Stuart Twin Victoria (Princess Royal) Mill Engine

Finally got around to using the 3D printed taper jigs today. Set up parallel with the bed, and checked both workpiece positions (they are too long to do in one pass):



Marked out the extents of the taper, and carefully made the first cuts:

un u





All seemed fine, so swapped positions and machines the other end to the same DRO z setting:



With the first pair complete, removed the jigs and replaced with those for the opposite hand, and repeated the process:





The time and effort in designing the jigs was worth it - in the end they worked perfectly and the whole process was fairly trivial. A few swirls to flat out, but nothing major.

Final job on the sides was to reduce the plinth widths by 0.65mm:



Finished parts:







Filed the rounds on the webs:



Test assembled:

A few questions:

1) The beds are slightly warped when tightened. I'd estimate perhaps 0.010" at a corner when the opposite corner is pushed down on a level surface. If I assemble with JB Weld on the cross-piece faces, then weight the sides down while nipping the screws up, presumably when the JB Weld has set, I can fully tighten the bolts (with Loctite retainer, and hopefully they would remain true, ready for the upper surface pads to be machined?

Any thoughts on this? Obviously if I can get away without skimming the lower surfaces, all the better because each one will need two setups to cover the length.

2) The end faces now need machining with their tapers. I think I'll use a spare 3 degree printed wedge for this:

The idea is to traverse the shell mill side to side, and feed down in increments. I'm not sure from the insert drawing whether it's designed to do this - i.e. with a 90 degree side faces. When I've tried it before on a test piece I seem to end up with scalloped face, as if the insert sides are tapered.

Any advice on this? Is it a matter of just incrementing in tiny amounts and flatting the resulting ridges?

3) Finally - the radiused internal corners. I know previous advice was to use JB Weld with a thickener. Having now used JB Weld, and finding it's a bit messy, is there any disadvantage to using white Milliput? I'm comfortable using it from plastic modelling experience, and I think I can sculpt it quite easily into radii with some basic tools and a damp cloth. Is Milliput robust enough for this application?

Thanks!

It should work the way you describe as just gently weighting/clamping the parts will mean the JBW fills a very fine tapered gap and the screws will just keep everything in place. On the Victoria I did recently the base was twisted on the underside and bowed up in the middle of the top so had to shim with feeler gauges and then skim the bottom flat then remachine the top.

I've found the same whan machining a vertical face with teh insert cutters, you may be better off just using a 10 or 12mm cutter with 5mm high passes followed by a very shallow full height pass. Either stop 1mm high and us ethe insert cutter to form teh 0.8mm fillet or just cut right down and do the fillets at the end with filler.

I use Milliput quite a bit for internal fillets as it's easy to shape with a ball ended tool and wet paintbrush and there is almost no sanding. If you do go with JBW you may find it slumping to the bottom of all the vertical corners if you do plus there is then the filing and sanding to do a thickener or letting it sit for a while after mixing should see it stay where you put it. Car body filler is another alternative, better adhesion than Milliput and easier to sand than JBW,

Couple of items filleted with Superfine white.

Edited By JasonB on 15/05/2021 19:47:53

Ok thanks Jason. I’ll use Milliput for the fillets.

I suppose the trick with JB Weld on the joints is to keep it off the screw threads so I can nip them up. I wonder if coating in Vaseline would do? I suppose no threadlock will be needed, since the counterbores will be full of JB Weld / Milliput eventually. Then again I could sequentially remove the screws after curing, then remove-fit with retainer?

What ball tool do you use for smoothing? Ball bearing stuck to a stick or something?

Ill try a 10mm end mill, and finish with the shell mill to get the same radius.

You can stick a ball onto the end of a suitable handle or just round over the end of a rod. I tend to use these which are from my figure modeling days. Top one is used to apply a run of Milliput then dip the ball in water and draw along the corner. With luck you will get a nice fillet and two strips of waste material which are easily picked off and put back onto the pile. Wet artists paintbrush for a final clean up.

