Engines > Restoration of Model Engines
Fitting a piston for F2C engine
Ramon Wilson:
Thanks Neil for explaining your processes on this and your other thread :ThumbsUp:
I don't think I would need to go to this level for the kind of engines I care for but I can fully respect the need to be so accurate for what you do. Your explanation of 'touch on' is an eye opener - true old saying you can learn something every day if you choose to, as is the sharpness of tooling.
I've been out this morning to give this method a try, albeit on brass and without doing anything to the lathe. I confirm that it does work remarkably well and will shave off micro amounts. I didn't achieve .001 but I did achieve a measurable .002/3 difference on my Mitutoyo mic.
Your comment on hand feeding is noted - I made a larger saddle handwheel some time ago to assist in slow feed like this. Works well but you do have to be careful that your left hand doesn't catch the cross slide handle - yep!
As I was trying that out it occurred that you would be using a high grade of silicon ali for your pistons - the plated liners though would be well beyond my aspirations. That said, like Jim Allen's posts it's really good to hear and see how a professional approach to these matters is carried out.
Measuring kit is the key word too. Save one Geneve clock I don't have the means to measure in microns save that on the Mitutoyo mics - a 'guess' at best, despite how careful the feel. Lapping then provides the means to get accurate fits but not to any defined measurement of course.
You have however been cause for much thought and whilst I think it will still have to be the old lapped cast iron in En1a for me I think I will be approaching matters with your methods firmly in mind
Many thanks indeed
Tug
dieselpilot:
Thanks for sharing. Do you also cut a taper at the top of the piston?
I would think a fairly good lathe(Hardinge HLV-H or Schaublin) is needed to make a useful piston like this, especially hand feeding. I have a PCD grooving tool that will take off such thin chips, 1mm wide chips float away. I don't think my old Clausing is in good enough condition to turn the OD of a ringless piston. The highschool students who beat on it previous to my ownership, crashed the chuck into the compound slide so many times it looks like a dog chewed the corner off.
With .01mm/rev and .4mm nose radius theoretical surface roughness well into lapping territory. Real results are completely dependent on the material and cutting edge. The only thing sharper than these sharp edge PCD is monocrystalline diamond. You can cut cast iron with diamond, but CBN is typical. Cast iron is much harder and probably more variable in properties than these aluminum, so the results may not be the same.
steamer:
--- Quote from: dieselpilot on January 11, 2019, 03:47:09 PM ---Thanks for sharing. Do you also cut a taper at the top of the piston?
I would think a fairly good lathe(Hardinge HLV-H or Schaublin) is needed to make a useful piston like this, especially hand feeding. I have a PCD grooving tool that will take off such thin chips, 1mm wide chips float away. I don't think my old Clausing is in good enough condition to turn the OD of a ringless piston. The highschool students who beat on it previous to my ownership, crashed the chuck into the compound slide so many times it looks like a dog chewed the corner off.
With .01mm/rev and .4mm nose radius theoretical surface roughness well into lapping territory. Real results are completely dependent on the material and cutting edge. The only thing sharper than these sharp edge PCD is monocrystalline diamond. You can cut cast iron with diamond, but CBN is typical. Cast iron is much harder and probably more variable in properties than these aluminum, so the results may not be the same.
--- End quote ---
Monocrystalline diamond is usually on Diamond turning lathes, which, although would hold these tolerances at a doddle, aren't really intended for ferrous alloys, but for the optics arena. 0.003" DOC in a diamond turning lathe is considered a very heavy roughing cut!...
PCD, is probably the sharpest tools available for conventional machining, but really require a tight lathe with good geometry. Sounds like Neil has his tuned well! A dead sharp tool with have the lowest cutting force. Combine that with a stiff well damped machine structure, and a conventional lathe can work very closely indeed. I have seen HLV's turn sharpy ink off before....it's always pretty cool to watch that.
Dave
Ramon Wilson:
--- Quote from: Ken Croft on February 01, 2019, 09:56:35 PM ---..................
Actually, The holy grail has been to NEVER EVER lap piston and liner together. They are always lapped separately.
--- End quote ---
Well Ken - quite a provocative and somewhat surprisingly definitive comment for a first post :o
All my engines have had piston and liner lapped together but that wasn't my idea - that came from George Aldrich - possibly the most experienced and knowledgeable exponent of piston/liner fit when alive and like yourself, one of the original 'Motor Boys'. For me George was, and still is, very much a true guru. His treatise on 'Cast in Steel' written in Model Airplane News on making/using cast iron pistons in a leaded steel liner goes into great detail how to carry out this very method and is one carried out on all the engines I've made so far with great success.
I'm aware of your (model engine) background Ken and it may be so in certain situations with other materials but I'm afraid I can't agree with you in making this such a sweepingly positive statement - it's simply not the case.
Regards - Tug
Jo:
Welcome Ken,
Nice to see you join the forum. Would you like to make an introduction post in here: http://www.modelenginemaker.com/index.php/board,3.0.html so other forum members know who you are and your background in making model engines :)
Jo
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