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Model Forum / General / Models / May 2008Thanks Guys...Its Done
Finally, I've finished my attempt at building Malcolm Stride's Nemett15S. Never really made anything significant before, but Malcolm's article was that detailed I thought it was worth a shot. It fired up and ran tonight a bit of fiddling left to do with the carb, but it works. I didn't stretch to spark ignition but left it as a glow - motor. It's not exhibition standard, there are a few careless mistakes, but still pretty pleased with it. Here are the pics of the finished engine: www.btinternet.com/~steve.withnell/lathe/Nemett/nem2.JPG www.btinternet.com/~steve.withnell/lathe/Nemett/nem3.JPG The files are big <2M. So almost exactly two years to complete, I'll write down what I learned that wasn't in the books when I get chance. Anyway, big thanks to John, Peter(s), Dave, Tony, Keith and everyone else that provide help along the way and of course "Nemett" for an excellent constructional article. Oh and a question - anybody know where I can get a copy of the magazine article with Bruce Satra's valve lapping tool? Thanks Guys Steve Congratulations Steve! If that's your first attempt at something 'significant' you should be very satisfied. IC engines, especially at model scale, are a lot more taxing than steam engines. I haven't got a copy of the article you request, but I have made both piston and valve laps by simply making a replica(ish) of the mating part from a soft-ish material (usually alloy for convenience) and lapping the component with an appropriate abrasive. I have some 1 and 5 micron diamond paste I acquired moons ago and which works well. Also simple old-fashioned 'Brasso' does a fair job. The lap works better if you create some grooves (two or three or so) perpendicular to the direction of surface movement. This allows the abrasive slurry to get between the surfaces and do its job. You don't need a lot of pressure holding the parts together. Richard > Congratulations Steve! If that's your first attempt at something > 'significant' you should be very satisfied. IC engines, especially at [quoted text clipped - 14 lines] > > Richard So if I made a dummy valve from copper or brass to the same spec as the "real" ones but left a long stem to get hold of, cut the perpendicular grooves to carry the abrasive then I should be good to go? I'm planning a more challenging motor and want to get a good seal around the valves from the off. With this one I made also sorts of duff assumptions about how to get a good seal - I went to huge trouble to ensure the valve seat and valve were cut at identical angles (wrong!) I tried lapping with the valve in situ (wrong!). I assumed you needed a broad seat (wrong!). So now I know how the experts do it, and it just the lapping trick I was short of to complete the toolkit. In the end I cut new seats and didn't lap them in to get a reaonable seal. Thanks >So if I made a dummy valve from copper or brass to the same spec as the >"real" ones but left a long stem to get hold of, cut the perpendicular >grooves to carry the abrasive then I should be good to go? Broadly 'Yes'. You would probably be better off using brass rather than copper as it's somewhat harder and a lot stiffer - you don't want it moving about or you'll end up with the seat pissed. Secondly, You will probably find the valve stem too small to be useful to 'drive' the hone with so use it as a pilot, but machine the hone so you have a rod left on the valve 'head' to drive it with. Depending on what you have available a small, light drill or possibly a drill press can be used. I have found a speed of somewhere around 500RPM (but it's not at all critical) works quite well for finishing pistons, less for valves as they're smaller and you don't want to rip off so much material. You need to make sure you have plenty of fluid with the abrasive. You are really using the fluid as a 'lubricant' which keeps the two surfaces floating out of contact and the abrasive is somewhat larger than the film thickness and thereby gets to grind the surfaces away. It also gets embedded into the softer surface and continues cutting a bit like a microscopic grinding wheel, which is why you must NEVER be tempted to use the sometimes quoted technique of running-in a tight engine 'with a bit of Brasso in the fuel'. It's curtains in an instant and it's utterly irreversible. For a valve you obviously want to arrange it so you don't get a load of it up the stem/guide. As for seat width, models can't be scaled to use 'scale' contact widths, it'd be a few thou (at a guess) to get sensible contact pressure. But the relatively cool running temperature of methanol and the likely running hours means it's not a major issue in practice. A very slight deliberate mis-match in angles between valve and the seat can help to make sure the contact is round the outer rim of the valve contact area so that gas pressure in the cylinder doesn't get under the edge and help to lift the valve off its seat. >I'm planning a more challenging motor I look forward to seeing your progress Rgds Richard > So if I made a dummy valve from copper or brass to the same spec as the > "real" ones but left a long stem to get hold of, cut the perpendicular [quoted text clipped - 10 lines] > short of to complete the toolkit. In the end I cut new seats and didn't > lap them in to get a reaonable seal. As someone who cuts valve seats in race engines for a living I'm not sure where you're getting the above advice. The valve and seat should indeed be at the same angle although OE