06-12-2017, 03:05 PM
Thanks to everyone for their input.
I think that in summary there is not some clever method of indexing the bosses that I hadn’t thought about.
The 2 favourites so far (in my estimation) are:
1. Hold the piston by the crown end in a rotary table, get the bosses level and then turn 90 degrees. I think I would add a stage by getting them level and then turning by 180 degrees and then measuring how level they are on the opposite side and then splitting the difference on any variance and then turning by 90 degrees.
2. Make a mandrel that fits inside the piston and cut a vee in the top that locates on the bosses. This method would enable much quicker setup once the mandrel has been made but each mandrel would only be suitable for one piston size.
ieezitin, to answer your specific questions. Yes the casting has no external reference points and is bigger in every external dimension than is required. There is a band around the inside base of the skirt that is a bit thicker than the area above it (below it in my upside-down photos).
Regarding machining sequence I think I would do this.
1. Hold the piston by the crown end in the 4 jaw and dial it in as close as a raw casting can be. and then machine the skirt flat. Dont take too much off yet. Also machine the inside of the skirt.
2. Measure from the freshly machined base of the skirt to the bosses to check that the bosses are cast correctly and that the piston had been dialled in correctly for the first machining operation.
3. Turn the outside diameter of the piston. (Note.Its not obvious from my photos but the top and bottom of the piston sides are slightly larger diameter than the area in the middle of the piston sides.)
4. Flip the piston around and clean up only the crown (again don’t take too much off yet) and turn the remainder of the outside of the diameter.
So far you have only been turning a cylindrical shape.
5. Bore the gudgeon pin holes using one of the 2 methods outlined in the first part of the post.
6. Once the gudgeon pin holes are bored then these can be used to establish the distance from the pins to the crown and then take the appropriate amount off the crown. Note it would be wise to establish the thickness of the crown before doing this.
7. Cut the ring grooves
8. Then the skirt can be brought down to the correct distance from the finished crown.
Note I have never machined a piston before so the above is just my thoughts rather than based on any experience.
This is a bit of a convoluted method that is OK for the two that I need to do.
John
I think that in summary there is not some clever method of indexing the bosses that I hadn’t thought about.
The 2 favourites so far (in my estimation) are:
1. Hold the piston by the crown end in a rotary table, get the bosses level and then turn 90 degrees. I think I would add a stage by getting them level and then turning by 180 degrees and then measuring how level they are on the opposite side and then splitting the difference on any variance and then turning by 90 degrees.
2. Make a mandrel that fits inside the piston and cut a vee in the top that locates on the bosses. This method would enable much quicker setup once the mandrel has been made but each mandrel would only be suitable for one piston size.
ieezitin, to answer your specific questions. Yes the casting has no external reference points and is bigger in every external dimension than is required. There is a band around the inside base of the skirt that is a bit thicker than the area above it (below it in my upside-down photos).
Regarding machining sequence I think I would do this.
1. Hold the piston by the crown end in the 4 jaw and dial it in as close as a raw casting can be. and then machine the skirt flat. Dont take too much off yet. Also machine the inside of the skirt.
2. Measure from the freshly machined base of the skirt to the bosses to check that the bosses are cast correctly and that the piston had been dialled in correctly for the first machining operation.
3. Turn the outside diameter of the piston. (Note.Its not obvious from my photos but the top and bottom of the piston sides are slightly larger diameter than the area in the middle of the piston sides.)
4. Flip the piston around and clean up only the crown (again don’t take too much off yet) and turn the remainder of the outside of the diameter.
So far you have only been turning a cylindrical shape.
5. Bore the gudgeon pin holes using one of the 2 methods outlined in the first part of the post.
6. Once the gudgeon pin holes are bored then these can be used to establish the distance from the pins to the crown and then take the appropriate amount off the crown. Note it would be wise to establish the thickness of the crown before doing this.
7. Cut the ring grooves
8. Then the skirt can be brought down to the correct distance from the finished crown.
Note I have never machined a piston before so the above is just my thoughts rather than based on any experience.
This is a bit of a convoluted method that is OK for the two that I need to do.
John