Next up is the rear frame. I thought that this was going to be the easiest of the 3 major frame components, but it turned out to be the hardest by far. Read the following and weep
The first feature that I decided to do was the rear portion of the frame. I started by blueing the stock up and doing a basic layout. Then I drilled thru right at the center of the radius that forms the junction between the bottom of the part and the back angled part. Since I modeled this I had the xy coordinates positioned the hole with the DRO.
Next the part was repositioned as shown and i chewed away at the material that needed to be removed from the back. I first used a 1/2" roughing end mill. You'll note that i stayed clear of the layout lines at this point.
Then I switched to a conventional 4 flute end mill and brought the bottom feature to dimension and tried to blend that portion to the radius as best i could.
Repositioned the part again and cleaned up the rear surface with a 1/2" end mill. I used an 8 degree angle block to set the part tot the specified angle from the drawing.
And here's the part on the drawing so you can see how it compares at this point to the isometric view.
Repositioned the part yet again in the mill vise and chewed away most but not all of the material that needed to be removed from the interior of the part. In particular I was careful to stay away from the rear of the part as that will need to be cut away at an angle.
And here's the part with most of the material from the interior removed.
Since there's a radius called for in the bottom corners, I decided to plunge in as shown in order to create those radii. And then clean up the remaining material on the bottom and sides. I thought that this might eliminate the inevitable chatter when trying to shove an end mill into a corner.
Then I reoriented the part in the mill vise and used a 4 flute end mill to bring the bottom and sides to dimension.
In this pix you'll see the start of where I went wrong. I thought I had calculated the depth of the plunge of the end mill when I formed the radii in the corners so that when the back was cut at an angle, everything would blend together smoothly. I was wrong!
The part is reoriented in the mill vise as shown. I put an 8 degree angle block under the part to match the angle previously formed on the back. The idea was that this ball end mill would remove the material on the interior back and would blend the edges and corners together.
You can see what actually happened. Somehow I plunged in too deeply and I simply couldn't clean up the interior rear corners.
After crying in my beer, I decided that there was nothing to do but proceed. Here I've reoriented the part 8 degrees from vertical in preparation for creating the 1/2 round opening at the rear of the part. Note that this radius doesn't start at the top surface but about 1/2" lower, but interestingly the center of the radius is at the top surface. I noted at least one other builder missed this and had to notch out the back to clear the connecting rod when it rotates. Here I've found the centerline in y but I'm visually lining up in x as there was really no other surface to reference from.
Then I used my boring head to advance the cut until I had the radius formed as per the drawing. That all went well.
And here's the part laid on the drawing. It matches well except for the terrible eyesore in the bottom rear corners.
Next I turned the part on it's side and milled the features on the right side, first using a 1/2" end mill to remove most of the material and to create the relieved area that will mate with the front frame.
Since the drawings call for a .032" radius at the base of the raised features, I purchased a 3/16" end mill with an .032" corner radius and used it to bring the raised area to the final dimensions. I'm not sure that this was necessary as afterwards you really can't see the radius. So if I were to do it again, I'd not bother with this step.
The interior corners kept bothering me so I tried to fix it up a bit. I smeared some JB weld in the corners and at the rear and then tried to use a small ball end mill to clean it up. I was somewhat successful, but if you look closely it's still ugly. What I should have done was to wait until this part was complete and then use Bondo as a filler with some sanding to smooth things out. That would have been much faster and probably better in the end.
So now onto the other side. I placed the part in the mill vise on some large parallels as you can see along with a 123 block between the part and the movable vise jaw in order to hold it securely. I also blued up the surface and scribed some layout lines to double check myself. Note that I located the upper right corner and set the DRO to 0/0 at that point as the reference for work on this side.
Then I milled away most of the material with a 1/2" end mill.
Then I changed over to the corner radius end mill and brought the two raised features to dimension.
And finally I used a 1/2" end mill to reduce the height of what will be the cam boss to the height spec'd in the drawing.
And this pix is actually a bit out of order but you can now see how the 3 major frame components will all go together, sooner or later!
There's still some work to be done on this part and it has to be joined to the front frame, but that's all for today.
Enjoy!
Mike