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 https://fluxworkholding.com/pages/flux-workholding-calculator-online-version

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

Symbol	Units	Name	Description
S₁	seconds	Fixture Design Time	Design and document a fixture in CAD
S₂	seconds	Fixture Programming Time	Program the fixture/soft jaws etc in CAM
S₃	seconds	Part Programming Time	Program all operations of the actual part
S₄	seconds	Physical Machine Setup Time	Load the fixture, tram, probe etc
L₁	seconds/run	Open Door Time	Physically open the door to the machine tool
L₂	seconds/screw	Loosen Screw Time	Loosen each fastener, whether that be for a machine vise, or a fixture clamp
L₃	seconds/part	Remove Part Time	Take each finished part out of the machine and set it somewhere else
L₄	seconds/run	Clean Fixture Time	Blow or brush off the fixture to remove any loose chips
L₅	seconds/part	Load Part Time	Load raw stock into the first station of the fixture for each part
L₆	seconds/screw	Tighten Screw Time	Tighten each fastener, whether that be for a machine vise, or a fixture clamp
L₇	seconds/run	Close Door Time	Physically close the door to the machine tool
L₈	seconds/run	Hit Start Button Time	Physically push the start button so the CNC starts machining
L₉	seconds/part	Transfer Part Time	Time needed to transfer each part in progress from one station to the next
M₁	seconds/part	Cutting Time	Time the CNC spends actually cutting, for all operations of a single part
M₂	seconds/tool	Tool Change Time	Time from the end of cutting with a tool, to beginning cutting with the next (cut-to-cut)
M₃	seconds/tool	Break Detect Time	Time the CNC needs to perform break detection by automatically probing the tool
F₁	parts/screw	Parts Per Screw	Number of parts that are clamped with the actuation of each screw.
F₂	parts	Complete Parts on Table	Number of parts completed each run of the cnc machine.
F₄	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 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	$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 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:

Traditional Single Station Vise: $1400
Double Station Vise: $1400
Flux Workholding: $400
Competing high-density vise: $1200
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.

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).

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