Author Topic: Nemet Bobcat  (Read 40398 times)

Offline doubletop

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Nemet Bobcat
« on: April 14, 2013, 08:11:32 AM »
Iíve been away a while playing trains but have now made a start on the Nemet Bobcat 15cc 2 cylinder 4 stroke I/C engine serialized in ME from August 2012. Iíve never made an I/C engine before but this is suitable for beginners so I thought Iíd give it a go. The construction articles are pretty well written and work through the fabrication of the parts with words and pictures. Just a case of following along really.

Cut to the chase and I got stuck into the cylinders, liners, crankcase, and nose, timing gears cover and in about 6 weeks got to here.





At this point the next thing was the crankshaft. I started on the crankshaft last 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 thought of fabricating the crankshaft with 12mm and 10mm silver steel and silver soldering it together. I realised 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 made some sort of jig for the soldering I should be able to keep it all in alignment. At least better than the piece of EN8 I purchased.
So I gave it a go and its come out OK so hereís my words and pictures for anybody wanting to do a fabricated crankshaft.

I used some of the off cut of the EN8 bar Iíd purchased to make 3 crank webs, 12mm silver steel for the main shaft, 10mm silver steel for the crank journals (ignoring the 10.5mm larger dimension on the drawings) and another piece of 10mm silver steel for the rear bearing journals.

In all a simple mill, drill job for the three webs and turning job for the three shafts.



The webs are both drilled/reamed 10mm for the main shaft and rear bearing. For the crank journals the two outer webs drilled/reamed 8mm and the inner 10mm.

Loosely fitted together.



The next question was shall I silver solder and risk distortion with the heat, and how would I jig the assembly, or shall I Loctite and pin. I decided on the later.

To be able to hold the job and add the curved top edge to the webs I drilled an extra 3mm hole in the middle web. To enable a through shaft to be used in the lathe



The lower bar is a temporary alignment bar to ensure the outer webs are properly aligned.

With the crank journal and temporary main shaft Loctite the two outer webs with the inner web flipped out of the way and clamp up to set.



Flip down the middle web Loctite and position and clamp to set.



When set drill and pin the three crankshaft webs with 2mm pins





The pins will be tidied up later

Modify the temporary main-shaft with a 3mm section in the middle



Reassemble



Drill and bin the temporary shaft to the main-shaft web 1.6mm. This will be removed later.



Turn the radius on the top of the webs



Remove the pin and temporary main-shaft ready to install the main-shaft and rear bearing journal. At this point I had been wondering what Iíd use as a jig to align the main-shaft and rear journal while the Loctite set. Then I realized I already had a jig.



Just donít get Loctite everywhere.

Now a case of pining the main-shaft and rear bearing journal with 2mm pins, trimming and filling the pins flush and drilling and tapping for the counter balance weights.



Hopefully that will be useful to somebody, I'm far happier with this version than I think I'd have been with the solid version. It took me the weekend to make, I feel I'd still be out there if I was doing the one piece.

Pete
« Last Edit: July 16, 2017, 08:45:36 AM by doubletop »
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline sbwhart

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Re: Nemet Bobcat
« Reply #1 on: April 14, 2013, 08:33:37 AM »
Wow:- you're making a nice job of this Pete:-  crank shaft came out well  :ThumbsUp:

Stew
A little bit of clearance never got in the way

Offline ths

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Re: Nemet Bobcat
« Reply #2 on: April 14, 2013, 08:50:07 AM »
I take it that the crankcase has been excavated on the mill. Good work on the crankshaft.

Hugh.

Offline mikegw1961

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Re: Nemet Bobcat
« Reply #3 on: April 14, 2013, 09:09:10 AM »
Peter

Nice job on the crankshaft. I am making the Nemett Lynx at the moment and have looked at the Bobcat and wondered if it would be possible to fabricate the crankshaft.

Thanks for the instructions and pictures, I now know its possible

Mike

Offline Jasonb

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Re: Nemet Bobcat
« Reply #4 on: April 14, 2013, 09:10:41 AM »
How will the crank be located endways? you don't seem to have the 14mm dia x 1mm long section to locate the webs away from the bearings.

