Another Oscillator!

[NickG]

After seeing Metal Mickey's post on his twin wobblers for presents (thanks for the idea), I thought it was such a good idea I decided to set about designing a simple oscillating engine that I can make very quickly (hopefully!). My eldest son is 2 years and 9 months old and is obsessed with my other oscllating engine so I think it would be a great Christmas present to give him his own ... and it won't matter if he breaks it! I also have a 5 month old son so I'd better make him one at the same time, the idea is to mount them on some nice wood with a brass plaque with each of their names engraved ... not 100% confident I'll get that done though, I am running out of time and haven't even started yet!

Anyway, thanks to Metal Mickey for the idea, below are some snapshots of the 3D CAD Model, I am reluctant to put the drawings up on here in case there is an error somewhere, once I have made one though, people are quite welcome to have copies of the drawings. I'm going to try to take photos of me building these two ... not done this before though so it could be a bit of a mess!

Hopefully I will make a start on them tonight ... will update on progress tomorrow, if there is any!

Nick

ps the cylinder spring, nuts, steam inlet and mounting holes are not shown - I must have got bored by that stage! Cylinder cover, pivot pin and crank pin are all going to be soft soldered in place.

 

[SignalFailure]

Nick, I'd thread the crank pin and pivot pins rather than soldering them as I think you'd have more chance of getting them perpendicular to the crank disc and cylinder.

How are you going to make the frame, bend it or mill from solid?

 

[Metal Mickey]

Nick, you are too kind! I do like the simplicity of look of the main frame. Nice. :bow:

Have you thought of using a larger piston or is there a technical reason for the small piston?

For the more experienced members (or anybody really) I would like to learn your views on small versus larger diameter pistons with regards to running with say an aquarium pump. I suppose its a question of volume versus pressure. I know that in steam the 2 cylinder chest has a small and large piston with high pressure steam going into the small cylinder then the exhaust from that sent to the large diameter cylinder.

To me that seems to say that a large area should perform better with low pressure? Or am I completely off track (excuse the pun). I have started a design of my own which will be a new thread for me, but this question is making me consider two different cylinder sizes........

Nick I think your doing a great thing and I will follow your design and build avidly. Good luck. Metal Mickey.

 

[NickG]

Paul, thanks for your comments. It's interesting what you mentioned about getting the crank pin and pivot pin perpendicular. I've made quite a few of these engines and always threaded the crank pin and pivot pin, however more often than not I've struggled to get them in perpendicular.

I think it's more difficult to get them perpendicular when threading as it's introducing another element of error, it's not easy to put a straight thread on a small diameter. In the past, results have been acceptable as there is the option to tweak it by gently bending it afterwards.

It was for this reason I decided to have a go at soldering them in ... this way I can drill the holes and make the pins with an accurate location diameter to fit into the holes. This may have the disadvantage though that it can't be tweaked afterwards ... once it's soldered it's staying where it is, bending it may crack the joint.

Hmm, not quite sure which way to go now, I might try soldering method, just because I haven't done it that way before. On my hot air engine, I machined the crank from the solid to ensure everything was square, but that was a labourious task and this engine is meant to be simple!

The frame will be bent around a block of wood.

I should point out that this design is based on the first ever engine I made when I was 14, which was in Model Engineer Magazine. The engine ran, but on 40 psi and snapped the crank pin!!! Differences were that it had a cylinder made from round bar which was drilled off centre in the lathe by putting packing under one of the 3 jaws in the chuck. Then transferred to the 4 jaw and the side faced off, I find it easier to use square bar. The other thing it had was a tubular bearing between the frame, soldered in place. I guess this stiffened the thing up too but I think it will be ok without.

My intention was to make a very small engine, I should have mentioned some sizes ... the bore is 3/16" and stroke 1/2". The bore is about as big as I can get with that size cylinder, as there needs to be room to drill the hole for the pivot pin without breaking through.

