Author Topic: "Robby" a Novel Mill Engine  (Read 24629 times)

Offline b.lindsey

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Re: "Robby" a Novel Mill Engine
« Reply #30 on: September 10, 2015, 02:04:36 PM »
Tim, just a quick question. After splitting the bearing caps it looks as if the hole is now larger. Are you planning to use a brass or bronze type bushing there for a bearing surface or just the aluminum?

You are making great progress btw and its looking good!!

Bill

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #31 on: September 10, 2015, 02:25:12 PM »
Hi Bill,  I reamed the holes to shaft size after splitting the bearings. And yes, the plan is steel shaft in aluminum plain bearings.

--TIm

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #32 on: September 11, 2015, 10:38:58 PM »
The Roberts mechanism has two identical arms which I'll be calling the "radial arms". I decided to make them of aluminum.
After sawing some stock a little oversize, I drilled and reamed 1/8" pivot holes in the stock. This was done on the Bridgeport.
I made three blanks as there was little to lose by making an extra.

I drilled and tapped two holes in a tooling plate on the CNC mill, the holes being spaced to match the holes in the arm blanks. The blanks will be clamped to the plate with shoulder screws which are a slip fit in the reamed holes of the blank.
Here's one of the blanks next to the tapped holes in the sacrificial plate. You'll notice I use the sacrificial plates over and over until there just isn't a good surface left to clamp to. I just make sure to de-burr the edges of any previous cuts so that the parts will clamp down square.


A roughing pass was taken at small depth steps to get within 20 thousandths of the final outline.


Then a finishing pass at full depth, followed by a pass on the top of the clamped part to provide clearance for the coupler link. The part was then flipped and re-clamped to run the same relief program for a matching clearance cut on the other side.


Since I had 3 blanks, I used the same tapped fixture holes and the same CNC programs to produce two more.

Here are the 3 radial arms, two of which have been cleaned up a little. The extra, if it escapes the clutches of the shop gremlin, can be placed on my desk at work.


Thanks for watching.

--Tim

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #33 on: September 11, 2015, 10:40:43 PM »
The triangular "coupler link" is perhaps the most visually distinguishing part of the engine. The link consists of two identical parts, one on either side of the radial arms. The two link halves also bracket the joint at the connecting rod and piston rod.

The twin coupler link halves are made of 1/16" thick cold-rolled steel. I sawed two triangular pieces oversize and clamped them stacked on the Bridgeport, where I drilled and reamed three pivot holes through both parts for a perfectly matching pattern.
In a manner similar to that used on the aluminum radial arms, I drilled and tapped holes in a sacrificial tooling plate and screwed down the two stacked pieces with shoulder screws through the reamed holes. All machining could now be done with reference to the large hole.

A CNC profiling operation was run


And then a finishing operation. Here are the stacked parts just after outside and inside profiles had been finish milled.


The parts after quick cleanup.


Thanks again for watching!

--Tim

Offline Art K

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Re: "Robby" a Novel Mill Engine
« Reply #34 on: September 12, 2015, 01:01:28 AM »
Tim,
This is a fascinating idea for an engine. I have to admit to going to Wikipedia to find the difference between Watts and Roberts linkage. Watts was straight forward, Roberts was very formulaic. If I hadn't seen your diagrams of the linkage they would have been speaking Greek. I will continue to follow along.
Art
"The beautiful thing about learning is that no one can take it away from you" B.B. King

Offline Dave Otto

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Re: "Robby" a Novel Mill Engine
« Reply #35 on: September 12, 2015, 01:20:40 AM »
Coming along nicely Tim!

Dave

Offline fumopuc

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Re: "Robby" a Novel Mill Engine
« Reply #36 on: September 12, 2015, 05:48:16 AM »
Hi Tim, I am also watching. An Interesting concept.
Kind Regards
Achim

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #37 on: September 12, 2015, 06:04:25 PM »
Thank Dave and Achim.  Art, it's good to see you watching.

To pin the revolute joints of the Roberts linkage, I made brass pins from 1/8" brass rod.
I have no process photos, but each pin was finished to length in a lathe collet and drilled and internally threaded on each end.
I made the lengths to be very close to the width of the link joint, this required removal from the collet to measure, then replacement and facing. A little error to the short side was acceptable for my purposes.

Here you see 4 pins with example screws and washers that will be used to hold things together. All joints consist of two outside flanges bracketing an inner. The washers effectively fix the pin to the outer flanges, so the inner rotates on the pin.


The following photos should give more of the general idea.





Thanks!

--Tim

Offline smfr

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Re: "Robby" a Novel Mill Engine
« Reply #38 on: September 12, 2015, 07:27:27 PM »
This looks fascinating, Tim! Following along with interest!

Simon

Offline b.lindsey

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Re: "Robby" a Novel Mill Engine
« Reply #39 on: September 12, 2015, 08:01:27 PM »
Nice progress Tim. If you can, in the next batch of pictures please put something in the picture for size reference. I am trying to figure out what size this little jewel is going to be.

Bill

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #40 on: September 12, 2015, 08:47:38 PM »
Simon, thanks for following!

Bill, this may help a little. I'll remember to throw in some references when I get back to the frame. That'll be more descriptive.



--Tim

Offline sshire

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Re: "Robby" a Novel Mill Engine
« Reply #41 on: September 12, 2015, 10:41:32 PM »
Tim
This is getting really fascinating.

Yes, I love form taps, especially for the smaller sizes. The people at Balax were very helpful, even though I'm quite sure my impact on their bottom line is microscopic.

Great work. Looking forward to seeing it at NAMES,
Best,
Stan

Offline b.lindsey

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Re: "Robby" a Novel Mill Engine
« Reply #42 on: September 12, 2015, 11:12:02 PM »
Thanks Tim....yes that helps a lot.

Bill

Offline Don1966

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Re: "Robby" a Novel Mill Engine
« Reply #43 on: September 12, 2015, 11:16:09 PM »
Nice start Tim and thanks for showing the size reference I would of thought it to be bigger looking at it without it. Still following along.

Don

Offline tvoght

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Re: "Robby" a Novel Mill Engine
« Reply #44 on: September 14, 2015, 05:01:27 PM »
Stan, Bill, and Don, thanks for your comments.

I made the connecting rod from a piece of 5/16" thick brass, 3/4" wide.
In the vise of the CNC, a quick program was used to profile the big end of the rod, which will have a removable cap.


Mounted vertically in the Bridgeport vise, I spot-faced the locations for the cap screws and center drilled.


I drilled the depth of the cap plus .040" or so to clear a 4-40 screw, then drilled to a bit more than screw length depth, and tapped with a 4-40 thread-forming tap.



I'm trying this photo to see if anyone can make sense of it. To align my slitting saw vertically, I laid a steel rule on top of the saw blade and bridged it over to the top of the work. Lit from behind, I could see when there was no light leaking under the rule to determine when the top of the blade was flush with the top of the work. I zeroed my DRO Z axis there.


The spindle was lowered and the cap sawn off.


Horizontal in the Bridgeport vise, I first found the edge with the cap removed, and then screwed the cap tightly on. I Drilled and reamed. The closed hole on the other end was drilled and reamed as well.


This is familiar if you've watched my previous operations. Ready to be clamped to the tooling plate with snug-fitting shoulder screws.



Roughing passes commenced.


Roughing passes done.


And after a finishing pass. A side profile (not shown here) is yet to be cut using the same program on both sides.


After cutting the side profiles, the small end was notched out for joint with the Roberts mechanism and piston rod.


Thanks for watching,

--Tim