manufacturers sometimes use a very small (less than 1 degree) difference between them to save having to lap them in and let them bed into each other on their own. I don't hold with that approach myself. The more accurately the two faces match from the start the better sealing and heat rejection you get. A good choice of seat width is 4.5% of the diameter of the valve head on inlets and use the same seat width as the inlet on the smaller exhaust valves which automatically creates a slightly higher percentage width to aid in heat transfer. This normally equates to about 5% to 5.5% of the exhaust valve head size. By seat width I mean the perpendicular width not the horizontal. For lapping you just need a stick with a rubber sucker on the end and fine lapping paste. However lapping in general is bad n'kay. It actually creates concave surfaces on both valve and seat as the majority of the grinding action takes place in the centre of the contact patch. You can see this very clearly when you reface a heavily lapped valve on the grinder and the initial contact is only on the inner and outer edges of the seat. Any valve/seat that needs a lot of lapping hasn't been properly cut and really the lapping operation is mainly to check the seating and not to correct poor cutting.  SignatureDave Baker Puma Race Engines >> So if I made a dummy valve from copper or brass to the same spec as the >> "real" ones but left a long stem to get hold of, cut the perpendicular [quoted text clipped - 24 lines] > cut and really the lapping operation is mainly to check the seating and > not to correct poor cutting. My position is all "learning by forum" not trained professional experience. In context these valves are 3mm diameter stem and 10mm diameter across the head. Some of these small engine designs don't cut a seat in the head at all but let the valve bed down on the edge of the bored chamber, others go for a 46deg seat and 45 deg for the valve. I was that concerned about getting a good seal, I followed what was described as the "rolls royce" method - I cut the seat at 60degrees then made a second cut at 30degrees, to end up with seat around little more than 0.6mm wide overall perpendicular, but with a concentric edge in the centre of the seat. So basically a distillation of a number of contributions to forums. Steve > Finally, I've finished my attempt at building Malcolm Stride's Nemett15S. > Never really made anything significant before, but Malcolm's article was [quoted text clipped - 25 lines] > > Steve Very nice! Any more details? Engine size/bhp/rpm? >> Finally, I've finished my attempt at building Malcolm Stride's >> Nemett15S. Never really made anything significant before, but Malcolm's [quoted text clipped - 28 lines] > Very nice! > Any more details? Engine size/bhp/rpm? It's nominally 15cc and the designer suggested in the constructional article that it would throttle between 2000 and 8000rpm. I used to fly RC aircraft about 20 years ago and this sounds a delight compared with those screamin two strokes - I've only about 12mm of pipe brazed into the exhaust manifold and it's loud but not screaming. I think big credit is due to the designer Malcolm Stride that the article was written with such skill as to allow me (I've no training except for school metalwork classes about 36 years ago) to scratchbuild this. The only parts I haven't made are the cap head screws, the bearings, the cam belt and cam belt pulleys and the glow plug. The fuel needle in the carb was stolen from my wife's sewing kit. Steve > Finally, I've finished my attempt at building Malcolm Stride's Nemett15S. > Never really made anything significant before, but Malcolm's article was [quoted text clipped - 24 lines] > > Steve Well done Steve a superb job that you should be very pleased with. Looks as if it could be useful in hauling about something of a reasonable size as well. Where was the article from? I've obviously missed it and it looks the sort of size (larger) that I could attempt as I could at least see most of it. Excellent first major project Steve and it runs as well, no stopping you now. Ashamed to report that my own first attempt at a 1.5cc diesel only produced a couple of "putts" and very sore fingers - attacking it with the electric starter just produced a bent conrod but I did have the excuse that I was young and impatient at the time - just much older now nothing else has changed. :-) Congratulations a fine and impressive achievment, now will you make it work or is it into the glass case for showing? Regards Keith looks a good job for a first project and finished just in time for th start of his next series on the inline twin ;) If I recall its a 15cc engine, head is about 60mm diameter to give a idea of size and the series was in Model Engineer. I was considering i as a second job to do while I'm making my 2" Fowler TE but will probabl go for the V twin 'Hoglet (http://www.pbase.com/captain_carl/image/67722872) that was in Mode engine Builder mag, bit more to see going on. Jaso -- jasonballam > looks a good job for a first project and finished just in time for the > start of his next series on the inline twin ;) [quoted text clipped - 7 lines] > > Jason That's the one - just published a video of it running: http://www.btinternet.com/~steve.withnell/lathe/Nemett/nemrun.wmv Hopefully this works, it fires up in Mediaplayer for me, doesn't mean it will for amyone else! Another nice V-twin is the "Vega" which I've be thinking about. No intention of building the inline twin Nemett not on the plan at all. I will take a look at the hoglet though. Pretty certain a steam engine is next. Partly because I've got all the bits and there is a bit of history to it. Steve On 26 Apr, 23:52, "Steve" <steve.withn...