Making Parts Flat

Commentary by Larry Crainich
President, Design Standards Corportation

A reader of this column called and told me he had a flatness problem with certain types of parts that his company was manufacturing. I asked him to send me parts, specifications, etc., for determination of a potential solution.

I also told him I wrote an article relative to flatness and would fax him a copy. The article was published in the October 1991, MetalForming issue combined with other topics and was very scant relative to flatness. So I thought I would expound on the topic.

The parts and specifications arrived. Without disclosing any proprietary information, a general description follows:

My initial reaction was, "what an ambitious undertaking." We all know, however, that enough talent, time and money can produce remarkable achievements.

All things being perfect, what causes parts to be produced other than flat? One is material being imperfect with a crown in it, or coil set that wasn’t straightened prior to processing. Another is stresses within the material that relax (and distort flatness) when the periphery is released from the parent material or even when holes are pierced within the confines of the part. To a lesser degree, some materials have memory that creeps into "out of flatness" over time.

Processing sheet metal also can result in out of flatness conditions. This, however, can be corrected.

If you are using a "bridge" stripper for piercing and are blanking the product through the die, you are inviting out-of-flatness conditions.

A spring-loaded stripper for piercing is mandatory for flatness. Be certain your cutting edges are sharp and with proper clearance so that stresses are not introduced during processing.

Cutting the outside periphery must be done while the blank is under flatness control. One way to achieve this is to hold the part under spring pressure (spring stripper) and blank the scrap. Another way is to blank the part onto a spring-loaded pad and return it to die level for ejection.

In extreme circumstances, a part can be blanked onto a spring pad, returned and pushed back into the stock and pushed through the die in a subsequent station. The piercing or internal needs can be performed after blanking and return. Now that the processing causes no detriment to flatness, we still haven’t resolved the problems caused by material.

If a part has a bow in it, you can "spank" it between two flat plates, release it and still have a bow. If you’re lucky, there will be less bow but it will still be there. A compensating form can produce flatness only if the bow and material characteristics remain absolutely constant.

What’s the solution? Stipple the blank! The word stipple in the dictionary has no relevance to what I’m talking about. My definition of stipple is a waffle-shaped impregnation on a piece of metal. See Fig. 1

If you take a piece of unflat material and squeeze it between two flat plates with one plate having a cross-hatch pattern to embed itself into the material, you will physically move material, destroy its memory and produce a flat part.

The stipple pattern may be one- or two-sided, depending on need for flatness and your customer’s permission to put a waffle pattern onto their product. The depth of penetration is relative to material thickness and degree of correction required. The greater the inconsistency of product coming into the stipple operation, the deeper the penetration to overcome same.

Experimentation is necessary to achieve desired results. The depth of stipple can be refined by the adjustment of the press ram.

MF