Author Topic: "Robby" a Novel Mill Engine  (Read 24625 times)

Offline tvoght

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"Robby" a Novel Mill Engine
« on: September 07, 2015, 04:09:52 PM »
The subject of this build is a kind of novelty engine. I don't know of any similar full-scale engine, and I can't think of any reason why one would (or should) have existed.

"Robby" resembles a conventional horizontal mill engine, but instead of a crosshead and guide, he features a straight-line motion to guide the joint where the connecting rod and piston rod meet.

The use of the straight-line motion was the usual practice in the days of beam engines, and in those days the mechanism most commonly used was Watt's linkage (an invention of which James Watt was rightly proud).

The linkage used here is known as the Robert's mechanism, and is chosen because it is different from the usual Watt's mechanism. Thus Robby's name.

I hope to build an engine that will be somewhat entertaining to watch, and might evoke conversation. In other words, an engine tailor-made to show at model engineering expositions.

The following sketchy drawing is a side view of the engine with the crank in 3 different positions to show the general motion of the Roberts mechanism (in blue). Note how the joint at the bottom of the triangular link guides the end of the piston rod in an approximately straight line.


« Last Edit: September 07, 2015, 04:15:20 PM by tvoght »

Offline crueby

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Re: "Robby" a Novel Mill Engine
« Reply #1 on: September 07, 2015, 04:41:48 PM »
Shold make for an interesting model!

Online Kim

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Re: "Robby" a Novel Mill Engine
« Reply #2 on: September 07, 2015, 04:42:02 PM »
Tim, that does look like it would have a lot of interest motion!  I'll be following along!  :popcorn:
Kim

Offline sshire

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Re: "Robby" a Novel Mill Engine
« Reply #3 on: September 07, 2015, 04:49:36 PM »
Good one,Tim. I'll definitely be watching
Best,
Stan

Offline mklotz

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Re: "Robby" a Novel Mill Engine
« Reply #4 on: September 07, 2015, 05:25:41 PM »
You could also call it a "Cheby" engine.  That's a form of the Chebyshev linkage used to produce straight line motion.

https://en.wikipedia.org/wiki/Chebyshev_linkage

[Although, I have to admit that "Cheby" just doesn't have the ring that "Robby"  does.]

Lots of uniqueness and wow factor.  Bound to be a hit at the shows.
---
Regards, Marv


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Offline b.lindsey

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Re: "Robby" a Novel Mill Engine
« Reply #5 on: September 07, 2015, 07:32:31 PM »
Nice Tim. Looks like it would offer lots of ways to bling it up some too. As Marv said...sure to be well received at shows. Let the chips fly!!

Bill

Offline Jim Nic

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Re: "Robby" a Novel Mill Engine
« Reply #6 on: September 07, 2015, 08:46:55 PM »
An interesting concept Tim.  I'll be watching with keen interest as this is the kind of "out of the ordinary" engine I like.   I also have a little self interest in that I hope that if your idea works well you could perhaps publish drawings.   :P
Jim
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Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #7 on: September 07, 2015, 09:11:04 PM »
Thanks crueby, Kim, Stan, Marv, Bill, and Jim for checking in. I'm gratified to have some initial interest.

Marv, in fact "Cheby" was a possibility as I explored my options.

My references classify the Roberts and Chebyshev as distinct linkages.
Not only are the radial bars crossed in the Chebyshev, the bar lengths are proportioned differently and the straight line motion is taken from the center of the coupler bar instead of from an extension therefrom. In fact, I eliminated the Chebyshev linkage because the only engine configuration I could come up with resulted in more parts to be made. It would have been even more interesting in appearance, though.

Having not been trained in mechanical engineering, I didn't realize what a fascinating subject linkages could be, although I do remember being intrigued when an ME roommate of mine in school had to build cardstock models of linkages for a class he was taking.

Anyone interested in linkages, and specifically straight-line linkages, might be interested in checking out a little old book which inspired me:

How to Draw a Straight Line by A.B. Kempe (1877)


https://archive.org/details/howtodrawstraigh00kemprich

The book is based on an interesting lecture given to science teachers by the author, and covers the various mechanisms known at the time to approximate the straight line. It finally concentrates on the Peaucellier linkage which can produce a true straight line as opposed to an approximation.

