much more than 1. above. So little prospect of matching up wheels and axles to minimise runout.
    4. On some axles, on close inspection, the back of the wheel insert was not pulled tight up to the shoulder on the axle when the screw was tightened. Wheels fitted to these axle ends always had a degree of free play. Easy to miss this. This turned out to have been because the screw was bottoming in the axle thread before pulling the wheel up tight. Filing about 0.5mm off the end of the screws fixed this.
    Still left with more runout than I'd like on most of the wheels. In the end I resolved to true them up the best I could and see how they looked when installed in a running chassis. I decided to keep each wheel associated with the same axle end and the same clock position on that axle end throughout assembly. To do that I cut a tiny notch in the square of one end of each axle with a piercing saw blade.
    axle end 1.jpg
    That indicates left hand side and top centre position of the crank. The position of the right hand wheel on the axle is then determined by the right hand lead (in the case of an 8F - and most other 2 cylinder locomotives, but not always). Each axle then gets a patch of coloured paint on each of its wheels. Leading axle is purple, then green, orange, magenta for 2, 3, 4 respectively. The axles will always go into the chassis in that order to minimise variation when setting up the rods (if there is some variation in crank throw, bearing in mind plastic shrinkage, it's as well to have them in the same place every time). Also one wheel can be fixed to an axle now and need never come off during assembly, so once made to run true it won't be disturbed again (see point 1 above). And to keep that lot straight in my head for the duration of the project I made a wooden holder to keep them in order on the bench.
    wheel holder 1.jpg
    To true them up I put them back in the lathe with the indicator and with the wheel firmly screwed onto it's respective axle end I simply twisted the wheel on the end of the axle to minimise the indicator reading. Impossible to do by eyeball, but easy enough with an indicator. Brutal, and I had wondered if I'd loosen the insert or damage the axle end, but apparently not. Warranty voided and then some if I break one like this.

    In the end I have them all less than 3 thou TIR. I have to accept that there may be some variation on the 'unfixed' wheels when I assemble them. That seems to be the best I can achieve, and probably the best the manufacturing process can achieve.

    There's another experiment I'll do when I have the time, and that is to try and bore the axle insert out in the lathe to a 3/16" press fit. It ought to be possible that way to get perfectly true wheels. Well, as true as the shonky lathe permits, but there are ways to minimise the shonk.
     
  2. adrian

    adrian Flying Squad

    Interesting to see what can be done with the Slaters wheels.

    The published working dimensions suggest 4thou. (min 1.228" - max 1.232").
    Working Dimensions and Tolerances - Scale 7 Group

    Having decided on split axles and insulated hornblocks I have gone down the route of cast iron wheels from Mark Woods.
    Scan b LMS 8F

    I will be interested to see how you get along.
     
  3. Ian@StEnochs

    Ian@StEnochs Western Thunderer

    Ian,

    The nature of the beast I'm afraid. However there are few alternatives which give the range of sizes and types so we have to make do.

    Many of my locos have Slaters wheels in some form, those on my 4F are from the S7 group and apart from turning one of the axles into a crank axle and removing axle burrs and rubbing the wheel back flat!, straight from the pack. There is a slight wobble which is noticeable, the loco vibrates, on the rolling road but is invisible when run along a track and it negotiates my iffy track ok. I always try for a bb dimension of 31.25, mid way between specification, so even if there is a wobble the wheel is always inside. Not always possible with Slaters S7 axles that I have which clock at 31.3.

    I have turned and reprofiled lots of Slaters wheels and this does result in a wheel which runs true initially but taking them off and on can be challenging as they don't always go back exactly as they were. I use a similar marking system to that you describe but there must be variations in the way the squares engage, tightness of the screw etc. Remember too that on wheels reduced to scale we are removing some of the plastic from the spokes, who knows just what stress is released and its effect on true running?

    The locos with cast wheels that I have use tubular axles and taper pin fix which is much more likely to give wobble free running. On a loco nearing completion I have used this system on a set of Slaters 6'9" wheels. The brass wheel centre was bored out and steel stub axles locktighted in. So far so good, they look a bit better than the screw fixing but time will tell.

