Ramon's ETA 15Ds

A member has raised a number of points about the drawing of the crankcase shown on page 159 issue 4474. Most can just be put down to drafting standards, several I saw the drawing a different way and explained to the member how I read them.

The one main item is what has happened to section B-B, this does contain two dimensions that can't be deduced from whats in Fig 2. I know it was on Ramon's original drawings so did it get lost between Greece and MTM towers?

J

Edited By JasonB on 02/03/2014 19:42:01

Ramon has provided the section that was missing from the magazine, here it is.

And I've also included a larger section A-A to show that the 25deg angle runs out to the edge of teh liner bore top edge.

J

Edited By JasonB on 03/03/2014 17:49:23

Hi there,

Following an email from Jason, I've just become aware of this post and the omisssion on the drawing.

This appears to have been an error on my part - the original drawing has it but somehow it has removed itself from the drawing submitted to ME. Though I checked and treble checked the drawings before sending them off my apologies for not spotting it. I'm not making excuses but noticing your own mistakes is difficult especially after long hours at the computrer staring at the same thing over and over. I make no apologies however for the fact that I'm more a machinist than a writer .

When it comes to the drawing per se - I have no background in drafting only that which I've picked up along the way and the use of AutoCad is relatively new too. I do understand about first and second angle projection but as an 'amateur' draftsman understanding and applying it sometimes doesn't quite gel. I hope readers will at least try to appreciate that I am trying my best in presenting what's been done - it would be nice to think that it's perfect but I think you would agree that would be a tall 'ask'.

The one thing I would like to say is that I would like to hear of any anomalies found - that way I can put them right and even send out new drawings should that be neccessary. With the best will in the world mistakes can and do happen - believe me I dont like making them - who does - but will respond to any input.

For those with an interest in the engine I wish you well with it - If you have any problems at all please contact me through PM on here.

I have sent Jason a copy of the corrected drawing which I'm sure he will post on here

Kind regards - Ramon

Hi Ramon/Jason

Thanks for clearing this up when Jason first posted the query I went back into my records and had a good head scratch, I'm not familiar with the working of this type of engine so failed to pick it up when I checked things through, so I guess I contributed to it somewhat.

Thanks Again

Stew

Ah! can I assume from that you were the proof reader Stew

Having checked through the drawings published so far I see I have omitted a dimension on the rotor disc.

The angle between the drive hole and trailng edge of the cut out should be 40 degrees .....

As a well known designer at a place of work would have said ' Whoops, sorry about that'

Ramon

Just to pre-empt anyone asking in part 4 of the series the Mk1 prop driver shown in Fig 6 should show an overall length of 15.8mm. The 19mm and 24mm ends can be assumed to have a 1.2mm length before joining the curved part.

The crankshaft for the Mk2 shown in Fig7, ignore the 16mm radius line shown on te rear of teh web as that relates to another version of these engines

J

Edited By JasonB on 14/03/2014 15:31:49

I think the 2 micron (0.002mm) fit limit for the crankshaft bearings is a bit ambitious. I would have a job to turn to that limit let alone measure it.

I think we should all be grateful that Ramon has given us some tollerances to aim for as that is something that is seldom seen on ME drawings as confirmed in a recent thread.

Where are you seeing the 0.002mm, I can only see 0.003-0.005 shown?

0.003 to 0.005 is 0.002 is it not?

Don't get me wrong i think his motors are brilliant and i may have ago at sometime. I was just thinking it must be a drawing error for a tolerance that tight which would be very difficult to machine too or am I missing something.

0.002mm? That's several wavelengths of light, you should be able to SEE the difference

Neil

Got you now Paul, I thought you were looking at +0.002 written on the drawing not taking the difference between the two. Yes its quite a narrow range and a bit tighter than most ISO limits which would be say 0.006mm to 0.01mm between the max and min on a shaft of that size.

These little engines do need to be made to quite exacting levels if you are to get good performance out of them particularly the fit of cylinder to piston. It's certainly not something you can do with a £8.99 digital calliper from Aldi/Lidl, but a good micrometer will measure down to that. 0.002mm is 0.0001" and my mic(non digital) has 0.0001" divisions though at that small an amount its very dependant on feel on the thimble. As I said above its something to aim for, whether we can hit the size is another matter.

Hopefully Ramon will see this an add his comments.

J

Hi Jason, Paul et al,

Despite that best will in the world errors, ommissions and downright mistakes can and do get made. I put my hands up - I'm not infallible!

I have updated the crankhaft drawing and emailed it to Jason who will hopefully post it here later.