I've often had this debate in my mind about piston bore vs stroke & volume. My feelings are the same, the compound engine does illustrate your point, if you're operating on low pressure, the best way to go is to have a larger area since pressure = force / area, therefore force = pressure x area. You're quite right though, it's a balance with volume, if the compressor you're using can't supply the volume required it won't work either! So to get one to work from an aquarium pump, I think you'd probably end up with something with a relatively large bore and short stroke.

Actually, as I'm writing this, I'm not sure if what I was about to say is correct, so am going to have to do some more maths on the subject. I was about to say that it's more beneficial to increase the bore than the stroke .. it is, but the down side is the swept volume increases more by doing that. So two engines with the same swept volume should have the same maxium torque at the crank, that's at 90 degree crank angle, not sure about all the other angles without doing a few calculations to confirm, can't quite get my head around it!

Nevertheless, there are still a few balances to be struck. having too short a stroke would mean the cylinder was not oscillating enough to get big enough inlet and exhaust ports. I believe the placement of these is also critical, if the ports are too far apart, there will be a period of time just after top dead centre where no air is getting into the cylinder. The inlet should be just starting to open just after top dead centre, this means the gap between the centres of the ports should be 2 x port width. If they are closer, there will be overlap so periods where air / steam is going from inlet straight out of exhaust. This distance is controlled both by the stroke (crank throw) and piston rod length.

Port sizes are also critical, not sure what the optimum is, just big enough the flow the air without restrictions I assume, or as big as possible within the particular design scheme.

I think it would be interesting to make two engines of the same swept volume, on with a large bore and short stroke and visa versa. I think they should perform roughly the same, however, the only disadvantage of the long stroke version may be more frictional loses due to having to make the cylinder oscillate more.

Does anybody else have any thoughts on this?

Should start building tonight .. honest!

 

[mklotz]

Nick,

One thing bothers me about your design.

The two holes for the crankshaft in the frame will have to be drilled after the frame is bent. Given the springiness of the frame, both while drilling the holes and the "relaxation" after, are an invitation to misalignment and consequent friction. You might want to consider making the frame from solid.

It's not good engineering practice to depend on threads for alignment. I would drill a pivot pin sized hole into the cylinder and then counterbore it to a shallow depth. The hole aligns the pin and the counterbore gives you a place to solder without interfering with the seal of the cylinder against the standard. If you intend to run the engine on steam at some point, be careful to use at least a silver-bearing soft solder lest the steam heat cause your engine to dissolve. If the engine will always be run on air then you can forget the counterbore and use Loctite to secure the pivot pin in the hole.

The crankpin can be threaded into the crank disk if you make it in the form of a shoulder bolt so the shoulder establishes the alignment and the thread merely holds it in place. Many of my engines are done that way so I know from experience that it's a workable solution.

 

[NickG]

Marv,

That is a very good point, I thought I'd drill it after the frame was bent, however, I think you're right, the springiness when drilling will cause the holes to become misaligned. This is probably why the original design had a tubular bearing. Might have to go down that route, the only reason I dismissed it was to have 1 less component.

Good advice with the counterbore, I was just going to counter sink slightly for the solder to filet into as there is a relief on the frame anyway. This engine will only ever be run on air though so would use loctite if I had any. I really need to stock up on some of the basic model engineering items!

Again, with the crankpin, It was just going to be a plain diameter soft soldered into place to keep it quick and simple, however, if I was doing an engine on a slightly large scale I would use your solution as it is a far better engineering practice. I am not usually one for soldering or loctiting parts and I designed my other osciallating engine to be assembled with without those methods, but on this occasion ease and speed are the name of the game.

I am slightly worried about the frame springing, however, I may get around this If I use the block of wood it was formed around to keep the sides apart whilst drilling. If not, I will have to make a bearing tube.

Thanks again for the advice.