    IMG_1430.JPG

    Another factor to take account of is the flexibility of the plastic centre, more apparent on large diameter wheels, where side force over time can induce a set. I found this on a friends loco, O gauge, where the model had been tightly wrapped in tissue paper and bubble wrap and stored long time in a box. The driving wheels, 4-4-0, had a distinct toe in. I always store my engines on their wheels and if wrapped very lightly.

    Ian.
     
  4. S7BcSR

    S7BcSR Western Thunderer

    Ian C

    Why did you not use the 8F wheels produced by the S7 Group? It would in all probability have saved you a lot of work but an interesting item all the same. The S7 Group have all the wheels for the 8F bogie, driving and tender.

    Rob
     
  5. adrian

    adrian Flying Squad

    With the steel inserts on the wheel and crankpin they look like they are the S7 group wheels.
     
  6. Scale7JB

    Scale7JB Western Thunderer

    Yes, the runout on Slaters wheel can be bad sometimes.

    However there is a fix. It seems to be the brass hub in the plastic that is causing the problem.

    The way around it is to rub the entire back of the wheel in a circular motion on a flat surface until the brass on the back of the hub is clean all over. Let me know if that doesn't make sense and I'll do a wheel later on with some pictures.

    JB.
     
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  7. Ian_C

    Ian_C Western Thunderer

    They are the S7 Group wheels. Sorry I didn't make that clear.
     
  8. Ian_C

    Ian_C Western Thunderer

    Ahhhh...so it can be done then. that might be the encouragement I need, thanks! I may practice on an old set first. Expensive if I get it wrong. There are some other reasons why boring out and using plain axles interests me. Although the Slaters countersunk cap screw is quite a clever way of holding the wheel on and looking like the end of an axle it doesn't quite convince when seen up close. The size of the hex is a little small for a hollow axle (and of course it's not round) and a little too big to represent the turning centre on a solid axle. Also there's a very visible circle around the cap screw where often the prototype axle end and wheel are almost flush. I think wheels are one of the 'signature items' on a model, if they don't look quite like the prototype then the whole model never looks quite right to me.
     
  9. eastsidepilot

    eastsidepilot Western Thunderer

    Quite agree with you Ian, I've bored out Slater's wheels often and fitted telescopic axles.
    I've also found some, not many, to be eccentric also but re-profiling soon sorts that problem.
     
  10. Threadmark: Crankpins
    Ian_C

    Ian_C Western Thunderer

    I've been gradually making up the motion for the 8F and thinking about how to fix the return crank to the driving crankpin.
    P5210032.JPG
    There's no advice in the instructions and the Geoff Holt books (Wild Swan) don't say anything much about the subject apart from a reference to being quick with the soldering iron. Not sure I fancy my chances of soldering the return crank arm to the end of the crank pin; crank parallel with wheel, phased correctly relative to crankpin and not melting the crankpin insert or clogging up the motion. Besides, how do you get the motion apart after the return crank is soldered on, apart from unsoldering of course? Here's one way of doing it.

    The prototype driving crankpin is 6" diameter, or 3.5mm in S7. Much larger than a typical model crankpin, but provides a better opportunity for fixing the return crank to it. The steel crankpin insert in the Slaters S7 wheels is threaded 10BA and provides a good, solid fixing for the pin. There's no reason to change that, so the end of our new crankpin should have a 10BA male thread to match. A 10BA thread has an outside diameter of 1.7mm so it's quite possible to tap a 10BA thread into the end of the 3.5mm crankpin to secure the crank. It should be possible to solder a 10BA stud to the back of the crank. Then we'd have a crank we can screw into the end of the crankpin and secure with a medium strength thread lock. We have to set the angle of the crank correctly relative the crankpin but the chances of getting the threads phased accurately to go tight at the right angle is about zero. If we assumed a worst case of getting the crank tighten 180 degrees from where we wanted it then we'd need to adjust the thread engagement of the crank stud by half a thread pitch, or about 0.17mm. We could put the crankpin back in the lathe and skim a little off the length or we could put a shim on the back of the crank to phase it the other way. So if we machine the crankpin to be a little longer than necessary we can adjust the phase of the crank relatively easily by carefully shortening the pin. Seems plausible, so the next step is to make one and try it out.

