Real basic rough draft but almost to scale. Here's most of the case. I think I want the backplate to bolt in like a wasp. It's kinda based on my favorite aspects of the wasp, VA, and CS engines. I love the CS case and liner, but it's kinda bulky. The VA is an unbelievable size case, but split. So it'll be in between that somehow. It will be a single exhaust port in an effort to keep the plane clean...

Dane,
The "spline" on the little cranks (Cox) are nothing more than a fine straight knurl. The hole in the prop driver is very slightly larger than the pitch diameter of the knurl. The metal needs enough room to flow without trying to compress the metal of the prop driver when it is pressed on the crank. The crank should be machined from something pre-heat treated so it is tough but there is no heat treat distortion. I don't know if I have any Stressproof or ETD 150 the right size. If nothing else it can be turned from a 1/2" socket head cap screw. The socket heads are about 38 Rc, not hard to machine with carbide tools.

Pat

If you're not familiar with it, this site is well worth spending a few hours browsing through. 
http://www.modelenginenews.org/index.html
Sadly, due to Ron Chernish's passing, the website is now only an archive, but all the info is still there. Adrian Duncan would be well worth talking to regarding your plans.

Rod.

dane,

You probably do not need more inspiration since you seem really serious to do your own 1/2 engine.  You might already be familiar with the work that Wayne Trivin did several years ago.
Keith

There are so many helpful ideas in the link you've supplied. I remember seeing those a while back, but now it means a lot more! His engine is FAR more advanced than what I have in mind to start with, but a lot of his processes are very helpful.
 Thank you very much Keith. Hopefully I'll see you again at a contest soon, and I'll bring whatever I come up with, with me!

Brother Dane

On CEF we have a 3D printer wizard...here is one of his way cool projects

https://www.coxengineforum.com/t14093-ball-bearings-you-say-3d-printed-bb-crankcase

I actually built all of the moving pieces for a Cox TD 09, plus the cylinder and head.  It was crap.  It wore out about 5000 revolutions after it was nicely broken in.  But it did that wearing out under it's own power!

I learned a lot -- starting with the fact that just scrubbing the parts with Dawn and a toothbrush won't get all the lapping compound out of steel.  I think I also learned not to use Mystery Metal in critical wear areas like pistons and cylinders.

It even got written up (without the bit about wearing out half a second after breaking in -- I sent in the pictures after the first run in-hand): http://modelenginenews.org/ed.2012.04.html

Based on all that -- I'd suggest finding some plans for an engine (there's a few "beginners engines" on modelenginenews.org) and just building to plan.  Or grab a clapped-out engine and make some new bits (unless you've done that).  Once you get that working, even if it's not strong enough to get the skin off of hot milk, then go and build an engine.

A bit of advice that I see over and over in Model Engine News is to start with an engine from about .15 to .35 displacement -- it's felt that's a good compromise between fiddly bits that are too small for the beginner to machine effectively, and materials cost.

These may be nice, too.  You need to search out the download link: https://rclibrary.co.uk/title_details.asp?ID=2122, https://rclibrary.co.uk/title_details.asp?ID=2139

This forum will be helpful, although they'll start by telling you that the last engine a beginner wants to build is a two-stroke: https://www.homemodelenginemachinist.com/.  They may be right -- but I don't want to build anything else!

Very cool, I'll check it out.

The reason I've decided to build my own is that I've built almost any engine part at some point or another with varying degrees of success. I've just never built everything for one engine at once! I'm just more of a 1/2a guy than anything else. So that's pretty much the reason for the size.

Danish,

I used 4140 for cranks. Machines just fine. Haven't gotten one to the running stage so I can't say anything about reliability.

A few different engines so far:  A modified 1/2A Dragon from Clough plans while in High School; a custom 1/2A motorcycle engine based on a Cox piston and cut down cylinder (made into a liner); and a 2x-sized Morton radial.

My Dragon used a 2024 piston with a tiny compression ring. I bought 4130 tubing to make the liner, but the lathe available did not have sufficient stiffness to bore the ID.

I made a fixture to offset-machine the crank pins in place, instead of using a 4-jaw chuck. Used multiple setscrews to dog the part to the fixture. It worked fine.