Thanks for watching!

--Tim
« Last Edit: September 07, 2015, 09:14:41 PM by tvoght »

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #8 on: September 07, 2015, 09:34:23 PM »
... I hope that if your idea works well you could perhaps publish drawings.  ...
Jim

Jim, I'd be more than happy to share what I've got when the whole thing comes together. The only problem is that when I make drawings, I take shortcuts as I don't consider anyone but myself would be using them. They aren't ordinarily dimensioned, for instance (and you wouldn't want them if they were dimensioned by me :-/).

Nevertheless, when they're complete anyone here will be welcome to them such as they are. I'm not inclined to take the time to pretty them up for public consumption. Oh, by the way, they're in 2D DXF format with all parts on one big sheet. To my credit, I do tend to correct them for last minute changes I make in the shop...

--TIm

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #9 on: September 08, 2015, 12:11:17 PM »
The engine frame is fabricated. The sides from hot-rolled angle stock, cross pieces from cold-rolled steel, and bearing blocks from aluminum.
The constituent parts will be epoxied and screwed together. The screw heads will be inset and covered over with epoxy to attempt the appearance of a one-piece frame casting.

First two lengths of angle stock were placed back-to-back in the CNC mill vice, and the angle legs which will form the frame base were faced off. I used bits of sacrificial aluminum to maintain a separation between the work and the vise.


The base portions of the two frame sides were profiled by CNC. Here is a preview screen shown by the LinuxCNC control software. Before setting the machine loose to do what it would, caution was in abundance, I studied the screen for a while to verify that the cut paths were what I expected.


Roughing of the profile proceeded in several shallow passes, leaving stock for a full-deoth finishing pass.


The roughing passes complete.


A finishing pass at full depth was done with a smaller endmill.


With the finishing pass complete, base mounting holes were drilled. The second photo shows the two frame halves at this point.



The completely profiled base legs were then clamped back-to-back in the vise, and profiling of the frame sides was commenced.
Roughing first:


After a finishing pass, both sides are shown profiled. Note also that holes have been drilled for cross-pieces to join the sides, The photo does not show well that the holes are counter-bored so that the joining cap screw heads can be hidden.


Thanks for watching.

--Tim

Offline Tennessee Whiskey

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Re: "Robby" a Novel Mill Engine
« Reply #10 on: September 08, 2015, 01:45:56 PM »
Tim, I'm in for the ride. Another beautiful frame made from angle iron. You and Chuck Fellows make them look so nice. This CNC stuff is really starting to stir my old gray matter :thinking:.

Cletus

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #11 on: September 08, 2015, 02:21:51 PM »
Glad to have you Cletus, have a seat on the porch right there next to the washing machine.

It's funny, I didn't realize that Chuck Fellows had been the inspiration for this frame until a couple of days ago when I saw him doing it on his new 2 cylinder. Then I remembered the frame of his Bessemer oil field engine, and that was where he had planted the seed... Thanks Chuck!

--Tim



Online Dave Otto

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Re: "Robby" a Novel Mill Engine
« Reply #12 on: September 08, 2015, 03:32:47 PM »
Hi Tim

I have pulled up a chair too and will be enjoying your new project. I like the idea of doing the mirror image parts with one tool path.

Dave

Offline Tennessee Whiskey

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Re: "Robby" a Novel Mill Engine
« Reply #13 on: September 08, 2015, 03:53:37 PM »
Thanks for the warm invite Tim, don't mind if I do.  I bet there is even a matching refrigerator on the other end of the couch  8). Don't worry,  I'm well behaved,  if my shoes are muddy I'll leave on the back porch by the deep freezer  ;). I'm with Dave,  the mirroring is sweet.

Cletus

Offline fumopuc

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Re: "Robby" a Novel Mill Engine
« Reply #14 on: September 08, 2015, 08:10:22 PM »
Hi Tim, no couch and no fridge close by at the moment, but I will following along too.
« Last Edit: September 09, 2015, 04:57:58 AM by fumopuc »
Kind Regards
Achim