    The crankpin is machined from steel rod, I'm guessing it's EN1A or similar. You really need to do it in a collet, it's far easier and more accurate than a chuck for these tiny things. Turn down to 3.5mm , then the threaded section to 1.7mm (for a 10BA die) and beyond that a short section at just greater than thread core diameter, 1.4mm will do. The 1.4mm diameter section acts as a lead in for the die so that it starts to cut easily and is lined up nicely before it starts to cut the real thread. Square up the shoulder, under cut the thread, reduce the thread to the correct length. Gets you one of these...
    8F crankpin 1.JPG
    Cut it off and put it back in a collet the other way round. I confess to cheating, I cut it off with a junior hacksaw rather than shred my nerves trying to use a parting off tool. Face it off and take it out of the collet to measure the length of the pin. Put it back into the collet and machine off to the right length, 4mm in this case (being the combined thickness of coupling and connecting rod bosses, plus some clearance, plus some extra length to allow for crank phasing). Dimple with a tiny centre drill then drill 1.4mm (tapping drill for 10BA) to the required depth. Tap 10BA with a taper then a plug tap. There's your crankpin...
    8F finished crankpin.JPG
    The crank is a laminate of an etched and a half etched overlay and they come to about the right scale thickness. It's a Stanier 4 stud fixing and the kit has two different overlays, one correct (ish) and one with a hole in the middle and the 4 studs too far apart. Goodness knows what that's for, so I used the correct (ish) one. There's a hole etched in the main thickness of the crank on the crankpin centre so we can use this to locate a stud. The stud is threaded 10BA to fit the crankpin, has a thin flange to solder to the crank arm and a small spigot to locate in the etched hole. Made this from brass. A picture's worth a thousand more words...
    8F return crank and stud.JPG
    Now we can assemble the parts...
    8F all the crank parts.JPG
    The starting angle of the threads in the wheel crankpin insert, on the crankpin and the return crank stud are uncontrolled and quite random so the angle that the crank arm ends up at relative to the wheel crankpin when everything is tight is also random. This doesn't prevent the model valve gear from working in a fashion ,but we should take the trouble to get the crank arm phasing and throw correct (trailing the crankpin by 90 degrees and with the end of the crank arm at a radius of 8" = 4.7mm on an 8F). We can do this by noting the angle between where the arm ends up and where is should end up. The pitch of a 10BA thread is 0.35mm, which takes a 360 degree rotation of the thread. 90 degrees would be about 0.08mm, and so on. We can use the angle error to calculate how much length to take off the end of the crankpin to get crank arm to sit at the right place when tightened up. Pop the crankpin back in the collet and reduce the length by the amount you calculated. Go cautiously, if you take off too much you either have to make a very thin shim or reduce the length nearly one pitch and have another go, and then you may end up with the pin too short. From this point on the crank arm and crankpin become specific to an individual wheel, so mark them and keep them together. We end up with this...
    8F crank assembled 1.JPG
    I plan to use a higher strength thread lock to keep the crankpin in the wheel and a lower strength thread lock to retain the crank arm in the crankpin. That way it should be possible to unscrew the crank arm to take the motion apart. I'm hoping that the cranks arms don't unscrew in normal operation. If the motion and valve gear move freely then the crank arm won't need to transmit much torque. Fingers crossed, we'll see.

    Sketches (sorry, not quite BS 8888) from the notebook if anybody wants to adapt it to their own circumstances...
    crankpin sketch.JPG return crank stud sketch.JPG

    I should add an observation about small taps and dies. I recently bought some budget 10BA and 12 BA taps and dies with this project in mind. They were 'carbon steel' and a real bargain. except they weren't a bargain when it came to using them. The threads and cutting edges were so badly formed that the resulting threads were very poor. I coughed up for some proper HSS taps and dies from Tracy Tools Ltd.
     
    Last edited: 28 May 2017
  11. Threadmark: Chassis jig
    Ian_C

    Ian_C Western Thunderer

    While I'm waiting for a couple of odd sized drills to turn up to complete the 8F motion I thought I'd make a start on the chassis. Getting chassis to fabricate properly with axles at correct centres and square across the chassis hasn't been something I've found easy with etched kits in 4mm. It's my first attempt at a 7mm locomotive so I want to get it right (and it's too expensive to get wrong!). I've checked out some of the chassis jigs you can buy these days. They look handy, but any that look like 'proper' engineering are expensive. Here's my simple version of proper engineering.

