Author Topic: Currin's LTD Stirling  (Read 1550 times)

Offline Hugh Currin

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Currin's LTD Stirling
« on: June 27, 2017, 05:59:06 PM »
So, this is a journey of the Traveling Sherline Show. Actually a dry run. I described the situation and equipment in this thread. There I mentioned the best way to check the equipment and select tooling would be to build an engine. So this is a log of that build.

I've wanted to build a LTD Stirling for a long time. This seems like a good opportunity. I looked around for a plan set small enough for the Sherline equipment and found the Egg Cup Stirling from Jan Ridders. He graciously sent me a set of plans. I generally work in imperial units (inch) and the plans are in metric so I re-drew them in imperial units. Can't re-draw something without making a few changes, but it is true to Jan's original plans. Too close to give out the re-drawn plans.

With luck it'll look something like this:


I've ordered the materials I don't have on hand and should get started soon. It'll be a slow build for a couple of reasons. First I'm using new equipment and need to learn it. Second I'll have to build some tooling as I go. One reason for the build is to determine what tooling to take along. Finally, things are a little crazy around here, shop time is limited. Hopefully I can get an engine in a couple of months.

You're welcome to come along. Any advise or suggestions will be welcome.

Thanks.

Hugh
« Last Edit: July 04, 2017, 09:18:46 PM by Hugh Currin »

Online b.lindsey

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Re: Currin's LTD Stirling
« Reply #1 on: June 27, 2017, 07:40:07 PM »
Hugh, that should be a good test project for the Sherline. Jerry Howells 1/2 scale Miser is a good one as well but a bit fancier. You may want to consider ball bearings for the crankshaft as they will reduce friction greatly which is critical in LTD Stirlings.

Bill

Offline mklotz

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Re: Currin's LTD Stirling
« Reply #2 on: June 27, 2017, 08:03:51 PM »
You may want to consider ball bearings for the crankshaft as they will reduce friction greatly which is critical in LTD Stirlings.

If you do use ball bearings on an LTD Stirling, it's a good idea to wash out all the bearing lubrication with solvent and use them dry..  Cuts down on the friction and it's not like an LTD is going to put much load on a bearing.
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Online Vixen

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Re: Currin's LTD Stirling
« Reply #3 on: June 27, 2017, 08:07:34 PM »
Hi Hugh,

Is this another coincidence? There are two more Jan Ridders' Egg Cup Stirling engines coming together very slowly in my shop. I seem to copy everything you do Hugh.

My Grandson, Lewis, is showing an interest in engineering and is determined to become an engineer one day. So most weekends we spend time together in my shop building simple engines. My objective is NOT to teach him just to be a model engineer but to give him the wider appreciation of all aspects of mechanical (and electrical) engineering. I am trying to provide him with those precious hands-on engineering skills that do not come with collage courses and books. He is keen and learns quickly, so I believe the time we spend together is well spent.

For Christmas, last year, I put the drawings of several of Jan Ridder's hot air engines into his Christmas box, From these, Lewis decided he liked the look of the small Egg Cup Stirling for his next project. We have made a start, and have completed two fly wheels. Unfortunately, due to his final school year exams, we have had to put the weekend sessions on hold. Exam results are more important than learning skills. We will get back on track any time now.

The first engine Lewis built was a simple oscillating engine, christened the 'Wobbler'. This engine was made entirely by hand on an Emco 4 lathe with a mill attachment (even smaller than your Traveling Sherline Show) The little engine runs well and has already worn the main bearings out, so these will need to be remade. More practice.

The plan for the Jan Ridders' Egg Cup Stirling is to broaden his engineering experience by doing most of the engine using CAD, KAM and CNC techniques. Hopefully he will learn to 'walk the walk and talk the talk' and be well ahead of his peer group at collage. So like you, Hugh, we have redrawn Jan's engine in AutoCAD, we have produced the toolpath code using DesKAM and have machined the flywheel on my Emco F1 mill using LinuxCNC. I know these are not the most up to date software and machines but the thought processes are the same and that's the message I am hoping to get across to him. How to think as an engineer thinks.

Hopefully he will repay me one day with a students low price copy of Solidworks.

The Jan Ridders' Egg Cup engine is very small and may not run too well on the low temperature differential provided by a cup of hot water, so I propose to make a few changes to allow it to be powered by a T light candle, to provide a little more heat energy.

Cheers

Mike
It is the journey that matters, not the destination

Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #4 on: June 27, 2017, 10:50:55 PM »
Bill & Marv: It's not as bad as it looks. Jan is using a point contact for the bearings:


The ends of the axle are pointed (60 deg included). It is then supported in set screws that have been spot drilled (90 deg included). The set screws in each post adjust the load/clearance. So theoretically there is a point contact for rotation. I think this is used in clocks and watches? It should work OK??

Mike: Great to hear you are bringing along a young machinist. It'll be a real advantage for his engineering career. Are you doing a build log?

Thanks.

