[PSUBS-MAILIST] casting parts

[Marc de Piolenc via Personal_Submersibles]

Well put.

There's an additional advantage now, which is that masters can be 3-D 
printed from CAD files, so the master pattern maker's services are not 
required.

Marc

On 4/19/2016 9:05 AM, Sean T. Stevenson via Personal_Submersibles wrote:
> 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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