Edward Koehn Co., Inc.

Buyer's Guide
For the Purchase of Precision Machined Products.

The Precision Machined Products Association (PMPA) 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.

PMPA members provide the best possible service and experience. Plus, they:

Bring accumulated engineering technology disseminated throughout the PMPA.
Offer engineering talent and suggestions for cost reduction, value analysis, component assemblies, and physical requirements.
Provide the advanced machinery and tooling as well as the highly-skilled operators required to produce the parts.
Eliminate the need for inventories of raw materials and high cost tooling, jigs, and fixtures.
Assume responsibility for delivery of specialized finishing operations, sub-assemblies, and high-quality parts.

Look for the PMPA "Symbol of Reliability" ¯ PMPA members will deliver the completed component to your door and eliminate many potential headaches.

WHAT	HOW	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	Order consisting of exact quantities increases cost. 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. Be open to supplier suggestions for material grade types.	(For more information, see PMPA's "Stainless Steel Buyer's Manual," and "Carbon Steel Buyer's Manual.") Free machining grades usually result in lower piece part cost.
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 tighter tolerance parts. 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 gaging which slows production and increases piece part cost.
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) metric standards 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 produce.
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.
SURFACE FINISH	Clearly specify finish no finer than necessary to meet functional requirements.	Finer surface finish may require special tooling, slower cycle time or additional operations and which could result in higher part production costs 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 (i.e. – PPAP, FEMAs, warranties, etc.) 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 critical features.
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.