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LITTLELOCOS MODEL ENGINEERING
Handy Lathe Charts
for the Home Metal-Shop Hobbyist
& Model Engineering Projects
The back plate starts out as a simple turning. The trick here is to remember to include a clamping fixture machined into the end of the plate. The fixture has a 1/2 hole bored for the Morse Taper Plug machined earlier. A 3/8 wide by 3/16 deep slot will be used for clamping.
Once the back plate is parted off, it is reversed in the lathe and clamped to the face plate centering with the Morse Taper Plug.
Once the back side of the plate is machined in the lathe, the workpiece is transferred to the mill and centered on the rotary table for drilling the mounting holes.
After parting off the crankcase, the fixture end is re-used for machining the ring gear. A 0.10 wide shoulder is machined so the gear is a light press fit and a 1/2-20 thread is tapped in the center. The gear is held to the fixture with a 1/2 bolt and washer for turning the outside diameter to 1.625.
The ring gear fixture is transferred to the rotary table that has been centered in the mill with the 1/2 Morse taper. The dead center is set over to the pitch diameter and used to set the zero-degree mark. For drilling the clearance holes for the 1-72 retaining screws, the gear is held by the press fit.
A little different method is used to machine Pips crankshaft. After accurately center-drilling both ends of the stock, the basic shape is roughed out, leaving things well over size. The rest of the part is gradually brought to final shape and size taking successively lighter and more accurate cuts. The crank pin is left unbraced throughout the machining process.
After bring the 3/8 and 12mm bearing journals to size and machining the timing gear blank, the part is transferred to the mill.
A special cutter has been machined from drill rod, hardened and tempered. This form cutter is used to cut the 20-tooth, 48-pitch timing gear. A commercial cutter cannot be used as it is about 4x the diameter needed to clear the crank web. It is ABSOLUTELY CRITICAL to set the height of the cutter to the centerline of the crankshaft mounted in the lathe. This is done with a Vernier height gage.
Here the gear is being cut with the form tool. Note the markings on the crank web. 7/8 od is the maximum size allowable for the cutter.
Once the gear is cut, the crankshaft is recentered on the crank pin. Rotating over 60 degrees, the first flat is cut with an end mill. Rotating back 120 degrees, the second flat is cut. Then the end-mill is left running while the part is rotated around the crank pin, cutting the rounded end of the crank web.
Now we are back to the lathe to bring the 5/16 diameter propeller shaft to size and finish things up. Although I die-cut the threads on the prototype, I recommend chasing them in the lathe at this point to improve accuracy.
DISASTER STRIKESThis is my first go at cutting the gear teeth. A temporary aluminum collet was used to hold the machined propeller shaft this facilitates the alignment of the crankshaft with the rotary table. A 7/8 od x 1/64 carbide slitting saw was used to rough the shape of the teeth, followed by the form tool. Two mistakes have been made at this point. First, the height of the roughing tool and the form tool was set with the milling machine down-feed dial. Second, the depth of the roughing cut was set by eye with a magnifying glass. The white line at 11 o-clock is a scratch about 0.002 deep caused by the roughing cutter. Bad news for a bearing journal. The form of the teeth was proper for one side only since the roughing cutter ended up 0.002 high on one of the two passes. Based on this experience, the roughing cutter was dropped and the height is set correctly with a vernier height gage. All is not lost though, I am now about 90-percent complete on the crankshaft for the one-cylinder version based on the same cylinder and top end.
The cam blank is machined in the lathe in a single chucking. It is then transferred to the rotary table and centered with the Morse taper plug with a split sleeve. The exhaust cam and holes for the planet gear shafts are machined in the first mounting, then the cam is flipped over to machine the intake cam.
A major milestone at last. The Crankshaft and cam are completed. Note that the final rounding of the cam lobes was done with a file. Later the cam will be hardened prior to having a sleeve bearing pressed into it. The planetary gear set gives a +4:1 ratio.
All eight rockers ready to be cut off like cookies. A small tool-setting guage provides a size reference. The total machined length here is just over two inches.
The rocker blank is machined in the mill to get the rough shape. It is then center drilled (oversize) drilled through, then reamed 3/16. A blank insert is center-drilled and machined in the lathe for a light press fit into the off-end of the rocker blank. It provides the center for the side nearest the rotary table. Eight rockers are macnined to shape, drilled and tapped in this set-up.
The machined rocker blank is mounted vertically in the mill vise so that each rocker can be cut of with the slitting saw.
Once cut off, the sides of each rocker are machined. This set-up consists of an angle vise set up so that when the rockers are clamped in the v-block, the back of the rocker is level. Rockers are clamped in pairs using a short length of 3/16 drill rod and a scrap of brass. To keep the large clamp out of the way, a smaller clamp is bridged from the top of a pair of step-blocks and the brass scrap. Each rocker is machined in turn to the same settings, then the clamps are reversed to machine the other side.
Here is another view of the set-up for narrowing each rocker, this time shown from the end with the clamp and packing piece.
To the left, we have 8 rockers, narrowed and ready to receive the tips (shown on the right). These are silver-brazed together followed by final filing to shape.
The Diffuser mounts to the backplate of the engine and is responsible for distributing the fuel evenly to all three cylinders.
At the is point, the diffuser blank has been turned in the lathe. It is mounted vertically in a fixture in the mill so that the holes for the diffuser counterweights can be drilled and tapped opposite of the Diffuser Drive Pin.
The rotary table is set up on the mill table with an adjustable angle plate to cut the diffuser blades with a 1/8 end mill. The blades are cut in successive stages until they are brought to their final shape.
The first stage is completed, cutting down the center of each space between the diffuser blades.
At this stage, the fins are complete. The drawing in the background is a LARGE-scale print used to work out the cuts for for the prototype. The final dimensions are shown in the drawings.
The diffuser has been flipped over and indexed on the fixture with a 3/16 pin. To index with the rotary table, the table is centered under the mill spindle, advanced to zero degrees, then offset 5/16 to drill and ream the fixture for the locating pin.
A 1/4 end-mill is reground with two concave radii at the tips to cut the rounded diffuser inlets. By cutting through the back of the diffuser, the bases of the diffuser blades are revealed. These are smoothed with a file to bring the inlet to its final shape.
This is a close up of the completed diffuser, showing the rounded inlets and the smoothed inlets at the base of the blades.
The completed diffuser is shown with its parts.
From left to right:
· Steel button to be pressed into the backplate forming a solid foundation for the diffuser assembly.
· 1/8-diameter sealed ball bearing to be pressed into the diffuser.
· Steel spacer to maintain the distance between the two bearings.
· The second sealed ball bearing.
· The diffuser, shown with the blades up. These will face the backplate of the engine.
· The diffuser drive pin will be pressed into the diffuser.
· The diffuser stud. This is pushed through the diffuser assembly then through the backplate holding everything together.
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