Making a Wobbler model steam engine

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Making an Oscillating Cylinder (Wobbler) engine Phil Oles September 2015 From plans available at: And modified where noted

LMS wobbler closeup.jpg

Layout and defining a reference datum This procedure is usually used for milling and drilling operations. A dye is applied to the work. A height gauge is used to scribe lines where the holes go. Some machinists position a starting (or spot) drill directly where the scribed lines cross but we’ll be using a more accurate technique.

We’ll be using layout marks to stay honest. Locate the actual datum edges using an edge finder which is accurate to within 0.001” when care is taken. Several styles of finders exist. An electronic one lights up when the tip makes electrical contact with the edge. When you’ve positioned the spindle over the first and subsequent holes, you can check to ensure the center or spot drill is hitting the “mark” on the layout lines.

Use a spotting drill to start the hole. They are short, stiff and won’t wander.

Spot drill.jpg

For lathe work, layout marks are not often used but as usual, there’s no rule against it. Anytime you want a guide to avoid making a mistake, it’s best to use them.

Order of making parts: As a rule, make parts with holes first, then turn shafts that go through them last.

Upright – make this part first if possible. Ream the crankshaft hole 0.250” or 0.251” so that the crankshaft stock is a close fit. You’ll know which reamer to use if you use a micrometer to check your crankshaft diameter first. Allow 0,.0005” – 0.001” for a good sliding fit.

Upright a crop.jpg

A reamer is used by first drilling a pilot hole in the work. Reamers shouldn’t cut more than a few thousandths if possible so choose the pilot drill accordingly (check wire drill sizes). Drill the pilot hole, then ream the hole using a slow speed (100 rpm – 200 rpm) and cutting oil.

Three holes are tapped in this piece, two for mounting to a base, one for the air inlet. Tapping is best done with the spindle directly over the pilot hole to ensure the tap goes in straight.

I made a simple tap wrench guide to keep the tap straight.

Tap guide .jpg

The guide is held in the drill chuck and the top of the tap wrench fits in the guide.

The spindle is lowered to the tap wrench and using the quill apply very light pressure downward to keep the tap straight while rotating the tap wrench. Advance ½ a turn, back off slightly to break the chip, then advance another ½ a turn. Use cutting oil to get clean threads.

The hole for the air inlet is a 0.125” through hole, and a 0.159” (#21 drill) blind hole for tapping 10 -32 threads. Drill through first with the 1/8” drill, then down with the #21 drill about 0.25”, thread 10-32 with a plug tap first, then a bottom tap.

Decide which, if any, of the 2 remaining holes in the face should be reamed.

Crankshaft - threaded ¼” – 28 (turn to 0.240” diameter to cut threads), This part must be made AFTER the web and upright bushing. It needs to be filed and polished so it goes through the upright and web before this assembly (crank web/crankshaft) is completed.

Take the ¼” stock, clamp it in the 3 jaw chuck and face both ends so that the OAL meets spec. Turn a length of about ¾” to a diameter of 0.240”. Place a ¼” – 20 NC round die in the tailstock dieholder and screw down the set screws snugly. Move the tailstock right up to the workpiece.

Make sure the lathe power is off at this point. Turn the tailstock handwheel to push the die into the workpiece. Using the lathe chuck key rotate the chuck towards you while advancing the tailstock slightly. Remember, as you cut threads the die wants to advance into the work. To prevent stripping the threads you must advance the tailstock to allow the die to keep cutting new threads. Cut at least 3 full threads in this fashion. You can finish cutting threads by clamping the work in a vise, then using a diestock finish cutting the full length. Note the piece of aluminum flashing used to protect the shaft from the vise jaws. Crank web – Make according to plans from steel or aluminum.

Crankshaft Assembly - Loctite the web onto the crankshaft, set overnight.

Clamp the crankshaft assembly in a ¼” 5C collet using the square block holder. Use channel pliers to tighten the locking ring. Make a flat on the center of the web hub, center drill then drill a 3/32” hole for a brass pin using a #41 drill (0.096”).

Insert a piece of 3/32” brass rod with some Loctite and set overnight. Clip off the ends, then turn the assembly on the lathe to remove excess brass pins. Caution, removing the excess brass is an interrupted cut and must be done slowly and carefully.

Now, drill/tap the 6-32 hole for the crankpin while holding the work in the 5C collet block or a suitable fixture.

Assembly 1.jpg

Cylinder – Make this part 1st or 2nd. Order of machining: Mill 1” x 1” square aluminum to exact length on plans Polish cylinder face with #200, then #500 grit paper Drill the bore to 31/64” Finish the bore with a ½” carbide end mill or reamer Drill the port hole Drill/tap 10-32 the pivot hole using plug and bottom taps. Caution: don’t drill through to the bore or the part will be scrap.


Piston Face both ends of the stock so the OAL meets spec

Drill a hole 0.144” diameter (#27 drill) from the end while holding work in a 5C collet held in a 5C collet block

Turn the minor shaft on the lathe to spec. This will create a burr in the hole that you can remove by pushing the #27 drill through the hole and twisting by hand.

Crank pin - a shoulder bolt A shoulder bolt is used to anchor a pivoting or rotating part in place.

Shoulders bolts.jpg

Start with ¼” hex stock (steel or brass) and turn a length of 0.600” to a diameter of 0.143” and without removing work from the chuck, make sure it fits through the piston hole, (check while on lathe), then turn a ¼” length to a diameter of 0.134” for the 6-32 threads. Using the tailstock dieholder start the threads. Cut off workpiece, then faced the head to about 0.10”. Put a slight chamfer on the head of the bolt using an E-5 lathe tool.

Pivot pin - a shoulder bolt The pivot pin is a shoulder bolt. Start with 7/16” brass hex bar stock faced on both ends for an OAL of 1.45”. Put a slight chamfer on the head of the bolt using an E-5 lathe tool.

Turn a length of about 1” to 0.187” and test it for fit in the upright without removing your work from the lathe chuck. Then turn a 0.250” length to 0.182” diameter ± 0.001” for 10 – 32 threads. Threads are cut using the tailstock die holder.

Flywheel – the flywheel is made from the stock provided. Start by facing both sides true. You’ll do this by holding the work in the 5” three jaw chuck. The max capacity for the inside jaws is 2”, so if stock is larger in diameter switch to the outside jaws. The flywheel width is not critical but should not exceed 0.6”.

With the flywheel faces trued center drill the flywheel and drill through with a #7 pilot drill used for ¼” 20 NC threads. Place a ¼” center cutting end mill in the tailstock drill chuck. Drill a register about ¼” deep.

Remove work from lathe, clamp in mill vise and center the mill/drill spindle over the through hole using the #7 drill. Tap the ¼” – 20 threads.

Flywheel and crankshaft

Base The base may be made of any material you prefer, metal, wood, Corian. I’ve provided a disk of cast iron that’s 2.5” diameter. Only the topside needs to be faced on the lathe using the outside jaws of the 5” three jaw chuck. When faced, place that side down on the parallels in the mill vise, then drill the two #10 clearance holes for the upright. Countersink the holes using a 3/8” end mill that is drilled down 0.280”.

Using the spring, push-to-connect air fitting and socket head cap screws provided, assemble the engine. You should get free movement of the flywheel through its full rotation. If you do, put a small drop of oil on all bearing surfaces, add some compressed air and your engine should run. If not, check with the instructor for troubleshooting.

The base should be heavy because when running this single action engine likes to walk. How’s that for a paradox?

V 1.0 September 2015