Nalon Viper 2.5cc CI Engine

[Ramon]

Hello Peterha

I always cut the slits with a standard hacksaw (or a junior one if very small - I have made them as small as 1/8 diameter but just the one slot in that case) On a couple of larger diameter ones made I cut three slots but most at two at 90 degrees by eye.

I was shown this tip many many years ago and the person who passed it to me turned the 60 degrees on a caphead so I've done it ever since. Some use a standard countersunk screw but though it works the angle is too obtuse in my view - the 60 degrees allows a very secure grip with little torque on the screw. I've done this as small as 8BA and the largest has been 8mm.

Some time back I bought a very nice set of mandrels at a show. Well made and very nicely finished - I've never used them! - mainly because they never seem to match in diameter to the work piece. They are so easy to make oneself, obviously the thread needs clearing out after slitting and once turned to required diameter hold a work piece extremely well and are always as accurate as the lathe itself.

I very rarely reuse a mandrel a second time for it's given turned diameter but recycle them downwards re-turning to new use each time. I roughed out quite a few from bar ends when at work but am now finally on the last one

Hope that's of use Peter

Ramon (Tug)

 

[mu38&Bg#]

The idea is to be as close as you can in size, so that expanding the mandrel only adds pressure, not distortion. In this case a small offset of the slots shouldn't matter. If the mandrel has to bend to make contact with the part, we would expect non uniform segments to bend unequally as well as make point contact on the part. Preloading the arbor prior to cutting (to allow some retraction) is typical to assist in removal of the parts and allow for some tolerance (if you've looking at expanding 5C collets for instance). When engagement length is short, clearance isn't really an issue, but tolerance may still be a concern if making many parts.

 

[Ramon]

Agreed - if you are making your own the expanding area needs only to be sufficient to provide the grip. The rest of the mandrel should be as close a tolerance as you can get and particularly so when doing the front bearing housings either on a case or a separate crankcase front - the plain part provides the location - the expanding part the drive. I've never felt the need to expand the mandrel slightly before final turning just bringing the screw up until it touches is sufficient.

If the mandrel is going to be used for support when porting is drilled or milled through a liner for instance, due clearance needs to be milled into the mandrel to allow for any burrs created as the part is removed from the mandrel.

Tug

 

[pat_pending]

Hi all,

I managed to get a bit of shed time this weekend so thought I'd share progress. I tackled the crankshafts and, after a few 100g of additional weight added to my scrap box, I'm pretty happy with the end result. Thanks for all the previous suggestion on this thread which helped a great deal. I picked out the tips and tricks that matched my skills and tooling.

Having run out of high-tensile bolts (my usual go-to for crankshaft steel stock) I ended up buying some EN24T off eBay and must say it made such a difference. Easier to machine, less showers of red-hot metal burning my arms when trying to machine away the threads and probably healthier as Im not sure breathing in vaporised galv coating is such a good idea! Anyways, I'm a convert.

I made one according to the plans which stated cutting out a crescent in the web as counterweight. After doing this I checked and the counterweight specified didn't even cancel out the wight of the crank pin let alone any attached piston and conrod. In the end i decided to make the second crank with a simple cutaway, This seems to counter the weight of the pin much better. We will see if it makes a difference in the running engines.

So heres how I made em..

Turn down to 1mm oversize (11mm) with a lice centre and thread the final bit M6.




Stick in mill and mark out the crank throw using DRO.

Mount in a piece of aluminium faced and reamed hole for the oversized crank journal.

use a wiggler to locate on the crank pin.

Tun the crank pin and polish for fit into reamed 5mm hole.

Cut away the crank web counterbalance.

Finish between centres and polish for a firm push fit for the bearings. This is suck a nifty way of doing it as you can take the crank out by simply moving the tailstock, checking bearing fit, slapping back in. Saves a load of time and ensures its running true. Thanks to Jason and Tug for the tip.

The 10 picture limit makes 3 solid days in the workshop seem unimpressive but I reckon I have some cranks that will do the job v nicely. Next up, I'll try to turn my attention to the front housing and conrods.

Thanks,

Patrick

 

[pat_pending]

And on the topic of crank web cut-outs, i was wondering what people's 'go-to' method for doing them is. This is my first time attempting it so I spent a good bit of time reading the piece on modelenginenews which details a number of different approaches.

http://modelenginenews.org/techniques/crankshafts.html
I went with what I thought was the simplest. Firstly do a quick sketch on Fusion360:

From this i figured out how high the pin needed to be held above the vice if the crank journal was resting on the top face.

Mill the top. Flip round to the other side and do exactly the same depth from there.