Nick

 

[mklotz]

Once you've bent the metal, the wooden form will only fit loosely in the engine base so the base will still be free to misalign while drilling. A bearing tube will definitely help. I understand your desire to simplify but remember Einstein's words, "Make things as simple as possible, but no simpler."

Every shop can benefit from Loctite. Their blue threadlocker is essential and their green "Press Fit Assist" (609 IIRC) will hold as well as solder. Purists may eschew the use of adhesives but I remember a quote of famed woodworker Sam Maloof, "If Holzapfel had had a router, he would have used it."

Consider machining your crankpin with a reduced diameter tenon that goes into the hole in the crank disk. That way the shoulder provides the orthogonality in the same way that my aforementioned shoulder bolt would. Clamp it in place when you solder (or Loctite) it so it doesn't droop under the force of gravity.

 

[SignalFailure]

It's not good engineering practice to depend on threads for alignment. I would drill a pivot pin sized hole into the cylinder and then counterbore it to a shallow depth...

Marv,

Surely there's almost as much margin for error there as you'd have to get the hole exactly perpendicular, the pin a precise fit and ensure that there's no distortion when soldering? You might also have a fillet of solder to remove around the bottom of the pin which might be fiddly!

Nick,

Have a look at the plans for the second engine down on the left-hand side of this page... http://jpduval.free.fr/Plans_moteurs_vapeur_p1.htm ... it would be easy to tun it through 90 degrees!

 

[mklotz]

Paul,

Yes, one needs to drill the hole square - but that's equally true if it's threaded. The counterbore is meant to provide a place for the solder fillet so it won't interfere with the sealing of the cylinder against the standard. Distortion isn't much of a problem with soft solder but, regardless, he says he'll run on air so Loctite can be used in place of solder.

 

[NickG]

Paul,

There is a clearance between the big end and crankdisc which will be big enough for the fillet of solder.

When the engine is made and tested, I will update the drawings to suit the practices I used to make the working engine, I may have to change a few things along the way, so will see what happens!

Anyway, I have finally made a start on it, however, I should point out that unless I have a very busy Christmas Eve in the workshop, this is going to be a late Christmas present! Please see some progress below, I may have gone into too much detail to start off with here so will have to review the way I am going to present this!

Well, first of all, I think this lot will need tidying a little!

This was partly to blame ... the caput starter motor from my Renault 5 GT Turbo, think I've sorted it but that's another story and another project! The rest of the mess on the lathe was due to a favour I did for someone at work ... turning down a 2" diameter lump of mild steel, counter bored at one end and cross drilled, then parted off. Except I couldn't part it off as I didn't have a good enough grip on it, so had to saw it by hand with a very used hacksaw blade! Anyway, no more digression from now on, I promise!

After / inbetween tidying up, I found some materials ... probably not what I state on the drawings but not too bothered. The drawings state what the materials should be, I use what I have! The brass plate is for the frame, mild steel for crankshaft, round brass for piston / cyl. cover, square brass for cylinder and I found some cast gunmetal bar which I will use for the flywheel. Copper pipe is for inlet / exhaust, bronze bar was going to be for crankpin / cyl. pivot rod but I wasn't thinking straight, I will use some mild steel. I would usually use silver steel for that sort of thing but not sure how well it will solder to brass and I find it's a pain to turn at very small diameters (as Marv says, I could use loctite but don't have any ... could even use araldite, that's strong stuff but need to make sure it stays where it's meant to!

I started on the flywheel as I'm not keen on sheet metal work, I find that boring!

Gunmetal chucked, faced and OD roughed. I noticed there are some blow holes even this far under the surface of the casting ... hopefully they would disappear as I got to finished diameter. The surface finish wasn't quite as good as I'd have liked as I was using a cutting tool with wrong radius and rake angle really, the reason for this was to try and use the same cutting tool all the way through to save set up time ... this could backfire!

Turned to finished OD and the boss machined, ready to centre drill, drill (I'm not superstitious but why did it have to be No. 13 drill that I needed before reaming!) and ream for crankshaft hole.

