Technical, Specification, and Design Guide

ELECTRICAL
Our supplier uses a proprietary electrical test to provide the best electrical grade MgO for high performance, high watt density heater applications.  The end product is a maximum performance in dielectric strength, insulation resistance, and thermal conductivity.

BASIC PROPERTIES
Melting Point (C)    2825 C

Theoretical Density (g/cc)  3.58 gm/cc

Fabricated Density    2.4-2.8 gm/cc

THERMAL PROPERTIES (of MgO crystal)

• Coefficient of Thermal Expansion13.9 x 10-6
• Specific Heat, 70 F (Btu lb-1F-1).22 Btu-lb-F-
• Thermal Conductivity
  • 20 C.480 (w/cm- C)
  • 800 C.085 (w/cm-C)

This guide is intended to help our customers design parts within normal fabrication limits for best value.  Most dimensional tolerances are based on ASTM spec E1652.  These specifications are designed to maximize value and minimize the inspection required to certify the parts.  Tighter tolerance can be offered as needed for engineering demands

O.D. (OUTSIDE DIAMETER)
Nominal O.D. sizes may be specified to the nearest .001” increment from 0.050” (1.25mm) to 1.250” (30mm) inches.  TATEHO OZARK has over 500 round dies in inventory.  Modest tool charges (about $200 per inch ($8/mm) OD size) may apply for parts over 0.5” if a nearby size is not available.

Non-round parts are made in shaped dies.  If a new die has to be cut, TATEHO OZARK maintains blanks which can be finished with good turnaround times, at a price of about $300/inch ($12/mm) dimension.  Only the maximum dimension or diagonal may be controlled by OD tolerances.

The OD size is considered the average of two measurements made at 90 degrees.  Variation above or below the limits caused by roundness may not exceed 20% of the overall OD tolerance.

I.D. (HOLE SIZE)
The minimum standard hole size is the lesser of 0.010 inch (0,25mm), or 15% of the OD.  Smaller hole ratios can be produced with less control of the hole position.  The Maximum hole size is dictated by the wall and web thickness criteria (see table below).  Closely spaced holes may result in “teardrop” shaped holes, or web knitting problems.

The hole size is defined as the largest pin gage size (to the nearest .001 inch (0.025mm) that slides freely to a depth of ten hole diameters.  This definition ignores the actual shape of the hole.  Slight teardrop shapes are accepted.

CONFIGURATION and BOLT CIRCLE
Standard configurations are defined by having equal web and wall spacings.  Symmetric designs are preferable, but not mandatory.

Thin webs and walls lead to misshapen holes due to the tendency to collapse into the neighboring hole.  Insulator shape cannot be guaranteed for wall or web thickness less than the hole size. The minimum size limit for wall and web thickness is 50% of the neighboring hole size.  Large holes may result in loss of roundness and potential for thin webs and/or walls. The following table indicates the suggested hole size for a standard geometry as a multiple of the O.D.

Any other configuration may be specified within fabrication limitations.  Center holes may be added, and unequal hole sizes may be specified.  Please contact us with your request for optimum design specifications for special configurations.

Default Tolerances- Many specifications overlap, are redundant, or ambiguous.  The individual features are described here.

Bolt Circle – The bolt circle diameter tolerance is the greater of 1) the OD tolerance for the part, or  2) +/- 0.005” (+/- 0,13mm), or 3), the product of 0.001” (0.025 mm) times the OD/ID ratio.

Wall thickness / hole position –  The wall thickness measurements of similar walls must be equal to each other within 25% and the difference between them must be less than 0.010” (0.25mm).

LENGTH – Any nominal length may be specified up to 12”. There is a premium for lengths under 2” and a larger premium for lengths under 0.5” (13 mm).  The premium for lengths over 8 inches (200mm) is 10% per additional inch (25mm) of length.  or over 8”.

There is another premium for L/D ratios over 30.  You may avoid some premium costs and obtain better value if you permit up to 15% of your order to be made at a shorter length (and a pro-rated price).

Default Tolerances- A minimum tolerance range of 3% is required to avoid a premium charge.

STRAIGHTNESS – Camber is inspected by rolling the insulators between plates of glass set at the maximum permissible spacing as determined by the specification and the nominal length of the parts.

Default Tolerances-  The default tolerance for heater grade products is .006 in/in (.006 mm/mm).  The limit applies to individual parts and lots are inspected 100% if necessary.

TWIST – Twist is overchecked by spot inspection of samples for the lot.

Default Tolerances-  The lot must average less than 4 degrees/inch ( 0.16 degrees/mm).  No individual parts in the sample can exceed 15 degrees/inch (0.6 degrees/mm).  The tolerance does not apply to parts below 0.200” (5mm).

SURFACE – Features that are cosmetic in nature are acceptable.

Default Tolerances- Per ASTM E-1652:

The maximum depth or lateral dimension of a surface chip, void, or fissure should not exceed 25% of the wall thickness.  (For MIMS cable application, the lateral dimension spec is 25% of the OD).

Shallow defects and positive surface conditions such as ridges, gouges, drag marks, spalls, etc.. are permitted within the envelope of the dimensional tolerances of the outside diameter regardless of lateral dimensions.

Particulate metal impurites are not permitted.  Fully oxidized metallic impurities appearing as orange or brown spots are permissible if less than 0.25% of the surface area in the aggregate.  Non-discrete discoloration appearing as bands or random patterns from forming gradients or firing atmospheres is permissible.

STRENGTH and HARDNESS – TATEHO OZARK designs the proper particle size to achieve the proper result for cartridge wind core applications or for multi-pass drawing into fine cable.  Please inform your application for the best recommendation.

For Cartridge wind cores, TATEHO OZARK recommends the following guidelines:

It is generally safe to vary from these tables by one level above or below, if the parts “feel” right.  Tubular designs are usually fired one or two levels harder.  TATEHO OZARK does not have a similar table for other heater component applications (No-Heat or Cold Cores) due to the variation in end use for different designs or customers.

Please contact TATEHO OZARK for more technical information on electrical, thermal, chemical, and strength properties of MgO extrusions.