What Changes When You Own a Second Fixture

Tool quantity is how many identical physical units you own plant-wide, and it is the only capacity a tool has: the pool for any day and shift is the quantity multiplied by that window's working hours. Raising it from one to two doubles the pool and lets two operations hold a unit at once, on any mix of machines. Entering a number you do not own removes contention from the plan and from nowhere else. EDGEBIC by User Solutions makes the comparison cheap enough to run before you spend money.

The arithmetic, in one line

For a tool on a given day and shift:

pool hours = quantity x the window's working hours, minus what other jobs have already booked

An eight-hour day shift and a quantity of 1 gives eight fixture-hours to share across the whole plant that day. A quantity of 2 gives sixteen. A quantity of 3 gives twenty-four. Every allocated machine-hour books exactly one hour from that pool, no matter which work center consumed it.

Note the last row. Zero is not "no constraint"; it is a hard stop, and deliberately so. That behavior has its own post.

What a second unit actually buys

Return to the two-mill example from one fixture, two mills: two five-hour jobs on two machines, both mounting the same fixture, both mills on an eight-hour day shift.

Quantity 1. Monday's pool is eight fixture-hours. The first job takes five, the second gets three Monday and its last two Tuesday morning. The second job finishes Tuesday 10:00.

Quantity 2. Monday's pool is sixteen fixture-hours and there are only ten hours of work. Both jobs get a unit each and both run Monday 08:00 to 13:00. The second job finishes Monday 13:00.

The second unit bought twenty-one hours of elapsed time on one job in one day. Whether that is worth what the fixture costs is a business question, but it is now a business question with a number attached rather than an argument about whether tooling is tight.

Notice also what the quantity-2 plan is: it is exactly the plan the shop had before anyone modeled the fixture. The difference is that it is now true. The original plan assumed a second fixture. The new one has bought it.

Quantity is units, not fits

This is the mistake worth stating twice, because it is easy to make and quiet when made.

A fixture that physically bolts to CNC-Mill-1, CNC-Mill-2, and CNC-Mill-3, but exists as one physical object, has a quantity of 1. Entering 3 because it fits three machines tells the scheduler three jobs can run at once. The plan then looks exactly like the plan you had before you modeled anything, and it collapses in exactly the same way.

You never tell EDGEBIC which machines a tool fits. There is no compatibility model. The fit is expressed entirely by which routing steps carry the requirement, so if only the mill steps name the fixture, the scheduler will never plan it elsewhere. Record the physical fit in the tool's notes field so your planners can see it, and keep the quantity as a plain count of objects.

The value entered on the tool answers one question: how many of these do we have? Nothing else.

Using the model to size the investment

Because quantity is a single number and a reschedule is cheap, the tool constraint doubles as a sizing instrument. The procedure is short:

1. Model the tool at the quantity you own today and schedule normally. Record the completion dates of the jobs that mount it.
2. Raise the quantity by one and reschedule. Record the same dates.
3. Repeat once more if the second unit helped.

The difference between runs is the marginal value of one more unit, measured on your real order book rather than on an average. Two patterns show up, and both are informative.

The dates pull in sharply. The tool was genuinely the binding constraint. The second unit buys the time you measured, and the case writes itself.

The dates barely move. The freed work immediately hit a different limit, usually the machines or the operators. A second fixture buys you very little, and the money belongs somewhere else. Finding this out for the cost of a reschedule is worth more than the reschedule.

You will often see diminishing returns clearly: unit two saves a day, unit three saves an hour. That inflection is where the buying stops.

Where quantity meets downtime

One subtlety. Downtime overrides quantity completely. On any date inside a tool's downtime range the pool is zero regardless of how many units you own, because a downtime range describes the tool being away rather than one unit being busy.

If you own three gauges and one goes out for calibration, that is not downtime on the tool. Downtime with a quantity of 3 blocks all three. The correct model for one unit leaving is to drop the quantity to 2 for the period, or to model that unit as its own tool record if its absences are routinely independent. Recording absences is covered in how to record tool downtime for calibration.

The one rule

Enter the number of units you would genuinely put into production at the same time. Not the count in the drawer, not the number of machines it fits, and not the number you wish you had. Everything the tool constraint is worth rests on that one integer being honest.

Over 35-plus years, User Solutions has watched plants argue about tooling spend without a number to argue with. The wider placement logic is in the scheduling engine guide, and you can price a second fixture against your own backlog in EDGEBIC.