I went ahead and drilled and reamed the bore for the crankshaft.
It's close, but there's a mismatch somewhere. Need to evaluate my techniques more. But in the meantime I'll soldier on.
You can see how the nearside is mostly aligned, but the far side interior surface looks off - result of that error made much earlier on that I mentioned.
And sure enough, once the bearing cap is removed, the hole is way off center relative to the locating steps, and indeed a little off height wise, but not by much.
I think it will run. I did not seem to get any "wandering drill" when I drilled all the way through the second side - I drilled the hole out in steps, leaving some stock to clean out with the reamer. So I am confident that the bore is centered height wise to the rails. I will measure that and check.
However, going to call this "acceptable (just)" and keep going. Not sure what to do next - maybe a few of the simpler components, like upper slide rails and spacers.
I checked the crankshaft bores with a gauge pin today. It's .002 oversize, which surprises me a bit given that the hole was reamed, and in a single set up without ever moving the table. Still, that's OK, I will make the crankshaft to suit.
The bore may not be as far off center as I had thought. Looking at it here with the gauge pin in place, the center of the pin seems to fall pretty close to the split line at the bottom of the bearing cap locating slot. I didn't need to chamfer the edges or anything to get the pin in - it popped right in from the top, so I think I'm not as far off as I thought. The only mystery is the difference in width of the locating slots. Maybe I'll just pretend that's an orientation feature.
This went very well, and now the rails are even about center of the bore in the end of the casting - they are also matched in width.
This means that the mating part will have to be adjusted, but I'm OK with that.
Speaking of the mating part............
I set measured up the casting and discovered it had a good amount of stock all round, and would accommodate the changes in dimension to match my rails (I did that before adjusting the rails), so set about getting some datums.
I set up as level as possible for a first cut - I got a run iut of very roughly 0.005 along the length of the part.
And took off the minimal amount of material possible.
Then on to parallels to clean the opposite side.
The two faces measured parallel as best as I can figure it, so I then used those surfaces to set up and register to clean the back side.
Then it was time to get on to the nitty gritty. I machined the width of the guides and the thickness in two set ups. I set up like this so that I could mill all around the guide tab on one side.
The result...........
This gave me a solid set of perpendicular surfaces to clamp the part square, and get registration to match the features on the opposite side. Notice how the 1-2-3 blocks are staggered - so that later on I can register off the 1-2-3 block face, and match the upper tab to the lower one.
As I machined around and got close to final thickness, I actually mounted a DTI to the quill, and measured the position of the tab face relative to the 1-2-3 block - and hence the lower tab face, so I was able to keep the tabs co planar within about 0.002" as near as I can measure.
I measured the part after machining - pretty much within 0.001 to 0.002 on all dimensions. So I had to do a quick fit check.........
It's perfect!
After so many bungled attempts on previous parts, I was thrilled and overjoyed at how this part has progressed. Everything is square, parallel and true, and it fits into the base casting just so well - it's a real boost. No wiggle or play, and it slides between the rails really well.
So back to the mill, and set up for the tapped hole.
After drilling and tapping that, I popped it back on the bast, along with my temporary aluminum packing, with a drill bit inserted into the tapped hole just to get a feel for alignment with the bore.
I'm very happy with this part - everything went according to plan, no drama, just enjoyable machining. I think this will clean up a treat and look very nice on the finished engine.
Couple of ops left on it - first I need to drill and ream the cross hole, then mill the slot for the connecting rod. But I don't have a 1/4" reamer on hand, so I'm going to wait a bit before finishing this. Should have one this week some time.
Continuing on - I realized last night that on my cross slide I probably hadn't assessed the center of the rounded boss properly with respect to the other final features on the casting - and sure enough when I did a measurement this morning, when I put the cross hole in at the correct dimension relative to the rear of the casting, it will appear off center relative to the cast feature. Haven't decided how to address it yet - I can actually shorten the casting and skim a little off the rear end, or attempt to re round the casting. Or leave it. It will be functionally fine. Probably I'll skim the rear of the casting.
Based on these experiences, and some excellent set up info on you tube, I have resolved to ask myself a new set of questions when writing out a plan for machining castings, to try and force myself to understand the casting before cutting anything.
Anyway, today I worked on the rail and spacers. I saw a technique using gauge pins to visually center the mill to casting bosses, as part of achieving a pleasing overall look as well as dimensional accuracy, so I tried that. The first rail didn't work out so well, and I altered the technique a bit for the second rail. Which frankly didn't help in the end, so I need to revisit this approach. Before machining, the underside of the rails were cleaned up on my little belt sander. That worked well at least.
EDIT: The following pics don't show every step. I cleaned the undersides of the rails first and got them flat. Then squared the side of the rail to my vise using a machinists square. I was playing with the gauge pin approach in the pictures, but then secured the part with the various clamps before finishing all the locating. Obviously I didn't attempt to drill with just the middle clamp, the parts would have ended up spinning across the shop, probably.
As you can see, I ended up off center one end.
I still ended up off center even after drilling assessing both bosses, and then splitting differences in X and Y location.
I tried a similar approach for the center hole. The first one didn't work out, but the second was a little closer.
Anyway, after all was said and done, and having made some simple spacers, the cross slide does go together and the slider slides without rattling, it actually fits quite well.
So, a few steps forward, a few back, but on average more steps forward than back. It can be frustrating at times, but I think I'll win out in the end.