Steam Cylinder Patternmaking

[Johnlanark]

Given the interest in patternmaking, as Edward suggested here’s a thread for a specific example.

Patternmaking is good fun and very rewarding . But it can be hard to find up to date and straightforward information on how to do it in the home workshop, Steam cylinder patterns seem to be a particular problem, and there is a perception that they are very difficult. For one-offs or small batch projects it needn’t be so. You will see that I can neither attempt technical drawings, or any woodwork beyond the very basic. The main tools I use are a small bandsaw and a disc/belt sander, also my metalworking lathe so look away now if you don't approve!

The cylinder is a start on a replica of a very nice vintage single engine I was surprised to buy on ebay. Surprised because there were no other bids! A very straightforward design of 2.25” x 3.5” and would suit an 18 to 19 foot boat.





The foundry will use the patterns to make impressions in sand in two part split moulding boxes, then draw them out. For each part, the patternmaker has to think about where the parting line in the sand would best lie, and how the pattern can be removed without damaging the sand. Simple suitable shaped patterns can be made in one piece, and the parting line will be placed at the widest point, like this vase shaped example.



The cylinder top cover is a good place to start.



It can be made in one piece. The casting will be machined all over, and the aim of the pattern is simply to give enough material, and perhaps help the workholding. I sketched a pattern layout. The finished machined size is in red, and the pattern outline in black. I’ve added machining allowance all over and shown exaggerated taper. The taper changes direction at the parting line.



Th pattern is three discs, made from ply. The discs were marked out with compasses, rough sawn to shape and finished on the disc sander. My table doesn’t tilt up the way, so a piece of wood lying on it gave the taper.



The three pieces were drilled ¼” at their centre and a steel pin pressed in to help location while the PVA wood glue set. The outside corners were sanded to remove sharp edges. Next, I'll fillet the inside corners with car body filler and the pattern will be ready for final sand and varnish or paint. I'll add a photo shortly.

Moving on to the valve chest...The valve chest on the vintage engine has an integral cast on cover. This is not ideal for access to check valve settings, so I’ll make the copy with a more normal separate cover. An oval boss at the bottom end takes studs for the valve rod gland. At the top end is a screwed in bronze support for the valve rod tail, however this is quite loose, so I’ll alter this too, by making it part of the casting in the replica.


The chest is 1 ¼” deep, which would be quite a deep hole in the middle to deliver by tapering the sides, so is best cored out. A core is a block of special sand that is made separately and placed in the mould once the pattern has been withdrawn. The pattern is made with projecting pieces to suit called core prints – these make impressions in the mould sand that the core slots into. This engraving shows a cored out bush in the mould.



The valve chest can be considered as a rectangular belt of metal wrapped around a large hole. I decided to split the pattern along the line of the ruler shown here.


To withdraw from the mould the chest ends will need a little taper. These faces will not need machined. The front and back mating faces of the chest where it joints to the cylinder and the cover will need machining allowance and taper.
The pattern was made from yellow pine, offcuts from my new boat's floorboards. The two halves were drilled and located together loosely with two quarter inch steel pins. The oval gland boss has still to be added where drawn on the end, and also the round valve rod tail support at the other end.



To make the gland boss, two rectangular offcuts of mahogany were dry screwed together, held in the four jaw chuck and a round neck turned. The oval is marked on the end.



The next step is to form the oval with filing buttons.
I'll progress the core box too and post this next, them move on to the cylinder. John

[Maltelec]

Another point of interest is that rough castings like what we require are reasonably rare compared to precision castings, such as metal signs etc.

It is quite amazing what detail you can get with modern day sand.

[farmerden]

Thanks John This is great stuff! I've got the patterns to my engine hanging on the wall. I've kept them more as forms of art. The question I have is -my engine is an original -how do you make the first? Do you mock up a wooden or clay engine and pattern from that? Can't wait for your next installment! Den

[steamboatjack]

yes this is the type of stuff we need in the forum, great.
although to be honest I prefer on most jobs to use contious cast bar rather than bother getting castings made, this is available in sizes up to over 12 inch in diameters, and in squares and slabs. ok there is extra machining but this is off set by not having to make a pattern and nowdays there is not a foundry on every doorstep like they used to be.
regards
jack

[Johnlanark]

Den - to make the first engine you would follow the drawings and lay out the patterns making your allowances for machining in the same way as I did for the top cover.

Minus six this morning but I managed an hour in the workshop progressing the valve chest core box.

Mr Joshua Rose advises (in The Pattern Maker's Assistant 1877) "Previous to baking, cores are so weak that they cannot be handled without being in some way supported. It is therefore as great a consideration to the pattern maker how the core is to be taken fron the box as it is how a pattern shall be drawn fron the mould." He gives this illustration of a box for a rectangular core. It is held together by hand while the sand is rammed in, and then pulls apart leaving the core behind, probably on a steel plate for baking. The core sand at that time would probably use horse manure as a thickener!


I built mine along similar lines, again using the yellow pine and half inch ply offcuts. It is intended to be made on this plywood base board, and has two retaining pieces screwed down to it. The moulder will lift one side slightly to free it, then remove the other pieces and invert the core on to a baking plate.




John

[csonics]

This is great stuff! Thanks for the post John!

-Mike

[Johnlanark]

Thanks Mike.

