Reorder point calculator for critical spares
Paste a year of issues and a lead time. Get the reorder point, the safety stock behind it and the min and max levels for one spare. Free, runs in your browser, nothing is sent anywhere.
Why a critical spare needs a reorder point
A critical spare is one whose absence stops a press, scraps a die or idles a line. Most of them are cheap. The ejector pin set, the shear blade, the gate insert, the proximity switch on the transfer. What makes them expensive is the day you reach for one and the bin is empty, because the replacement takes six weeks and the die is down for all of them.
Spares are also harder to stock than production material. Usage is lumpy, two in one month and none for three. Lead times are long and uneven. Nobody watches the bin daily. So the reorder point cannot be a round number somebody guessed at the last stock take; it has to come from how the spare is actually used and how long it actually takes to replace.
How the reorder point is calculated
The reorder point is the demand you expect while you wait for the replacement, plus a buffer for the wait running long or the demand running heavy.
Usage entered per week or per month is converted to a daily rate first, taking a week as 7 days and a month as 30 days. The result is rounded up to whole units, because a spare is not issued in fractions and rounding down would put the trigger below the demand it protects.
Safety stock, two ways
The worst-case method asks for two figures a toolroom lead usually knows from memory: the most of this spare you have used in one period, and the longest the supplier has ever taken. Safety stock is then the worst you would use over the longest wait, minus what you would normally use over a normal wait. It is blunt, and it is honest about lead time being the thing that goes wrong.
The service-level method needs a usage history, twelve months of issues from the spares register is ideal. From that it takes the mean and the standard deviation, converts both to a daily basis, and sizes the buffer as Z times the daily standard deviation times the square root of the lead time in days. Z is set by the service level you choose, the fraction of the time the spare must be on the shelf when asked for.
When you paste a history the service-level figure is the recommendation and the worst-case figure, if you also entered maximums, is shown beside it. The service-level method treats lead time as fixed, so when the supplier is the unreliable part, the worst-case figure is the safer of the two.
The sample spare, worked through
The Load sample button fills in an ejector pin set for one die with twelve months of issues, 2, 1, 3, 2, 0, 2, 4, 1, 2, 3, 1, 3, a 45 day lead time, a 95% service level, 3 on the shelf and a pack of 6 per order. The history averages 2 a month with a standard deviation of 1.13. Lead-time demand is 3 units. The service-level safety stock is 2.27 units, the worst-case figure from a maximum of 4 a month over 60 days is 5. The reorder point is 5.27, rounded up to 6 units, with a minimum of 6 and a maximum of 12. With 3 on hand the verdict is to order now.
Why spares carry a maximum as well
The minimum is the reorder point. The maximum is the reorder point plus one order quantity, which is the most you should ever hold. It matters more for spares than for production material because a spare outlives its usefulness: when the die retires, every unit still on the shelf is scrap, and money spent on a shelf of pins for a die that runs twice a year was money the toolroom needed elsewhere. Holding to the maximum keeps the buffer and stops the hoarding.
Spares your own toolroom makes
A manufactured spare has a lead time too, it is just your own. From ordering the raw material to a finished spare on the shelf might be three weeks of toolroom time that the toolroom does not have when the die is already down. The reorder signal therefore belongs on the material, not the finished part, and the quantity that counts is the finished stock plus what could be made from material already in hand.
In VoraTool a spare carries its lead time in days, a minimum and a maximum quantity, and its current quantity. Bought-out spares run on the stock minimum and maximum with a movement history behind every change. Manufactured spares carry a separate material reorder point that raises the signal when the overall quantity, available plus not yet processed, drops to it. This calculator gives you the numbers to put in those fields; VoraTool keeps them on the record of the die the spare belongs to and shows the signal when it trips.
Questions about reorder points for spares
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