Author Topic: Offenhauser Inline 4 cylinder, Might Midget Model Engine Build, 1:4 Scale  (Read 1806 times)

Offline propforward

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Amazing work - heck of a project.

 :praise2:
Stuart

Forging ahead regardless.

Offline Admiral_dk

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Very impressive  :praise2:

Still following your amazin build   :cheers:   :popcorn:

Per

Offline eccentric

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Offy – Cylinder Head – Part 2
Cylinder Head – Part 2

In this second installment describing the machining of the head, I show the steps to machine the two non-vertical/horizontal surfaces–the face the intake and exhaust manifolds mount to, and the face the cambox mounts to.

First I use the CNC router to machine the two bevel surfaces on each side.  Why not use the router to complete the machining on these beveled sides?  I couldn’t given the limitations of my little CNC router.  The work piece is 4.125″ long and the vise on the CNC router can only open to 4″.  My mill vise to larger, but I can’t use it on the CNC router because it is too tall and I don’t have enough Zed clearance. I reasoned that this part needs to be clamped from the ends since these are the only two vertical surfaces the vise can bear against.  I looked at a couple of fixtures, but the rotational forces of clamping the part would result in an unreliable work holding situation.

I can probably manual mill the features faster anyway.  I decide to start with the simpler face first, the manifold mounting surfaces.  The four port holes were machined in an earlier step, so already exist.  There are only the 7 threaded holes for mounting the exhaust/intake manifolds and the 4 holes for the pins securing the valve cages.  These are shown below.



I have been creating the drawings for the parts as I build and machine them; this way I can find issues with the print.  I find a couple of missing dimensions and hole callouts.


There is a fixture required to align the beveled surface I am working on.  I decide to 3D print it.  the fixture carries light loads during the milling operation and no clamping force.  Designing and printing the fixture part is much quicker than using a piece of aluminum for a one off fixture.


I do not rely on the plastic fixture alone.  An indicator is used to verify and tweak the clamping to insure the surface is properly aligned with the mill table. You can see that a sheet of notebook paper was used as a shim to bring the part in perfect alignment.  I use a square to confirm perpendicularity and an aluminum rod on the moveable vise jaw to insure only the primary face of the vise jaw is aligning the part.

As mentioned before the four large port holes were drilled in an earlier operation.


Once the part is secure in the vise and aligned to the mill, spot drilling, drilling and tapping the holes is routine.


Machining the cambox face is more complex because in addition to drilling and tapping holes, I need to drill and ream the large holes for the valve cages, then machine the oil collection channel.


All of the machining on the head prior to today’s operations used the center of the part as the origin.  This way any variation in the outside dimensions of the part will be spread evenly on all sides.  If you look at the print in the picture above, you can see all the dimensions are referenced to edges.  This could result in slight miss alignment as tolerances would be biased to one side.  I realized this before I machined the Cambox surface and created the print using the center of the surface as the datum.  I doesn’t really matter what is used as a datum as long as the machining operation on all of the faces use the same ones.

Below I spot drill the four holes for the valve cages.


Then drill them undersize.


Then ream them to final size.


Then the threaded holes that secure the Cambox are spot drilled.


Drilled….


I use a 3/8″ roughing mill to machine the 3/8″ oil trough.  I would prefer to have used a finishing end mill..but I don’t have one.





There is one final operation cutting the oil trough from end to end with a 3/16″ flat end mill.


Finally I tap the threaded holes.


Then flip the part over and repeat for the other side.


Offline Roger B

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Splendid  :praise2:  :praise2:  :wine1:
Best regards

Roger

Offline Kim

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Excellent explanation of your setups and order of operation.  I'm enjoying following along with your build.  :popcorn: :popcorn:
Kim

Offline eccentric

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Offy – Timing Gear Tower
The timing gear tower houses the majority of the timing gears and is mounted on the front of the engine as shown below.


The timing tower assembly consists of a front and rear half, each holding 6 ball bearings for the gear shafts. The quality of the gear mesh is determined by the alignment and precision of the gear tower inside machining which includes not only the bearing positions, but the alignment of the screws holding the assembly together.  For this reason, all of the inside machining of both halves was done first and done in a single set up.   There is a fair amount of machining required on the outside face of the timing gear tower front half.  This was done as a secondary operation with the work piece mounted on a fixture using the screws for alignment to the rear features.


Inside Detail of the timing gear tower assembly

I start with the inside of the rear timing tower first as the back of the part is flat with minimal machining.  Then I perform basically the same machining on the inside of the front timing tower.


Once the machining is complete on the inside of the rear gear tower, I secure it face up on a fixture block that has been prepared by machining flat and the screw holes drilled and tapped.


Since I needed to machine the complete front face of the gear tower, I had to machine in two separate operations because the securing screws were in the way.  I secured the part to the fixture with four screws as shown in the photo, machined half, then moved the mounting screws to the area just machined and completed the secondary operation.


I had an error in my tool path and I crashed the end mill into the part, this resulted in a blemish on the face of the finished piece.

Machining of the face was accomplished with a 1/4″ end mill, a 3/16″ ball end mill and a 1/8″ ball end mill.


Likewise when I machined the countersinks for the mounting screws, I had to move the screws around so the work piece remained firmly mounted.


Below is an inside view of the two gear tower halves.




I will finish the part with bead blasting and then hand sanding to give it a used cast aluminum look, like I did on the front cover.  When asked about the blemish, I explain that I machined a Carnation flower into the face as a sort of makers mark.


Offline eccentric

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Offy – Block Side Plates/Crankcase Sides
Machining the sides of the crankcase was straight forward, but a little nerve wracking as the number of hours invested has grown and the anxiety of messing up has increased proportionately.  Below the crankcase is being rough machined.  The crankcase is assembled with the dummy crankshaft, bearings and most importantly the crankcase gaskets.

Notice how the crankcase is mounted in the vise.  The datums being used are the top of the crankcase flat against the primary vise jaw, the forward face of the crankcase covered in layout fluid, and the opposite side of the crankcase mounted down flat against the vise.  An aluminum round is used to press the crankcase top against the primary vise jaw to insure this datum is in alignment with the mill.



Below the  ball end mill has completed the finish milling of the crankcase side and an 1/16″ end mill is being used to “drill” the holes for the crankcase breather plate mounting holes.



There are two cylinder block covers that mount to both sides of the block.  The one on the left side is simply a flat finned plate, but the one on the right has a small water jacket pocket and the fitting for the water flange.  After machining the features on the surface, I spot drill the mounting holes.  I have not had good luck actually drilling holes on my small CNC router, there is not enough Z height to get a drill chuck mounted.  I kludged one up using a standard drill chuck in a collet, but the run out was atrocious.   So, for small holes I spot drill, then final drill on the mill, or for larger holes I will simply mill them out with an end mill.



The block side plates are held in place with a large number of 0-80 socket head cap screws, I drill the .070″ holes with the side plate mounted in its final position on the block in the mill vise.  That is, I match drill the holes in the side plates and the block at the same time.  I had to be sure to install the .020″ head gasket and the .010″ block to crankcase gasket to insure the spacing was correct.  I tap the holes in the block and I drill out the holes in the side plates to a .089″ clearance size.




Above is a picture of the engine as it stands now.

Offline Admiral_dk

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Looks really good - you should be happy so far  :ThumbsUp:

Per

Offline Zephyrin

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Hi,
impressive model so far, clever and instructive machining setups, great thread !
the recessed bolts on the lower crankcase half would require some patience to install !