Yes some sort of release agent will allow then to be used to line things up but still turned when you need to tighten. I'm not keen on Milliput for filling over screw heads, usually opt for body filler, U-POL Easy was nice but now hard to get in smaller tins so have gone over to U-POL rapid but it lives up to it's name and is also a bit runnier so watch out.

Thanks Jason - I'll get a set of those FWIW, they only seem to be a few quid off Amazon.

Assembled the beds with JB Weld on the mating faces as a liquid shim. At present everything is snug tight and lightly weighted down on a flat surface. Coated the screws in Vaseline, so hopefully once everything is set true and level I can remove each screw, and fully tighten with retainer:





Next chance I get to work on this will be Wednesday. Hopefully by then the JB Weld will be solid enough to put full load on.

I got home earlier than expected today, so I spent a couple more hours on the beds. First off, remove and clean the screws, and replace with some retainer added:



There still appeared to be some residual twist in them, but a couple of passes over some abrasive paper on the granite slab got them perfect. I think the issue may have actually been a couple of small dings on the undersides. Still, the JB Weld gave a smooth finish to the joints, which will hopefully be invisible once painted.

Next up was to clamp to the printed wedge. I used a stop at the back to make sure it wouldn’t get pushed back with vibration:



Checked alignment to my mark-out:



Double-checked with a dial gauge on each end. It was within 0.004”, so good enough for this part:



Made about 10 Z passes with an end-mill, and incremented in X until I got to the line:



Finished the last couple of mm with the shell mill, which gave me the corner radius:



Still needs the plinth width reducing, which will get rid of the slight discontinuity, but so far so good:







Still another three to do, but enough for tonight.

Continued with the remaining three end tapers this evening:



Then setup and trimmed the end plinth edges to match the sides:



I was happy with the result, with both bases ending up exactly the same overall outside and inside dimensions, and their top faces being within 0.004”:



...but when I put them back-to-back, the middle cross pieces didn’t quite match. There was about a 1.5mm discrepancy in the distance between them, side-to-side. Somewhat puzzled, I eventually checked the draw tapers, and found that on one base, both pieces had one vertical side, and one 3 degree side, instead of two 1.5 degree faces. The other base was fine:





As previously mentioned, getting the jigs right was confusing, so we must have ended up mixing the sides up when cutting the second faces.

So with heavy heart, I decided to dismantle the offending side, and re-make the two pieces. I assumed it would be next to impossible to separate them with the JB Weld and the Loctited screws (with their laughably soft caphead hexagons), but I was pretty shocked at how easily everything came undone:



So a bit of a setback, but some progress overall I think.

You could enlarge the holes in the side plates slightly and just tilt the cross pieces.

You probably had some vaseline left in the holes which may have stopped the Loctite bonding.

I thought about enlarging the holes, but the counterbore diameters are a very close fit on the heads, so they'd have to be milled to a slight slot. The tops of the cross pieces are kind of flat in the middle too, so they'd also need correcting, and tilting them would give a gap at one side at the base.

I think overall it'll be quicker to make two new pieces, but be extra careful with orienting them on the wedges. It's very easy to get mixed up and confused with the sequence of machining to get equal angles each side. At least it is for me. Pure luck that both on the other bed were fine I suppose.

Re. the Loctite, I did thoroughly clean the holes with brake cleaner aerosol, and washed the screws, but I guess not enough. It wasn't like they were loose, but just didn't take much torque to undo them. I'll use araldite next time, and also the body filler should prevent them from loosening (not that they would).

Re-made the two spacers. I know what went wrong, I think I got mixed up when swapping them on the 1.5/3 degree jigs, and ended up cancelling the taper on one side and doubling it on the other...



Full disclosure though - on one of them I turned the z-handwheel the wrong way, so that dig-in needed filling with JBWeld. I let that one go because a) it’ll be painted, and b) it’ll be pretty much invisible when the engine is assembled. At least it looks happy:



Also made a start on the mounting lugs which will be screwed into pockets milled into the bases:





Next job will be to drill the strips and radius the ends.

So this is taking a lot longer than anticipated, but I’m wary of cutting corners. The whole thing sits on these beds so they have to be right despite looking pretty simple.

