Economics of CNC Workholding

By Michael Sargent, Founder of Flux Workholding

The One Goal

In the world of production CNC machining, there is one goal:

Maximize throughput while minimizing cost per part.

The more high-quality parts that you deliver every month, and the less they cost to produce, the more profit your business will make.

In this paper, we will:

    • Show how to mathematically maximize throughput in CNC Milling by choosing the optimal workholding technique for your situation.
    • Reveal the single most important factor in increasing throughput, and how any shop can exploit it.
    • Provide the simple tools needed to minimize your total machining cost per part, available for free download at

      The Problem

      To simplify the problem of minimizing cost while maximizing throughput, let’s first consider the costs which do not change much regardless of how advanced your machine shop is. In most businesses, and CNC manufacturing is no exception, it is difficult to reduce fixed costs such as:

      • payroll
      • lease payments
      • insurance
      • miscellaneous overhead expenses

      Similarly, variable costs are largely outside of our control. These costs do not change much on a per-part basis:

      • raw material
      • cutting tools
      • coolant
      • subcontracted finishing

      However, we have tremendous control over one thing: Throughput - the rate at which parts are completed.

      While your variable costs scale with quantity, your fixed costs scale only with time. Therefore, if you can produce more parts in a given period of time, not only will your cost per part be lower, but you will also produce more parts (more profit). This compound effect is what leads to happy economies of scale that make life worth living as a business owner.

      Defining Terms

      In production machining, cycle time is the amount of time, in seconds, required to produce 1 part.


      Throughput (units produced per second) is simply the inverse of cycle time.


      Luckily, cycle time is easy to measure and estimate, if we're careful. Intuitively, every machinist knows the steps involved in machining a part. The three main phases are:

        • Setup – the time needed to program and physically setup the CNC machine
        • Part Loading – time in between machining runs, where the operator loads/unloads parts.
        • Machining – the time the CNC machine actually spends machining parts.

          The Time Breakdown

          Let’s break down each phase into its smaller time-consuming steps.

          Setup (performed by the programmer):

          • Design the fixture (in CAD)
          • Program the fixture (CAM)
          • Program the part (CAM)
          • Physically setup the machine (in the real world)

          Part Loading (performed by the operator):

          • Open door
          • Loosen screws
          • Remove finished parts
          • Clean the fixture (blow off and/or brush)
          • Transfer multi-op parts between stations
          • Load fresh stock material
          • Tighten screws
          • Close door
          • Hit “Start” button

          Machining (performed by the machine tool):

          • Cut (including positioning moves)
          • Change Tools
          • Perform break detection

          Adding It All Up

          The total production time for a batch of parts is just the sum of all of these steps, added together each and every time they occur in a production run. You do setup once, but then you must load and run the machine many times until the required number of parts are produced.

          Once you know the loading time and machining time for every repetition of pushing the green start button, you can calculate the total production time.

          However, every single variable above depends on how you fixture your parts! To determine how workholding technique impacts production time, we considered the following parameters.

          List of Parameters







          Fixture Design Time

          Design and document a fixture in CAD



          Fixture Programming Time

          Program the fixture/soft jaws etc in CAM



          Part Programming Time

          Program all operations of the actual part



          Physical Machine Setup Time

          Load the fixture, tram, probe etc



          Open Door Time

          Physically open the door to the machine tool



          Loosen Screw Time

          Loosen each fastener, whether that be for a machine vise, or a fixture clamp



          Remove Part Time

          Take each finished part out of the machine and set it somewhere else



          Clean Fixture Time

          Blow or brush off the fixture to remove any loose chips



          Load Part Time

          Load raw stock into the first station of the fixture for each part



          Tighten Screw Time

          Tighten each fastener, whether that be for a machine vise, or a fixture clamp



          Close Door Time

          Physically close the door to the machine tool



          Hit Start Button Time

          Physically push the start button so the CNC starts machining



          Transfer Part Time

          Time needed to transfer each part in progress from one station to the next



          Cutting Time

          Time the CNC spends actually cutting, for all operations of a single part



          Tool Change Time

          Time from the end of cutting with a tool, to beginning cutting with the next (cut-to-cut)



          Break Detect Time

          Time the CNC needs to perform break detection by automatically probing the tool



          Parts Per Screw

          Number of parts that are clamped with the actuation of each screw.



          Complete Parts on Table

          Number of parts completed each run of the cnc machine.



          Number of Operations

          Number of machining operations for a single part.



          Total Parts in Batch

          Total number of parts being made in a production run



          Number of tools

          Number of tools used (or tool changes performed) during the machining cycle.


