Machining a Piston from a Casting

[Zoot Suit]

Hi, I have a question that I have been pondering regarding machining a piston.  It occurs to me that this operation has been done millions of times before so I thought I would ask on here as I am sure that some of you guys will have done this before.  Note that this is a first for me.

I have what we in the UK call a vintage motorcycle and you guys in America call an antique motorcycle.  It is about 100 years old and the original pistons are cast iron.  I need to replace the pistons to suit larger bores and pondered as to whether I should go for more modern aluminium pistons or cast iron ones as per the original.  I have opted for cast iron ones.

The castings have been sized to suit the new bore sizes whilst maintaining the same wall thickness as the original pistons. 

The machining operations themselves are simple enough but I do have one query regarding boring the wrist pin/gudgeon pin holes.

The holes obviously need to be through the centre of the internal cast bosses.  Locating the centre of the boss in a line from the bottom of the skirt to the top of the crown seems to be straightforward.  However I have been pondering on the best way to locate the centre line in the direction 90 degrees to this.   Obviously these lines have to be translated onto the outside of the piston.

Any ideas on this question would be greatly appreciated along with any tips on workholding.

Here is a picture of a new casting alongside one of the original pistons.





Thanks,  John

[ieezitin]

I don't think the tolerances are that close on the outside of the piston so chucking it in the 4 jaw machine od up to the jaws then reverse and clock in again.


if that dont float ya boat Chuck up your new casting in a 4 jaw with the piston head facing the chuck which means you can see down into the underside of the piston. Now bore into the wall just stopping at the gudgeon pin bosses, now you have a reference point.

Make an expanding mandrel that fits into the bored hole, now you have a one shot deal on machining the piston, ring slots and drilled hole for the gudgeon pin. The mandrel needs to have the screw that expands the boss activated from the back end of the mandrel.

With the mandrel you can take it from the lathe to the mill for drilling ..

Anthony.

[Zoot Suit]

Thanks for the reply ieezitin, I understand what you are saying (I think) but I cant exactly understand how it will help me.

I understand that the rough casting needs to be machined in the first instance to get a datum to reference from.  Once the crown and skirt have been cleaned up in the lathe then they will provide a reference to get the depth of the bosses from the bottom face of the skirt.

Also I can see that a jig will help to hold the piston without deforming it but will a mandrel locate the bosses to avoid the situation in the below picture when transferring from lathe to mill?  Also, the pistons are approx 3 1/2" in diameter, what would you make the mandrel out of bearing in mind I only need to do 2 pistons?  Do I need to buy some 4" stock?

The gudgeon pin will need boring on, I assume, a milling machine.

What I want to avoid is illustrated by this sketch.




The question is, when transferring to the milling machine what is the best way to index the bosses.  I should mention that a friend of mine has said I can use his Bridgeport (but it doesn’t have a DRO).

I did think about holding the crown in a rotary table held vertically on a surface plate and then getting the top of both bosses at the same height.  The issue with this is I don’t have a surface plate (yet).   Is there another way that I am not seeing that doesn’t need a surface plate?  (I am planning to get one eventually but its not at the top of my list yet.

Thanks,

John

[Zoot Suit]

I have just been pondering this while in the shower and a mandrel that has a vee in the top (going accurately through the centre line) and a flat surface on the other end would be a simple way of indexing it.

Also, I could do like I said above regarding getting the bosses at the same height but instead of doing it on a surface plate I could just do it off the mill table.

It just needed a bit of thought. I am sure that someone who does this regularly would think its obvious.

Maybe I am over thinking it but the reason I am "over thinking" it is because I have already had 2 pistons machined by someone else and they have ruined them (one of the things wrong with them is that the gudgeon pin holes are not bored correctly) . I am now waiting for the foundry to call me to tell me that my replacements are ready.

