Moriya Stirling Engine Build

[rmac]

So far, so good. Here's my Neanderthal tool and the result of a test in a piece of scrap. There was lots of chatter, as expected, but I think with a bit of tool refinement, this is going to work.

-- Russell Mac

[NP317]

Looks like a GO.
Chatter? Meh. No one will ever see it except for you, prior to assembly.
But I understand the O.C. mindset.
RussN

[rmac]

The tool required for creating the cut by lathe would have unusual geometry, so you'd overcome the small radius. It will be prone to chatter and breakage, even in aluminum.

I'm still inclined to try the trepanning idea just for fun. If that fails, then I'll use the mill and the rotary table. I agree that that approach offers a better chance for success.

Well, that was fun all right. I got lots of chatter, as predicted. And the tool snapped off on the first part after gouging out about 3/16" of the required 5/8" depth, as predicted. One ignores Harold's advice at his own peril.

Anyway, now I'm at the point between "If that fails" and "I'll use the mill and the rotary table." It looks like the only end mill I have that's long enough to work is 3/16" in diameter. So I need to check if I can fudge the design and make the groove a little wider, or if I need to get another end mill.

On the plus side, the chattering did make an interesting pattern on the bottom of the groove.

-- Russell Mac

[NP317]

Looks like the tool have too much positive rake?
Turn it into a turbine...
RussN

[Harold_V]

I made mention of the geometry of the tool you'd need. Your design doomed you to failure because of its length. You had to clear the long projection, and that wasn't necessary.

I hand grind all my parting and grooving tools, although I'm now better positioned to use my #2 Cincinnati cutter grinder, so that may become my new normal. Mean time, give this some thought. You already have to grind a tool, so why grind one that's so long and flimsy? What I'd do in this instance is start with a blank, maybe 3/8", but more likely ½". Instead of grinding the cutting edge perpendicular to the sides of the blank, I'd grind it offset, say 15° or so, so the tool is mounted at an angle (it now doesn't have to clear the projection), yet the cutting edge is parallel to the face of the part. That way you can't have to clear the projecting and you'll shorten the tool tremendously. The setup you provided is just too "springy" to expect anything less than what you experienced, although you certainly could have succeeded with some luck.

Given the part is just aluminum, not stainless, you can make the project go a little easier if your choice of material is 2024, say in the T351 or T4 condition. I'd keep it wet with kerosene or WD-40 while cutting.

Thinking of the tool you used, any time you can eliminate a projecting tool (like the boring bar holding your trepanning tool), you're better off. May not always be possible.

H

[Harold_V]

Looks like the tool have too much positive rake?

I don't think so, but it definitely has too much front relief. Reducing that feature will limit a tools ability to self feed, which, in this case, encouraged chatter. The long slender tool and the extra long projection sure didn't help.

H

[NP317]

Looks like the tool have too much positive rake?

I don't think so, but it definitely has too much front relief. Reducing that feature will limit a tools ability to self feed, which, in this case, encouraged chatter. The long slender tool and the extra long projection sure didn't help.
H

Thanks, Harold. I was hoping to learn more from you!
RussN

[rmac]

You already have to grind a tool, so why grind one that's so long and flimsy?

I agree that long and flimsy is bad, but I don't see how my tool was any longer or skinnier than it had to be. The groove I am trying to cut is supposed to be 5/8" deep, and my tool was sticking out of the boring bar just a hair more than that.

Then as far as skinny goes, the tool couldn't be any wider than the intended groove, and it had to be narrow enough in the other direction (from top to bottom) so it didn't rub on the outer radius of the groove.

So unless I'm missing something, it seems like the business end of a tool ground as you suggest would still look about the same, except that it would be attached to end of a big tool blank instead of being clamped in the boring bar.

