Constructing the Nemett Bobcat

[JasonBModerator @jasonb]

Actually I can't see in the text that it says tap M3 it just says the holes in the cylinder will be drilled later or are you reading ahead of us David. I assume the holes will be spotted through from the heads for those that use that method.

EDIT forget that I have seen they were spotted in an earlier installment and tapped M3

J

Edited By JasonB on 19/12/2012 17:54:01

[David Clark 13Participant @davidclark13]

Hi Kwil

The designer (Malcolm Stride) wrote the text as well as did the drawings.

regards david

[Martin-tygParticipant @martin-tyg]

Gentlemen,

interesting comments re drawing accuracy. While not being a mechanical engineer, I do read quality mechanical drawings on a daily basis. The Bobcat drawings are, at very least, adequate. Given some study, there is no reason to go wrong. If YOU actually produce a set of drawings for something as complex as the Bobcat appreciation of the complexity will soon arrive. We, as amateurs, are probably working outside our professional area, but apply our technical skills to both manufacturing the parts and interpretation of the drawings.

we should be grateful that Mr Stride took the trouble to publish his drawings and not complain about trivia.

Martin

[KWILParticipant @kwil]

I am sorry that Martin considers such matters as trivia.

Inspite of the fact that Malcolm Stride wrote the text as well as the drawings, errors exist and we should comment on these as they arise for the guidance of others who may well have lesser skills than Martin. I for one am not complaining, merely highlighting what may be a problem.

We can all be guilty of creating errors when we write or draw and it is not easy to edit/review your own work, because you "see" what you expect to see and not what is there in black and white. Peer review or Editing by others is always the safest way, in the past there were professional readers whose job it was to try to eliminate faults.

When drawings are published bit by bit, it is not always easy to take an overall view or perform a review of the parts and the necessary interfaces that will arise during the construction of the "model".

[Martin-tygParticipant @martin-tyg]

I do not really want to continue this conversation on this forum, and I do not want to "have the last word", so lets leave it there.

best regards

Martin

[doubletopParticipant @doubletop]

Now the series is complete and I've read and re-read I’ve decided to give the Bobcat a go. I’m waiting for my "suitable aluminum block" to arrive from my ally emporium and then I'm off. This will be my first I/C engine.

At risk of re-igniting the perennial debate, and I'm not condoning errors, but you know the drawings and text are going to be wrong somewhere so it's a case of live with it and pay attention. Surely it all contributes to the intellectual challenge.

Pete

[John Stevenson 1Participant @johnstevenson1]

My only complaint with this engine is that you need the text and drawings.

You can't work from drawings alone unless you have some experiance because none of the drawings tell you what material and quantities needed.

These are all in the text or gleaned from photographs. 3 pages in you get a Spec list but no bill of materials.

The drawings have the usual text box at the bottom right telling you scale, drawing number etc but not material and quantity .

These have been edited by MHS because they have included thier copyrite legend so not all the blame can be laid on Malcolm Stride

[JasonBModerator @jasonb]

The other thing to bear in mind is that if Malcom had decided to sell the Bobcat drawings like he did those for the Lynx he would likely have included this info on his drawings and spec. This is one reason why a few details were left out of the Lynx articles so you had to buy the drawings to be able to build the engine.

MHS just took the easiest route and printed what they had available.

Good luck with the build Pete, keep us upto date and post some phots

 

J

Edited By JasonB on 20/02/2013 09:52:42

[KWILParticipant @kwil]

Well said and very correct JS

[David Clark 13Participant @davidclark13]

Hi John

All I did was change the copyright to MyHobbyStore Ltd.

Nothing else was changed.

Drawings are untouched.

The same with the printed book, all I did was change the copyright and I think I removed some blank pages.

It took about 2 years to purchase these from the solicitors so they did not get lost for ever.

regards David

[KWILParticipant @kwil]

Some idea of progress to date. Cams in embryo form, first polishing prior to separation and hardening. Whole box of piece parts as well, rockers, rest of gears, rocker shafts and stands, cam followers etc.

Edited By KWIL on 20/02/2013 11:20:53

[Anonymous]

Looks excellent, but then I wouldn't expect anything less!

Regards,

Andrew

[doubletopParticipant @doubletop]

Not major; Parts 16 & 17 Cylinder Jackets "……..Turn the outside to 38mm diameter for approximately 40mm" should be "……..Turn the outside to 38mm diameter for approximately 25mm". Otherwise you won't be able to make the mounting flange 40mm dia as it starts only 34.3mm from the cylinder head end.

