Quality Measurement: Driving to the Finish
Moving gaging to the point of manufacture is the process engineer’s dream. It is what drives productivity on the shop floor, puts quality in the hands of the machinist and quickly weeds out process problems before they become catastrophic. This is not only true of dimensional hand tools and gaging, but of surface finish gages, as well. Different, but similar to dimensional tools, surface finish hand tools are more important than ever with today’s strict manufacturing requirements.
There is a strong similarity between dimensional handheld gages and portable surface finish gages. With dimensional handheld gages there are two classes: general purpose hand tools and dimensional gages that are more dedicated to a specific task. Each has a specific purpose.
Hand tools, such as calipers and micrometers, are great for versatility and can provide good results for many departments, from incoming inspection to dimensional part inspection on low-volume runs. For best results they require a skilled operator and time to ensure they are aligned properly and applied with the proper gaging force.
But if making a high volume of parts with tight tolerances, and the machinist does not have the skills of a trained quality inspector, then dedicated dimensional gaging is the better choice. These gages are made for a specific dimension so they are faster, easier to use and produce results with better performance and gage repeatability and reproducibility (GR&R). With a dedicated gage, virtually any operator can easily inspect machined parts and make decisions about their quality. They can even collect the results for process analysis and control.
Portable Surface Finish GagesBeginning some 25 years ago, portable surface finish gages-some small enough to fit in a shirt pocket-brought a new level of part control to the manufacturing floor. Bringing surface finish inspection out of the lab and onto the floor was important for several reasons.
For the previous 50 years, part dimensional tolerances had been shrinking. As a result, surface finish and form irregularities were eating up an ever-increasing portion of overall dimensional tolerance, a trend which continues today. Also, it became clear that surface finish played an important role in the performance of the part. The unique characteristics of a surface could determine whether the paint stayed in place, lubrication held or leaked, or how much noise was produced when surfaces moved against each other.
With proper locating and fixturing, these small portable surface finish gages could measure surface finish parameters easily and usually with the touch of a single button. Like dimensional hand tools, the general-purpose portable surface finish gage provides easy to use and reliable results for many applications. Simply place the pocket-sized gage on the part so that its built-in rest pads locate it in place, press a button and the results pop up, ready to be compared to the requirements.
Dedicated Portable Surface Finish GagesBut sometimes it is not that easy. Surface finish parameters can be applied to virtually any surface on the part, from inside a small bore, on a short land, around a hole, to an outside diameter (OD) between two walls or even on those side walls. These all have to be measured at a high rate of speed and in a dirty manufacturing environment by a semi-skilled operator.
Now, these parts could be taken to a special surface gaging station. They could be located in a position where the portable surface finish gage could access the part, and the gage placed gently on the part with the operator taking time and care to locate the sensitive probe on the feature without damaging it so a measurement could be taken. But this process would take a lot of time.
In the interim, many additional parts would have probably been made, and given the delicate nature of surface finish probes, a goodly inventory of broken probes produced, as well.
Thus, as with the dedicated handheld dimensional gage, the need for fast and reliable surface finish measurement on such parts has created a whole family of dedicated gages, designed to put the fast, reliable surface finish analysis in the hands of machinists at the point of manufacture-often while the part is still fixtured in the machine tool.
Nowhere have these requirements become more aligned than in the automotive industry. Automotive has special needs that make dedicated surface finish gages a necessity. These include a high volume of parts, the need for fast and reliable measurements by machinists semi-skilled in dimensional measurement, and more and more requirements for surface finish checks to ensure part quality and eliminate the high cost of part recalls.
For gaging purposes, automotive engine components can be classified by portability. There are parts that can be brought to a gage and those where the gage has to be brought to the part. Pistons, connecting rods, cam shafts and transmission gears, for example, are all small and easy to bring to a dedicated gaging station where an operator can quickly assess the required surface finish.
Then there are components that are not that easy to move around, such as engine blocks, cylinder heads and crankshafts. Moving these from a machine to a work bench is hard enough, but trying to precisely align a portable surface finish gage onto a short land without damaging the sensitive probe is nearly impossible. Thus, the need for dedicated hand gages to assist the operator in making these important checks. Systems are available for each of these major components.
The Engine BlockThe engine block is by far the largest and perhaps the most complicated part of an engine. There are literally thousands of dimensional checks to be made on blocks, along with many critical surface finish and surface waviness call-outs. These surfaces include cylinder and crankshaft bores, along with deck, pan rail and front faces. It is clear how the finish in these applications is critical. Cylinder bores need specific finishes to balance retaining oil and smooth axial movement, while decks and other mating faces need to be flat and smooth to provide good sealing.