    Fundamentally all we need is four rods pretending to be axles exactly the right distance apart, parallel to each other and square to a flat surface. A metal plate, 4 holes and 4 lengths of 3/16" ground silver steel seems like the simplest way of doing it. I managed to get an odd end of 3"x 1" extruded aluminium bar locally, probably HE30 or similar. It's reasonably flat but you can see a little daylight under a ruler in some places. I had a friend at work with a milling machine skim it both sides and drill and ream the holes at the correct centres for a 7mm 8F (thanks Paul !). I want the axle rods to be a good push fit in the plate so there's no free play, but I also want to be able to remove them. Turns out that intermediate sized reamers are available 0.05mm increments and 4.75mm is about right for a light push fit with ground 3/16" rod. Simple, and as accurate as a modern machine tool will allow, which is plenty good enough for this job. Sure, it's not adjustable but I can just have some more holes put in at different centres for another loco (if I ever finish this one). Yes, the slab is long enough to take five holes for a 9F.

    I made two sets of rods in the lathe. One short set to use for making up the coupling rods, and one longer set for chassis assembly. Here's the short set...
    chassis plate 1.jpg
    The ends of the rods are turned down to the diameter of the holes in the coupling rods, 2.3mm for axles 1,2, 4 and 3.5mm for axle 3 (see earlier in the thread for details of the driving crankpin). And here they are pressed into the slab...
    chassis plate 2.jpg
    There's one downside to having a aluminium brick under your chassis; it'll suck all the heat out of any soldering work. I cut a spacer from a bit of tufnol sheet to provide some insulation. On the MOK chassis the horn blocks are located to the chassis by tabs that project through the chassis plate and are removed after soldering. The spacer allows the tabs to project through the chassis and keeps them clear of the plate.
    chassis plate with tufnol pad.jpg
    I was expecting some conflict between the exact centres of the jig and the assembly of the etched parts. Amazingly they match exactly. Horn blocks and bearings just slide down the rods and push exactly into the slots in the chassis. Gobsmacked. You could have just pushed the tabs into the slots, soldered it up and got the same result. I imagine the etch design was done on CAD so it ought to be correct, but the masking, registration and etching are spot on. That gives me some hope that the coupling rod centres are good too (they're etched on the same sheet, which helps). Congratulations to Dave Sharp at MOK and whoever he uses for photo etching. Here are the main chassis plates, the horn blocks, the compensating beams and the bearings all set up for soldering...
    chassis plate ready to solder.jpg
    After some messing around with a 40W soldering iron that wasn't quite up to the job I went and found an ancient iron in a box in the shed that's been around for years, and of great and mysterious calorific output. Mysterious because there's no marking to indicate the power. You just plug it in and it gets very hot very fast. 60W? 100W? Dunno, but you could heat the room with it. It's enough to do this job neatly. After a bit of cleaning up and a dip in the ultrasonic tank (and it looks like a 7mm 8F chassis will fit in the tank - bonus) here's the result...
    chassis and hornblocks.jpg

    A hint on removing excess solder - a triangular scraper is far quicker and tidier than files and/or emery. With a sharp scraper and a bit of practice you can shave off any amount of solder really quickly and leaving hardly a mark. Not that I ever end up with excess solder of course. Just saying.
     
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  12. simond

    simond Western Thunderer

    A hint on removing excess solder - a triangular scraper is far quicker and tidier than files and/or emery. With a sharp scraper and a bit of practice you can shave off any amount of solder really quickly and leaving hardly a mark. Not that I ever end up with excess solder of course. Just saying.

    I agree with him...

    Best
    Simon
     
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  13. Threadmark: Coupling rods complete (nearly)
    Ian_C

    Ian_C Western Thunderer

    Another day on leave. The problem of how to spend the day was solved by a packet dropping through the letterbox containing a 2.3mm drill. That was all that was standing in the way of coupling rod completion (nearly). Here's what I ended the day with...
    8F coupling and con rods 1.jpg
    Being a first loco in 7mm,and S7 to boot, I'm anticipating some mither around coupling rod and crankpin clearances behind the motion and slidebars. On the prototype the clearance between the leading crankpin and the rear of the slidebars is nominally 11/16". That's about 0.4mm in 7mm. The prototype had recessed crankpin nuts on the leading 2 axles, the return crank on the third axle and a normal crankpin nut on the fourth, trailing, axle. Seems to me that my odds of success would be greatly improved by having some kind of recessed crankpin nuts on axles 1 and 2. the S7 Group sells a set of crankpin nuts to complement their Slaters S7 8F wheel set. I bought a set. Thing is the set provides normal crankpin nuts for all the axles apart from the driving axle. Not gonna work in S7 is it? And I have read on a forum somewhere that other folk have had clearance problems to overcome as a result.

