It's been quite a while since I've built an engine. I'm in the mood for a static live steam display, but had a heck of a time deciding which engine to build for it. The material I have available to make a boiler is a bit limited at the moment, and some unforseen expenditures means my hobby budget is down to nil; I have to work with what I have in stock.
An Elmer would be an obvious choice, but as the last engine I built was an Elmer, I felt in the mood for something else. Running through my plans, I glanced a couple of times over the "Potty Popcorn" plans that Stew (sbwhart) kindly emailed to me, but it was a bit big. Seeing as I'll only be able to build a smallish boiler; the boiler would never cope with running it.
While looking at a printed version of the engine overview, it hit me! - The print-out is only about
2/
3 actual size, and about the size I need. Going through the plans, I think I can build it with what I have available in terms of material and taps & dies, it'll be small enough to run on the boiler I can make, and would make a good-looking display at the end of the day - I think.
Before I get
- I know I said live steam, but I'll post the engine build here, and once it's complete, I'll divert to the "Live Steam" board for the boiler build and the rest of the stuff. After all, I do need to have an air running engine first
Enough nattering - here goes my version of Stew's "Potty Popcorn" in
2/
3ds scale. As always, suggestions, critique and questions are most welcome
The cylinder block started out as a piece of 25mm square aluminium 23mm long:
That was marked out on one of the sawed faces to have an 8x8mm offset from two sides, clocked up in the 4-jaw, faced of, and drilled and reamed through for an 8mm cylinder bore. This was now the reference face for the rest of the cylinder machining - and will be the connecting rod end of the cylinder. The finished cylinder is not symmetrical, so this choice needed to be made early during the machining process. It's best to face the connecting rod end and make the bore in one step, as that will mean the cylinder head with the packing nut will fit nice and true to the bore:
After that I flipped the workpiece in the chuck and faced it to length. No need to true it up concentric to the bore here; close enough is good enough.
Off to the mill, and I milled down the excesses to the correct dimensions for the valve face and the cylinder foot. The cylinder foot is on the upper right hand side and the valve face on the lower right-hand side in the photo:
Back to the mill again, and I located all the edges and then set the DRO to 0 on the center of the exhaust port - then drilled holes from readings to start milling the port slots from; a 2mm hole for the exhaust port, and 1.5mm holes for the steam ports. The holes later on allowed easy milling of the ports by providing a space to plunge the cutter in:
Then I milled the ports to size, and also drilled the 1.6mm holes (to be tapped M2) for the steam chest bolts. from the plans, the ports should go down quite deeply in order for the steam passages to meet up with them at 90
o angles - I didn't fancy milling slots that deep as it's slow going with these small mills, so the steam ports were only milled 3mm deep and the exhaust port in the middle 4mm deep:
The workpiece was rotated 90
o on the X axis to present the base, and the exhaust port hole drilled through, as well as two 2.5mm holes to tap M3 for mounting the cylinder. The one offset to form one side of the mounting base was also milled out:
On to the steam passages. As mentioned earlier, If made to Stew's plans, these are easy to drill at 90
o angles and no special setup needed. I needed to get the
right correct angle, and in my usual lazy fashion I laid out the angles on the side of the workpiece, eyeballed the scribed lines to the side of the vise jaws, used a small mill (2mm in this case) to flatten the edge at the cylinder bore corner, center drilled it, and drilled the 2mm passage using a "wiggler wire" (in this case the back end of a small drill) to feel when I broke through into the port on the valve face. The second photo shows the scribed lines I used to line up to more clearly; the first photo was on the "dark side" of the vise:
You can click on the last photo to see the "bigger picture"
Some rounding over followed - 8mm drill bit through the cylinder bore and incremental turns - some filing required afterwards, but much quicker than setting up on the rotary table or dividing head to do, and as this is cosmetic, no great need for super accuracy:
After a couple of minutes with files & emery, things looked up, and I was ready to start tapping holes:
I wonder who has spotted the obvious problem here... It only hit me at that point that I'd forgotten something VERY important
....
... I'm an Idiot
3 ...
A cylinder all nicely finished up, and no holes to mount the cylinder heads... Those should have been done before any rounding over
.
OK,
break, and a bit of
. The rounded over bits would make work holding difficult, and it would take quite a bit of time to set up and clock in each side of the cylinder to drill the holes - the DRO's bolt pattern function is easy to use, but in this case setting up will be time consuming (and labour intensive - did I mention I'm lazy?).
Plan hatched, and a bit of 12mm aluminium turned down to fit the cylinder bore, then drilled deeply and threaded using the taper tap from my M6 set to allow about 4 full threads at the start, and roughly split using Mr. Junior Hacksaw:
Mr. Outta Focus dropped in unannounced while taking the photo.
A short section of all-thread 6mm rod was hacked off the length I have, and some slots crudely sawed in the top to make a screw from. A 6mm grub (set) screw would have been better, but I don't have any, and a bolt or cap screw's head would be in the way. Back to the mill, and rotary table center zeroed in on the DRO, then the collet chuck with the arbor mounted, and the cylinder block on that - clocked in using a DTI before tightening the make-shift screw finally to lock th cylinder in place:
- That Outta Focus guy just didn't want to leave.
Clocking the cylinder like this may sound like a long and tedious process, but it's not. I set the RT to 0
o, and also adjusted the mill Y axis to give me a 0 on the DRO. The cylinder was mounted with the valve face just eyeballed to approximately square to the Y axis, then I moved the X axis to engage the DTI to get a half-scale deflection reading on it ("40" on my DTI). Then cranked on Y to just before the DTI would slip off the edge, and rotated the cylinder on the arbor to get the same half-scale reading on the DTI and tightened up the arbor locking screw. A final check with a full traverse across the valve face, and the DTI stayed pretty much on the same reading. Q.E.D.; it took me
much longer to type this up than to do it
After poking the missing holes, I decided to call things a day. The result of another six hours of my miss-spent youth:
I'll tap all the holes in the next shop session.
Regards, Arnold