Successfully parted off (or is it? You can see component underneath bed!) under power cross-feed. I have found setting the feed to slowest and spindle speed about 200rpm works best for me. Gives a nice constant feed, better than I can do by hand.

At this point I was considering leaving the flywheel as it was because I knew I'd have problems putting it back in the chuck (my 3 jaw is very worn). However, I decided to go ahead and try to make it how I'd designed it as there were some chatter marks from the parting tool so I at least needed to skim it. Luckily the first place I gripped it, it appeard to be running ture!I started trying to counterbore but ran into problems. There wasn't enough clearance around the tool so as I got deeper, it started catching the edge of the hole. So my plan had backfired, I needed to change the cutting tool! This chafing also had the effect of moving the flywheel in the chuck ... d'oh!

After grinding trying another cutting tool and messing it up even more, I decided to grind one properly which gave good results.

I was quite chuffed until I looked at the rear ... the chuck jaws have quite badly marked the boss (can't see on this photo), I've dressed with a needle file and it'll do, but I always get this problem, does anybody have any advice concerning this? Other than that, fairly pleased, just need to drill and tap the hole for the grub screw. I've put 6ba on the drawing but not sure whether this may be too big, might go for 8ba.

I think I've made this far too detailed ... all this for a flywheel! :-[ From now on I'll just post a couple of pics on each component with less waffle! Oh, forgot to say, there is still some very slight evidence of blowholes so it obviously isn't a great piece of gunmetal.

Crankshaft and frame will be next, not sure whether I'll get anything done tomorrow though!

If anybody has any suggestions on the way I'm posting I'll be happy to take the advice as clearly I haven't done this before!

If I don't get on here tomorrow, Merry Christmas to all, I hope everybody has a good day.

Nick

 

[Maryak]

I was quite chuffed until I looked at the rear ... the chuck jaws have quite badly marked the boss (can't see on this photo), I've dressed with a needle file and it'll do, but I always get this problem, does anybody have any advice concerning this?

Nick,

Two possibilities, one don't tighten the chuck too much and take light cuts, two cut up an aluminium soft drink, (or better beer after emptying in the approved manner),can and wrap a piece around the part to be held in the chuck jaws. Electrical plastic conduit is also good near the same diameter.

that's three :

Hope this helps ??? ???

Best Regards
Bob

 

[SignalFailure]

Quick reply as last minute xmas shopping required (!).....

I wrap a bit of oily paper around stuff to avoid marking it.

Looks like a good start to me Nick.

 

[NickG]

Ah, good ideas chaps, thanks, will definitely try those!

 

[SignalFailure]

I've also used electrical tape and even thin wet and dry paper!

 

[mklotz]

On the matter of the chuck marking the workpiece when clamping on machined surfaces...

Bob has already described the most expedient solution - never throw away an empty soft drink can. The material therein is ideal for all sorts of shimming and protection uses.

Just for completeness sake, I'd like to add two somewhat more complex ways of managing the problem.

With a bit of ingenuity you can cut shapes that will fit over the chuck jaws from light copper sheet. Mine are equipped with tabs that hold them in place even when there is nothing being clamped in the jaws. This technique is particularly helpful when clamping non-cylindrical parts in the 4jaw - especially for those of us who only have two hands.

A collet chuck is the ideal solution to this problem but that can get expensive. However, you can make your own collets which is a particularly good idea if making a number of identical parts. Machine a top-hat shaped piece of aluminum and bore it out to the size of your workpiece. Then slit it lengthwise with a hacksaw or endmill so the chuck jaws can compress it. The "brim" on this impromptu collet will prevent it from sliding back into the chuck jaws.

 

[Metal Mickey]

Nick, for mine I used aluminum and bored it out to suit the boss, like a tube really and then cut 3 slots at 120 degrees at each end to half way ish, with the slots at a different start point. Sounds horrible from here but it is quite simple to make. You can tighten you chuck then as much as you want.

