ETW's Wyvern Gas Engine

[Jo]

Rod,

Whilst I agree with John on the need for Datums, deciding what and where to place them on castings can be challenging. The problem is that whilst the dimensions of the casting should be right, they rarely are. There are lots of things that cause this: shrinkage due to the shapes of the pattern can drag the cast metal, worn patterns cause everything to be undersized, over enthusiastic people with angle grinders at the foundry.... .

So you need to find a reference that enables you to get the part out of the casting, then you work from that to identify a suitable datum for further work. Aim to get the bits that need machining out of the metal, you can always build up the cosmetic features using a filler or something. In the worse cases don't be frightened of altering measurements but make sure if you do that you also adjust their mating parts. 

Jo

[tangler]

John, Jo,

Thanks for the comments.  Castings are indeed, tricky chaps.

The important thing with the bed plate seems to be to get the axis of the main bearings at right angles to the cylinder axis and at the same centre height.  I've already assured myself that I can get the drawing dimensions out of the casting "envelope".  Fitting the bearings caps is really just adding to the casting, it is only then that the real machining starts.  The plan is to machine the front bearing face for the cylinder jacket and then to use this as the datum to set up boring the main bearings at a right angle.  I'm not sure how I'm going to do this yet.  If it all fits I may use the mill as a horizontal borer but otherwise there are a couple of other options.

cheers,

Rod

[b.lindsey]

Rod,

I looked at the website to get a feel for what the final engine will look like....very nice I must say.  John and Jo both make good points as to castings but you seem to have a bead oh how to proceed and I will certainly enjoy following along on the journey.

Bill

[Bogstandard]

Rod,

No problems.

I just wanted to let you and others know that sometimes life isn't as easy as it seems if you want a good result.

BTW, Jo raised a very good point about over fettling in the foundry. On my last set of castings from Hemingway, after almost completing the engine, I had to get a friend build up the front of the main upright cylinder casting by 1/4" with ali weld, so that the cylinder head would fit in the correct position. They certainly had a gorilla on duty that day.

I always prefer to have no fettling done at all, at least then, it is only your fault that causes the problem.


John

[tangler]

Making and fitting the bearing caps.  They come as a cast stick of 2 so I separated them with a hacksaw (this could probably have waited till after the next stage).



Then a light skim across the bottom



Each cap was then snugged down on to a length of gauge plate (ground flat stock), acting as a parallel, in the milling vice.  A best guess was made as to where in the casting the centre is located and the DRO zeroed





holes were spotted with a centre drill at 7/8" centres using the DRO and then spot faced with a 1/4" slot drill



centre spots drilled out to 4BA clearance on the pillar drill (I feel more comfortable doing this with small drills than on the mill - the downside of not having a quill)





Now to drill the holes in the bed plate.  Mounted on the tilting table and levelled (if I dare use that jargon ) using a DTI



I used the DRO to put spots at the correct spacing



Although it's inconvenient to move the job to the pillar dril to enlarge the centre spots to 4BA tapping...



at least it's close to the tapping guide!



Temporary attachment of the bearing caps to the bed plate, ready to be trued up with the rest of the casting



I'm pretty comfortable that the caps are where they should be relative to the base and front of the bed plate.  The picture demonstrates what John and Jo mentioned earlier: the casting isn't as symmetrical and accurate as one might hope so the nearer cap looks misplaced.  A bit of work with a file will sort that out prior to painting.

[tangler]

Apologies folks, I managed to take an afternoon's worth of pictures with no card in the camera



I did use the mill in horizontal mode, there was just room to get the job and my boring head in the Y travel.  I milled the front face where the cylinder locates and then zeroed the table with the horizontal axis.  I was then able to lower the table to give the 1 3/8" height to the cylinder axis.  There wasn't room to get a drill and chuck in front of the casting so I started the hole off with a 1/2" slot drill and then opened it out to 5/8" with the boring head. 



The casting was rotated through 90 degrees and set up absolutely square with the aid of DTI on the newly machined front face.  Using the DRO I was then able to bore the main bearing hole through the casting and cap at exactly cylinder centre height and 4" from the front face as per drawing.  I also faced the mounting for the cam shaft bearing on the nearside main bearing support.  The casting was rotated through 180 degrees and the procedure was repeated with the boring head at the same offset for the other bearing hole.



