Shop Doc – Never This Rusty

Today’s Machining World Archives February 2009 Volume 02 Issue 05

Dear Shop Doc,

Recently a customer contacted us stating he had received several thousand pieces, all of which had rust. I’m using a water-soluble coolant for small-machined parts. The parts are machined, dropped into baskets, processed through a corncob dryer and packed for shipping. We haven’t changed the process and products for years, so why are the parts rusting now?

Never this rusty

Dear Never this rusty,
The first thing you need to do is ask yourself the following questions about your operation: Can you verify the concentration of your coolant? If the concentration dropped below the recommended level, rust could occur.

Do you use treated or softened water or is it from the municipality? Excess salt from softened water systems can create rust or high levels of chlorine from city water that may require increased coolant concentration levels.

Has your source for metal changed recently? Perhaps the metal had some surface rust present before processing.

Did the metal supplier change the mill oil he uses? Variances in mill oils can create rust. What day were the parts manufactured? Perhaps they were manufactured on Friday and sat over the weekend, prior to being dried and cleaned?

Look at your employee records and verify that the operators who typically process these parts were present on the day the rusted parts were produced. If a fill-in or temporary employee handled them that day he or she may not have followed the standard procedure.

Did you change suppliers for your cardboard boxes? Cardboard can be acidic due to the way it is manufactured. If a change in suppliers was recently made perhaps the acidity level was increased.
Has anything changed within the plant, such as positioning of fans, vents or placement of parts?
Are the parts near an area with high forklift traffic? Forklifts use propane gas as fuel and the exhaust is highly corrosive.

Did you calibrate your refractometer? To calibrate, place the water you use on the device and look through the eyepiece. The line should indicate zero. If not, adjust accordingly.

If these questions don’t reveal your problem, you can send some processed parts, a sample from the sump and a sample of the water that is used for dilution to a lab. They can evaluate the coolant for microbiological activity, tramp oils, foam, rust protection and concentration. If bacteria are present it can deteriorate thrust inhibition package.

In your specific case it’s possible that when the operator used the corncob method for drying parts he only processed them for half the standard time. After processing, the operator packaged the parts and sealed them, creating a humidity cabinet. You may be able to decipher this by looking at water stain inside the packed rusty parts. If that’s the case, you should consider posting the processing time above the machines so that every operator knows how long the parts are to be cleaned.

Mike Pelham
International Chemical Company