Regarding the tolerances on the crankshaft - Yes I agree they would be too much to expect to be able to turn to - they are after all more realistically a grinding allowance but they are on the drawing as a guide to the kind of fit required for the bearings. If you check the text you will see reference is made to 'polishing' the diameters down in the areas shown so as to give a light hand push fit. The bearings do not want to be over tight on the shaft as this can lead to brinelling and ruined races. I do not have access to cylindrical grinding - all the machining of the engines made so far has been carried out on a Super7 lathe and my quite old Linley milling machine and all this kind of work is done by hand using nothing more than wet and dry carborundum paper stuck to a flat surface (a parallel) with double sided sticky tape. By having the paper stuck to something flat a far greater degree of control is able to be exercised and using parafin as a lubricant prevents the paper loading up though it can lead to the sticky tape gradualy releasing.

Where measuring is concerned I use nothing more than a Mitutoyo 0-25mm mic which reads to 0.001mm on it's vernier scale. Whilst that is somewhat variable at such fine measurement it does never the less give you some indication as to where you are at such fine limits - the rest is then down to 'try and see'.

As Jason says these engines do require close limits particularly as above and as said on the piston and liner fit but all are achievable using basic kit with a degree of forethought and careful approach.

Once again, I hope that's useful, allays some fears and gives a degree of confidence for any prospective builder of these small type of engines if not specifically the Eta's. If I can be be of assistance to anyone then please do PM through this forum or MEM

Regards for now - Ramon

Edited By Ramon Wilson on 16/03/2014 19:22:07

Edited By Ramon Wilson on 16/03/2014 19:23:44

Thanks for the insight Ramon and here is the revised Mk2 Crankshaft drawing, click for a larger view.

Having read the paper version of Part 4 there are a couple of other items to mention.

1. Ramon describes the machining of the prop nuts, these will actually be detailed on drawing/Fig 9 in a forthcomming issue not Fig 6 as stated in the text.

2. The description of cutting the prop driver and collet tapers should read "set the topslide to 10degrees" which will give the included angle of 20deg as shown on the drawing. Not 30 and 60 deg mentioned in the text.

J

Edited By JasonB on 12/04/2014 11:33:42

Hello Ramon, Can i ask what is the Milling Machine you are using and what lathe?

I have a larger bridgeport HMT mill and I am thinking about downsizing. thankyou mark

A late question, of a general nature...

I am ignorant of all aspects of model IC engine design, and would like some enlightenment.I have read the construction articles, and Mr. Wilson has fitted the piston to the liner by lapping to a close fit. In other IC articles, I have read that the piston should be fitted with a clearance, to allow for expansion at working temperature, with rings to take up the clearance. My question is, why is piston expansion in the hot engine not a problem for Mr Wilson's engine? Can someone tell me about when expansion has to be allowed and when not?

I don't hope to attempt to match the machining precision required here, I'm simply interested in the design aspects.

Thanks

Paul

As the Iron piston and steel liner have similar expansion rates there is no need to allow clearance.

However on an engine with say a iron liner and aluminium piston the piston will want to expand at a far greater rate than the liner so you need to allow for this and to take up the gap when cold piston rings are used.

J

The ETA 15D is a diesel engine which require a very high compression- maybe 15/1. This cannot be obtained with rings fitted due to leakage. The piston must be lapped to fit the cylinder. Glowplug and petrol engine work at a much lower compression and are made with ringed aluminium pistons or can be unringed. Lapping is a black art and you will find that many modellers have had to make two three attempts before achieving desired compresssion. Rings are not too difficult to make with fine grained cast iron but require heat treatment to remove brittleness and to give some springiness.

Thanks, Jason.................however I would have thought that the piston would run hotter than the liner, so there would still be an expansion problem? but see below...

Thanks, Thomas, for your explanation. I guess that the requirement for a close lapped fit in a diesel engine is why the piston has to be of iron and not aluminium? And evidently the expansion of the hot piston is not a problem, since these engines do run without seizing.

If there are any further comments I'd be interested to read them.

Paul

The steel liner/iron piston does not have to be limited to the compression diesel engines, I have done a glow engine in the same way and that runs fine.

The way that the pistons are lapped by starting from the bottom of the liner and gradually working them in does tend to produce the slightest of tapers so the piston is not as tight over the whole of its stroke which does ease the fit. The fact the cooler air/fuel mix is passing through the crankcase and around the bottom of the piston also has a slight cooling effect that you would not get on say a small 4-stroke where the mix enters through the head.

J

PS I'll see if I can get Ramon to add a comment as he does not visit here very often.