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Old 03-06-2011, 05:08 PM   #1
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Default Poppin Stuff

I finally got the first chips cut on my first project in with my new to me tools. I chose as my first project the Poppin air-cooled vacuum engine by Dr. J. R. Sentf from Live Steam November 1980. The internet has hundreds of plans but I chose this for its well documented build steps and many fine examples on this board and the internet to use as references.

I bought a fine tool collection from a friend that was retiring for what I thought was a very good price and my wife thought would break the bank, so the better part of this build is going to be a ‘run what you brung’ project, so if you see weird compromises or steps away from the plans of Dr. Sentf that is what is going on. I am as far from an expert on machining as anyone I know so please feel free to give tips where you see this build going astray.

Here goes.

The plans call for Meehanite bar which I had no idea where to get or what it was, but in my introductory post I asked and was informed that it was basically grey iron available anywhere and so I went and got a 12” stick of 1” grey iron class 40 from Enco. So the rod came and I whacked off 2 inches in the band saw and went to work.

I have been practicing cutting fins on some old cold roll I had laying around while waiting for the grey iron to arrive and they all looked terrible fat and not the least bit like the stuff I saw being build to I Goggled ‘cutting cooling fins model engine’ and came up with this great tutorial It gave me the two clues I needed to cut good fins; grind up a special cut off tool to the desired fin spacing, and then to move the combined distance between fins and you come out right in the end. I ground this down to .047 with as close to the following clearances suggested in the tutorial as I could without a tool grinder

I faced off the grey bar and center drilled.

This is cutting the fins. I decided to use the compound for these cuts as suggested in the tutorial above because my carriage does not have a measurement and I haven’t figured out a good place to stick my dial indicator yet on my old Atlas. As this is my first project, I checked the cross slide and compound against a dial indicator and was satisfied with the accuracy. For the first cut I touched the left side of my fin tool to the shoulder, then calculated the width of my tool and the desired first width and started cutting, plunging .125 for each cut. After the first cut I simply moved .080 each cut and ended up with perfect fins. I thought for sure that I would break some but all was well. Well, minus the fact that it all worked so well I forgot to count and ended up with an extra fin and a slightly longer cylinder than the plans.

I tried boring the cylinder but I have two problems the first is my excellent tool buy did not come with a boring bar and the second is I have no inside measuring devices. They are now on my list but will try to work around them for this go. I tried using a cutter that is labeled threading tool that looked like it would do the trick and it cut just fine, but when I unchucked the bore was larger by .005 on one end than the other. As I have no inside measuring tools but my tooling did come with a nice set of reamers I think I will finish the bore with a 5/8 hand reamer.

Not a bad first days work.

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Old 03-07-2011, 08:20 AM   #2
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Default Re: Poppin Stuff

Hi Rogbo,

Looks like you're are off to a good start. Your fins look great.

The slight taper in the bore you made might be due to the spring of the cutting tool. Once you reach the required size, if you take a few extra passes at the same setting it should remove the taper.


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Old 03-07-2011, 12:24 PM   #3
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I've also made that engine and a Jan Ridders internal Valve flame licker before it. The only way I could get the bore right on both engines was to ream it too. I used an adjustable reamer for mine, the exact size of the bore is not that critical as you'll make the piston to suit.

Can't see the pics from this computer but will check tonight. Good luck, it's a lovely engine to build and run.

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Old 03-08-2011, 02:05 AM   #4
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So I was not as happy with my fins on Sunday morning as I was on Saturday night so I got back at it and made another one. The first thing I didn’t like was the fins were a to narrow and the grooves to big. I double checked my grooving tool - a ground down parting tool and it was closer to .050 than the called for .047. I reground it and after a couple failed attempts where I had to grind it way back and start over I got a nicely ground .046 cutting bit.

Below are a couple of shots of the second cylinder and the newly ground tool.

Hand reaming to 5/8 as my thread cutting boring bar came up short but was fun to use. I drilled to 9/16 then bored out out to around .610, then used the reamer to go to 5/8.

Here are a couple of shots of the two cylinders. The one on the left is the first one and it has skinnier fins and an extra one because I was having such a good time I forgot to count. For the one on the right I used a thinner cutting tool closer to the specs in the plan. I find it more aesthetically pleasing.

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Old 03-08-2011, 05:55 AM   #5
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Second cyl does look better than the first, can tell your having fun.

Will be watching

"Nothing is more irritating than that which exposes the pedigree of ideas"----Lord Acton----
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Old 03-08-2011, 07:31 PM   #6
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Default Re: Poppin Stuff

Those are looking real good Rogbo, especially the second one. You have already pregressed well up the learning curve on one of the more difficult tasks (i.e. cutting fins). Looking forward to more.

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Old 03-10-2011, 03:05 PM   #7
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Thanks all for the feedback. The board and its members are a fantastic source of information. In the seventies and eighties I was a mechanic and you had to be related to someone or be an apprentice to get this kind of knowledge found here.

Next on to the pistons!
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Old 03-14-2011, 05:14 PM   #8
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This weekend’s machine shop time was mostly an exercise in frustration. My Jet Mill motor started smoking, my two pistons went south, and my tool post fell apart. I did get my Harley motor that DeadEx crushed in shipment torn down though and my Buell Blast Piglet back on the road.

For whatever reasons my calculations were all off on my drills, cuts, reams, and bores. I just figured out how to get a dial indicator on my movements and while hokie, it works. The class I took at Laney College in Oakland – great Machinist program by the way - had big LaBlonde and South Bend lathes that were well suited for putting a magnetic base dial indicator on, but my little Atlas not so much.

I think everything through on paper and calculate prior to cutting like we learned in class, so all my bad cuts are doubly disturbing.

My first piston jumped out of the lathe during parting and got a big nick on it. Even the photo machine was off yesterday as you can see from the blurry pictures.

Here is my tool post dissection. Mean to build or by a quick change once my wife recovers from my purchase of the lathe and mill.

So as I said my measurements and best thinking were all off today and the piece of gray bar I thought would produce 2 pistons was two short for the second. I thought I would write my own dimensions for a short piston and practice the cutting on what I had left. All worked out better on my second piston except for my parting off dimensions – UGH!

So a weekend of good practice and lessons learned but no pistons.

If anyone has advise about how they mount a dial indicator on work on the Atlas 12 or other small lathe please let me know.


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