Precision Machined Products

Precision Machined Products Buyer's Guide

PMPA The Precision Machined Products Buyer's Guide* has been prepared as an aid to those responsible for specifying and purchasing screw machine products and turned parts. Its aim is to promote the understanding of the many factors which affect the costs of components produced by the Precision Machined Products Industry.

For a handy reference chart click here Metric Chart

*Abridged version. Reproduced from the PMPA Web site.

WHAT	WHAT / HOW	WHAT / WHY
QUANTITY	Check annual requirements and advise supplier. On re-orders, advise supplier of any changes from the original prints.	With an annual contract the supplier can minimize: tooling, setup costs and material costs. Greater flexibility for the most economical method of manufacturing is allowed.
OVER AND UNDER RUNS	Ordering exact quantities increases costs.	Allowance must be made for overage in production which must be prorated in the quoted price. Over/under run quantities in the Precision Machined Products Industry may vary based on bar length, part size and equipment used to produce the parts.
DELIVERY	Allow sufficient lead time for normal planning. Provide your supplier with the best forecast you have.	Quick deliveries involve premium material and labor costs. A longer/better forecast permits more economical manufacturing runs.
PACKAGING	If special packaging, labeling or bar coding is required, advise the supplier of this when the request for pricing is made.	Unusual packaging requirements are expensive due to special material and labor. If advised in advance, a supplier may be able to provide alternative, less expensive packaging.
MATERIAL	Design and application permitting, specify standard sizes of free machining grades.	Standard sizes cost less and are more readily available. Free machining material yields greater production, improved finish and closer tolerances.
TOLERANCES	Specify no closer tolerance than absolutely functionally necessary. Be sure that the tolerances are shown on the drawing and are practical and consistent with the application of the part.	If required, suppliers of precision machined products can and will make the parts with close tolerances. However, higher costs can be expected the closer the tolerance requirements.
DIMENSIONAL RELATIONSHIP	For slotting, cross drilling, milling, broaching, etc., do not specify definite relationships between dimensions unless required. Be sure all dimensions are shown clearly and add up.	Specifying a close relationship requires expensive fixturing and slows production.
CONCENTRICITY	Concentricity as a dimension should be specified to the maximum allowable limits.	Concentricity of dimensions often determines how a part will be produced. Holding closer limits than required can cause considerable problems in manufacturing and inspecting.
INTERNAL DIAMETERS	Hole diameters should be specified to the maximum allowable limits.	Suppliers of precision machined products stock standard drills, reamers and plug gages. Special internal diameters can be produced where required with special tooling.
SHARP CORNERS	Unless there is a functional need for a sharp corner, specify chamfers or radii at all intersecting surfaces.	Sharp corners nick easily and require special handling. It is also more economical to produce a part with "broken corners."
THREADS	Specify standard threads, American National Standards Institute (ANSI) Unified Inch Screw Threads (UN and UNR Thread Form) or other ANSI standard threads. Use class 2a or 2b fits where application permits. For I.D. threads choose the least percent of thread that gives the required strength.	Special threads require special tools and gages. Closer fits may require additional operations. Full threads are harder to tap.
BURRS	Do not specify burr removal unless necessary to part function.	Burrs are a natural result of any machining operation involving intersecting surfaces. Burr prevention and removal may require additional machining or second operations.
FINISH	Clearly specify surface finish no closer than necessary to meet functional requirements.	Finer surface finish may require special tooling, slower cycle time or additional second operations and special handling and packaging methods.
QUALITY	Be as detailed as possible with the quality requirements. Specify which characteristics are of greatest importance to product performance. Acquaint the supplier with any internal quality control requirements, so that production quality methods will be consistent. Indicate initial sample requirements, specialized data requirements and inspection routings as part of the request for quotation.	This will allow the supplier to focus on what is important and to follow parallel or compatible quality procedures.
GAGES AND GAGING EQUIPMENT	Wherever possible, part design should avoid the need for special gages.	Special gages and gaging equipment add to part cost. Customers may supply special gages to eliminate additional gage costs.

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