I'm not planning to try this again, since it seems iffy even under the best of conditions. But if I was, I'd consider two things: 1) adjusting the front relief angle as you mentioned to RussN. I was sort of guessing at that angle, and I can see now how too much relief would induce chatter. 2) using better material. The tool that broke was literally ground from a masonry nail (some folks call them "cut nails"). Probably not the ideal choice for a tool bit.

As always, thanks for your interest and help!

-- Russell Mac

[rmac]

Turn it into a turbine...

That's exactly what I thought when I first saw the picture. Make seven more of these and build a B-52!

-- Russell Mac

[Harold_V]

I agree that long and flimsy is bad, but I don't see how my tool was any longer or skinnier than it had to be.

I stand corrected. It was difficult for me to determine the length of projection, so I assumed that it was long enough to clear the tip of the part being machined.

So unless I'm missing something, it seems like the business end of a tool ground as you suggest would still look about the same, except that it would be attached to end of a big tool blank instead of being clamped in the boring bar.

Likely so, although the tensile strength of tool steel exceeds, by a huge margin, the strength of carbon steel, from which you made your cutting tool. That may or may not make a difference, but when you work on the edge, anything you can do to increase stability is usually a good idea. That would include adding depth (height) to the tool, although the gain may be small. You would also create side relief by grinding a radius instead of a flat (or hollow grind). Any metal removed beyond that needed for clearance reduces rigidity. The gains may be small, but they may spell the difference between success and failure.

I'm not planning to try this again, since it seems iffy even under the best of conditions. But if I was, I'd consider two things: 1) adjusting the front relief angle as you mentioned to RussN. I was sort of guessing at that angle, and I can see now how too much relief would induce chatter. 2) using better material.

A shame, as staying the course would be a great learning experience. We gain strength by failures like this. A change to 2024 aluminum, alone, may spell the difference needed to succeed.

By the way, I took a close look at your picture. It's hard to determine if I'm seeing a sharp edge on what would be the left side of the tool when looking at the picture. The slightest rounding of the edge would be a serious problem in raising cutting pressure beyond reasonable limits. When you use a tool like this, it should be honed to a keen edge, with limited clearances on all features. Excessive positive rake can be troublesome, but not if well balanced with front clearance. When you get the geometry right, the tool will perform perfectly well, assuming you pick the proper speed and feed. Take a very close look at your tool, even with magnification if necessary. There should be NO signs of contact anywhere but at the actual cutting edge, and it should NOT display the remotest sign of negative rake (in this case created by rounding of the edge). The cutting edge should be a distinct sharp point, displaying no sign of rounding. If it is shiny, it is likely rounded.. That is usually a deal killer.

Just sayin'

H

[rmac]

I'm not planning to try this again, since it seems iffy even under the best of conditions.

A shame, as staying the course would be a great learning experience. We gain strength by failures like this.

You're right. A better plan is to go ahead and make these engine parts using the mill and the rotary table, and then to continue with the trepanning and tool grinding exercise later when 1) I can experiment with scrap rather than parts that already have some work in them, and 2) I can positively identify the material. (I got the stock I'm using now from the 6061 remnant pile at my local supplier, but it's not actually marked so I'm not 100% sure what it really is.)

Edit: Here's a later thread where I played around some more with trepanning: http://www.chaski.org/homemachinist/vie ... 4&t=110660

-- Russell Mac

[rmac]

Progress Report #8:

Hot off the press, parts for two displacer pistons, done except for the grooves discussed earlier. I'm waiting on a long eighth-inch end mill before I can finish that part.

I think I'm gonna get pretty good fits when it's time to press the ends into the tubes, but I'm worried about one thing. Especially with the stainless steel tube, when I push the ends into it just a little bit, there are gaps in the marks on the aluminum ends where they rub on the inside of the tube. I have to guess that this is because the tube is not perfectly round.

I wouldn't worry about it, except that the completed assembly is supposed to be airtight. Seems like the easy fix is to smear some JB-Weld on the parts before pressing them together, as a bit of insurance against leaks. Any reason not to do that? Might something other than JB Weld work better?

-- Russell Mac