Hugh
« Last Edit: July 04, 2017, 09:20:21 PM by Hugh Currin »

Online crueby

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Re: Currin's LTD Stirling
« Reply #5 on: June 27, 2017, 11:49:26 PM »
Bill & Marv: It's not as bad as it looks. Jan is using a point contact for the bearings:
The ends of the axle are pointed (60 deg included). It is then supported in set screws that have been spot drilled (90 deg included). The set screws in each post adjust the load/clearance. So theoretically there is a point contact for rotation. I think this is used in clocks and watches? It should work OK??

Mike: Great to hear you are bringing along a young machinist. It'll be a real advantage for his engineering career. Are you doing a build log?

Thanks.

Hugh
The main shafts on clocks are stepped ends through a barrel shaped hole typically,  this type of point bearing is used on jeweled bearings in watches. I think.

Online Vixen

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Re: Currin's LTD Stirling
« Reply #6 on: June 27, 2017, 11:52:04 PM »
Hi Hugh

I brought up my eldest son to be a fine aircraft engineer, sadly we lost him a couple of years ago. Now, I am fortunate to be able to act as mentor to one of the next generation. It is may way of giving something back.

As to a build log, that will be up to my young protege. I will encourage him to join this wonderful MEM community of ours. However, I imagine he would find the prospect of addressing our community to be very daunting. To a young lad, reaching for the first rungs of the ladder, the established members of the forum must appear like the unapproachable 'Gods of Old'. Maybe the discussions in the '4000 Strong!' thread will find a way to overcome these barriers and make it less intimidating for the younger generation to contribute.

I have seen graphite rod used to make ultra low friction crankshaft bearings for an LTD Stirling engine. Graphite rod is readily available on evil-bay. I never fell compelled to build an engine as per drawing, if a small change can lead to an improvement, it is worth a try. If it does not work out, you can always follow the drawing.

Mike





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Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #7 on: July 03, 2017, 02:02:55 AM »
So, I have the materials needed for the Egg Cup Stirling.


Mostly from McMaster Carr since Online Metals didn't have 12L14 Leaded Steel. I decided to get leaded, free machining, steel because it machines so nicely and the cost is reasonable at this size. The plastic bag contains a short length of 1/4" graphite for the piston. Haven't worked with this before, I hear it machines well but is messy.

You'll notice I've already broken the rules. I used my large horizontal bandsaw to cut the two AL disks shown. Sure didn't want to attack them with a hack saw. I will have to figure out a cut off method for the Traveling Sherline Show. I'll also slice off pieces of the large steel round for a flywheel and the acrylic tubing for the displacer cylinder. More rule breaking.

I've been slowed down. I need to make some tool holders for the Sherline Mill. For a CNC mill I need several holders in various sizes. SO, I'm trying to become friends with the Sherline CNC Lathe. I need to bore, groove and thread to fit the Sherline spindle. Then mount on the spindle and ream for specific tooling (1/4" & 1/8"). Might as well learn enough to make them on the Sherline CNC lathe since I need several.

So I'm still on task but slow. There's also a lot of things going on around the house. And I though retirement would bring more shop time. Think again!

Thanks for following along.

Hugh
« Last Edit: July 04, 2017, 09:21:37 PM by Hugh Currin »

Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #8 on: July 16, 2017, 04:52:41 AM »
I've been slowed down. I need to make some tool holders for the Sherline Mill. For a CNC mill I need several holders in various sizes. SO, I'm trying to become friends with the Sherline CNC Lathe. I need to bore, groove and thread to fit the Sherline spindle. Then mount on the spindle and ream for specific tooling (1/4" & 1/8"). Might as well learn enough to make them on the Sherline CNC lathe since I need several.

Still planning to build the Stirling, but the above problem is still present. I posted details in another thread. If anyone has insight into cutting specs on a Sherline please chime in.

So I'm on task but slowed down at the first step. Thanks.

Hugh

Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #9 on: August 15, 2017, 12:36:27 AM »
I believe I have the Sherline equipment tooled up. But I've run out of time. I'll be tied up through September with little shop time. Hopefully I can get a good start on the Stirling beginning October, but life is going to get real busy around then. I haven't forgotten the project though.

But I'll bet you all had forgotten about it. :-) If you didn't thanks for hanging in there, I'll get it off the ground yet.

Thanks.

Hugh

Online b.lindsey

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Re: Currin's LTD Stirling
« Reply #10 on: August 15, 2017, 12:40:03 AM »
We'll be here when you are Hugh. Looking forward to it!!

Bill

Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #11 on: December 18, 2017, 04:39:15 AM »
Finally, some progress on the LTD Stirling. We're in Brenda AZ for awhile and able to set-up the Traveling Show. A meager start but thought I'd let you all know I'm somewhat on task.

The first task was to square up the displacer cylinder. I wanted these complete to check the sizing of the upper and lower displacer plates as they are made. Simple turning with a three jaw chuck and HSS insert tooling.


