And we're on to the cylinder in today's pix. This part has some challenges due to the references required and when they become possible to measure from. So, some of the steps and setups have to be repeated in order to get to various final dimensions, at least the way i thought about it and did it.
First pix is the raw casting, just after I worked it over a bit on the belt sander. This was to just take off the worse of the rough spots and get flat enough surfaces to be secured in the vise. Notice in the print, the first tricky dimension. Note that 2 dims are referenced from the center of the valve rod hole, that is the top of the valve chest and the bottom of the slide valve surface. But that feature can't be done initially since the part is very difficult to hold at this point. Also note that i used a permanent marker to note the front and rear of the cylinder as the raw casting is symmetrical but the operations make it non-symmetrical as the valve rod boss is removed from the top but left on the bottom. This matters as the inlet port needs to end up on top and the outlet port needs to be on the bottom when finished.
So the first thing I did was to hold the cylinder by the ends using some AL packing against the moveable vise jaw. Then I used a 1/4" carbide end mill to just lean up the face of the valve chest and the bottom of the valve chest. There was no attempt to get to the stated dims at this point, I just wanted to get some flat surfaces.
Now since the valve chest face is flat, I oriented that face toward the fixed jaw and used some packing against the moveable jaw. Then I squared it up as much as possible using a machinists square on the side and indicating across the top keeping in mind that the casting is still quite rough.
One end of the cylinder finished. This isn't done to an exact dimension, but rather I milled it down until the surface was smooth and the valve rod boss more or less disappeared. At least is LOOKS like the print at this point, but its not yet possible to hit an exact dimension here.
Now we have 2 flat surfaces!! So, now the cylinder is flipped over and can sit flat on a parallel on the bottom and flat against the fixed jaw. AL packing is still used on the moveable jaw side. So milling can commence! And of course now it's possible to get the cylinder OAL to spec. That's a 1/4" carbide EM spun "pretty fast"!
Measuring the length of the cylinder using a depth gauge to get the OAL to the print spec.
Part shown against the print just for reference:
Next is finishing up the valve rod packing gland area. This involved drilling thru then creating a recess for the packing material and drilling and tapping 2-56 for the small fasteners used to hold the packing gland in place. This is my typical setup for tapping small holes, a spring loaded tap follower and a very small tap wrench.
Now back to finish off the valve chest area since we have the required reference feature. First I oriented the cylinder in the mill vise now having 3 flat surfaces! I used a DTI get the cylinder oriented as flat as possible.
Then I used a depth mic measuring to a 1/8" rod inserted thru the valve rod hole. That way I could figure out the required distance from the center of the rod hole to the top of the valve chest and from that feature to the bottom.
Then taking the top of the valve chest to the required dim using a 1/2" EM.
Taking the bottom of the valve chest to .438" from the top, but of course this is really referenced from the center of the valve rod hole. I used a 1/4" carbide EM for this operation. Also I had set the centerline using the 1/8" rod thru the valve rod hole, as this really need to be the center of the part from this point forward.
Next was milling the rather deep exhaust and intake ports into the bottom of the valve chest. This is the start of the exhaust port using a 3/16" EM. The tricky thing is that it's hard to see what's going on down deep in the valve chest plus I frequently used a vacuum and/or air blast to clear the chips quite often.
Then doing the inlet port using a 3/32" carbide EM. Note that I wrote the movement of X on the vise as I really, really didn't want to screw things up at this point!
Last operation at this orientation of the part was to drill and tap the holes for the cover plate. Pretty straightforward, just moves using the DRO in x,y. Centerline was still established from the previous operation.
The cylinder against the print trying to show the operations completed so far.
Next comes the cylinder boring operation. The tricky part is to figure out where the cylinder center should be, as the rough cored hole isn't at center nor is it round or smooth. So what I did was to indicate the valve rod packing area hole to make sure the spindle was exactly on the Y axis centerline as I'm taking that feature as the center of the cylinder's world. X should also be fixed at this point.
Then I used a DTI ball point just for a reference, and swept the OD of the cylinder to get cylinder as close as possible in Y.
Then I moved the DTI ball point to close to the existing cast center hole and swept that area to see how close to center is actually is/was. It turned out to be not good, but not terrible either and it's oblong with a one very rough side. But there's plenty of material so it should clean up just fine.