Cylinder bores were probably the first feature to have dedicated gaging designed for measuring surface finish. Characteristics include bores of various sizes, which can be relatively deep, some up to 8 inches. The prints may call out specific depths and locations for checks, and with up to 12 cylinders in some blocks and four in a vast majority of high-volume applications, there are a lot of bores to measure. Therefore the gages need to be highly portable and easy for the operator to align and set to depth. The design also has to protect the sensitive probe so it does not get damaged when bringing the gage to the part.
The dedicated portable surface finish bore gage is much like an expandable tri-bore gage. The tri-bore has a certain amount of size adjustment, but can be set to a specific size. This allows it to lock in place and helps provide repeatable readings with no operator influence. The indicating device also is protected by a transfer mechanism so there are no locating forces applied to it when the gage is placed onto the part.
The surface finish cylinder bore gage borrows the same principles. To give it adjustability to lock into various sizes, interchangeable plates are used to achieve the right measuring range for the diameter. Setting the gage with the right set of blocks allows easy entry into the cylinder bore, and once in place, a manually controlled air cylinder expands the sizing blocks so they lock the gage into position. The clamping force is such that an operator can virtually pick up the block with the gage, although this should only be done by serious weight lifters.
But the gage’s cylinder has another, even more critical function. When compressed with no air applied, it holds the sensitive surface finish probe in a retracted position. This means that during insertion the probe is protected inside the body of the gage. After the operator is satisfied with the gage location, he applies the air, locks the gage into position and the probe extends so he can make the surface test. When done, the air is released, the probe retracts and the gage becomes free, allowing easy removal by the operator.
Other bores in the block are not as large as cylinder bores but also require shop floor surface measurement. Here another tack can be taken. Again borrowing from the world of precision dimensional hand gaging, a take-off on the fixed mechanical plug can be incorporated. With this type of gage, the mechanical body of the plug is made to measure a specific bore size. Thus it is fast, self centering, and there is no operator influence.
The same concept can be used for a portable surface finish gage. In this case, a plug body, made close to the bore size being measured, incorporates a surface finish probe. But unlike a fixed mechanical plug, which always has its contacts touching the bore, the surface finish version retracts the probe until a mechanical transfer mechanism releases the probe when the plug is in its final measuring position.
Also, like a snap gage or fixed body bore gage, masters are part of the gaging routine since they are comparative measurements. With surface gages, a reference standard is used to verify the system and probe performance. In this case, a holder for the reference specimen simulates the part and places the reference specimen at the proper location to easily perform calibration throughout the day.
The HeadDeck face surface finish and waviness is important because it prevents oil from leaking between the mating block and head surfaces. Often specific locations are called out on the part prints for surface checks. This concept is similar to building air tooling where the jets are set to specific locations and orientations to ensure that the operator measures the parts at the proper place every time.
For these requirements, a special template is made that sits on the part to be measured. The template can only sit on the part in one orientation, and provides the operator with the locations to align the matching surface waviness gage.
As with the cylinder bore gage, the main factors for use are ease in positioning and fixing the gage location, and probe protection. The waviness hand gage does all of these. The template sets the proper location, mating pins ensure it is locked in place, and a mechanical transfer protects the probe until the gage is in location and placed on the fixture.
The CrankshaftSurface finish is critical on a crankshaft. Rotating at thousands of RPMs, and with bearing and crank surfaces and their end faces in metal-to-metal contact, there is tremendous potential for wear. Surface finish helps determine the life of these products.
Looking to dimensional hand tools yet again, the variable snap gage is one of the most shop-worthy gages used in the precision manufacturing environment. It can be used over a long size range, but is set to the specific size being measured. A similar concept is used when measuring surface on crank journals. The surface finish gage has to be adjustable for various sizes, has to lock in place, and protect the probe.
Air operated, the dedicated journal finish gage allows for setting the locking jaws to the approximate size, then with manual air actuation, the gage locks itself into position and releases the probe to contact the part. As with all these gages, the probe does not make contact unless the gage is fully locked into place. The key difference with this gage is that it has a traverse probe operation so that the probe moves horizontally along the crown of the part.
Just as important as the bearing and crank journals are the thrust faces they ride against. Another snap gage concept is used to position the surface finish probe here, and with the same thoughts in mind: easy to use, no operator influence and protected probe positioning.