How to accurately and consistently machine a die cast component?

This is sort of a general question. I'm not working on a specific project that requires die casting followed by machining.

I'm wondering how you accurately and repeatably machine a series of "identical" die cast components to add high-accuracy holes, surfaces, features, etc? The die cast surface is coarse and uneven, so even from part to part I would expect it to vary slightly. Or do you build into the casting mold a feature/flat surface that you reference when going to machine? I imagine you can just face off a certain face on a mill but how do you know exactly how much material to take off from part to part?

Part of what made me think of this is based on watching engine block manufacturing videos. None of them go into the nitty gritty (Nacho Libre, anyone?) of how they set up the cast block on the CNC, or any of the details between casting and machining.

Thanks for the feedback!

Engine blocks in cast iron are very much in error could be 1/16 or 1/8" and more error so the first thing is to mill or drill and ream a locating reference then every thing is made to those reference points.

Yes there is precision die cast and lost wax process that can make very close parts.

michiganbuck said:

so the first thing is to mill or drill and ream a locating reference then every thing is made to those reference points.

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So that makes sense, but how do you know that even the ream or milling is consistent if the error on the casting could be 1/16-1/8"? Like if you know to mill .025" off of a reference surface, how do you know that .025" gives you the same reference surface relative to say the cylinder bore center point. Or if you have a cast blind hole that you ream, how do you know the bit or reamer will center itself exactly the same each time on that cast hole?

Never worked on die castings specifically, but the castings I worked on during my time at Sikorsky had tooling points included in the part design. As michiganbuck said, these were first machined to establish a reference location. The parts had cast pads for work supports, and IIRC, we machined the tooling points and zero locations on one side along with any critical features (usually in a VTL with milling capabilities to locate critical bores at the same time as the tooling points), then flipped it into another fixture that set the part on the now-machined tooling points and went from there. Those tooling points were carefully placed to provide maximum support for the part and used the 3-2-1 rule (3 pads on the bottom for Z, 2 in Y, 1 in X). The fixtures themselves used tooling balls or ground reference flats mounted to the pallet as the work offset reference rather than picking anything up directly off the part after the first op.

mazzotta.al said:

So that makes sense, but how do you know that even the ream or milling is consistent if the error on the casting could be 1/16-1/8"? Like if you know to mill .025" off of a reference surface, how do you know that .025" gives you the same reference surface relative to say the cylinder bore center point. Or if you have a cast blind hole that you ream, how do you know the bit or reamer will center itself exactly the same each time on that cast hole?

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The casting is typically designed with enough material left in critical areas that it doesn't matter, and important holes are machined into solid material rather than leaving a rough bore that you try to finish later (unless it's very large, like a cylinder bore, in which case you'd use a tool that doesn't follow the existing hole). If everything is referenced from the same points, you don't need to worry about errors in non-critical areas of the casting, though the more precise you can make a casting, the lighter it will be overall if weight is a concern.

QQT: [how do you know that .025" gives you the same reference surface]
Sand casting an engine block not anything is going to be trusted to be .025 close.

You don't so the error, shrinkage, cope and drag parts fitting error all have to be considered so there is enough finish stock for machining, and the just as cast areas might fit into where they have to go.

Hi mazzotta.al:
You wrote:
"but how do you know ..."

Short version is....you don't.
As a process engineer, you choose your raw stock for the attributes you want, if one of those is a complex shape, a casting is often a decent start point.
But you have to accept the limitations of the stock you choose, and one of them is that castings are not super consistent...it's just how they are.

So if it must be both very complex and very precise you have an expensive part on your hands.
You can either machine it from a raw lump, or you can machine it from a "near net shape" blank like a casting, and leave material you can machine away everywhere you need precision.
If you need precision all over the part, it can certainly start as a casting, but there won't be much of the raw casting left when you're finished.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

mazzotta.al said:

This is sort of a general question. I'm not working on a specific project that requires die casting followed by machining.