This went well. I had four blanks so squared them all.


To machine the upper and lower displacer plates I needed to use the 1" riser blocks for the Sherline lathe. I hadn't used these before so decided to align the headstock. Put a 1/4" piece of round stock in the 3 jaw and a test indicator in the tool post. First measuring near the chuck, taking center as half way between largest and smallest readings. Then move towards the end of the 1/4" bar. Again taking the middle of low and high readings as the bars center. Adjust headstock to zero at outboard end and re-measure. A few go arounds put alignment within a thousandth or so.


Then using the three jaw chuck I faced and then cut the recess for the displacer cylinder in upper and lower displacer plates. I faced the disks manually (CNC jog commands) but wrote a g-code program to cut the recess itself. The Sherline lathe takes light cuts. I found for this set-up it liked to take 0.005" per pass. That's a lot of passes so I found it easier to write the program that type in MDI codes one at a time.


The plan is to use soft jaws to hold the upper and lower plate for finishing. This for the OD, thickness and machining details of the upper plate on the mill. To machine soft jaws one wants the jaws pushed against the scroll as it will be in use. Grabbing the ID of the plates requires the jaws the be pushed in for machining them. For this I put a twist of wire around the jaws and adjusted the chuck outward to tension the wire. Finally, I faced the jaws and then cut a step in them to hold the displacer plates.


I've done a little more but no pictures yet. The soft jaws hold the plates well for the most part. I have three sets of plates (3 engines) so I'll machine all of them. With luck I'll end up with one or two complete engines. One plate didn't want to catch on the soft jaws. I think, for some reason, the recess is tapered. Next session I'll try to correct this and finish the plates.

I need to get some chip control first though. Some kind of tent to catch chips. Soon as that's set-up I'll try for some more progress.

Till then, have a Merry Christmas and a Happy New Year.

Hugh

Online Kim

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Re: Currin's LTD Stirling
« Reply #12 on: December 18, 2017, 06:09:08 AM »
Interesting technique there with the wire to hold the jaws in position to radius the outside diameter of the soft jaws.
Kim

Online b.lindsey

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Re: Currin's LTD Stirling
« Reply #13 on: December 18, 2017, 11:28:03 AM »
Glad to see the travellin' shop now in action Hugh. Looking forward to more updates as things happen. Enjoy that warm Arizona weather :)

Bill

Offline Hugh Currin

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Re: Currin's LTD Stirling
« Reply #14 on: January 13, 2018, 03:59:56 AM »
Here is a little more progress on the Stirling. The soft three jaw worked well holding the plates for thinning.


When I cut the aluminum disks they didn't seem to be way oversized. But after cutting to diameter and thickness on the Sherline, they were. It takes many passes and a bit of time to take these to size on the Sherline. But the set-up worked well and I was able to thin them all to size. Per the plans the lower plate had an inset of 0.075" for the plastic displacer cylinder while the upper plate had 0.050". I grabbed the wrong plates and thinned for the lower plate. But I was able to modify the design to account for the error. I even like the new dimensions a little better. So, the lower plates were completed.

The upper plates have a pocket cut out. This is for better heat transfer through the plate and to hold ice/water on this plate to better power the engine. I converted the Sherline to its mill configuration and put on a tooling plate to hold the blanks. I found that I'd loaded CamBam on my laptop but didn't transfer the post processor over. So, it took some time to modify CamBam's LinuxCNC post to my liking. I machined a 0.025" recess in the tooling plate to hold the upper plate. Two tie down holes were tapped using jog and MDI commands.


The program to cut the basic pocket used a 1/4" end mill (slot drill), then a 1/8" end mill to finish up the corners, and a spot drill to chamfer the inside edge. I completed the 1/4" end mill pass. Watching it progress I found the 1/8" end mill was programmed to take a full depth, 0.100", cut to finish the corners. I thought this would cause problems so stopped after the 1/4" end mill pass. Looking at it I liked the way it looked. So I stopped at this point calling the pocket good.


I completed, nearly, by reaming the 1/4" hole in the center and 4-40 threads for the pillars via jog and MDI.


I liked it so much I cut number two the same way.

But since this is for fun, I re-wrote the code to include 1/8" passes and and inside chamfer. To get the chamfer tool, spot drill, to clear the clamps I rotated the part, in CamBam, 90 degrees. If I'd only been thinking correctly from the start. This code took longer to run but I do like the results a little better. Here are the three parts.


The one on the left has the full pocket and inside chamfer. It's also sitting on the displacer cylinder with lower plate in place. The other two are to the right, one upside down to show the bottom.

I need to thread the hole for the power cylinder 3/8-24. Didn't think this was a large tap till recently, but with the Sherline it is. I have a large tap handle at home that would work but would also be too tall for the Sherline mill. I do want this taped hole to be square so will have to figure out a bushing or something to hold it. More minor tooling fun.

Thanks for coming along. Stay warm out there.

Hugh