[PSUBS-MAILIST] casting parts

[Sean T. Stevenson via Personal_Submersibles]

There are a few reasons:

1) Economy of process. Creating a casting entails creating a master pattern, usually by a pattern maker at the foundry or associated with them. Once this is done, additional parts can be obtained for the cost of an additional heat / pour.  For multiple similar parts (six identical hatches) this is cheaper than machining them all from billets.

2) Economy of waste. Machining from billets obviously is less material efficient, as much of the billet ends up as chips on the machine room floor. With a casting, extraneous material exists in the form of risers, flow channels and flash that must be trimmed, but in many cases these wastes can be cleaned and remelted in the furnace for reuse.

3) Suitability to size.  Some parts which would be awkward to handle or difficult to machine without access to suitably large machines can be more readily cast at lesser cost.

4) Efficacy of form. The casting process allows you to readily incorporate large sweeping or compound curvatures and other features to reduce local stress concentrations or improve aesthetics, without necessitating multiple machining operations or tooling change outs to do so. Often, recreating a cast part with machining operations exclusively is prohibitively expensive. The design process is different for each - with machined parts, you need to think about machine envelope, repositioning, cutter clearances and interferences, tooling, avoiding impossible operations, and so forth. With castings, there is greater freedom of form, although you have to avoid thin-walled parts, apply draft to parallel edges to facilitate mold release, and consider material flow into the mold and how the part will cool to avoid warping. Often, combining processes gives you the best of both worlds: casting a blank to get the overall desired shape where surface profile is generally not critical, and then
subsequently machining specific faces or features to establish critical dimensions.

This is the approach I am taking - casting the transition rings from the hull shell thickness to the hatch seat thickness to get that smooth large curvature (and a sexier looking part, IMO), where the contact interface is machined in situ after stress relieving the hull, and then also casting the hatch blanks and machining their critical features (contact interface, O-ring grooves) into the blank.

Sean



On April 18, 2016 4:11:09 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi Sean,I am curious why you wouldn't machine your hatch from a heavy
>disk, it would be one piece and any shape you want.Hank
>
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