[PSUBS-MAILIST] casting parts

[Sean T. Stevenson via Personal_Submersibles]

Yes, but only to create the necessary machined surfaces and features. You wouldn't need to hog large volumes of material out of a billet, for example.

Sean


On April 18, 2016 7:57:17 PM MDT, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,
>
>I would think you would still need to machine this hatch part after in
>comes from the foundry, no ?
>
> 
>
>brian
>
>--- personal_submersibles at psubs.org wrote:
>
>From: "Sean T. Stevenson via Personal_Submersibles"
><personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>Subject: Re: [PSUBS-MAILIST] casting parts
>Date: Mon, 18 Apr 2016 19:18:07 -0600
>
>Actually, I would think that casting would be a useful process for a
>hands-on guy like you, Hank. You could probably do your own patterns
>from plywood and Bondo and then deal with a foundry without having to
>pay for the pattern making.  Just do some research first - you need to
>accommodate shrinkage, and the draft requirement.  Also, you do need to
>100% NDT the completed castings. Unlike billets / plates, voids can be
>quite common in cast parts, and these need to be detected and corrected
>for our application.
>
>Sean
>
>
>On April 18, 2016 7:05:01 PM MDT, "Sean T. Stevenson via
>Personal_Submersibles" <personal_submersibles at psubs.org> 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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