Stirling Engine : Laura

Which tool is used to make the recess cuts in the cylinders of the Stirling engine ? On one cylinder they are 5mm deep and 2mm wide. On the other cylinder they are 1mm deep and 2mm wide. I could use two parting off tools : one with the cutting edge on the left and the other with the cutting edge on the right to end up with an even cut but I feel there might be a better way ? My parting off tools are 1.5mm wide and do not work with a flat cutting edge ; they must have an angled cutting edge to work.

Brian look at how you have your tool mounted and ground, As I have shown elsewhere with exactly the same tool ground square across the end I can easily cut fins.

But remember, before I out an angled edge on this tool I was having problems parting off : the cool kept digging in.

NOTE : that was before Hopper worked on the lathe so it might be possible to take a ''flat'' recess cut now.

Edited By Brian John on 14/11/2015 09:05:08

Was that before Hopper worked his magic on it, may be OK now. Another option is to grind up a narrower tool to put less load on the lathe and take several stabbs at the groove

Yes, I just remembered that was before Hopper worked on the lathe It may be okay to do that now with a flat edge.

My first parting tool was ground up from a bit of a 12" x 1" industrial power hacksaw blade, needed it to part off 16 bits of 2" diameter phos bronze to make radiator nuts for a Lanz Bulldog tractor, worked like a charm. This sort of tool would cut fins for a hot air engine quite well.

Ian S C

Brian, rather than try to use two parting tools together to make a wider groove, try using one parting tool to make a groove to the required depth, then move the carriage by however much you want to widen the groove by and make another cut. Keep the carriage locked during the cut.

Either way you are going to end up with an odd shaped bottom to the groove if you use one or more cuts from a tool with an angled end

I put the parting off tool on the grinder and gave it a flat cutting edge : all went well, no problems at all. I made the recesses a bit wider apart than I should have so I ended up with five recesses instead of six ! That last recess will now be the parting off as that will give the correct overall length of 27mm. I may do it all again tomorrow as I am not sure if it will make a difference to the running of the engine.

Edited By Brian John on 15/11/2015 06:20:26

Edited By Brian John on 15/11/2015 06:24:14

Edited By Brian John on 15/11/2015 06:25:34

Brian,

That's looking MUCH better

... Bet it's good to have something that actually qualifies as a lathe.

MichaelG.

Brian, the number of fins won't matter at all.

When cutting fins and things like that, I put a tiny radius on both corners of the tool so that the bottom of the slot does not have a sharp corner, it's not really important, it's just the way I do it.

Ian S C

A closer inspection of the plans and I realised that the recess cuts are not deep enough : they need to be 5mm deep so I did that today. I will cut off that first recess and that will give me the correct overall dimension of 27mm. The parting off cut has been marked close to the chuck.

There is a recess in the face to be made for the muff (part 27) to hold the glass tube (or metal tube). I will also have to drill and bore out to 14mm. Parting off to give 27mm overall length will also have to be done. What is the correct order of operation here ? I was going to drill first, part off, then cut the recess in the face but perhaps the boring should be left until last ?

I am also a bit puzzled about the smaller cylinder (part 26). Why have they put the cross hatching on this diagram and what is that ''2'' on the left ?

Edited By Brian John on 16/11/2015 06:20:47

Edited By Brian John on 16/11/2015 06:21:16

Edited By Brian John on 16/11/2015 06:22:03

Cross hatch indicates the metal that has been cut through to give the sectional view

"2" is the depth of the milled slot between the long hole along the bottom and the main bore.

I would part off a little over 27mm and then reverse in the chuck and face to correct length just incase the parting tool wanders

Given the lightness of your lathe I think I would part off, face to length, turn back round in the chuck, do the bore then the recess which will keep overhangs to a minimum and make it easy to run the boring bar right out the end of teh hole no risk of crashing it into the end of a blind hole.

I have parted off, faced off to 27mm length and drilled out both cylinders to 8mm : starting at 3mm then going up by 1mm increments. I have to wait for the 9.8mm drill bit, 10H7 reamer and the small boring bar to complete the boring work. It usually takes one month from China and one week from the UK.  I may make one more of each cylinder to do some experiments with the boring bar and reamers when they arrive.

I have found that it is best to have two separate parting tools : a flat edge for recess cuts and an angled edge for parting off. I will also buy that stub drill set as per the link above ; I can see now why they will be very useful in future.

NOTE : yes, I know that some chatter marks are visible in the recesses of the larger cylinder. This piece of brass was purchased from the scrap yard. It was badly bashed about and I think it had become work hardened. It was not as easy to work with as the smaller piece of bar stock ; I could feel the difference when turning it.

Edited By Brian John on 18/11/2015 06:20:50

Edited By Brian John on 18/11/2015 06:21:55

Edited By Brian John on 18/11/2015 06:25:18

Brian, as you are using a HSS parting tool, sharpen one end for grooving, and the other with an angle for parting.

Ian S C

Yes, I could do that but I already have two blades. My blades are this shape :

**LINK**

Is that curve in the end necessary or desirable for a parting tool ?

You could just grind an angle on the other end and have both styles available Brian. Each has it's advantage. The plain end remains at the same height when it's reground. The other with back rake doesn't. The rake may make the tool grab on some materials but can leave a better finish on some when used for say grooving. It also means that the tool will cut into the metal more easily than the plain end.

The problem people usually have with parting off tools is chatter and noise which causes them to ease off on the feed which will make things worse. Myford 7 users often have other reasons for that happening though and a rear parting tool helps as the cutting forces are reversed.

Should mention noise and slides being too loose too.

John

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Edited By John W1 on 18/11/2015 14:12:22

I always make sure everything is tightened up before parting off or making recess cuts. I think I will try the other end of the tool without the rake when I make another of each of the large and small cylinders. It will be interesting to see if there is any difference.

Your ordinary slide setting for turning should be fine Brian, maybe locking the saddle but with the style of bed you have and Hopper's work you probably wont even have to do that.

John

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Brian, I tend to lock the saddle when parting, I do it on a much heavier machine. The curve you mention is on the top? If so that is to give you the top rake, as ground should do for most materials, it will be averaged , so best for steel. Ideally brass would have a flat top, and aluminium a much greater rack angle.

Ian S C