John

[ieezitin]

John.

look at my crap-o-cad its a vision on one way to do this.. i don't see this being a straight forward set up without making a fixture..

a great problem to solve though in work holding and machining to square.. let me know your thoughts  


Anthony..

[Zoot Suit]

Anthony, what CAD program did you use? :)

On a serious note I see what you mean now but my initial question still stands .

In order to mill bosses B & C to provide a reference for the fixture you need to be sure that you are at 90 degrees to the axis of the pin. You need to get that right so that you end up with my "Yes Please" rather than my "No Thanks". The centreline of the pin is the key.

Like I said, you have to establish that before you can machine the bosses.

John

[ieezitin]

John

its very hard to communicate this problem using this posting method so forgive me if i have not the correct understanding of it..

going back to the diagram once you have faced off the top of the piston you have a Zero point to work from, placing the flat surface of the piston on the mill table allows you establish a center line of it where you can bore a small lip into the now bottom of the piston thats looking straight up, this bored lip acts as a reference for center of the piston also a register for your fixture.

In the same sitting on the mill you now can mill off the gudgeon pin faces ( B & C ) now you have established a square using the known center of the register lip ring. You also have established a zero point or zero degrees in reference to bosses (B & C) mark this point on the fixture.

now to bore the hole for the pin: the fixture seats into the milled boss surface's now if you put the fixture on a chuck or V-Block it can be bolted to a angle plate on the mill or even better a spin indexer, now you can find three important  things.. center of the piston from the fixture, distance from the top of the piston to the center of the gudgeon pin, plus You know where you are in space in angle of degrees....

By doing all this work you have established a center in X Y Z... by knowing all three you can achieve anything you need to do to it... hope this makes sense.

you will spend more time in making the fixture than machining the parts this is always the case but you gain repeatably by getting two equally machined parts and the luxury of knowing where you are in space at anytime.

This is just my view of machining this there are many ways to achieve this some maybe even more simple than mine as they say there are ten ways to skin a cat...

Anthony..

[Zoot Suit]

Thanks ieezitin, that’s helpful reply.  However I think I must be a bit dim though because I am still missing a stage.

I get all of the stages in your explanation and most of it was along the lines of what I had in my mind before I posted this question.

However I still dont follow one stage of your post.  Before milling B & C you need to make sure that the face of B & C is at 90 degrees to the centre line of the bosses.  You skip over that bit and that is the bit that is the whole point of my question.  

If you look at my "No Thanks" sketch then if you machine B & C at 90 degrees to the red lines then you will end up boring the holes so that they are not coaxial to the bosses.

In effect I am trying to avoid this.




Which is what the previous person (NOT me) did to my first set of castings.

Getting the bore to go through the diameter of the piston is simple but it needs to go through the centre of and coaxial to the boss on each side of the piston.

I totally get that setting parts up to machine and making fixtures is quite often the biggest part by far and the actual machining bit is a small bit at the end.

I am still pondering a mandrel with a vee on one end.  Or holding the piston at the crown end in a spin indexer and getting the bosses parallel to the table and then turning through 90 degrees.

When the new castings arrive I will see see what i can do.

John

[hermetic]

I have been pondering this, and trying to work out why this process, which has been done millions of times in industry is so hard in this case, and I think it is perhaps the casting should have had a core in it to give the gudgeon pin bosses a position on the outside of the piston. would it be possible to make a vee block to fit inside the piston and on to the gudgeon pin bosses, but also bee deep enough to fit the lower half of the block in a machine vice, clamp in position on the piston crown, and drill and line bore. Best done on a horizontal boring machine, but maybe possible on a lathe with sufficient centre height. I am intigued, what is the bike?
Phil

[Zoot Suit]

Hi hermetic.  If you look above, one of my thoughts is to make a fixture with a vee in it to locate the bosses.

I agree, its been done hundreds of millions of times before and I assumed that there was a simple answer that I was overlooking.

The bike is a 1920 Harley Davidson "J"

John