I never throw them in the scrap bin as its amazing how many times you make things around the same sizes.

Mike

 

[NickG]

Thanks guys, these are the practices I really need to start adopting to start producing some good quality work!

 

[NickG]

??? :'( Just spend an hour writing an update and it didn't post, now all work is gone!

 

[NickG]

I can't possibly remember what I wrote and am so annoyed right now but here are the key points!

I have made more progress, finished flywheel, made crankshaft and made cylinder.

Flywheel - changed from 6ba to 8ba for grub screw ... I thought 6 would look too big. I used a hex head bolt as don't think I have a grub screw but it looks OK.

Crankshaft - Simple turning, parting off and drilling for crankpin ... well it should have been. I used the tailstock centre but think this has given it a very slight taper, prob my tailstock isn't aligned properly. Shouldn't cause a problem though. Another annoyance was not being able to get into the work piece enough with the cutting tool to get the square faces. I should really grind up a standard set of tools and put aside associated packing pieces as it would save a lot of time, would love a quick change tool post with holders! Another problem came when parting off, I tried to be clever and set a very slight angle on tool to get a good surface finish on the crankdisc face ... big mistake as it started to chatter on the other side. At which point I got scared and got the hacksaw out! Soldering the crank pin in was OK, however I remembered why I don't like it, it's hard and time consuming to clean components up afterwards. I should have got some loctite or reverted to threading.

Cylinder - This went well by comparison. Marked out brass, squared up and milled to length in my new (well, unused since I bought in summer) chinese milling machine. At this point I realised the handwheel graduations are quite accurate so used those to get coordinates for holes, confirmed by the marking. For the pivot pin I centre drilled, opened up and finished with 1/8" end mill to get a flat bottomed hole. The mill has a DRO on the spindle so used that ... not sure whether it's accurate or not, will have to try measuring! Then flipped the cylinder vertically in vice, got coords, centre drilled, drilled No. 14 then reamed 3/16" at a slow speed with plenty of oil. The port face was then lapped on emery cloth, then with oil, then oil stone, this has always worked well enough for me in the past. Some of you will notice that I did all the drilling work with ER collets, that is because the chuck supplied has a min dia of 5mm! It's huge! Probably a good thing though as the cheap chinese collets (about £30 from ebay for chuck, collets and carriage from china!) kept everything running nice and true.

Here are pics:

Turning crankshaft down to size


Test fit of flywheel


Reversed in chuck to face disc after sawing off!


Finished crankshaft, looks OK.


Set-up for soldering, not good practice but worked for these small components


Finished crankdisc and crankpin assembly after cleaning up


Crankshaft / Flywheel assembly


Set-up in milling machine for drilling holes, had previously squared ends and milled to length


Flipped vertically in vice to centre drill and drill bore


Reaming bore


Finished cylinder after lapping. Countersink on pivot hole was done with no 1 drill by hand for solder to run into. Countersink on port was a mistake after centre drilling too far. THis may cause slight overlap of ports at TDC but don't think it'll affect it much.


Well, I better go, will be in trouble with the wife for writing all this twice! Just got the frame, piston, pivot pin and cyl cover to do ... oh and the base .. I always discount the base and always mess it up, making the rest of the engine look worse!

Hopefully over the coming few days I will bring you the final installments and fingers crossed a running engine!

Thanks for any interest, I am aware this is pretty insignificant by comparisson with a lot of projects on here!

Nick

 

[ksouers]

Thanks for any interest, I am aware this is pretty insignificant by comparisson with a lot of projects on here!

Nick

Nick,

NO project is insignificant. Lots of people are interested in others projects. They do read them. One never knows what bit of insight might arise out of a project. For example, your bit about chamfering the port. I had never considered how that might affect timing before you brought it up. Now I will.

As for the rest of the project, can't wait to see it finished and running. Nice work.


Cheer,
Kevin