Quick Reference Guide
Casting Design
Near Net Shape
Casting a part very close to the required end shape so that
it requires almost no machining. Pentz produces castings
to the closest to near net shape in the industry.
Reverse Engineering
In the casting industry, reverse engineering (RE) is the
process of using an existing part as a model to create
a CAD file to produce either a prototype or production
parts. At Pentz, the CAD file is created from highly accurate
digital images produced by laser scanning or by CMM measurement
of an existing part. The CAD files may be used to produce
prototype models from SLA or FDM prototyping methods or
CNC machined tooling for production parts. Find out more
about our Reverse Engineering
Process.
Datum Points & Planes for
Castings
A datum is the origin of dimensions. It is a feature on a
part from which other dimensions or points are referenced
to specified limits. The use of datum plane designations
and locating points are critical factors in casting design.
Careful consideration should be give to where datums are
placed. They directly impact the ability of tooling, machining and inspection to meet requirements. We strongly recommend
contacting us early in the design process to assist you
in establishing datum locations. Contact
our casting engineers for additional information about
datum points and planes for castings.
Draft for Patterns & Castings
The term draft can apply to either a pattern or casting.
Pattern Draft is the taper allowed on the vertical surface
of a pattern to permit withdrawal of the casting from the
mold. Casting draft is the actual draft angle on the casting.
It is determined during the design process. Normally draft
begins at the parting line. Draft angles vary up to
5° with 3° typical on most patterns.
Parting Line and Parting Line Tolerances
The mark or line on a casting formed at the junction of cope
and drag section where the mold parts. Its location and
draft are determined during the design process. It is normally
removed by grinding and or sanding. The tolerances across parting
lines are added to basic linear tolerances. See Dimensional
Tolerance Tables.
Cast In Features
Features of a casting formed by the pattern, mold and cores
rather than machined as a post-casting operation. Typical
cast in features include walls, fillets, ribs and bosses.
There are some general design guidelines for these features.
The designer should, whenever possible, create tapers in
features that widen as they get closer to metal feed points
in the tool. In addition, when joining thicker and thinner
features it is generally preferable to use a gradual taper
to make them of uniform thickness where they intersect.
Fillets should be used at intersecting surfaces for improved
flow characteristics in tooling and to provide for more
uniform distribution of stress in service.
Cored Out Features
Features formed by cores inserted into a mold. Typically
they are hollowed-out interior or cavity features in a
casting which could not be formed using a pattern or by
machining. Tolerances for cored out features should be
somewhat looser than tolerances for features formed by
the pattern.
Metal Inserts, Cast in Part
Metal inserts may be cast into parts by placing the inserts
into the mold before pouring the alloy. Cast in insets
are typically used when they are to function under very
heavy stress loads. Most inserts are made of steel or iron
that is machined or cast. Sufficient metal thickness should
be allowed around the insert to encapsulate it sufficiently
to prevent it from cracking the casting.
Finish Allowance
Additional cast on material (machine stock) that is to be
machined away from the surface of the casting. Finish allowances
should be specified on the part drawing using commonly
recognized symbols for finish allowance Pentz can assist
with this. See Dimensional
Tolerance Tables for Typical
Machine Tolerances.
Surface Finish “As Cast”
"As cast” finish is typically 200-500 RMS depending
on alloy, size of part, molding process and tooling quality.
Pressure Tight / Vacuum Tight
The ability of a casting to be impenetrable to gas or liquid
under pressure. With our processes there is no need to
impregnate the metal with special coatings to achieve
pressure tight/vacuum tight performance. Our processes are
engineered to produce pressure tight/vacuum tight castings
using rotary degassing, flux injection and special certified
alloys. Click here to see pressure tight/vacuum tight applications.
Corrosion Resistance
Corrosion resistance is the resistance of alloys to loss
of volume, strength, or surface appearance after exposure
to various environments. Contact a Pentz
engineer for help
in selecting the aluminum alloy best suited to meeting
your corrosion resistance requirements.
Basic Linear Tolerances
Basic linear tolerances refer to dimensional tolerances that
are attainable within the same mold half. Additional tolerances
for machine allowances, coring and parting lines should
also be considered in dimensioning the overall casting.
See Dimensional Tolerance Tables below.
Dimension Tolerance Tables
View Dimension
Tolerance Tables (PDF) for
tables of basic linear tolerances, additional tolerances
for dimensions across
parting
lines and typical
machine
finish allowances for green
sand and permanent
mold processes.
Pattern Shrink Factor
The allowances made in the pattern to compensate for changes
in volume of metal during casting. Rates of shrinkage
vary with the type of alloy cast, tool design and casting
process
used. Pentz calculates the shrink factor required in
the pattern for you.
Lettering and Graphics
Letters or graphics can be cast in as raised or recessed
images. The image may be created by several methods. The
most common method is to use customer-supplied art work
to create a photo-etched plate that is assembled into a
pattern. Another increasingly popular method is to use
data from 3D CAD files to CNC machine a 3D image directly
into the pattern.
Letter or graphic images should be oriented at or near parallel to the parting plane, whenever possible. Cores can be used to incorporate lettering or graphics that are oriented perpendicular or vertical to the parting plane.
RMS (Root Mean Square)
Standard method to specify the texture/roughness of a surface
of a casting finish.