Little '36 Midget Build

[Jasonb]

If you are the type who reads the back page of a book first then go straight to this post

http://www.modelenginemaker.com/index.php/topic,8882.msg208172.html#msg208172

For those still following along I first became aware of this engine when one popped up as a suggested video on You-tube and I quite liked the look of it. The Video said that the design was in a 1936 issue of Popular Mechanics which detailed making the patterns and a second issue had the working drawings and machining description. After a bit of googling I found the first part on the web but not the second, however the two parts were also published in Popular Mechanics "Shop Notes" from 1938 which I managed to buy an old copy of.



Now you know how I feel about some castings and that combined with a newly acquired CNC mill made the decision to cut from solid an obvious choice and while I was at it a reduction in size from the original 1 1/4" bore down to my usual 24mm bore and a complete metric design.  As shown in the magazine there were some rather crude details and lack of refinement so I also set about changing just about every part yet still staying true to the original layout and the look of an old engine not a clunky barstock engine. A few evenings work in Alibre and the basic design was ready.



The parts that needed the most work and that would make or break the project were the two crankcase halves so I started with them by exporting my design from Alibre to F360 where I used the CAM to create the codes for cutting the crankcases.

<a href="https://www.youtube.com/watch?v=IDQwAdshtOQ" target="_blank">http://www.youtube.com/watch?v=IDQwAdshtOQ</a>

Rather than make nesting jaws to mill the inside I decided to use the lathe, firstly bringing the case down to thickness with the tailstock giving support as there was not much to hold during the interrupted cuts.



Followed by boring out the space for the crank and an undersize bearing housing. You can just see that I also included a small locating recess for a spigot on the other half, the original did not have this but I felt it would help keep the two halves lined up.



The rear half was milled out with a few changes to the CAM to take into account what I had learnt doing the front half, it is basically a mirror image with the addition of a boss for the vent.



This was then turned in much the same way except a spigot was left rather than the recess.



I also started cleaning up the machining marks with needle & reiffer files and Emery cloth.



The screw holes were reamed to 4mm and some dowels made with M3 tapped holes each end. These also help keep the two halves lined up and are again another feature of my own. It was now possible to hold both halves in the lathe to bore both bearing housings at one setting to ensure perfect alignment.

[mnay]

I had this one on my bucket list, so i will be following along
Mike Nay

[mnay]

I finally had the sense to follow the link and see that it is already built.
Nice upgraded engine.
Mike

[Craig DeShong]

Hi Jason

Interesting engine, but you take all the fun out of the thread by waiting till it's done to post. 

One of joys on this board is "acting the expert" till every thing goes to hell-in-a-hand-basket and you have to eat crow 

Still, and an interesting model.  I'll be following along.

[Jo]

Would Surus consider CNC machined barstock as a casting  I think it would need the Ramon treatment.


I don't have the first article from the 36 Popular Mechanics build notes but I do have the second 

Jo

[Zephyrin]

Hi,

A few evenings work in Alibre and the basic design was ready.

I still hope to become able to do that in a few evenings...some day

very appealling engine and a nice thread and pictures, thanks to share.

I do have the 2 parts in a PDF, both quite interesting, lots of detailed  technics in patterns making and in tooling for the build.



I also add link for another one "ejusdem farinae" from "popular mechanics" oct & nov 1946 issues.


the point with these old articles is that you have to wear a tie to do the machining...

[Jasonb]

Would Surus consider CNC machined barstock as a casting  I think it would need the Ramon treatment.

With your newfound love of working the brown stuff on your sewing stand I though you and the little chap would knock up some patterns

Don't worry Craig, I will include the part that went wrong.

Thanks for the PDF's but you do miss out on looking at all the old adverts which are just as enjoyable as the articles

[Jo]

With your newfound love of working the brown stuff on your sewing stand I though you and the little chap would knock up some patterns

The brown stuff is getting a good work out morning and evening  Surus has been checking if any of he needs to do a bit of heat treatment to normalise his existing castings now that the opportunity has arisen again but he is still waiting for those CHUK Castings.....


The UK Elf and Safety rule has made getting castings done a bit too expensive. The last single 10cc crankcase casting set they wanted over £100 a set for    My supplier clamed he thought I would want one   

Jo

[Jasonb]

I thought he had enough equipment to pour his own, a quick click of the scale button and a pattern could be CNC'd out easy enough it already has the draft angles which you don't get on many barstock replicas.

My Muncaster flywheel man has said he can cast in most metals if supplied with a pattern 

[Jasonb]

The next job on the crankcases was to mill off the last 0.5mm that I had left on the cylinder flange. To get things lined up I bolted a 20-40-80 block along the X-axis and clocked that true and with the ends of the main bearing and cam bearing housings against that the case was true in X&Y. A couple of 15-30-60 blocks supported the two mounting lugs.



The top was then opened up by drilling and then taken to final diameter and depth with the boring head. The four cylinder screw holes were also drilled and tapped M3 at the same setting.



Leaving the crankcases for the moment A piece of cast iron for the cylinder was turned down to the diameter of the fins and then held by this reduced diameter to face the end and bore out to 24mm



Before cutting a spigot on the end to locate in the crankcase.