I agree with Pat that straight knurling could produce the splines on Cox cranks. However, if you are not familiar with knurling, be aware that it puts much higher tool loads on a part than most other machining. I don't know for sure if Cox knurled or broached these pieces. You may need a specific sequence for success as well. For example, N-condition rough machine, knurl, heat treat, then grind.

Carbide inserts may be a bit bulky for smaller parts. Also, they often do not have positive rake, so the tool pressure is higher. Might make it tougher to prevent chatter and hold tolerance on small parts.

Sounds like you are having fun!

The Divot

Very cool, I'll check it out.

The reason I've decided to build my own is that I've built almost any engine part at some point or another with varying degrees of success. I've just never built everything for one engine at once! I'm just more of a 1/2a guy than anything else. So that's pretty much the reason for the size.

Oh, well then -- carry on!  And let us see the results!

... I agree with Pat that straight knurling could produce the splines on Cox cranks. However, if you are not familiar with knurling, be aware that it puts much higher tool loads on a part than most other machining. ...

You can get knurling tools that clamp the knurls onto the part, instead of pushing against them.  So instead of the force coming from the cross-slide and being opposed by the part, the force of the top wheel is opposed by the bottom wheel.

It's said to be gentler on the parts and the lathe -- and certainly looks like it.  I can't compare: I have a cheap lathe, so when I got knurling tooling I just got the clamp style.

Dane.  That sounds like a great project!  Not a fun project, but a great one anyway.

I think the splined shaft on Cox engines is a poor idea.  Great as long as you don't bugger up even the smallest part of it.  Once a spline is even slightly damaged, it will then require massive number of Newtons to get it back on.  Once wrecked, the drive washer is next to impossible to get back off.  Others have suggested a taper shaft.  Great.  That can't get stuck.

Dane.  That sounds like a great project!  Not a fun project, but a great one anyway.

Thanks Floyd. I figured I've done enough case, muffler and engine reconditioning for friends and family I should just combine it all into making one.

So if everything goes according to plan, I cut a crank blank from this grade 8 bolt. I'm completely winging it here. Looks goofy, but I'm trying to make the excess of the crank part of its own offset fixture. I only have a 3 jaw on the chuck now. I found a 4 jaw on Ebay... I'm watching it.

Danish,

Looks like lots of fun.

That machine ought to have plenty of horsepower to turn 1/2A cranks! Based on the picture, I'd try a tool with a smaller tip, if you can.  Try to have clearance behind the cutting edge so you are not dragging an edge. Hone the edge that is doing the work. You can stiffen the setup by running the live center into the bolt head. (You may be giving up some stiffness chucking on the threads, as well.) If you get rid of any chatter and get good tool geometry it is a lot easier to get the tolerance and finish you want.

A soft wheel or diamond hone would help dress the edges when using brazed carbide inserts.

I'll have to go look for these "self-supporting" knurling tools that guys are talking about. I haven't used one of those before.

McDivot

PS--The FAA got on me years ago with my excessive crank departures. They apparently rated my Mouse racing by whether all of my departures were on-time or not. I finally got to the point--after a lot of work--that all of my departures were on-time. One every heat. Usually right where they wanted it after the pit stop. So oddly, my government rating went up, but my scores (DNF) did not.

If I had to choose one cutter for general work, I would take a boring bar, as stiff as possible, with insert shaped like in picture below. They are easy to grind sharper when needed, and the geometry gives a small relief angle to both x and y directions. It improves the surface quality.

CCGT, high positive?
Normally high positive is for non-ferrous.

I have the lathe in the video. Also I watched the video again and saw a few things I probably shouldn't be doing, and some things I should add. Time for a reeducation!

... or you could go with a threaded hole and stud arrangement which would cut down on material removal. ... 

An advantage to this arrangement is that when you bork the plane, there's a good chance that the stud will bend but not the crankshaft.

I deleted the part where you mention 12-32.  If Dane is building an 049 then he probably doesn't want bigger than a #6 thread.  Cox uses a 5-40 on theirs, and the Norvel 049 and 061 engines use M3.5 (Edit: or M3).  If it were me I'd probably use a 5-40 just because of the sheer weight of all the Cox stuff I have around.

Dane, these may help.