@btinternet.com> wrote: > Finally, I've finished my attempt at building Malcolm Stride's Nemett15S. > Never really made anything significant before, but Malcolm's article was [quoted text clipped - 24 lines] > > Steve Well done Steve a superb job that you should be very pleased with. Looks as if it could be useful in hauling about something of a reasonable size as well. Where was the article from? I've obviously missed it and it looks the sort of size (larger) that I could attempt as I could at least see most of it. Excellent first major project Steve and it runs as well, no stopping you now. Ashamed to report that my own first attempt at a 1.5cc diesel only produced a couple of "putts" and very sore fingers - attacking it with the electric starter just produced a bent conrod but I did have the excuse that I was young and impatient at the time - just much older now nothing else has changed. :-) Congratulations a fine and impressive achievment, now will you make it work or is it into the glass case for showing? Regards Keith It is Model Engineer running every other issue from May 2006, which is when I started it. So all that advice on setting up collet chucks did actually deliver! Many thanks Keith. I've now got this running consistently, and this afternoon it started "first flick" but it does need an electric start, which if you get a hydraulic lock will cause serious dmage as you found out! I've now got some video clips which need editting down to web sizes so will get those up as soon as I make some time. I did wonder about building one of those big vintage cabin wing RC planes, but I'm kidding myself to think I can make the time at the minute! Now I have a broad feel for my lathe and it's had some serious use, next task is a full tune up and to add a digital scale to the tailstock, counting turns on a handwheel with the lathe running at c3000rpm with a 0.7mm drill bit in the chuck and trying to get to a 17.5 mm deep straight hole is something I can do without! I have the bits to add a rev counter to the spindle without too much effort, but JS advises the effort doesn't really pay back. So I might just calibrate the speed knob. Once I've tuned up the lathe and mill, I plan to re-build great grandad's steam engine. I've enough bit's to do a rebuild and it shouldn't require as much effort as this motor did. It's pretty big, about 5 inch stroke x 1/3-4 bore, single cylinder horizontal engine. Steve > It is Model Engineer running every other issue from May 2006, which is when > I started it. So all that advice on setting up collet chucks did actually [quoted text clipped - 26 lines] > > - Show quoted text - Thanks Steve, I'll think I have a couple of issues from that time so I'll have a look. You are right a big old Piper Cub or something would look and sound just right. Now if I could just find the time to finish that time machine I could get all of these planned projects finished; the one issue with this hobby is there is never enough time but always too many things to make and mend. I have thought about a spindle tacho as well but have came to the same conclusion as JS (wise old owl), suitable cutting speeds are normally within quite a wide range and we often change them anyway to suit the circumstance so absolute accuracy is not really essential. On my Myford I borrowed one of those cheap digital rpm gauges and calibrated (in the loosest sense) my rpm pot. Although I marked it at the highest speed belt position I also did it with the drive belts in all possible speeds and made up a chart for future reference. To be honest I have only ever run the machine with the belt on the high speed setting since but I'm sure I could find the chart if needs be :-)) Look forward to seeing and hearing the video Best regards Keith On 27 Apr, 17:23, "Steve" <steve.withn...@btinternet.com> wrote: > It is Model Engineer running every other issue from May 2006, which is > when [quoted text clipped - 30 lines] > > - Show quoted text - Thanks Steve, I'll think I have a couple of issues from that time so I'll have a look. You are right a big old Piper Cub or something would look and sound just right. Now if I could just find the time to finish that time machine I could get all of these planned projects finished; the one issue with this hobby is there is never enough time but always too many things to make and mend. I have thought about a spindle tacho as well but have came to the same conclusion as JS (wise old owl), suitable cutting speeds are normally within quite a wide range and we often change them anyway to suit the circumstance so absolute accuracy is not really essential. On my Myford I borrowed one of those cheap digital rpm gauges and calibrated (in the loosest sense) my rpm pot. Although I marked it at the highest speed belt position I also did it with the drive belts in all possible speeds and made up a chart for future reference. To be honest I have only ever run the machine with the belt on the high speed setting since but I'm sure I could find the chart if needs be :-)) Look forward to seeing and hearing the video Best regards Keith Here it is - http://www.btinternet.com/~steve.withnell/lathe/Nemett/nemrun.wmv I've got the tacho head installed, so I just need to plug in a frequency counter to do the calibration. I've only two belt ratio's so a dual calibration on the knob should work for me without needing a chart. Regards Steve |
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