Super Newbie - Elbow Engine UHMW, HDPE.

[Novian]

I dont have a good enough camera to show you, but trust me, the walls of the bores i've made on the other parts are smooth as silk, using regular HSS bits.

 

[Novian]

Got the Flywheel holes bored and used a counter sinking bit to de-burr the edges.

Will drill out the centre hole once I pick up a 2" Forstner bit. Hopefully the flywheel will have a tight fit over the 2" UHMW rod that I'm using for the revolving cylinders.

 

[Novian]

For the pistons, I'm not sure if the ends where they attach to the elbows should be slightly above the cylinders, or completely flush. I'd really appreciate your opinions. Also, do i want the pistons to reach the very bottom of the cylinders or just slightly above?

Thanks.

 

[steamer]

I dont have a good enough camera to show you, but trust me, the walls of the bores i've made on the other parts are smooth as silk, using regular HSS bits.

Hi

I'm sure the walls are smooth, but that isn't a guarantee of squarness. Honestly, I'd try the coat hanger ;D

 

[Captain Jerry]

Indexing the holes for the pistons will be tough. You may want to match drill them together, so any error is duplicated in the other part and hopefully cancelled out......but I haven't built this engine....say guys, who has ?

Dave

I have not built this particular engine but I HAVE built a similar engine using less than accurate machines. I t was not until I came to understand that truly square is difficult to achieve and not really important. The real important factor is parallel and it can be achieved easily on any machine. Do Not! rotate the stock to index the cylinder bores. If you maintain the orientation and move the blank stock into position under the drill bit, then all of the cylinder bores and the center shaft bore will be parallel. It is not critical to maintain the cylinder bores on a precise radius and it is not critical to have the cylinder bores angular offset position absolutely equally spaced. Cheap compass, protractor, and eyeball position is good enough as long as the bores are all parallel. You must mark the cylinder positions and the matching faces when you cut the cylinder blank in two. The only time that square is really important is that the valve faces must be square to the shaft to have a good seal and pressure transfer. It is also not absolutely necessary that the pistons be bent to exactly 90 degrees as long as they are all bent to exactly the same angle. Ninety degrees is a good goal but not as important as consistency.

I am not advocating bad practice and sloppy work but with the tool limitation that you face it is important to keep the important factors at the front. Without a rotary table or other precision indexing method it is almost impossible to drill a perfect circular array. But you can be sure that everything is parallel.

I am more concerned with the stability of the elbow bend after it is removed from the bending jig. Most materials will have some spring back when bent and in some plastics, the spring back can be slow to appear, days or even weeks later the angle may still be moving. I have not worked with this particular material but if it turns out to be a problem, you might be able to stabilize the bend by using a metallic core.

I think that this project can be completed as you plan. I am watching with great interest and moral support.

Best wishes for success
Jerry

 

[Captain Jerry]

It took me a few minutes to find this but I wanted to show that I have some familiarity with bent-axis engines. This engine has a 72 degree bent axis but almost any angle, including 90 degrees can be made to work.

Jerry

[ame]http://youtu.be/hOOzFG1canw[/ame]

 

[steamer]

Capt,

Parallel to what ...each other?

Won't they need to be parallel to each other in both planes?
???

Dave

 

[Captain Jerry]

Dave,

Thats exactly what I mean. If the holes are drilled without rotating the stock about its center, then all holes will be parallel in all planes. Even if there is a slight misalignment from square of the spindle/table. on the other hand, assuming that there WILL be some misalignment, if the cylinder stock is rotated around a central axis to locate the cylinder locations, the error will be doubled, not cancelled.

I hope I am am making this clear, but probably not. I will make an illustration if it would. My Crap-O-Cad system is down so I'll have to resort to something else.

Jerry

 

[Novian]

Thanks for all your suggestions, hopefully they will do me some good.

 

[Novian]

Now ain't that a beautiful, super precise flywheel... :-

 

[steamer]

Looking good!

Dave

 

[Novian]

Thanks. I was wondering which would be the best way to mount the Flywheel to the cylinder column. The column is 2" diameter, should i buy a cheap 1 15/16" forstner bit to bore out the flywheel and provide an interference fit? Or should i use a 2" diameter bit?

 

[Tin Falcon]

Early am here. I did not read all the posts so could be missing something. first of all you have a hole for the intake exhaust ports. you need to machine a crescent for each the space between the two needs to be a bit bigger than bore diameter.
Second it looks like the jig you set up allows for different length pistons you only need one length.
Again if I repeated something or am missing something sorry.
Tin

 

[Captain Jerry]

Mr XCV (or can I call you Assassin?)

I would get the material before deciding on the bit and be sure of it actual diameter. Then instead of a Forstner bit bit you might try an adjustable circle cutter. The kind with a pilot bit and an adjustable beam that holds a HSS tool bit. Those things work great on HDPE. I have used them to make instrument panel cutouts in marine applications with great results. Fitting the flywheel to the cylinder needs some thinking. A 1/16" interference fit might put enough pressure on the cylinder to compress the cylinder bores. On the other hand, I don't know of any adhesive that will work on HDPE. It might need some kind of mechanical fastener, pins or screws.

Jerry

 

[ddmckee54]

Going back to the cylinder bores for a minute, couldn't you drill them oversize and sleeve them with thinwall brass tube from the hobby shop? It's available in a boat load of sizes, should satisfy the smooth as silk requirement, and should press into the UHMW fairly easily.

Just a thought,
Don

Long time lurker

 

[Novian]

Tin Falcon:

That jig is just for the elbow parts of the pistons, i just bend the 1/4" rod, and then drill holes into the 3/8" rod, and push the 1/4" rod elbows into the 3/8" pistons.

Captain Jerry:

There is no glue capable of gluing UHMW together, that's why i chose this stuff, since it has machining properties of Metal. It can be welded though. If i can't weld it (testing with scrap pieces) then i'm thinking i could put some slices of tubing on either side of the flywheel and using some set screws to fasten it.

I do have one of those hole cutter kits, where you take out all the blades except the size you need. I also have a 2" on its own cutter, but it isn't accurate enough compared to the clean edge that a forstner bit will cut.

Gotta get back to it,

Ian

 

[Novian]

Completed jig for elbows:

 

[Novian]

Elbows to attach to pistons

 

[Captain Jerry]

Captain Jerry:

I do have one of those hole cutter kits, where you take out all the blades except the size you need. I also have a 2" on its own cutter, but it isn't accurate enough compared to the clean edge that a forstner bit will cut.

Ian

Thats not the kind of hole cutter I was thinking of. Those are just a hole saw and the cut is pretty ragged. I was thinking of a beam type fly cutter like the one in the attachment. It uses a single HSS cutter and can produce a fine finish if properly sharpened an used in a drill press at the correct speed. Its primary advantage is that the radius is infinitely variable, like a boring head. It can produce a hole that exactly matches the diameter of the cylinder, even if the cylinder stock is slightly over or under.

You are right about a forstner bits ability to produce a fine finish but the hole is one size, take it or leave it.

Jerry

 

[Novian]

Completed the elbows and pistons, they work with press fit since they're really tight, so no glue or plastic welding required.
THe good thing with UHMW is that it has a very low melting temperature, i can pull out a cigarette lighter and make angle adjustments to the elbows as i need to.

How tight the 1/4" rod fit into the 1/4" counterbores in the 3/8" rod means that i will need the holes in the cylinder blocks to be slightly larger than 3/8".