Drill an offset hole in some round stock, then make a slit from hole to the stock edge, across the widest part of the remaining stock.  If you make the hole a slip fit or light press fit on the crank, then when you tighten up the holder in your 3- or 4-jaw, it'll tighten the holder on the crank.

In my case I didn't care about getting the stroke exactly right, so I just kind of threw things together.  With a 3-jaw you set the crank throw with the hole location in the stock.  With a 4-jaw (or a 3-jaw and shim stock) you can use a dial indicator with enough range (or if you use an indicator plus the readout on your cross slide) to get the stroke more accurate.

This was for a TD 09 rebuild; I was aiming for exactly 1/2 inch stroke, plus or minus a mile.

Ah, see that's what my half @$$ed picture was alluding to. I believe that would be the simplest thing for me to do without restoring to buying a bunch of stuff. I am not tooled up for this... I'm working on it. And I will be. But a little at a time.
Thanks for the pics

If you think your jaws will hold the collet straight, then your offset pot chuck will work. Not sure how you would make one without a 4 jaw.


Motorman

In my top picture that's a 1" diameter hunk of aluminum with a piece of stock roughly 1/4" think behind one jaw, to offset the aluminum round.

Dane, I just took another look at your sketch, and it looks an awful lot like an Owen's Mate, with plans in the Motor Boys International plans book.  You could do worse than take inspiration off of that engine, I think.

Awesome. Thanks for the tip. I'll look it up!

For an even better finish, round off the end of your tool.  I can't remember the right name (the real machinists will), but there's a round-nosed tool profile that's specifically made for nice finish cuts.  The idea is that a sharp-point tool is going to make a 'V' groove; a round-nosed tool is going to make a lot of really shallow overlapping 'U' grooves.

Well through my desire to learn the proper ways to do things... I happened upon this. I just so happen to have a compound tool reset so now I can accurately cut my crank taper!

Now the question... can I make the prop drive and cut the taper with a sharp reamer, and then just copy that angle on the shaft? Or is there a pre existing known angle I should be using?

Tim, I'm really liking this Owens Mate. It's a good inspiration for ideas.

Well not a monumental failure, but not flyable. My intake window is not big enough. I purposely made the window and the crank center slightly undersized. That sounded reasonable, because I think it broke originally due to how thin the crank was. But, doesn't really draw fuel as well. Once I got fuel to it to lubricate the crank, it feels normal. Nice and loose. But it runs. So there's hope for the next one.

Well not a monumental failure, but not flyable. My intake window is not big enough. I purposely made the window and the crank center slightly undersized. That sounded reasonable, because I think it broke originally due to how thin the crank was. But, doesn't really draw fuel as well. Once I got fuel to it to lubricate the crank, it feels normal. Nice and loose. But it runs. So there's hope for the next one.

It sounded like you could pull a small, relaxed, sport free-flight with that, if you could get it to hold a needle setting.  Something designed for a 1950's 049, or a TD 020.

Flying engine first, fire-breather second...

You could put in a smaller venturi to match your smaller crank -- they'd still be undersized for the transfer ports (assuming everything was nicely balanced), but you'd have better needle response, and maybe something that would run.  You could just do a quick & dirty insert with a piece of wood, for a test.

Did you check the intake timing?  A smaller window would reduce the intake time, which up to some point (that I don't know, but I'll bet it depends on the porting) should make for easier starting and lower-speed tractability -- but if you put it too far over one way or another it'd mess things up.  It would be good to check your work on that.  An A/B comparison of the old and new intake timing may help you figure things out (or you could post it, we seem to have experienced engine modifiers here).

You could modify the crank easily enough.  You could open up the window with a Dreaded Dremel Tool, or you could figure out a work holder for it in your mill.  Opening up the hole would only take chucking the crank in a 3-jaw.

Are you saying it was a terrible engine before?  So you haven't lost much?

You may have also have introduced a crankcase leak -- I would think (someone correct me if I'm wrong here) that you'd want the crank to be just a bit tighter than a good running fit initially, to give it a chance to wear in.  So if you start with the right clearance for "after it's broken in" then it may be leaking now (or will be).

My approach (and I'm trying not to push -- really!) would be to experiment with getting the most you can out of that part, for the learning experience, and then move on to the next one.