Preferred is to have inner diameter dimensions and their tolerance listed on the drawing. If outer diameter dimension is given, consider an average of 25% to 30% material thin-out from the starting raw material thickness. This is typical for spin forming.
Sidewalls thin depending on type of material. height of sidewall required and how much pressure is needed to form raw material to mandrel. Sidewalls will vary typically 25%.
Avoid sharp corners. Formed radius should be specified at no less than 2-3 times the material thickness if possible. The radius in corners can equal the metal thickness in some situations. Secondary machining can sharpen the radius if necessary, however this will weaken the corner. Corners and radius will vary typically 30%. The thickness of the bottom will be the thickness of the original blank.
Don't over specify tolerances. If a uniform wall thickness is critical, be sure to specify it on the drawing. Secondary machining may be necessary if you need a specific thickness of dimension on a part of a sidewall. Don't specify it on the whole part. Be sure to specify critical points with tolerances. Don't just give overall tolerances on the print. The tolerance you specify dictates where emphasis will be placed when tooling is developed. Be sure to evaluate the cost versus tolerance -Tighter tolerance increases cost.
WHAT TOLERANCES CAN BE HELD
In some cases a +/- .005 is possible on parts under one foot in diameter. From 12" to 24" inside diameter a normal tolerance should be +/- .010. 2' to 4' +/- .020 and 4' to 8' +/- .030. Roundness, parallelism, flatness, perpendicularity is directly related to the diameter of the part configuration. Type and thickness of material part is manufactured from a tolerance of .010 for every foot in diameter and can be obtained in most cases.
IMPORTANT - Any secondary work that is required on any given part such as drilling, milling and welding will affect the tolerance that can be maintained. The engineer should state whether measurement is to be restrained or unrestrained.
Symmetrical shapes are normal, however parts can be sectioned to achieve a wide variety of geometric shapes. Conical, hemispherical, cylindrical, re-entrant, parabolic, venturi, flanged and dished shapes are specialties.
Any formable alloy or metal can be spun, especially any material that can be formed by deep drawing, formed on a press, hydroformed or formed by fabrication on a press brake.
Spun parts can range from a fraction of an inch up to ten feet in diameter. The thickness of materials range from .015 to .190 in most materials. Part configuration determines the material thickness our machinery can handle.
This plays a major role in the success in manufacturing by way of metal spinning. Tools cost less due to its simplicity of being machined from steel, tool steel and case hardened steel. For shorter runs and prototypes, in most cases, composite materials can also be used. Delivery is quicker due to ease of tooling, therefore lead times are shorter. Ease of design changes lower overall cost from prototyping through production. NOTE: Most tool dimensions can be made smaller at any time.
The metal spinning process actually improves the raw material metallurgical properties by re-aligning grain structures. Tensile strength can be improved, allowing thinner materials to be used. Overall part integrity is actually better with spinning than any other process.
Metal spinning is a process performed by highly skilled craftsmen using a blank which rotates between a chuck or mandrel and a clamped tail stock of a spinning lathe. The blank is formed with spinning rollers, with ample pressure applied by the spinner, the blank is formed over the chuck with the dimensions of tool (mandrel) corresponding to the inside dimensions of the part being formed.
Each job stands on its own merit. If tolerances are tight, tooling may cost more or secondary machining operations may be performed. The Designer/Engineer should specify what is needed.