Calculating Scrap %

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    Mr Gue

    I have a product that includes 6 internal processes. I want to know what the scrap is but this is my challenge
    Process A 500 parts are loaded 50 are scrapped = 450 for next process
    Process B 450 partsare loaded plus 35 of the day before = 485 loaded 80 are scrapped = 405 for next process
    this similar situation happens in the consecutive processes, the last one is inspection and packing
    How would you calculate total produced and scrap %
    Any takers?
    Your help is appreciated



    I would not try to find one metric since it would likely hide too much information.   Assuming you (and management) would use the metrics for CI, you should consider calculating several different metrics.   Certainly the overall scrap rate is great, throughput yields are nice and a metric that shows the scrap rate at each step individually would be nice.   These are all very easy to calculate.   Be sure to graph the results over time looking for trends.   The effort you (and management) put into this should be proportional to how costly the scrap is to your company, you may have to set up some different data collection schemes.   Just develop some that seem right to you.   Then make them inputs to management planning.   If your management planning works properly, they will morph over time into what is truly beneficial to your company.


    Bee Guy

    Here’s what we do…it may not be “correct”, but it works for us. A scrap % at the first operation doesn’t have the effects of other operations. How much scrap, why, fix it! Other than that…
    Scrap is reported at each work cell or station throughout production, along with parts produced and direct hours. Knowing the “standard”, these numbers are plugged into an Excel profit model that takes into account fixed and variable hourly costs associated with each operation. The model calculates, by operation,  % scrap, cost per piece, cost variance from standard (profit/loss), cumulative scrap cost, contribution margin, and gross margin.
     Pertaining to scrap, we get the value added, not just the %. From there we can look deeper into causes and relationships. We don’t worry about absolute totals and percentages.


    Troy Lord

    The calculation I use in it’s rawest form is one
    minus the parts shipped divided by total parts in.
    Using the numbers in the question 500 parts started
    and last number provided of 405 gives a scrap rate
    of 19%. (See the 35 parts below)There is also cost, as I assume cost is added at
    each step of the process, therefore I weight the
    scrap percentage to aid in project selection. The issue of parts not finishing the process each
    day is an issue that I have struggled with in the
    past, it varied depending on the process. What did
    not vary is how we implemented a method to track.
    Let me explain. The total parts into the process
    was new reset at the start of 3rd shift every day
    (As this was already considered the start of data
    collection for other process’s), as was parts
    shipped. Though there was output variation with
    each process due to manufacturing, the method stood
    up to scrutiny over time. The process I speak of
    ran daily though.



    I would calculate it more from a statistical standpoint.  Both operations are independent of eachother.  You have 450/500 or .9 and you have 405/485 for .835.  (.9)*(.835) = .75.  1-.75 = 25% of the parts are  scrapped.


    Don Strayer

    Look up how to calculate RTY (Rolled Throughtput Yield).  That’s the basis of the formula Adam suggests.

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