Anyway, drilled and countersunk the strips, and Made a start on milling their recesses. Marked out with verniers first, just as a sanity check for the DROs:



I needed the feet strips to fit precisely to a depth where the 0.8mm fillet in the plinth ended, that way I can replace it with Milliput, hopefully with seamless joints. So pocket depth and strip thickness needed to be identical:



Milling:



Drilling and tapping for the securing screws:



Finished:



Checking the fit:



Next job was to radius the ends of the feet. I wanted them precise, and identical, so the boy and I made some 13mm diameter steel filing rollers:







Worked great, so the feet were then split ready for fitting:



Final job today was to make the throttle bellcrank bosses out of aluminium:







They go in the pre-drilled holes in the bed sides, and will be pressed in with retainer, and then have a fillet of Milliput applied around the joint.

Filled the counterbores with body filler and flatted back. They’ll probably need a bit more work before they’re invisible under a coat of paint:



Next job will be to fit the lugs.

Fitted the lugs , they are a press-fit into their recesses, plus I applied some JB Weld to fill any residual gaps. The excess material on the inner faces will be filed and flatted back:



Also fitted the bellcrank bosses in a similar way:





They will have their faces milled back to within a couple of mm of the bed faces when I machine the top pads, then I'll apply a fillet of Milliput all around..

Question: When I'm machining the top surfaces to leave the cylinder, crosshead mount and main bearing pads, should I drill and tap the mounting holes as well? I want them to be in the centre of the pads, so I guess while the beds are clamped accurately to the mill table, it would be OK to do them at that stage? Only slight doubt is the cylinder feet holes, which will be positioned as set on JB Weld, and secured with a bolt. I wonder if I could somehow use the bed pads themselves as a jig for when I fix the cylinder to the feet? That would guarantee the feet are in the right position. My thinking is this would be done before any cylinder machining, ie the feet would have their bases machined and the mount holes drilled, then once set and bolted, the feet would be the datum for cylinder boring, and facing.

Or am I over-thinking this? Just seemed a good way of guaranteeing the cylinder and feet would be a perfect fit, and would also provide the first datum for boring.

Thanks.

Can't see any reason why not to do it as you describe

Ok I’ll see how it goes...

This evening I filed off the excess lug material:



And then flatted the lower surfaces with wet and dry paper to remove any slight burrs. I’ve pressed a spare bit of granite worktop into service as a large surface plate:



Now ready for marking out the raised pads and setting up on the mill table:

Quick question regarding the attachment screws for the feet to cylinder:

If I deliberately drill all the way through the foot and the cylinder, and then use screws that protrude into the cylinder (JB Welded in place), could I then file the excess screw length off and machine the cylinder as normal? I think I’ll end up breaking through the cylinders at some point, so might as well make it deliberate and incorporate it into the process.

The only potential issue is can think of is some kind of differential expansion if run on steam. Then again if it’s just steel screws and feet, and iron cylinder, maybe its ok?

Should be OK

Made a start on machining the mounting pads:



And drilled an tapped the mounting holes at the same time, while everything was aligned:



Also side-milled the throttle boss a fraction narrower than the base footprint:



The mill capacity is too small to do it In one piece, so currently setting up again for the other end:

I hope those small threads are going to be ok in aluminium. I initially tapped the first cylinder foot hole 3BA instead of 4BA by mistake (the tap seemed tight and so luckily I didn’t get too far; I think I got away with it). I’ve made all the threads almost as deep as the bed sides to get maximum engagement. Hopefully they will be fine.

So what’s a good method of matching the machined depth now Ive moved the part? I did the first half by just touching the upper surface with the tool, and incrementing down to 1.5mm.

trouble is, the top surface is stock, so might be a bit wavy. I think I heard of a technique where you somehow stick some paper to the surface you want to match, then lower the tool until it moves the paper, then move the tool down by the paper thickness.

Any tricks to get it perfect? Obviously I can easily blend any tiny mis-match, but it would best to avoid that.

Thanks.