Like a lot of parts I make one was used to gauge the size of the other, I think this fit is about right.



The cylinder was again reversed in the chuck and with tailstock support the slots between the fins were added with an insert parting tool



The last turning job now that I did not need anything too solid to hold by was to machine the waist between fins and mounting flange



As I wanted the practice on the CNC I used that to give the flange it's square shape with rounded corners and also drill the mounting holes.

<a href="https://www.youtube.com/watch?v=j9r4BFrJWjc" target="_blank">http://www.youtube.com/watch?v=j9r4BFrJWjc</a>



That gave me an excuse for a test fit.



The last machining job on the cylinder was drilling and tapping for the head fixings, the spiral flute taps work just as well on crumbly materials as those that produce long swarf. Just needs a bit of honing to complete.

[Jasonb]

Luckily I was able to keep the same 21/42 tooth count on the timing gears by using MOD0.8 which gave a suitable PCD distance. So first job was to machine the blanks, this is the larger gear that has had the faces recessed.



Unfortunately when cutting the teeth on this one the 4mm central hole was not upto holding it still and the gear crept on thearbor, it was also quite chattery. So another blank was cut but this time left flat faced and an angle plate placed against the blank so it could also be clamped to that with a toolmakers clamp and all went well. I also used 32DP cutters but cut to the MOD0.8 dimensions as there is so little difference between the two and I had the 32DP cutters.



The smaller 21T gear was not a problem having a 10mm hole though I did butt the angle plate up against the arbor to stop any deflection.



The large gear was then recessed and some weight reducing slots added more for looks than performance. Also in the photo are the two silver steel (drill rod) tappets and some bronze guides that I decided to include rather than having them run in the plain crankcase holes as shown on the original design.

.

The crankcases were then held in the mill vice and with a  10mm dia carbide cutter shank in the main bearings to support the small gear and the large gear mounted on the camshaft blank held in a collet the backlash was set and the DRO zeroed so that the holes for the camshaft bearings could be reamed in the exact position required.



All seemed to run smoothly enough .



With the holes done I split the crankcase halves so that the  hole could be located on the mill and a recess opened up to give clearance for the cams.

[Jasonb]

I made a couple of changes to the cylinder head, the most obvious is a separate post for the rockers to pivot on rather than one cast integral to the head. The other difference was to omit the way that the valve guide extends into the valve chambers and to retain the length of guide these were extended further above the head which also meant I could machine a step to help locate the valve spring.

I started by facing some cast iron bar in the lathe and forming a shallow spigot to locate in the bore. Then over to the mill and two holes were drilled and tapped into the valve cavities which allowed me to screw the head to a block that could easily be held in the CNC mill's vice.

<a href="https://www.youtube.com/watch?v=A3ymCvgD5-s" target="_blank">http://www.youtube.com/watch?v=A3ymCvgD5-s</a>

I could have set a finer step down on the contouring cuts which would have needed a bit less finishing but was happy to need a bit more filing as that would give a bit more variance that gives a more "cast" look.





I always prefer to bore my valve cavities, ream the guides and machine the seats with a boring bar all at one setting and find this needs very little lapping and results in a good sealing valve. To do this a simple jig was made up consisting of a bit of bar with 4 offset tapped holes and 4 standoffs, it was then just a case of screwing the head to this to do one valve and then taking it off, rotating 180deg and screwing back on to do the other.





It was then over to the manual mill to drill the exhaust and inlet passages as well as adding the M2.5 tapped holes for the carb and exhaust flanges.



An angle gauge was used to set the head in the vice so that the spark plug face could be skimmed and the 1/4" x 32 threaded hole for the Rimfire plug added.



The last thing to do was remove the contour line sleft by teh CNC with teh aid of needle files and that was the head complete.

[Johnmcc69]

Some very nice looking parts there! I also like the lathe fixture for machining of the valves. Good thinking.
 
 John

[Jasonb]

Thank's John

The crankshaft started out as a length of EN8 steel that was faced to length and ctr drilled for the throw and shaft centers before milling out most of the waste around the pin which is a lot faster and easier than turning it away. I left it for a week to settle after this but it did nor show any signs of having moved.



Then the pin and inner webs were turned, I used a boring bar for the left hand web and an HSS knife tool for the right, the pin was completed with a modified parting tool that has had the middle of the cutting edge ground away with a Dremel which stops any chatter as the tool is worked from side to side as a fine cut is put on.



I then rough turned the ends down using a holder that takes the "other two" corners of a CCMT insert while holding the web in the chuck.



Final finishing was done between centers using a DCMT insert which again helps reduce chatter and will also take off a fine cut.

<a href="https://www.youtube.com/watch?v=LpVtM35GFtM" target="_blank">http://www.youtube.com/watch?v=LpVtM35GFtM</a>



I also turned and rough bored the flywheel at this stage but more of that later.

Last jobs for the lathe were to screwcut a thread on the end and then cut the taper for the flywheel, doing it at the chuck end makes it easy to bore the flywheel at the same topslide setting.



Finally back to the mill to shape the webs to give some counterbalance.

[scc]

    enjoying quietly and hopefully learning..........Terry