Finally, with the mill back in vertical mode, the caps were tidied up, oiler mounts faced and the hold down bolt pads spot faced.

The seatings for the bronze bearings still need to be machined but I want to wait until the crankshaft is made before finalising the gap between the bearings and fitting the bronze bushes.

[tangler]

The cylinder liner locates the cylinder jacket assembly in the hole in the bed plate so I've tackled this next.  The liner is made from a length of 40mm OD cast iron.  I decided to use the 4 jaw SC chuck to give good support as there is 4" overhang.  I parted off to length (leaving enough to make the piston rings) and used a left hand tool to turn a bit truly round for mounting in the chuck for turning the OD and the bore.



Turning the OD was straight forward and there is a 1/16" flange at the headstock end that will locate in the cylinder head



I drilled a pilot hole with a 5/16" drill and followed this up with my favourite "opener out", a 3/4" drill with a 2MT shank



I followed this up with my biggest drill, a 1" blacksmith with 1/2" shank



Boring came next.  The hole needs to be about 4" deep.  So I set up as shown to give as much support to everything as I can.  I used my Gibralter toolpost (courtesy of Tubal Cain and Hemingway) and set the fixed steady as close to the cross slide as I could.



 I have a choice of 3 boring bars that can reach in 4".  The one on the left has a TC tip.  It chattered horribly at all feeds and speeds.  The middle bar has an inset HHS tip and this vibrated even more.  Fortunately the one on the right seemed to do the trick.  This is HSS but, judging by the marks left by the toolpost screws is much harder all through than the shank of the TC tool and is therefore stiffer, even though I imagine that the HSS is welded to a steel shank.



The final task is to machine the chamfer that will ease insertion of the piston with rings



Turning completed



The finish feels fine but, as you can see, is made up of very fine chatter marks.   Careful measurement shows that the headstock end (cylinder head) is 1/2 thou smaller than the other end.  Lapping should sort out these issues.  ETW suggests lapping once the liner is mounted in the jacket so that will come later



In the past, on smaller engines, I've used an aluminium split lap mounted on a taper and some valve grinding compound (fine at one end of the tin and coarse at the other).  Is that the best way forward or are their better methods to get a truly smooth and parallel bore?

[b.lindsey]

You might consider a brake cylinder hone to get some of the worst of it out and then switch to the lap. The hone may help with the slight taper as well.

Bill

[tangler]

Cheers Biil,

I have wondered about these http://www.pollymodelengineering.co.uk/global/technical-notes/cylinder-hones.asp from time to time but have always been dubious about whether they will make the bore truly round and parallel.  Seems easier than lapping if they do.

[b.lindsey]

The one I have is like the lower one in the picture. There are some good pointers on that link as to using them and like any tool, if misused the results will show it. I think the main thing is to keep the hone moving up and down the length of the cylinder evenly and with plenty of lubrication to keep the stones from clogging up and as noted NOT letting the pivot go beyond the end of the cylinder.  Maybe others will chime in here with more experience using these. I have only done it with two cylinders but with good results.

Bill

[Bogstandard]

Tangler,

The ones from Polly are way too expensive.

You can get a 4 in 1 set from Sealey for around £20, and they work great for honing out bores to parallel and roundness.

I have used mine many times, and they do a great job.

http://www.amazon.co.uk/Sealey-VS029-Cylinder-Hone-4-in1/dp/B000RO7THQ/ref=sr_1_3?ie=UTF8&qid=1366859414&sr=8-3&keywords=cylinder+hone

John

[tangler]

Bill & John,

Thanks guys, I'm convinced.  Ordered from John's link with "1-click" .

It's great here, I love it,

Rod

[Jo]

  Ok I have just read the guidance notes on the Polly website and it says to rotate the hone only in a clockwise direction: a Lathe chuck rotates anticlockwise, so does a drill chuck. Am I missing something? 

I have always used my hones in an anticlockwise direction  .

Jo

[tel]

With the hone in the tailstock chuck it is, in effect, rotating clockwise, even tho it's not moving

[Jo]

That means that the cylinder will need to be in the chuck. I often hold the cylinder in my hand  or rest the bigger cylinders on the cross slide:



Jo