This is the result pre finishing between centres and polishing etc. Most importantly the counterweight compensates the crank pin (which it didn't seem to using the crescent specified in the plans. Perhaps i didn't remove enough material.).

 

[methuselah1]

And on the topic of crank web cut-outs, i was wondering what people's 'go-to' method for doing them is. This is my first time attempting it so I spent a good bit of time reading the piece on modelenginenews which details a number of different approaches.

http://modelenginenews.org/techniques/crankshafts.html
I went with what I thought was the simplest. Firstly do a quick sketch on Fusion360:

View attachment 123115

From this i figured out how high the pin needed to be held above the vice if the crank journal was resting on the top face.

View attachment 123116

Mill the top. Flip round to the other side and do exactly the same depth from there.

View attachment 123117

View attachment 123118

This is the result pre finishing between centres and polishing etc. Most importantly the counterweight compensates the crank pin (which it didn't seem to using the crescent specified in the plans. Perhaps i didn't remove enough material.).
View attachment 123119

You know, I''d dearly like to make one of these, and others, but although I'd have gladly paid for the Motor Boys' disc, unfortunately I only discovered M.E.N. after Ron had died, so there was no-one TO pay.

I have enjoyed great support from this group; Gail Graham came to my rescue with "the book" (as well as help with my "Lobo Pup") never mind Les Stone's help with an FTPT and other, very kind members; I am at a loss as to how to obtain the disc designs. Would the remaining Motor Boys allow limited distribution? Does anyone have one to sell? Please help- I cannot be the only one in this situation!

-Andrew Curl (UK)

 

[IanN]

And on the topic of crank web cut-outs, i was wondering what people's 'go-to' method for doing them is.
:

Hi Patrick,

Once you have dimensioned drawings to work from, my preferred method is to:

1) Mark out the two lines on the crank web

2) Hacksaw along lines

3) Tidy with a file

All the best,
Ian

 

[Jasonb]

Andrew if you want the Viper drawings they can be found on Adrian's web site, link at the bottom of this page
https://adriansmodelaeroengines.com/catalog/product.php?cat_id=33&pid=155

 

[Ramon]

Andrew

A link to the book (disc) designs for free down load are actually on here courtesy of Ken Croft, one of the engine boys.

A quick google found this Go to RCLibrary : Motor Boys International Model Engine Plan Book title : download free vintage model aircraft title

That should help

Pat - there are several ways of relieving the crank web - don't know if you saw this on here
Super Tigre G32 1cc diesel - a 5cc version

Scroll down about halfway on page five and into page 6

Ramon (Tug)

 

[methuselah1]

Jasonb, Ramon, thank you!

 

[pat_pending]

Hi all, I managed to get a few hours in the shed this week and got the front housings done for both engines. I'm pretty happy with the result. Making 2 engines I have ended up putting all my 'first try' pieces on one and all the better bits on engine 2. Still both looking OK. Something I'm not entirely happy with is the matching of the shape of the chamfers to those of the crank case. I think there must be a better way of doing it e.g making a larger bit of aluminium in one piece and then machining crank case, housing and back plate from them thus guaranteeing the mating shape (provided all the other operations are accurate. One to try for the next engine.

Bit of a tidy-up in the workshop then I reckon I'll have a pop at making some conrods.

Thanks,
Patrick

Bolted together and the holes matched PERFECTLY . I think centering the mill on the bore and measuring from there seems to work much better than my previous attempts of measuring from an edge. Thanks Tug for the tip.

First I squared the stock and made some plug gauges (approx .025mm under size vs. the bearings).

I went for a test fit in a piece of scrap aluminium before risking anything on a machined part. The rear bearing slid in really easily when the test piece was heated to about 250c with the heat gun (not too sure the temperature gauge on those is that acurate tho ). Once cooled, the bearing seemed to rotate nicely without any rub etc so I decided to go ahead with this fit.

Next i centre marked the aluminium blanks on the mill using the DRO and centred it on the 4jaw in the lathe.

Next the rear bearing seat was bored and the through hole reamed.

Next the part was rotated and centred using the indicator on the hole. Crossing my fingers this will be accurate enough. We will see. The front bearing seat was then bored. Next up I made a fixture to hold the par for profiling.

Then profiled the front housing outers:

 

[Ramon]

Good progress Patrick but if you get the front and rear bearing in line concentrically and parallel with each other on a four jaw set up you will be a lucky. I'm not saying it can't be done but there is no way of knowing exactly where the rear housing is in a four jaw set up. For the tolerances required for a twin bearing front end it does have to be exact. The crankshaft with rear bearing in place should slide straight in to the front bearing once fitted. Does not have to be out by much to introduce running issues.

Much better to fit the part to an expanding mandrel made to a good fit in the rear bearing housing and expanding in the shaft hole for grip. Done all my engines like that and not had misalignment.

