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 highquality 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 https://fluxworkholding.com/pages/fluxworkholdingcalculatoronlineversion
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 perpart 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 timeconsuming 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 multiop 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
Symbol 
Units 
Name 
Description 
S_{1} 
seconds 
Fixture Design Time 
Design and document a fixture in CAD 
S_{2} 
seconds 
Fixture Programming Time 
Program the fixture/soft jaws etc in CAM 
S_{3} 
seconds 
Part Programming Time 
Program all operations of the actual part 
S_{4} 
seconds 
Physical Machine Setup Time 
Load the fixture, tram, probe etc 
L_{1} 
seconds/run 
Open Door Time 
Physically open the door to the machine tool 
L_{2} 
seconds/screw 
Loosen Screw Time 
Loosen each fastener, whether that be for a machine vise, or a fixture clamp 
L_{3} 
seconds/part 
Remove Part Time 
Take each finished part out of the machine and set it somewhere else 
L_{4} 
seconds/run 
Clean Fixture Time 
Blow or brush off the fixture to remove any loose chips 
L_{5} 
seconds/part 
Load Part Time 
Load raw stock into the first station of the fixture for each part 
L_{6} 
seconds/screw 
Tighten Screw Time 
Tighten each fastener, whether that be for a machine vise, or a fixture clamp 
L_{7} 
seconds/run 
Close Door Time 
Physically close the door to the machine tool 
L_{8} 
seconds/run 
Hit Start Button Time 
Physically push the start button so the CNC starts machining 
L_{9} 
seconds/part 
Transfer Part Time 
Time needed to transfer each part in progress from one station to the next 
M_{1} 
seconds/part 
Cutting Time 
Time the CNC spends actually cutting, for all operations of a single part 
M_{2} 
seconds/tool 
Tool Change Time 
Time from the end of cutting with a tool, to beginning cutting with the next (cuttocut) 
M_{3} 
seconds/tool 
Break Detect Time 
Time the CNC needs to perform break detection by automatically probing the tool 
F_{1} 
parts/screw 
Parts Per Screw 
Number of parts that are clamped with the actuation of each screw. 
F_{2} 
parts 
Complete Parts on Table 
Number of parts completed each run of the cnc machine. 
F_{4} 
ops 
Number of Operations 
Number of machining operations for a single part. 
T 
parts 
Total Parts in Batch 
Total number of parts being made in a production run 
C 
tools 
Number of tools 
Number of tools used (or tool changes performed) during the machining cycle. 
The Equation
After adding up the timeconsuming 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 SteadyState 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 F_{2} “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 lowestcost 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 HighDensity Vise
 Custom highdensity 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 
$500 
$35 
$0 
$35 
Double Station Vise 
$500 
$35 
$0 
$35 
Flux Workholding Vises 
$95 
$10 
$0 
$10 
Competing Small Vises 
$250 
$30 
$0 
$30 
Custom Fixture using Fixture Clamps 
$100 
$100 
$0 
$100 
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 reinstalled with a repeatability of less than .0001 inches (12 micron). Thus, the “repeat run” cost can be higher than shown for other workholding systems, if soft jaws have to be recut 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:
 Traditional Single Station Vise: $1400
 Double Station Vise: $1400
 Flux Workholding: $400
 Competing highdensity vise: $1200
 Custom highdensity 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.
Conclusions
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 fluxworkholding.com/pages/documentation), 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! (sales@fluxworkholding.com).