Machining a Piston from a Casting

[hermetic]

Looking at the above picture, it strikes me that as long as the gudgeon pin hole is perpendicular to the bore and the con rod bush, a little axial displacement in the bore is immaterial. Of course if it is NOT perpendicular, then it won't work!! I assume this is not a two stroke, and axial position of piston in cylinder is not critical to port positions and ring pegs? I have none to check on, but I am sure I have seen this defect on mass produced replacement pistons! the lack of a core in the casting, which would give a position on the outside of the piston leaves you relying on the concentricity and uniformity of the gudgeon pin bosses inside the piston as a centering point, and at the end of the day, they are only as accurate as the patterns.

Phil

[ieezitin]

ok I see what your saying.

you have a casting of a piston that has no reference points to it, its fat in every dimension but there is a piston hiding within it, its cast that way so as you know there is room to remove material from every facet to machine out a finished piston head, which poses a problem... you have to start somewhere right.. so in studying what you have you have to decide what are the critical references within the shape to make a start from, references B & C is a great place to start because you hit center and you hit square for the pin. Everything will be dimensionalized from it.

i see in the picture the boss housing relative to its bore appears to have a wall thickness difference, it doesn't make it wrong if the bore was put in true to the center of the piston head, it could have been cast slightly wrong ... casting are not exact within reason... or they bored the gudgeon pin hole not 90 degrees to center i dont know.

going back to a starting point, once the flat surface has been machined and once the piston head is on the mill table the bosses can be eyeballed to get the best line of attack to square the edges. You do however need to know the head thickness that should correlate to a distance from the head surface to the center line of the gudgeon pin bore as this related to how much you take off the top of the piston relative to the pin bore.

do you have a detail drawing of the piston head, if so send me a copy and i will give you the machining steps to machine it.... its very hard to cover the variables without knowing the facts to deal with..

all the best   Anthony..

[hermetic]

OK Zoot, just re read the thread, you can't drill and machine one boss, then index round and drill the other, they must be drilled then bored in one pass from one side of the piston, or the holes will not be in perfect alignment. Just looked up "boring gudgeon pin holes in pistons" on youtube, the blanks start off with a roughly positioned hole which is bored from both sides at once, but on a cnc machine that level of accuracy is built in! In old manual machining , all bushes or bearing positions that are to recieve a shaft are line bored to ensure alignment and concentricity. It will have to be pretty damn close, even for a Harley;-) (My brother has a road king)

[f350ca]

If you can mount the piston on a rotary table as you said, start with the pin horizontal. Use a height gauge to get the bosses flat, then rotate 90 degrees. Centre and bore.

[Zoot Suit]

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

[TomG]

I would do as Greg suggested with the dividing head, or just eyeball the basses perpendicular with a square, Either way, start out with a smaller hole to check location before boring the holes to finish size. The location of cast bosses cannot be relied on.

Tom

[Zoot Suit]

Thanks Tom. I was already erring towards that method already as I only have 2 to do.

Now I just need to wait for the foundry to call me to say the castings are ready.

John