Inspection Measurement

Proper and logical inspection procedures assist in maintaining optimal tolerance control. In powder manufacturing, the raw materials should be of a select grade. For atomized powders, the molten metal can be checked for proper chemistry before pouring and changes made to the melt as needed. Production lots of powders are usually tested and may be certified for screen analysis, apparent density, flow rate, compactability (density achieved by pressing at a specified tonnage), sinter size change under a specified condition, and chemical analysis for basic constituents. If the powder meets the agreed-on specifications the user should be assured of a reasonable consistency. It is obvious that tighter control of the powder results in better press-and-sinter tolerances, and this is an ongoing project with powder manufacturers. An audit by the parts producer of incoming powder certifications from time to time would help to ensure the required quality. Tests of flow rate, apparent density, and screen analysis are easy enough so that this should be done on a routine basis. Accurately controlled pressing of a tensile bar, a transverse bar, or a suitable ring that is then sintered in a laboratory furnace under standard time, temperature, and atmosphere conditions and then tested for strength, hardness, size change, and so forth is a very good way of ensuring powder control as well as a reference for sintering conditions.

Inspection procedures at the press should take into account the following variables.

Normal Variables. Weight and thickness are normal variables, and their running tolerances may be set by part requirements or by a process capability study that would yield statistical process control (SPC) limits for these parameters based on the material consistency and the equipment capability. This procedure indicates whether the equipment can, in fact, produce the required tolerances and yield a set of control limits that could indicate a process going out of control even if the attributes are well within the print specification range.

Weight can be measured manually by the operator or automatically by the pressing system. Electronic scales are the most practical, and some can be coupled to SPC monitoring systems. Automatic measuring by the press system can give a warning to the operator or go as far as making adjustments to the press fill. If the weight of a part is allowed to vary ±1% and the thickness is allowed to vary ±0.5%, the density could vary ±1.5% or a spread of 3%, which could create problems. See the section "Compacting" in this article for a discussion of causes of weight variation.

Thickness can be measured by any of the standard measuring devices such as verniers or micrometers or dial gages (analog or digital), or laser devices. Although standard micrometers are very common, electronic digital types provide the best readability and minimize reading errors. If measurements are done manually, care must be taken to make the procedure as easy as possible and virtually "foolproof" for the operator. If there are several thicknesses to be measured that are more than a turn or two of the micrometer spindle, several micrometers or a dial indicator should be used. If a dimension, such as a step, that would be the difference of two normal micrometer or dial readings or require a depth micrometer is to be measured, a dial indicator referenced to one surface would give more accurate results with less chance of operator error (Fig. 14). For long runs of more complex parts, multipoint gaging setups can be created that measure many features at once and statistically process the data (Ref 8).

Fig. 14 Simple measurement of a step

Slow-Change Variables. Slow changes in dimensions occur with wear on the tooling components. Where tolerances are nominal, a carbide die and carbide cores can produce millions of parts before wear is a problem. Tool steels containing large percentages of finely dispersed carbides (usually made by powder metallurgy) can do exceptionally well and be less prone to cracking. Most standard tool steels have a much shorter effective life. Therefore, the frequency of inspection for dimensions affected by tool wear depends on the tools themselves as well as on the powder used and its density. Although thickness and weight should be checked frequently, die and hole sizes need much less frequent inspection at the press. Steps in a part produced by either several different punches, or a step in the die, are subject to thickness variation and need frequent control. A step produced by a step in a punch face varies with punch wear and needs less frequent checking.

Sudden changes in dimension can be caused by the breaking or cracking of the punches or dies or of other tool support items. Experience on any particular job indicates those dimensions sensitive to this so that logical inspection can be set up. Often, when a part of a tool breaks or cracks, it can be heard or the press may indicate the problem by sound or by some change in those features normally checked.

By understanding the processes and how the variations or failures can occur, good tolerance control can be achieved efficiently, resulting in better P/M parts at lower cost.

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