9 Tips for Avoiding Last-Minute Surface Finish Issues

July 25, 2019 Joshua Jablons

surface finish requirements

How to Make Sure Your RFQ Doesn’t Skip Any “Finishing” Steps

When it comes to completing a request for quote, no detail is too minor to include. But being human, even the most conscientious of us can miss small but important details, such as surface finish requirements.

Picture it:

You’ve created an RFQ that seems to provide everything a supplier could possibly need to deliver an accurate and timely cost estimate for producing, say, a small metal part.

Your RFQ includes the part’s outside and inside diameters (OD and ID), length, and tolerances. It includes an engineering drawing done by the part’s designer. The RFQ even specifies, in detail, the material you want to use and its source.

What could possibly go wrong?

1. Make sure your purchase order agrees with your RFQ.

Suppose when you go to create your purchase order — or when your supplier is reviewing an order that you’ve just placed — you discover the RFQ left out a small but vital detail.

For example, perhaps the part must have a polished or rough surface finish, or some other feature that requires a secondary operation.

That small omission means:

  • Revising your RFQ to include the missing details
  • Having your supplier requote the job to include the added time and cost of a secondary operation
  • A delay in starting production — something that could further add to your cost if your deadline is tight

Whenever a purchase order does not reflect what was quoted, there can be issues. But this is especially true when differences between the RFQ and the PO require adding steps to the production process.

2. Take care not to omit any secondary operations from your RFQ.

As you do an RFQ, keep in mind which requirements add another process to production. This might include features such as:

  • Squareness
  • Parallelism
  • Perpendicular end cuts
  • Minimum or maximum end breaks
  • Specific chamfers

Metal finishing processes, from adding a radius to polishing, sandblasting, and tumbling, add extra steps to a part cutoff operation.

If you add passivation to a purchase order, it not only means having your supplier requote the job to include the additional secondary operation. It also means the added step of determining which method of passivation — nitric or citric — is required.

Even if the secondary operation will be performed by a third party, there may be related details that your manufacturing partner needs to know. For example, perhaps the parts will need to be packaged a certain way for efficient shipping to and handling by the third-party supplier.

3. Don’t assume your supplier knows how the surface finish relates to your part’s end use.

As a manufacturer or a part designer, you’re looking at the surface finish of a part in terms of performance. That is, you’re concerned with how the finish affects end functionality and/or how the part will fit with other components when your product is assembled.

From your supplier’s point of view, surface finish is either a side effect of the manufacturing process — OR it may be an important feature that determines what processes will be used to produce a part and achieve a particular finish.

If the finish is critical to your part’s function and/or fit, you can bring these differing points of view together by including the end use of the part, as well as its surface finish specification, in your RFQ.

At a microscopic level, even the “smoothest” surface has a texture, made up of peaks and valleys and their direction on the surface (also known as lay). Processes such as abrasive cutting, grinding, lapping, and milling affect these surfaces, as do tool selection and condition, feeds and speeds, and other factors

Understanding the end application of a part can help your supplier make important decisions about how to achieve the dimensions, tolerance, and surface finish you need.

4. Tell your supplier what standards you use.

Different standards might apply to defining a characteristic such as surface finish, according to your application, business, or industry.

For example, you might base your requirements for surface roughness on ASME B46.1 from the American Society of Mechanical Engineers or on ISO 4287 and ISO 4288 from the International Organization for Standardization. These standards include:

  • The terminology used
  • The characteristic’s parameters
  • Procedures for measurement

By specifying your standards in your RFQ, you can help to optimize for manufacturability and make sure you and your supplier are in sync.

5. Consider the effects of the processes you request.

Sometimes a customer requests a quote for a specific diameter, but the subsequent purchase order adds the requirement that the surface finish must be centerless ground.

Here, the trouble is that centerless grinding requires starting with a larger diameter to compensate for the amount of material removed in the process and ensure the final part is of the correct diameter.

So, this request not only adds a secondary process —  it also has an impact on the raw material that must be ordered.

6. Include an accurate and helpful engineering drawing.

If there is a drawing, include it with your RFQ. Providing a complete, detailed drawing goes a long way in getting an accurate quote and ensuring there are no last-minute surprises, for you or your supplier.

It is equally important to make sure the drawing clearly reflects what you want and need. For example, an inside radius requirement — such as on the head of a screw with a taper — can be confusing on a drawing.

Here at Metal Cutting, we’ll often receive a drawing that indicates a corner, but it is unclear exactly what is needed. Should it be flush? Not flush?

7. Include the parameters for quantifying surface roughness.

Did you know there are more than 100 quantitative parameters that can be used to describe and compare surface finish characteristics?

Each parameter has pros and cons, and is specified by engineers based on how it will affect part performance. Sometimes, two or more parameters or a ratio of one parameter to another is needed to fully describe a surface.

Thankfully, there is a short list of parameters that are commonly used and capable of meeting most needs in the machining world. For instance:

  • Roughness Average (Ra) is a calculation of the average length between all the peaks and valleys (or the average height) from the mean line of the surface.
  • Mean Roughness Depth (Rz) is calculated by averaging the height of the five highest peaks and the depths of the five lowest valleys on the surface.

Other common parameters include Root Mean Square Roughness (RMS) and Maximum Roughness Depth (Rmax).

Ra Pros & Cons

Ra provides a simple value for pass/fail evaluation and a good way to monitor process stability, making it the most frequently used surface parameter. In fact, if a drawing does not specify otherwise, Ra is considered the default parameter for surface finish in the United States.

However, Ra can’t distinguish between different types of surfaces having the same Ra value. Therefore, additional parameters might be used with Ra, such as Rp (Maximum Peak Height), Rv (Maximum Valley Depth), or Ry (Maximum Peak-to-Valley Roughness Height).

(Read more about Ra surface finish requirements.)

Rz Pros & Cons

In much of Europe, Rz is preferred over Ra as the parameter for surface roughness.

Because Rz averages the extremes (highest and lowest peaks) of the surface, Rz values may not accurately describe the average surface finish. However, Rz is more sensitive than Ra to changes in a surface finish.

You can refer to surface finish charts for more information about Ra, Rz, and other surface finish parameters. 

8. Don’t wait until it’s too late to specify surface finish details!

Sometimes an additional surface finish requirement is discovered after a part has already been made, creating complications along with added costs.

In the best-case scenario, the part can be reworked to provide the needed surface finish. Of course, that is after revising the quote and determining the extra time and cost for adding another operation to the production process.

Other times, the part can be reworked — BUT there are limits on what processes can be used without risking other important features of a part. For example:

  • If a smoother surface is required, a polishing step can be added, but this may increase the end radius and possibly decrease the diameter of the part.
  • Conversely, if a very rough surface is required, a sandblasting step can be added, but this may increase the diameter of the part.

In the worst-case scenario, the part cannot be finished as needed and the entire process has to be redone, from requoting the job to remaking the part. That means wasted material, time, and  labor, and significant cost overruns.

9. Including everything up front improves the odds your supplier can help.

For a supplier like Metal Cutting, when customers let us know up front about any secondary operations their parts will need, there are often things we can do to make the process easier and more cost effective.

For example, if the process is one that Metal Cutting is equipped to perform, we can streamline production and save you the time and cost of using a third-party supplier.

If a third party is required, Metal Cutting can properly package and ship the raw material or parts direct to the other supplier to perform the secondary operation.

But no matter the complexity of your RFQ or the number of steps in your production process, making sure your supplier is informed is a crucial step in optimizing manufacturability so you can achieve the results you need.

 

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