    So, recessed crankpin nuts then. Some challenges on the way; making new nuts with a 12BA thread in them, shortening the S7 Group crankpins or making some new ones, making a counterbore in the coupling rods to house the nut. After a day's work it ends up like this...
    8F recessed nut 1.jpg
    The nut is turned from a bit of nickel silver bar. I used NS because I wanted the nuts to match the rest of the motion. Don't know why I bothered, because the bar turned out to be more yellow than the etched rods. Looks like brass in the end, and it wasn't fun to work with either. Drilling a 1mm hole down the bar to tap 12BA was particularly patient and nerve shredding work and mercifully I didn't break a drill or a tap. I eventually solved the mystery of getting the parting off tool to work properly so I can now slice them off at exactly the right thickness. The counterbore means that the brass bush needs to be shorter, easier to make new ones than try to shorten the ones supplied, and the plain diameter on the crankpin needs to be shorter and the thread extended down to the new shoulder. I had sketched up a replacement crankpin to machine from scratch but hadn't quite worked out the practicalities of making the thing so chickened out and modified the existing ones which turned out to easier then I'd feared, albeit requiring some delicate work with the 12BA die. In passing, the way I held the crankpin to extend the 12BA thread was to clamp the 10BA (wheel) end in a 1-2mm collet in a collet holder held in the bench vice. It is possible, with gentle tightening of the collet nut, to get a good enough grip on the pin to hold it against the die and not to damage the 10BA threads.

    The hole in the rod is 2.3mm through, simply because that's the diameter of the brass crankpin bushes that come with the S7 crankpin set. The laminated rods were drilled through 2.3mm with the new, sharp drill, trusting that the drill would find centre, which it seemed to do. The prototype recessed nut is quite a large diameter and extends out nearly to the diameter of the boss, and although I'm not exactly replicating the original arrangement I want the model to look similar. Therefore the counterbore diameter is 4mm. So here's the next challenge. Where can you buy a 4mm counterbore tool with a 2.3mm pilot? Answer = you can't. I'd considered using a 4mm slot drill but experience suggests that you need the work rigidly clamped to make the drill go where you want, and an off centre counterbore would look pants and cause clearance problems around the nut. Besides I couldn't see how to clamp the rods effectively. While searching for small counterbore tools I came across a couple of model engineering forums (fora?) where folk had made their own custom counterbore tools. One method seemed easy enough with the tools at my disposal so I had a go. Here's the result...
    counterbore tool 1.jpg counterbore tool 2.jpg
    The tool is made from a length of 3/16" silver steel, axle material actually. Turned to c/bore diameter, drilled out pilot diameter, flats milled across to form two 'pegs' or cutting teeth, teeth filed to provide some rake, hardened (cherry red & water quench), teeth stoned to a sharp edge, pilot rod loctited in the hole. Sometimes you get lucky, and the pilot rod is the shank of an old mini drill bit that ended up in the scrap box. Exactly 2.3mm and Ti-N coated as well. Tested on a bit of aluminium first before attempting the rods. Worked very well, much better than expected. Very carefully worked out which ends and faces of which rods needed to be counterbored (no easy remedy if you get a c/bore in the wrong place), rods held on a piece of aluminium plate with a 2.3mm pilot hole drilled in it so the pilot is guided first by the hole in the rod and then by the hole in the plate before the cutting edges start to work, rod (and plate) held down by fingers (you get some sense of what's going on when you're providing the restraining torque with your finger tips), drop of oil on the rod, slowest speed on the drill, hold your breath and out comes a ribbon of swarf. Taken easily a bit at a time until the counterbore was 0.8mm deep and the nut tightened down on the brass bush and left the rod free to rotate with a little side play. Scrapping a rod was something to fret about but it worked OK in the end.

