Benson engine

[Jasonb]

Last couple of bases that I have done I just freehand ground the corners of a blunt milling cutter with a small concave 1/4 circle on the corner of each flute works a treat.

If you get the size right you can run your CNC at the same path, eg if the moulding is 2mm wide then profile with a 6mm cutter and modify a 10mm cutter, in both cases the path of the ctr of the tool will be 3mm from finished edge. This makes things easy when doing it manually like I have to.

[kvom]

My plan is/was to use a 1/8 corner rounding bit, but positioned both lower and further in than if I were just rounding the top.  This gives a molding effect.  I did some experiments with it on some scrap to get the parameters.  However, I needed to remove 93 thou of thickness from the .375 stock, so I'll have to see if that leaves enough material.

I am aiming for a convex form for the outline.  A concave form would be easy using a ball mill.

[Jasonb]

Yes the "quirk" is easily added by comming lower and further in with the cutter or if following the same path with a home ground one you use a larger cutter.

Does the extra thickness have to come off the top or bottom? would not hurt to have the base thicker.

These two just have a top quirk

[kvom]

If I had wanted to machine the base as drawn by Mount then I would have needed 1/2" stock to start, and even then it would be a fraction short.  Since I am making it from two pieces the lower part thickness isn't critical.  In terms of the other part dimensions, only the difference between the height of the cylinder boss and the column boss matters.

I started with .375" stock and machined off .093" to create the cylinder boss, leaving .282.  The trial I did on the scrap has the bit descending .225" from touching the top of the stock, so I'm good there.  Laterally I'm moving in .13" from touching off the side of the tool.  Assuming the barrel has a 1/2" diameter, then in CNC terms it would be specified as a tool with a .24" diameter for an outside profile.

[kvom]

Before running the corner round bit on the base, I tried out my calculations on a bit of aluminum.



The calculation based on the tool's diameter was a bit off, but I just used a series of negative roughing (overcut) passes until it looked OK.  So with a final decision to use a .08" overcut and using .02" of radial cutting on each pass, I ended up with this:



My F&S software doesn't handle corner rounding bits;  I used 1100 RPM and 10 IPM.

[zeeprogrammer]

Looks to me like it came out swell. 

[kvom]

Last part of the base today.  After drilling all the holes in some 1/4" brass, the boss for the column in cut out.  Note the use of 3 small tabs to hold the part inside its parent stock.



Then over to the Bridgeport to mill to thickness using a face mill.  This op removes the tabs.



In position on the main base.  I need to order some 3-48 screws to fasten them together.

[Tennessee Whiskey]

Well that's coming along nicely Kirk.


Eric

[crueby]

Great stuff. I like how you handled the tabs from the original piece.

[kvom]

No photo, but made the second eccentric, this time using some extra stressproof rod I'd bought during the Joy engine build.  I believe the first one was 12L14 from the way it machined, but that steel is liable to rust.  I may just remake that one from the stressproof as the whole part takes less than an hour now that I have the programming.

Next time I'll start on trial two of the conrod.

[kvom]

Third try at making the conrod in brass.  All is well when the shape is freed from the stock.  I left the "crossbar" in between the arms to prevent distorting them when clamped in the mill vise.



All was well on the extra manual milling until it came time to remove the crossbar.  As soon as it was severed, the endmill grabbed the nearer arm and bent it slightly.  I was able to manually bend it back, but whether it's precise enough remains to be seen.



In any case, I turned the bearings and did a test fit.



At this point I'm thinking I should just make a 4th one from steel.  I have the process down pretty good by now.   

[kvom]

Fairly short day in the shop as I was watching the golf on TV.  Rather than another try at the conrod I decided to attempt the crank.  Its profile fits withing a 1" circle, so I chose to CNC it using some 1" round rod.



Cut off on the lathe with a parting tool, then taken to proper thickness on the bridgeport, then finally the smaller boss on the CNC mill.

[kvom]

In today's episode I attempt to build the bearing blocks and main bearings in a manner such that the crankshaft will turn smoothly when all assembled eventually.  After preparing two pieces of 1/4" brass  that I cut from a larger piece of scrap, they measured.75x1.3".  Both got a reamed 3/8" hole 1/2" from the bottom edge, which is the edge against the fixed jaw.



Then with both pieces aligned with some drill rod and parallels, the mounting holes (for 3-48 tap) were drilled.  As long as the matching holes in the entablature are drilled correctly the bearings should align closely.  I used a center-finder on the drill rod to center them.



I then used the CNC mill to profile the sides, and then various manual milling ops brought them to nearly finished status (still have to do the tapping).



I then turned the main bearings on the lathe from some bearing bronze, using the bearing blocks to ensure a fit.  Family shot afterwards.



The caps will need to be done sometime soon.

[kvom]

The bearing caps are quite simple in the CNC world.  First op is to machine the profile in some 1/4" brass .  In this case, .30" DOC for a final target of .25" width.



Over to the Bridgeport to mill to width, drill 3-48 clearance holes, and spot face to clear the screw heads.



After tapping the mounting holes in the bearing blocks, the assembly using 1/4" long screws from American Model Engineering.



The 3-48 screws seemed closest to the metric screws Mount specifies, but in this case I think in retrospect that 2-56 might have been a better proportion.

[zeeprogrammer]

Looking good Kirk. I've been following.