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G.W.R Metro Tank in 5" Gauge - Part 5
by James Holden
Engineering Gibberish. [Adventures of Jammy the Journeyman]
Cylinder Block and Arrangement~~Steam chest.
This is made from gunmetal casting solid or hollowed out centre, overall dimensions 2"x 7/8"x 2 5/8"lg. First and formost set vice and clock up, clamp casting with its widest edge against the the back face of the vice and m/c top ' just enough to clean up'! reclamp job by putting m/cd face against vice back jaw and skim flat area with fly cutter this I find is generally the best for a large spread of surface metal to be removed, chuck for square off previous face if within tolerence set parrellels and turn over and skim opposite side too 7*500"--000" to +002" with one side left to finish still using the parrellels m/c final face to *875"-000" to +002".
Stand on its head with parrellels still supporting the casting and by using a square set the work in the vice with the aid of feeler gauges *002" is fine for this chack when square m/c face.With all sides m/cd, one method is to chain drill the centre out then by using a abra file locate inside the hole breaking trough each hole and with a little tap knock out the centre allowing for m/cing after, by using a endmill finish to scribed line a smaller slotdrill can now be used indexing down from top 1/8 dia will be all right for this job, once finished remove to marking off plate and set up with aid of angle plate, vee blocks or parrellels, V.H.gauge, hammer 1/2lb ball pain centre punch, Scribe and mark off all holes,using datum centre lines around the steam chest the two 3/16"reamed holes on 1/2"centres are for the bosses which support the valve guide shafts.
Next are the three holes on top,centre hole 1/4" dia the other two are tapped 4BA and are on 7/8" p.c.d.these bolt down the main steam entry flange. Back side 1/4"x 40T threaded hole for spring loaded relief valve preset for pressure in steam chest without blowing off. The two holes in the bottom are for drain off valves manual cab working for blowing condensed steam [water] out of the chest 3/8"x 40T tapped.
Holes through the steam chest are for studs pulling the two halfs of the cylinder block together, drills n* 43 n* 34 respectively 8BA & 6BA, all holes now covered in steam chest.
Moving on to the cylinder blocks, two gun metal casting, start by m/cing the bolting face once skimed up turn round and use parrellels m/c opposite face to size of 1 3/4"-000" to -002"clamp widest area in fixed jaw of vice using only one parrellel m/c across flange top leaving a witness in 'do not over m/c' turn over and complete the operation, size should be 1 7/8" plus, remove and stand on its end length ways,square off flange face using engineers square and feeler gauges are the biggest slip that can be found using part of the surface of the vice to bring square, once set fly cut top face turn over and m/c opposite face to size 2 1/2" working to a high level of tolerance,+000"to +002" this is important.
Turn block on its side with flange in horizontal position and m/c a path out at centre of block for exhaust port flange making note of m/c depth for second block.mark out using v.h.gauge stand block on its end. [capfaces] and run centre line all the way round split the differance 7/32" a side exhaust centres, "centre slot is 5/32"x 5/16"lg turn casting onto its m/cd base and scribe up 15/32" thus bisecting both lines,centre pop all holes mill out slot to a depth of 1/8" then by using angle vice slotdrill through into back face, drill and tap 5BA holes either side for bolting flange.
Once blocks are m/cd remove to marking off plate with all necessary items,angle plate, vee blocks. v.h.gauge, hammer, punch. scriber, dividers'.this will be marking off equiptment in future reference'! put centre line datum all around the block, stand on the base and scribe a line *750" were line bisects lightly pop, a wooden dowel tapered to the size of the rough bore at this stage will do,tapped in gently till flush with m/cd area pick up on bisection with dividers and set too 1/2"p.c.d.scribe line,'use marking out blue this will aid you to see the line'.another way is to put marked boxes either side of centre line using v.h.gauge.
Once marked transfer to the centre lathe with a face plate attachment with angle plate bolted too,this will support the cylinder block by clamping through the slotted holes,are if making one's own drill & tap the plate for the clamps, when setup bring the d.t.i.fastened in the tool post onto the front face and clock up then run down the flange edge when satisfied,bring in the scribing block knock chuck out of gear and spin,pick up marking out for 1"hole, to line hole to scribing block gently loosen the clamps which hold the angle plate to the face plate tapping lightly in the right direction so to bring the p.c.d. into a position were the point touches all the way round.' this operation takes a little bit of getting use to'.rotate the chuck slowly if counter belance is needed it should be opposite to the cylinder block, alter the speed of the chuck a little faster this will tell you weather are not the p.c.d. is running out if not you must try the scribing block again till satisfled tighten all clamps and using a boring bar take light cuts while the bore becomes concentric measure with v. caliper, finish to size with the help of 1" plug gauge, the GO and NO GO determine what fit is required this is known as limits & fits try your skills and keep it to GO! a good m/cd finish is needed here to avoid honing out the bore,discretion is needed here you can be over the top with accuracy size and size will be to tight when the steam warmed up the cylinder causing the piston to seize *002 clearance is plenty the piston ring will conpensate for the extra few thou.