Then I started to bore using my Criterion boring head and as short a bar as I could use to get thru the entire depth. You can see that start of the cut cleans up the bore pretty well, the difference between sides turned out to be about .020"
Then boring and measuring until the 1.000" target is achieved.
I also gauged the hole with a 1" plug that I had on hand from a previous project.
Next was drilling and tapping the cylinder cover holes 5-40. I did this with x,y moves on the DRO after centering up the cylinder. I drew up the pattern with the required PD and hole placement using CAD as I don't have a hole pattern function on my rather old DRO. Same operation on each end of the cylinder.
The cylinder shown against the print. More ops still to come!
I lapped the cylinder to get it as round as smooth as possible. I used Timesavers abrasive material and a 1" brass lap progressing from course to medium to fine.
I turned the lap as slow as possible in my lathe while hand holding the cylinder. It always amazes me how much difference lapping makes. You THINK you have a smooth, round hole, but lapping improves it greatly. I went thru this process as I intend to use an o'ring for the piston seal.
Next comes the scariest part, at least to me! One of my fears, is that by this point in the process, I've put some much time into the part that screwing it up during the final ops would be very painful!! This is drilling the inlet and exhaust ports between the cylinder and the valve chest. The print calls for these holes to be drilled at 20 degrees so a bit of a tricky setup is needed. I don't have any sort of any compound sine vise but I do have a Kurt vise mounted on my mill and I have a small toolmaker's vise. So the first thing was to get the toolmaker's vise mounted in the Kurt at 20 degrees. I used a 20 degree angle block, inverted it in the toolmaker's vise and indicated it flat.
Then after setting up the toolmaker's vise at the correct angle, I used a tooling ball inserted in an adapter that fit the cylinder.
Then indicating the ball in order to find the 'center' of the cylinder, which of course is really the center in Y but in X is center as included, but not when flat. Soooo, recalling high school level geometry and trigonometry, it is possible to find the required dimension of the print, which is specified when the part is flat.
I didn't show how the operations started, but once I found the required dimensions, I used a 1/8" end mill to create a flat spot at each location in order to start the drilling operation. This is necessary so that the drill starts straight. The drilling operation is shown. The drill need to end up in each of the inlet ports. The same operations need to performed on each end of the cylinder, but I don't show the operations on the other end.
Just to prove that the holes are correct, I shined a light from each end and it can be seen in the ports within the valve chest.
First end:
Second end:
I didn't the operation but the prints call for the inlet ports to be enlarged fully to 1/8" width and 1/8" deep. I didn't show the operation, but this just involves placing the cylinder in the vise in the flat position, find center, establishing the cutter depth and making the cuts. Shown is the finished cylinder end, same on both ends.
At last, we're at the very last operations on the cylinder!! This is drilling/tapping the inlet and exhaust ports. The prints call for 1/4-40 and I'd never run into this particular thread before. Some searching revealed many standards for model pressure piping and I became ever more confused the more I read. It turns out that I very possible got some bad taps from a vendor (I'll document more issues separately in the appropriate forum) but I DID end up with taps that worked that are the METP (Model Engineering Taper Pipe) also known as MTP (Model Taper Pipe) in 1/4-40 size. These are of course taper type taps so tapping depth is critical. So after I sorted this all out, here are the pix.
I placed the cylinder as required in the mill vise and then inserted a 1" rod thru the cylinder and indicated it to make sure the bore of the cylinder was horizontal. I did this to assure that the to be drilled ports landed in the correct locations.
Since there's no real reference to these port locations, other than 'center of boss', I inserted a 1/2" rod in the mill spindle and visually centered the rod over the port boss.
Then I skimmed the boss with an 1/2" EM taking about .005" off just so as to clean up the surface.
Then drilling thru with a #5 drill.
Then tapping with the 1/4-40 METP tap. I checked the depth frequently to assure the taper was not too deep nor too shallow.
Each port completed and shown with some brass fitting from PM Research. Going forward I'll probably make my pipe fittings as I did also purchase the require dies.
Using a light to make sure the exhaust port is drilled properly.
And finally, smoothing the valve chest bottom face using some fine emery cloth and using the slide valve for pressure.
Whew! I got thru the entire part with no major mistakes! This took several days as I spent a LOT of time thinking about the order of operations and how to do certain things. Choosing the inlet/outlet threading method also took some time as there seem to be sooo many standards and approaches plus getting some bad taps initially also slowed me know.
In any case, took a look and enjoy!
Mike