I'm wondering how you accurately and repeatably machine a series of "identical" die cast components to add high-accuracy holes, surfaces, features, etc? The die cast surface is coarse and uneven, so even from part to part I would expect it to vary slightly. Or do you build into the casting mold a feature/flat surface that you reference when going to machine? I imagine you can just face off a certain face on a mill but how do you know exactly how much material to take off from part to part?

Part of what made me think of this is based on watching engine block manufacturing videos. None of them go into the nitty gritty (Nacho Libre, anyone?) of how they set up the cast block on the CNC, or any of the details between casting and machining.

Thanks for the feedback!

Click to expand...

Don't assume die castings are all wonky. They can be pretty precise castings just not in comparison to a finished machine component. In large scale castings, there are reference points cast into the part that are used for locating during machining. The casting mold will be configured to hold not just that reference hole but the adjacent surface to a pretty tight tolerance allowing machining to be performed from those points. In some instances, the machine operation will include a spindle probe to locate a tight bore or surface relationships. In most instances, you will machine any portion that needs to hold a relationship to another feature.

I've done machining of castings for military parts before, which had some pretty tight constraints. Typically the print will specify datums from which everything will be measured, and you can use these to locate your intermediate reference features. Alternately you may need to do a "best fit", and machine your reference features based on that. Bear in mind that if you fixture on your intermediate reference features you're adding a small but real amount of inaccuracy, and if you then need to machine additional reference features from there for a later operation those inaccuracies stack. One part I recall started with a combination of datums and best fit to turn a bore and shoulder, then fixture on that bore and shoulder twice for two milling ops.

There are investment mold castings (unlike die castings) that have incredible accuracy +-.001" on surfaces. I used to make helicopter parts that were cast with investment molds in stainless steel. They normally had a couple of flanges and one floor that all dimensions referenced from and all dimensions were held to that.
I would suspect that die castings could be very accurate if done properly but I'm no expert although the entire casting trade has made major improvements in the last several decades.
Dan

Roboman01 said:

...The casting is typically designed with enough material left in critical areas that it doesn't matter, and important holes are machined into solid material rather than leaving a rough bore that you try to finish later (unless it's very large, like a cylinder bore, in which case you'd use a tool that doesn't follow the existing hole). If everything is referenced from the same points, you don't need to worry about errors in non-critical areas of the casting, though the more precise you can make a casting, the lighter it will be overall if weight is a concern.

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This. I ask my customers to leave at least .100" per side on any machined features to allow for cleanup. Granted this is for larger sand castings. Some castings we have them add specific reference points, ie 3 dimples to locate off of with bullnose dowels so we can do it all in one op on the rotory. Precision castings you can get away with much less stock.

As others suggested above, design in cast features for initial machining locators. Have all these features on one side of the die cast mold parting line to avoid inevitable errors due to die casting mismatch across the parting line.

Hi ,

My job is machining projects for die casting. I design 20+ fixtures per year for batches between 1k-50k. Die cast parts are generally very precise compared to sand casting.There are many considerations to make: generally I use positioning points on 1 side of the casting. Dimensional variation in the casting is in the thickness in the open-close direction.
If holes are cored , you have to possibilities: either you put in a drill/ reamer that follows the hole, or use an endmill at the theoretical position to the Datums. surface milling is in itself not difficult if your part has clamping ears.Beware that the part can deform a lot when you unclamp it.

machining a casting

I didn't see it mentioned, but it is also a good idea to verify the casting has material in all the correct places before beginning, or planning, the machining. I worked with small batch castings and I was provided with a casting model and a machined/finished model. This let me easily compare the two and do some quick caliper checks to verify I could set up the part like this, machine .05" off here, here, and here, then flip/rotate onto a fixture that used those machined points as the new datums from which to finish machining the rest of the features.

edit: another thing is to work off of 3 points, and/or a way to 'level' a casting so as not to introduce clamping stresses when machining...