Pete

[Neil CraigsParticipant @neilcraigs43644]

Now trying to start the beast but its not playing fair at the moment get a few pops out of the carb but will not fire consitant I am thinking about using the machined inlet manifold just to is if the shorter distance to the cylinder head will make any difference the pipe manifold is 22mm long and the machined one is 15mm long

[doubletopParticipant @doubletop]

Thanks Neil a timely post. I was doing the cylinder heads today and couldn't work out why the flange on the liner was 1mm and the rebate on the head was 0.5mm. There is supposed to be a gap. Somit looks like I'll have 11mm heads or no gap

Pete

[JasonBModerator @jasonb]

Its a feature you see quite often on IC engines. Makes sure that the head seals against the liner, the Firefly is the same.

You could probably get away with skimming 0.2mm off the head rather than the full 0.5mm

J

Edited By JasonB on 16/03/2013 07:49:42

[doubletopParticipant @doubletop]

Jason;

I can see that now thanks. this is a first for me so I'm new to this sort of detail. A 0.2mm skim it will be.

So far I've done the carb, cylinder liners, cylinders and I'm halfway through the heads. The rational for doing the carb first so I had it when I needed it rather than rush through it at the end in the eagerness to get the engine running. A bit like buiding the tender first when doing a loco.

Pete

[doubletopParticipant @doubletop]

I've made a bit of progress over the past week or so. finished the cylider heads, crankcase, crankcase nose, sump, timing baseplate timing case.

now just a case of making the bits and bobs to go inside

Pete

[doubletopParticipant @doubletop]

I started on the crankshaft this weekend and wish I hadn't bothered. I'd purchased an expensive piece of EN8 'key steel' that was 15mm x 30mm and set about fly cutting it down to 12mm. I've now got an expensive EN8 banana that's less than 12mm in the middle. I now realise that I maybe should have skimmed it on both sides or even heated it to red heat and let it cool slowly before I started. I could go and buy another piece but I'd be worried that in the machining it would bend even more.

So I'm thinking of fabricating the crankshaft with 12mm and 10mm silver steel and silver soldering it together. I realise that the throw is only 10mm and but by putting a 10mm shoulder on the bearing ends and the big end journals are 8mm that's 1mm to play with. If I make some sort of jig for the soldering I should be able to keep it all in alignment. At least better that the piece of EN8 I purchased.

Any thoughts?

Pete

Edited By Doubletop on 07/04/2013 08:55:20

Edited By Doubletop on 07/04/2013 08:57:39

[Sub MandrelParticipant @submandrel]

Normalising should solve your banana problems, just make sure it cools slowly enough to avoid local hardening.

Silver steel is no tougher trhan mild steel, so why not use cheaper, easier to machine precision ground mild steel?

When you jig up the crankshaft, don't cut away the main shaft betweenm the throws until after soldering.

Neil

[doubletopParticipant @doubletop]

Neil

This weekend my confidence got knocked a bit, after the crankshaft disaster I started on the cam blanks and that didn't go well either, although I did recover them in the end, but two days to make 4 cam blanks, not good. I'm just concerned that I could get another expensive disaster if I went for more EN8.

I have a length of 12mm and 10mm silver steel and with some of the off-cuts of the EN8 to make the webs I can give the fabricated crank go before I can get to the steel supplier. I had thought of running through the main shaft but it can't be done as the thicker part of the big end journals is 10.5mm, although I'm going to make then 10mm, and the rear bearing is 10mm so there would be 2 x 10mm holes in the middle web 10mm apart. e.g no spacing between them.

Thinking about it a bit more pinning the joints would hold it all in place for the soldering. Then extending that thought pinning the joints a loctiting it all together wouldn't present any heat problems. The cams are loctited so why not extend the technique to the whole job?

Pete

[JasonBModerator @jasonb]

I'd make the crank from EN8 round stock and machine the webs rectangular as one of the last jobs as its unlikely to distort them.

If you do go for a built up crank then I would solder then pin once its soldered.

J

[doubletopParticipant @doubletop]

Jason

I had thought of using round stock but didn't think I had anything suitable. But since you have suggested it checked the diagonal dimension of the webs and they are 26.39mm. I have some 26 mm EN36A and doing a quick dimensioned sketch in Visio it would just about do. The question would be suitability of EN36 for the job ignoring its normal use for case hardening?

Up until now I've just had 'stuff'' in stock in the general material categories, (steel, silver steel, brass, copper, aluminium, bronze etc) to make things with. The first time I go out of my way to get the specific material it lets me down, but it just happens this bar has EN36A stamped on the end of it.

Why pin after soldering?

Pete

[Sub MandrelParticipant @submandrel]

Pete,

I'm waiting for a 4 1/2" length of bright drawn MS.

Hopefully this will become the crankshaft for my Suffolk 4-stroke.

Neil

[Sub MandrelParticipant @submandrel]

Opps… should have added 'to cool down after heating to red heat" somewhere in there

Neil.

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