          The Equation

          After adding up the time-consuming steps of Setup, Part Loading, and Machining according to equation (3), we get the following equation predicting the total time needed to complete a production run:

          The following equations then follow:


          For continuous production which may proceed indefinitely, the setup time eventually becomes negligible, so we can concern ourselves primarily with the Steady-State Throughput and Steady State Cycle Time:


          Now we can reliably estimate the cycle time as a function of how the part is fixtured, and various other parameters. Don’t worry, we have a spreadsheet for this!

          If no upfront investment in tooling is required, the expense of machining 1 part can be estimated by multiplying the cycle time by the “shop rate”.

          For example, if your Net Cycle Time is 1 minute, and your shop rate is 60 dollars per hour (1 dollar per minute), then your expense associated with machining that part is 1 dollar.

          Effect of Part Density on Throughput

          The single most important factor in increasing throughput is typically F2 “Complete Parts on Table”, the number of parts completed each run of the cnc machine. Increasing this number will often increase throughput and result in lower cost per part, but not always!

          There is in fact an ideal number of parts to machine at once, which will result in maximizing your profit. If more was always better, everyone would be running CNC mills with the largest travels possible, and that’s clearly not the case.

          On the other hand, sometimes the ideal number of parts to machine at once is more than what you can physically fit in your particular machine, in which case filling the table with parts might be the best you can do.

          Effect of Fixturing Technique on Initial Workholding Investment

          Investing in high density fixturing will often increase throughput, but the benefit does not always outweigh the cost. The lowest-cost solution for machining 10 parts will rarely be the same as that for 100 or 1,000 and beyond, and the cost of workholding tools varies tremendously from one type to the next. If we want to minimize cost per part, we must calculate the required upfront investment somehow.

          Let's determine an approximate cost per “station”, where a station is used by 1 operation of 1 part. For example, 1 single part that has 2 ops would require 2 stations to machine the part completely. Or if you’re machining 10 parts that have 4 ops, you would need 40 stations on your machine table.

          The cost per station will depend on the type of workholding tools that you use. Consider a few possible scenarios:

          Possible Scenarios

            • No existing fixturing exists
            • You have vises of some type, but need soft jaws
            • This is a repeat run, for which fixturing has been made before
            • This is a repeat run, but small revisions have been made to the part

              Types of Workholding

                • Traditional 6” Vise
                • 6” Double Vise
                • Flux Workholding vise
                • Competing High-Density Vise
                • Custom high-density fixture

                  Based on these possible scenarios, and types of workholding, we compiled a representative table for the typical cost per station, taking into account the purchase price of various vises, soft jaws, fixture clamps, and pallets.

                  Typical Cost Per Station

                  No Fixturing Exists

                  Vises Already On Hand

                  Repeat Run

                  Revisions Needed

                  Traditional Single Station Vise





                  Double Station Vise





                  Flux Workholding Vises





                  Competing Small Vises





                  Custom Fixture using Fixture Clamps






                  We can see that the Flux Workholding vises have the lowest cost per station for every scenario. When performing your own calculations, we urge you to do your own research and determine the cost per station for your own situation and for the various competing workholding systems available to you.

                  Please note: As far as we know, only Flux Workholding vises and soft jaws can be removed and re-installed with a repeatability of less than .0001 inches (1-2 micron). Thus, the “repeat run” cost can be higher than shown for other workholding systems, if soft jaws have to be re-cut to meet tolerance requirements.

                  Calculating Fixture Investment

                  Consider an example situation where you need to machine a 4 operation part. If you already have 10 Flux Workholding vises, you can machine 40 stations at once at a cost of $10 per station, or $400 total. This would give you 10 complete parts per run.

                  In comparison, we estimate your cost just for consumables using the various methods:

                  1. Traditional Single Station Vise: $1400
                  2. Double Station Vise: $1400
                  3. Flux Workholding: $400
                  4. Competing high-density vise: $1200
                  5. Custom high-density fixture: $4000

                  Calculating Total Machining Cost per Part

                  Now let’s put everything together to determine the total machining cost per part.

                  Keep in mind that this cost does not include variable costs such as material, coolant, and cutting tools. Those costs tend to be similar regardless of the workholding methods used.


                  Using the equations in this paper, you can decide for yourself which method of fixturing makes you the most money. The easiest way to do that is to put all of this info into a spreadsheet, and that’s exactly what we did, so you don’t have to!

                  Using our Flux Workholding Calculator (available at, we think you will discover a few trends:

                  • Putting more parts on the table is almost universally a good thing (but not always for short runs).
                  • The ideal number of parts to run at a time may be substantially more or less than what you would first guess. Taking 5 minutes to run the calculator can easily save you hundreds or thousands of dollars, even if you never use a Flux Workholding product.
                  • Flux Workholding is the best partner to help you maximize throughput, make the most money, and invest the smallest amount possible upfront for your next production run.

                  If you have questions, comments, or would like assistance with using the calculator, send us an email! (