Moving on to the cylinder end caps straight forward m/cing for these gunmetal casting,grip in three jaw chuck leaving just enough to m/c flange knock up reasonably true using scribing block and m/c front face then skim top dia to 1 1/2" then m/c up to flange to a dia of 1"-000"to-002" x 1/16"lg chack with micrometer and depth gauge, bring in centre drill and open up for 3/16"reamer, tapping size drill is now required for 3/8"x 32T at a depth of 5/16"of a inch, turn cap round gripping on shoulder m/c down face to length and if needed top the remaining waste off. clean out thread, and deburr.
Back caps do not carry the gland housing and are blanks m/c in the same as before, soft jaws can be used here all that is necessary, m/c the jaw concentric before a start is made. A little jig can now be made to locate over the cap flange, holes to be marked off on a 1 1/4" p.c.d.with jig inserted in the dividing head two of the holes are parrellel to one another 3/8" either side of the centre line the remaining six are equally spaced into twelve holes but only half way round the jig this is because of the front end caps and the position in relation to the steam chest bosses, drill the holes clearance for 6BA. and 8BA, respectively and by using same drills spot through into blocks using the end caps as the templates before using tapping drill sizes and to a hole depth of 3/8" of an inch when first hole is tapped bolt cap in position and continue with all the rest, complete all the end caps, when finished remove to milling m/c but first mark off the three slots in the back of the cylinder block these two are 1/8" wide and one 9/32"x 11/16"lg and to a depth of 3/16"of an inch. An angle vice is needed now, m/c a 1/8"path on the edge of the bore and cram in as many holes as possible working out the angle so as to break into the two slots on the back face of the block the centre slot is drilled and breaks out on the exhaust port flange.
With steam ports drilled file all burrs off the front lip, all operations now complete, bring back to the marking off plate pick up on datum centres and run all threaded holes in for bolting to main frames with marking off equiptment allready covered.
Moving on to the piston valve glands, these are made from gunmetal or ph/bronze, set up in three jaw chuck using knife tool face and turn top dia to 7/16"turn up to a shoulder 3/8" x 3/8"lg introduce screwcutting tool english form 55*degrees touch work with tip and set index to zero, slow speed of chuck to around 80 revs per min quick change box set to 32 t.p.i.making two passes at *010 thou and then to finish at *024" finally die for correct form and size. Centredrill into face,drill and ream 3/16"dia turn round and face up all that is remaining are the tommy bar holes pitch these off on the dividing head, centre pop for six holes equally spaced for n*48 drill, do exactly the same for the valve rod glands except for the reamed hole in this case 1/8"dia.
This is a gunmetal casting with lug supports for the slide bar connections, start by m/cing the flange faces to size stand up in vice in milling m/c using wide jaws this will give extra heigth also use spacing blocks to avoid lugs m/c to size of 4*250"-000"to -002"turn on its end squaring up flange with square and feelers then m/c across the top.With three sides m/cd remove to marking off plate first mark off for the valve spindle guides, use marking off equiptment start by scribing a datum line 1 1/4" from base turn onto flange and scratch another line half the total width were lines bisect split the differance 1/2" centres 7/16"ream, working out from datum centre line 1 1/8" centres for cutouts were both lines bisect this the exact position for the cross head in relation to the motion plate by making a template of the cut-out the shape can now be determined mill are chain drill out,knock centre out with thin chisel and file the remainer to suit templates, the cut-outs when finished allow the crosshead to pass through and the right hand allows the crosshead with the pump mounting bracket to pass through, finally m/c across top side of lugs making sure that each one or parrellel to the prevous m/cing mark out remaining holes these are in the flange for bolting to the mainframe plus fixing the valve spindle guides to the motion plate 3/4"centres n*34 drill for 6BA tapping, the motion plate is now complete and can be assembled in the mainframes.
Moving onto the valve spindle guides.
These are made from ph/bronze bar anything over 1 1/32" can be used, 30m/m is ideal this will leave around 1/32" extra a side and will help when filing in the radius, using lathe attach 3 jaw chuck face up with knife tool and m/c across top dia to a size of 7/16" x 17/32"lg centre drill and open up to with drill to 19/64"for 5/16" reamer turn job round if 3 jaw is running true enough turn top dia to 7/16" leaving flange size of 3/32" and from shoulder to face a length of 5/32" for location in motion plate, remove to dividing head and mark centre line around front flange face and turn through 90*degrees and split the differance centres 3/4"centre pop then by using dividers set for 1/8"radius scratch round full circle using ruler working from centre to edge of radius scribe line this should look like a diamond shape now, saw and file in the radius to lines, drill the remaining to holes n* 34 then fasten in to the motion plate. The slide bars these are made of silver steel or mild steel case hardened m/cing by using 4 jaw chuck and scribing block pick up the points of the 1/4"x 1/4" square bar and m/c to dia of 1/8"x 5/16"lg turn round and m/c to over all langth of 3 23/32" making sure all 4 bars are the same use v.caliper.
This completes chapter 5.