There is a good youtube video where a cored pattern is moulded: http://www.youtube.com/watch?v=wG9JzeZYPi4

I overlooked posting the layout for the valve chest.


Today I carried on with the valve chest, which has more detail than most. I shaped the valve rod gland boss with filing buttons made up with holes to turn on wood screws.




The boss for the valve rod tail support was turned from two pieces of mahogany cut overlength and dry screwed together at the ends. Held in the four jaw chuck and with tailstock support straight into the wood at the joint, the boss was turned to the 3/4" size and a gluing piece reduced to .6" which seemed as small as it was reasonable to go without danger of it breaking.


The inner end waste was sawn off and the end roughly rounded on the disc sander, to be finished by sandpaper later, then the waste at the other end was also sawn off.


Back at the lathe, the valve chest was mounted on a faceplate angle plate, drilled from the tailstock then opened out with a boring tool to fit the gluing pieces on the two bosses.


The boss halves were glued into the half of the chest that has no locating pins. While the glue set, the chest was weighted down on a flat surface protected by a piece of thin shopping bag. I may back up this glue with a couple of panel pins.


Then the other halves of the bosses were glued to their chest half and the whole assembled with the locating pins doing their job. Pieces of the shopping bag prevented the two halves sticking at the gluey bits. Eventually, the pattern is looking something like the finished part.


Is this too much detail?
John

[csonics]

John,

That video and the others in it's series are great! Makes it look so easy. Thanks for sharing you expertise! I've learned a lot today.

-Mike

[Johnlanark]

Here is the cylinder top cover filleted and varnished. The parting line is at my index finger.

Here is the steam chest with car body filler fillets added to the two bosses. The core prints are identified with black paint. The pattern is sitting beside its core box to help visualise how the core will fit.

Mr Horace Purfield on fillets (Wood Pattern Making 1906)


This post is a bit of a holding exercise. I have the cylinder pattern well through, but will have to redraw my layout legibly to post as the first step! John

[Johnlanark]

My workshop temperature is back to almost bearable, so I got back to torturing innocent offcuts of wood into a cylinder pattern.

The steam cylinder is a fairly standard slide valve type. It has two end flanges and a rectangular valve chest, with rounded filler pieces either end of the chest giving covering metal to the inlet ports. Steam cylinder patterns are almost always made split along the line of the valve face, on the midpoint of the steam and exhaust ports, as indicated by the ruler here.



Bill Hall wrote an excellent article in Funnel No 111 on his pattern making for a replica Thornycroft engine. It included photos of the mould which Bill has very kindly allowed me to show here – thanks Bill.



At this point, the outside shape pattern halves have been moulded, the boxes split and the patterns removed.
- Down the centre can be seen the circular core for the main bore, resting in prints at either end. The bore needs to be cast smaller than the finished size
- - Near the bottom, an S-shaped steam inlet core rests in a slot in the main bore core, and a print at the valve face. The second steam core has not yet been fitted – it is a mirror image of the one shown and its slot can be seen in the main bore core, and its print just seen at the valve face.
- -The exhaust core is fitted into a print at the valve face. The part of this core that can be seen is rectangular shaped, however it will also have a round section pointing downwards into a print in the bottom sand. This print is probably tapered to give end location and support.

The Thornycroft has a more complex shape. Ignore the core at the upper left which is to do with a flange and boss on the outside, and the core on the right which forms an integral valve chest.

I made layouts of a section with cores and their prints, and an elevation of the valve chest with the exhaust core and print, looking the other way. The finished bore is 2 1/4”, so the core will be 2” to give 1/8” machining allowance all round. The bore core prints extend 1” beyond the top and bottom flanges of the cylinder.

The molten metal shrinks as it cools in the mould. For cast iron the rate is 1/8” per foot. This cylinder is 5 1/2" long, so it will shrink approx 1/16” in the length. I took account of this in the allowances at the ends.




One way to make cylinders is to turn them from solid wood, and there is a thread going on over at home model engine machinist where you can see this being done, starting at post #8.
http://www.homemodelenginemachinist.com ... pic=7380.0

I prefer to build up cylinders by staves (or strip planking if you like), over a central spindle. On the lines described by Horace Purfield.



With this quite small cylinder I decided to make the main bore core prints at the end from solid. Two pieces of beech that used to be a chair leg were trued and the usual locating pins fitted. I made the spindles just over 2” wide, to be turned down to the 2” core size. Extra thickness was glued on to the core print areas at the ends, to bring the height up to just over 1” each piece. Metal turning pieces were screwed to both ends and the bobbin turned between centres. The jaws of the chuck are not tight, just acting as drivers.





The four half round flange patterns were made from ½” ply. They need to be tapered away from the parting line both sides. The outer side has both taper and machining allowance.


I tapered my ply on the belt sander, using the laminates as a guide. This left the ply under-thickness, so I glued thin ply on to make this up again - it also restores a smooth surface. I bought a mixed pack of thin ply from 1.5mm- 3mm from a model boat supplier on ebay some time ago that is useful for this type of building up and for detail work.





The pattern halves were built up by gluing pieces of wood for the chest, referring to the layout, and strip planking the body. The valve chest needs both taper and machining allowance.




I've stuck this together mainly with Araldite 90 second setting for speed. Still to add the thickenings at the steam ports. Then I'll do some filleting with my west epoxy boatbuilding glue to strengthen it. Then on to the core prints and boxes. John.