Casting/sand cast design techniques

I've worked a lot in plastic part, injection mold and die cast design. Now
I'm trying to apply some of this to the sand cast process. I worked with
someone recently who has a particular way of doing it which instinctively
looks inefficient, but I'm unclear how best to improve it. His process is
as follows:

- build casting (engineered part) w/o draft
- build "template" (casting with core areas filled in)
- build "core template" (modeling empty space in casting)
- build "pattern" by cutting casting at the PL - add draft here
- build core patterns to make cores
- create halves of cast from drafted patterns
- assemble halves of cast and cores
- create a test part from halves of cast and cores
- compare test part to original cast part design

Intuitively, I want to start with a casting that has the draft on it, and
create the halves of the cast and the cores from there, then make the
patterns from that. Basically I'm trying to work like I do with injection
mold process.

How are other people manufacturing sand cast parts doing this?

thanks in advance...

matt

 

Matt,

You're right, the process is almost identical. The difference is that the
casting "should" have two configurations, as cast, and machined. The casting
model should represent the real world just like any other process. All of
the cores/cavities should be taken directly from the model. All feature
changes, or additions/subtractions to the cores/cavities, should be made to
the model, not the tool components.

It sounds like your friend is very knowlegable of old school pattern making,
and He's trying to model this process. No big deal if that's what he's
comfortable with, but it is convoluted. Problems may arise if someone else
has to work on the data. Also, design changes will be much more prone to
error.

Regards

Mark

 

Exactly right. He's an old timer to be sure, and I think he got his
start in the wood pattern-making business many moons ago, moved to board
drafting, then Cadkey, then Pro/E, then SW. I assume he developed this
way of working from his Pro days.

The only problem I see with trying to do it the same way as you would do
a plastic part is that on the casting, if you have areas where you need
to use draft to add material at the PL, SW can't do that. Well, it
*can* do it using Step Draft, but only on one side of the PL, not on
both.

I've put in an enhancement to see if we can get a draft feature that
will add material to both sides of the PL, based on a neutral plane at a
given distance from the PL.

The old timer's method gets around this limitation by essentially making
the two sides of the PL separate pattern parts, so he can change draft
however he likes without worrying about the other side. The
disadvantage here is obviously that you have to keep track of what
you've done where to make sure the PL matches on the patterns for the
two sides.

Any ideas about that?

matt.

 

Matt,

Can't you use multi-bodies to some advantage for this ???

Regards

Mark

 

hmmm

yes, I suppose you could. Split the part at the PL, make the draft, and
join it back together. a bit of a kloodge, but it would work.

If anyone else wants to chip in, I welcome more suggestions!

matt

 

Matt, contact a fellow named Bernie Mares. You can find a post or two
of his in the Google archives of this newsgroup. He owns a casting shop
in Portland (OR) and can tell you reams about the approach he takes and
the reasons for it.

'Sporky'

 

Agreed on this as a first step, but I figure once and include the
draft, shrink factor and rough stock from the start.

Setting the cores and coreprints would be part of the first step, this
is actually building the inside features of the part as well.

The parting line is developed in the same first stage, everything is
related from the parting line, the cope and drag halves of the
"pattern/mold halves" as well as chucking on a finished part for
workholding and machining etc. You must know where there may be a bit
of flash or possibly grind. Shrink factor must also be added, if you
forget to figure shrink the project will be doomed.

Still part of the first operation, the cores form the void that will
become the inside features of the casting.

I create the casting with draft, shrink and all features. The parting
is established first, draft works from the parting onto both halves.
I build a seperate set of surfaces inside the casting that represent
the machine stock that is to be removed, remember, you are making a
casting that becomes the machined part. In sand castings, often you
may need to build a few thou clearance into the parting depending on
the squeeze pressure available on the equipment.
I spent 18.5 years building patterns etc for a cast iron foundry.
Food for thought anyway,
Michael

--






Michael Gailey
Artistic CNC Mill, Router and Engraver Programming
3D modeling for Product Design and Development
http://www.microsystemsgeorgia.com/toc.htm

 

I have a related die casting question:
From a tooling standpoint (from the toolmakers view on life, i.e. machining
the EDM tooling) is it better to put radii in the sketch so that the radii
decreases with "pocket" depth (I have been cut-extuding with a draft angle),
or is it better to add the radii after the fact (and end up with "constant"
radii)? The design I'm working on has about one inch deep pockets so I would
like to start with a .062 corner radii (in the "sketch"), but the radii at
the bottom of the pocket ends up very small (.005). Would I be better off
adding radii "after the fact", or increasing the starting radii in the
sketch, or.....?
Jim W.