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August 3, 1998

4 Min Read
The meaning of commercial tolerancing

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Editor"s note: This article is another in a series by Bill Tobin of WJT Assoc., in which he describes molders' relationships with customers.

All of us have heard the phrase "commercial tolerancing" However it is viewed, it can be simply interpreted as getting what is asked for. Here are some examples.

Packaging

When your client orders 10,000 units, that's what he wants. But most purchase orders give a tolerance of ±5 or 10 percent, or whatever is the closest fraction to a full box. Thus 10,000 units may be 10,100 or 9,950. With small parts we tend to pack by part weight. Larger parts or more stringent cosmetic requirements require layer packing. The pieces per box are what is important. This is what the customer needs and this is how we are paid. However, have you ever audited your box counts? It may be an eye opening exercise.

With an audit, you may discover you have been over- or under-shipping your clients and billing them for a number of pieces that doesn't represent what was shipped. Not only is this bad business, it also reflects poorly on your inability to control your own operation. To eliminate this, institute a two-part program.

  • Have both the person who molds and packs and the quality inspector sign his or her name and mark the quantity on each box.

  • Ask your client to periodically check the amount indicated on the box compared to what is actually inside. This will directly assign the responsibility for accuracy and allow you to implement corrective measures.


Quality

When the phrase "parts to print" is thrown out in a meeting, it is almost like trying to describe the beauty of a sunset over Maui--nice when you experience it, but almost impossible to describe. Too many customers think if the part is to print, it must work. Usually the reverse is true. When the part works, nobody cares about the print. This is fine until you encounter a dimensional audit. Commercial tolerancing is defined by SPI in several of its publications. However, the more important definition comes from the process of product development.

When your part has gone through functional testing and been proven to work, go into a dimensional audit. If the part works, it must be to print. The only problem is the numbers on the print don't reflect this functionality! While it sometimes seems like a root canal without the benefit of drugs, it is important to extract a drawing change from the client so the functional part and the print match each other.

Your production window of varying process conditions and dimensions that allows you to make a functional part now comes into play. It is more common than many molders would like to admit that the final engineering change (which should be the release to production engineering change) looks at the dimensional audit of the three pieces submitted and allows the tolerances to be moved only so that they are at the upper or lower end of the measured part. This puts the molder in a position where if the part is off slightly in one direction the part is not to the print specifications.

It is important to keep in mind that the purchase order usually has words to the effect of "produce parts to print XYZ." This is the legal document and the basis of rejection. If you cannot get the print changed, go for a letter of release signed by the buyer based on a functional approval. While this is Quality by Hostage, it is better than nothing.

Resin Shipments

While this doesn't happen often, it is simply good material control to weigh your skids when they are received. Weigh a few empty ones to get a true weight of the skid and gaylord. Check to see that what you ordered, what the paper- work said you received, and what you were billed are all the same amount. If not, this is the time for
an enthusiastic discussion with your material supplier. Make sure when material is returned to the warehouse it is also weighed and logged into the material inventory. Nothing loses money faster than to find you are 200 lb short of material and have to pay air freight to get it in to
fulfill your customer's purchase order.

As a supplier it is a sign of incompetence or outright theft to misrepresent what you are selling. Accurate part counts, a good definition of an acceptable part, and good material control are all requirements of ISO. If you're not doing this now, you may be needlessly losing money.

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