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The High Cost of Estimating Cycle Times

The High Cost of Estimating Cycle Times

Whether it is milling, turning, swiss or a combination of, shop owners ask me every day if the Paperless Parts system will match their shop’s cycle times. At the root of their question is a pain in their quoting process: estimating cycle times is one of the biggest time sinks when quoting. It is one of the ways most shops over-engineer their quotes today (see our series on Overengineering the Quote). 

Estimating cycle times is hard. Every shop including machine tools/cutting tools, estimator, programmer, and machinist is different and each of these play a significant factor in determining cycle times. I’m here to go into why it is difficult to get right and to propose a better strategy for dealing with estimated cycle times.

Variables That Contribute to Cycle Times

Part geometry  – This is the parametric complexity of the individual features of a part. The size and angles of each feature play a big role in determining the required tooling and machine that is needed to produce a part. 

Material – Materials have different levels of hardness that can take longer or shorter to machine. The material often determines the tooling and cutting strategy used to utilize those tools.   

Tolerances – These are often called out on a drawing and need to be considered in addition to the standard cycle time. The tolerance requirements dictate the tooling choice, measuring requirements and finishing passes required for a specific feature. 

Surface Finish – Similar to tolerance, this is outlined on a drawing and drives the number of finishing passes and the type of tool required to achieve the desired end result. 

Available and Selected Tooling – There are 75K+ different tools available in MSC Industrial’s catalog alone. Each of these tools has a slightly different flute configuration, optimal spindle speed, and material removal rate. Whether they are internally cooled or externally cooled also plays a factor in the ability to cut chips efficiently. The number of tools you decide to use and the pattern in which you use them plays into the time that it will take for your machine switch them while running the program. 

Machine Tool – The type of machine you are using dictates a lot of things like: achievable spindle speed, accuracy, tool change over time, direction of cut, and machining strategy. All of these impact cycle time and often the machines in a job shop are not standardized and during the quoting process it’s hard to predict capacity requirements and where a job will actually get run. 

Machining Strategy – There are thousands of machining strategies and approaches. Are you using tricordial milling or high speed machining to remove material quickly, are you ramping in to pockets and at what angle, is the goal to conserve tool life or hog out as much metal as possible as quickly as possible? All of these impact how quickly you are going to cut a part.

Fixturing/Workholding – How a part is fixtured and the number of parts in a fixture plays into the cycle time as well. If you are working a single component, did you order the stock large enough to set the parts high in the fixture and hit the maximum features with the first setup? If you are working with a tombstone, how many parts are you trying to make in a single pass and what tool changeover efficiencies are you going to get from cutting them in unison? 

CAM Software – Not all CAM software is created equal. This plays heavily into whether or not your programmers are able to drive the most optimal cutting strategy and utilize the tools with the latest understanding of optimal spindle speeds. 

Machinists – Anyone who has actually cut parts knows that the spindle speeds in the CAM program and the actual spindle speeds that cut a part can differ quite a bit based on the machinist running the job. More often than not, unless you are using some sort of intelligent tracking system for MTConnect, you are likely unaware of the actual cycle for a job. 

As you can see, the possible permutations of variables that impact cycle times are nearly infinite. This makes it challenging to compute instant cycle times from the geometry of a part.


How Paperless Parts Comes Up With its Cycle Times

Paperless has analyzed actual CNC runtimes of thousands of parts of varying degrees of complexity. We use dozens of attributes and features extracted by our CNC interrogation algorithms to train a machine learning model that can predict runtime for a part in seconds. 

How does it work? 

We look at depth and diameter of holes, pockets, different types of surface area (profile cuts, surfaced faces, and pockets), overall removed volume, and our proprietary model for part complexity. Each of these attributes contributes some runtime to our estimate. We also apply multipliers based on material to account for materials that are harder to mill. Finally, based on the part’s complexity, we estimate how accurate that runtime prediction is likely to be by categorizing each part with High, Medium, or Low confidence.

Facts and assumptions:

  • Aluminum 6061 (cycle times scaled for hardness with aluminum as a base)
  • Standard tolerance on all faces (+/- 0.005”)
  • Stock material is the part’s bounding box plus 0.25” in all dimensions
  • Tool set can drill all holes without heliboring
  • Part is cut with minimum possible setups for a 3-axis mill
  • Profiled cut maximum depth: 2”
  • End mill maximum length: 1.75”
  • Surfaced area (i.e., faces required end mill work) add 0.6 seconds per square millimeter of surface area

As you can see there is a lot that goes into this calculation and it isn’t always going to be right or a fit for your shop. Instead, it was designed to be a consistent baseline for use during the estimating process to gage whether or not further engineering was required. 


What does this mean for your shop?

Knowing that for more complex parts an experienced estimator needs to be in the loop, shops should be selective about the time spent engineering estimates. This means you cannot afford to program every part that gets quoted. Your business needs some criteria to determine when you are going to sharpen the pencil to engineer a quote and when you are going to send out a very quick quote padded with extra margins because speed to quote and lead-time will win out over price.

This decision comes back to knowing your customer and the type of job you are quoting. For prototyping jobs where margins are larger, speed trumps cost almost every time. For estimators that means erring on the side of a padded price and less accurate estimate of cycle time in order to get the quote out fast. For long-run production jobs, it makes sense to spend a little bit longer figuring out an accurate estimate of cycle time so you can ensure your price and delivery are both profitable and achievable.

Interested in learning more?

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This article was written by Jason Ray

Jason Ray is the Co-Founder & Chief Executive Officer at Paperless Parts. He drives the company’s product vision, while building relationships with manufacturers and partners. Before Paperless, he served as an officer in the US Navy and led the implementation of additive manufacturing technology. Jason holds a BA (Trinity) and MBA (Babson).