    The end result...
    8F recessed nut 2.jpg
    Not exactly true to prototype, but looking similar. And anyway, it's a mid sixties 8F so you'll not see it under a coating of brake dust, ash, road grime and oil. This approach does reduce the rod/crank pin bearing area on the leading two axles, there's 1.1mm left, I'm guessing it won't be a problem. It'll be some time before I discover if I have 0.4mm clearance to the slide bars, or another challenge.

    Oh yes, 'nearly' complete. There's still the pinning together of the forked joints to work out.
     
  14. daifly

    daifly Western Thunderer

    Ian
    Could I make a request that you post your attached images at full size. It's an absolute PITA having to wait for the thumbnail to load in a new window while reading your narrative.
    Thanks
    Dave
     
  15. adrian

    adrian Flying Squad

    Thanks for posting - bookmarked for reference when I get started on my 8F.

    That looks very useful - again another solution filed away.
     
  16. Threadmark: Coupling rods complete (really)
    Ian_C

    Ian_C Western Thunderer

    Like the chassis, the etched rods are spot on for centre distance. Having drilled the crankpin holes through to the final diameter they dropped straight onto the rod setting pins in the baseplate. I'm still amazed that etching, soldering up and drilling through produces parts so dimensionally accurate. The holes for the rod articulation pins lined up perfectly too, so no messing about required.
    coupling rod setting 1.jpg
    (full size, just for you daifly;))
    The articulation pin holes are 1.6mm so I could have used a length of 1.6mm wire, but since I have the Wild Swan LMS Locomotive Profiles No.8 - The Class 8F 2-8-0s I can see how it's done on the prototype and I'm afraid that wire just won't do. Measuring, sketching and off to the lathe again to make some little rivetty pin things.
    coupling rod joint pin.jpg
    The pins are steel. I left a 1mm diameter pip on the head that I filed to an approximate hexagon, just about visible in the photo. The opposite end of the pin is drilled out 1mm for a about 1mm deep to make kind of tubular rivet end that can be spread out gently with a centre punch and a hammer.
    coupling rod joints 2.jpg
    The final job is to drill 0.6mm into the top of the oil reservoirs to take some 0.6mm wire to represent the corks. Couple of hints here. The rods are laminated from 3 thicknesses, so if you cut a notch in the middle laminate where the cork will go it will fill up with solder during the lamination process. Then it's much easier to centre spot and drill down this deposit of solder than virgin nickel silver. For work this small I usually make the centre spot with the end of a scriber as it's easier to position than the end of a centre punch. A vigorous poke with the scriber is often enough to start a small drill but the scriber divot can be used to locate a centre punch if you want a bigger crater. The wire is just a push fit into the hole, no messing about with solder or glue at this stage. To make 0.6mm brass wire a force fit in a 0.6mm hole gently squeeze the end of the wire in pliers or a vice to flatten and widen it slightly. Force it in with pliers, snip off and file down to whatever height you think an oiling cork is in 7mm.
    coupling rods complete.jpg
    Bit of an epic, lots learned on the way and I know they exactly match the axle centres. Very happy.

    There's a bit of me wondering if I couldn't make a steel set from scratch, hewn from a billet of Baron von Krupp's personal stock. They'd be easy enough to model on CAD. Anybody out there with a tiny CNC mill?
     
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  17. adrian

    adrian Flying Squad

    Funny you should say that - but that's my plans for the 8F. I'm currently making a set of fluted steel rods for my 3F - I will post soon but as I don't have a milling machine I'm just filing the bosses to shape by hand at the moment so it'll be a few days before they are finished.
     
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  18. Martin Shaw

    Martin Shaw Western Thunderer

    Ian C wrote
    From bitter experience anywhere from 3/4" to flush, usually nearer the latter. That's a max of 0.4mm at scale.
    Very useful set of tips above Ian, filed for future use.

    Regards
    Martin
     
    adrian likes this.
  19. Ian_C

    Ian_C Western Thunderer

    Sounds like practical footplate experience! So maybe I'll file a couple down some more.
     
  20. Ian_C

    Ian_C Western Thunderer

    Postscript to the rivetty things. Was looking for some other doodads in the kit parts this afternoon and whaddaya know? I found some rivetty things supplied with the kit. Very similar to the ones I made. No mention in the 'structions and I'd probably not have guessed what they were before this. Oh well, I have plenty of spares now.
     
    farnetti likes this.