Hope you are lucky though - fingers crossed for you

Tug

 

[pat_pending]

Good progress Patrick but if you get the front and rear bearing in line concentrically and parallel with each other on a four jaw set up you will be a lucky. I'm not saying it can't be done but there is no way of knowing exactly where the rear housing is in a four jaw set up. For the tolerances required for a twin bearing front end it does have to be exact. The crankshaft with rear bearing in place should slide straight in to the front bearing once fitted. Does not have to be out by much to introduce running issues.

Much better to fit the part to an expanding mandrel made to a good fit in the rear bearing housing and expanding in the shaft hole for grip. Done all my engines like that and not had misalignment.

Hope you are lucky though - fingers crossed for you

Tug

Bugger. Wish I'd thought of that and makes sense now you mention it. Ill try and do a test fit tomorrow. 10hrs work potentially in the scrap bin.... i guess thats how you learn + makes the end result that much more satisfying.

P

 

[Ramon]

Well it's not as bad as you might think Patrick and yes you can learn from it - not only the mistake but how to rectify it.

Fit the front bearing and try the shaft and rear bearing - if theres any form of misalignment don't force it just remove the bearings. Turn a ring of ally say .5mm bigger than the front housing diameter with an inner diameter smaller than the bearing intended

Make a mandrel as suggested and hold the front housing by it then bore out the front bearing housing to take the ring - Loctite that in place then turn to finish size. If you use a retaining compound and you get the fits right you should be able to turn the inner surface within 20 mins or so.
Not ideal on a new build but It works - reclaimed several commercial engines doing this - both front and rear bearings.

Give it a go - you've nothing to lose

Tug

 

[Peter Twissell]

Re. Alignment of chamfers, I will either machine the chamfers on the parts after they are otherwise finished and bolted together, or make the chamfers on the end parts a bit smaller than those on the crankcase, so there is a clear intentional difference.

 

[pat_pending]

OK, so it seems lucky on one front housing and the other seems tight when i insert the shaft into the bearings. Close but no cigar.

I will put the working front housing on my #1 engine (with all the other iffy bits) and try to make the other one perfectly using an expanding mandrel.

Now if only squaring a piece of stock to size before i even get to machining wouldn't take me an hour!

 

[Ramon]

Morning Patrick, now off for a day in the garden for a change but before I go ........

The mandrel wants to fit the rear bearing housing as a plug gauge would - the front portion of the gauge is the part that does the gripping - a 2 BA/4mm cap head would be fine for the expanding screw. The part is pushed fully back to seat against the front face of the mandrel

I prefer this way as all machining is done with reference to the rear bearing housing itself.

It can be done by holding the outer diameter of the rear bearing housing in a clamp plate bolted to the faceplate but again you are not referencing the bearing housing directly. The mandrel method is as good as your lathe is and the fit of the part to the major diameter. Once it's out of the lathe it can only be used (to the same degree of accuracy) for some thing smaller

I should add it's important that the rear housing and shaft clearance in the part are done from the same end and at the same time

Hope that helps some - Tug

 

[pat_pending]

Thanks Tug.Those pics are very helpful. Will get making one asap.

 

[Ramon]

This might be of use to others Pat so repeating it here.

Hi Pat - just come in for a coffee!

Make the ring first, say .5 mm larger than the housing and about 2mm wall thickness, face one end square to the bore and chamfer the outer diameter. Set the part on the mandrel and bore until the ring slips in easily - about .03/.05 larger than the ring - no more. Do this gingerly as you do not want to have the two surfaces gall. Use the 638 and Loctite both parts. Hold the ring against the front face of the tailstock drill chuck and use that to push the ring in square and to the bottom. Don't go for a tighter interference - the Loctite could go off before you are fully home and you do not want to use force in any way in case you disturb the mandrel. Let it cure for a bit - if the fit is as above it should be machinable within 20/30 mins but no sooner. Take very light cuts and take a very fine final cut off the rear face to ensure it's square. I wouldn't fit the bearing for 24 hours so the Loctite can really cure

Despite what it says on the product information Loctite has a very long shelf life so if it doesn't go off you've got too big a gap!

Good luck - Tug

 

[KenC]

OK, so it seems lucky on one front housing and the other seems tight when i insert the shaft into the bearings. Close but no cigar.

I will put the working front housing on my #1 engine (with all the other iffy bits) and try to make the other one perfectly using an expanding mandrel.

Now if only squaring a piece of stock to size before i even get to machining wouldn't take me an hour!

View attachment 123303

Have you seen the video by Joe Pie... on squaring blocks on the mill.
I found it helpful and I always do it this way.
Ken