Should I use use center of stock for molds origin?

[BillGEGHV]

Hi All,  Im getting ready to machine some molds that will be used for cold cast urethane.  Molds will be milled in 6061-T6 and are quite complex as far as the geometry is concerned.  There are only 2 small flats in the entire mold, with the rest of the mold pocket being organic surfaces.  Each mold will key together with precession dowl pins, and there are 2 molds for a L&R part when cast.  Not sure it matters but im on a Tormach 1100M. 

Im wondering what my best option is for setting my origin. I usually use the Upper left for my origin but am questing if I should use the center of the stock for my origin. 

Here is a screenshot, sorry it's not so clear as i can not show the product.  

[HughesTooling]

I always work from the centre because the centre is always the centre no matter how you set the part up. If you can machine the part in one go from one side it might not matter as much though. On the moulds I make they might need to be machined from both side and on different machines. I've seen some expensive screw ups when someone's used different corners when they've flipped a part over and rotated the part 90° and lost track of the corner they where supposed to use!

Mark

[leo.castellon]

I also use center of stock in case you have to go back and make a modification to your mold, it will be much more accurate this way. 

LeoC

[BillGEGHV]

Thank you, guys, for the input, center does make more sense.  

1 more question, after a mold 1/2 is machined If and when I need to go back in months later to re-machine something all I need to do is probe the outer walls to get my origin to center, but this time it would be off-machined surfaces so accuracy should be dead on correct?  The challenge I have with these molds (probably not unlike many injection molds) is that there is really no square geometry in the pocket that makes up the actual part to probe off However there are several precision holes for key pins and bolting the molds together.  There are also some pri-points which actually are 90-degree pockets. 

OR am I better off probing off some of the holes, which leads me to my next question? Can you probe 2 or more holes to establish an origin?  

Just want to make sure I get this right, I am making 20 of these molds. 

[leo.castellon]

The way we used to make molds is to make the mold bases first. Square up the blocks, drill and ream the alignment holes, machine the pry slots. Then pick a pair of bases, machine from center of the block, kiss the top surface and then machine the interior. You could use the center of one of your alignment pins as your reference also, just sweep it in with a test indicator. Just pick a method you feel comfortable with and be methodical in your procedures and you should be fine. 

LeoC

[HughesTooling]

I usually work from ground blocks so as long as I centred up accurately it's not a problem coming back later. Sometime if I think I might have a problem I've milled 4 short pads to centre up off.

Mark