What do the bearing surfaces clock like when you put it up between centres? I think I would have been tempted to leave the diameters a fraction oversize and then skim them down after assemble with teh crank between ctrs.

J

Offline doubletop

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Re: Nemet Bobcat
« Reply #5 on: April 14, 2013, 10:20:56 AM »
Thanks Stew I'm quite pleased with the way the whole job is going. Generally I getting to grips with 'fits' everything goes together with a slight push, nothing too sloppy.

Hugh - you've noticed; to be honest I couldn't be bothered with sitting there hour after hour using the DRO 'pocket' function in MNC mode. So I subbed it out to a mate who has CNC'd his SX3. It took him less than 1/2hour to draw up the cavity and then the best part of 5 hours while the machine ground it out and we had a cup of tea and a chat. If laser cut frames are allowed then getting one of the team to help out the boring bits is also OK in my book.

Jason - I had my money on you spotting that. I'm going to turn up two 1mm thick bronze washers 1x 10mm and 1x 12mm ID. I'd forgoten to add it to the write up. It simplifies the construction and would suggest its the way to go even when doing the solid crankshaft. There's then a bit of scope for adjustment, if its required.

I grappled with how I was going to put it together in alignment and what I've done seemed to work out OK. I had played about with the V blocks but the only common diameter is the two 5mm long bearing surfaces and it didn't seem to work (thought; is packing a V block up to adjust for different diameters workable? I'd need to work out the geometry and confirm) I hadn't thought of clocking it up as you suggest, I'll give it a go tomorrow. But as I said to Stew the fits are pretty much spot on so using the job as a jig has probably turned out better than making a jig.

regards all

Pete

?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline swilliams

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Re: Nemet Bobcat
« Reply #6 on: April 14, 2013, 11:07:38 AM »
I like it Pete and it's looking great.

Will be following along.

Steve

Offline vcutajar

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Re: Nemet Bobcat
« Reply #7 on: April 14, 2013, 11:24:40 AM »
Will be following your progress Pete as it looks like it is an interesting project.

Just an idea from a newb :- if hot rolled steel is used to machine out a crankshaft, would this avoid the tendency of bending whilst machining?

Vince

Offline John S

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Re: Nemet Bobcat
« Reply #8 on: April 14, 2013, 11:32:25 AM »
Weird looking crank, what does the central web do ?
Could it have been left out ? Sorry just asking as not read the words and music. I suppose I should.
John Stevenson, Nottingham , England

Offline steamer

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Re: Nemet Bobcat
« Reply #9 on: April 14, 2013, 03:14:59 PM »
Nice build Pete!....
Dave
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Damned ijjit!

Offline Don1966

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Re: Nemet Bobcat
« Reply #10 on: April 14, 2013, 03:24:53 PM »
Looking good Pete, I will be following along as well.

Don

Offline Bearcar1

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Re: Nemet Bobcat
« Reply #11 on: April 14, 2013, 03:26:51 PM »
You go Pete  :ThumbsUp:  A well documented progression of how you fabricated the crank with lots of good photo images to back it up, well done. This looks like it will turn into a very attractive engine. I can't wait to see what is coming next.


and five hours for a spot of tea? .....c'mon now......  :ROFL: :ROFL:




BC1
Jim

Offline zeeprogrammer

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Re: Nemet Bobcat
« Reply #12 on: April 14, 2013, 04:57:55 PM »
Very interesting Pete. Education helpful too. I like seeing crank work and that came out great.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline John S

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Re: Nemet Bobcat
« Reply #13 on: April 14, 2013, 06:08:22 PM »
OK looked at the drawings and answered my own question.
The web in the centre is to carry a balance weight, 3 are fitted in total, one on each web.
John Stevenson, Nottingham , England

Offline Jasonb

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Re: Nemet Bobcat
« Reply #14 on: April 14, 2013, 06:34:30 PM »
It could have been done with just the two webs but they would have needed larger weights so either the engine would get longer or need a bigger crankcase, having the three just uses up the space between the cylinders.

J

 

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