Quote:
Originally Posted by SylantBill
I have been using a 12 voly 50 amp car charger and having different results each time. I can't figure the 720 rule because a lot of my parts are screws and nuts and pieces under 1 sq inch. How does someone figure the 720 rule with these small parts.
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A current controlled (CC) power source is essential to good repeatable results.
Also, if you check the 720 rule calculator, you'll see that (depending on the current density you are using) 12 volts probably isn't enough.
At any rate, you'll need to accurately measure and hold the current (amps) that are going to the part (just let the volts do whatever they need to in order to maintain the amps).
Carefully measure the surface area of 1 part, and multiply by the number of parts in the batch. For smaller runs, it may be beneficial to add a sacrificial part. This will help even out any small measurement errors that are multiplied out by the number of small parts.
For example, you have 20 bolts that are .6 sq. in. each.
.6 x 20 = 12 sq. in.
Convert sq. in. to sq. ft... 12 sq. in. divided by 144 sq. in. (144 sq. in. in a sq. ft.) = .08 sq. ft. (rounded off).
Run these alone at 6 amps per sq. ft. current density = .48 amps for 120 minutes for 1mil.
This is a very small amount of current, and depending on your PS, may be impossible to control. Adding a sacrificial piece will increase the surface area, which will be easier to control:
Add a 6in x 6in x 1/8in piece of aluminum... 6 x 6 = 36. 36 x 2 (both sides) = 72. add the 1/8in edges... 24 x 1/8 = 3.
12 sq. in (20 bolts) + 72 + 3 = 87 sq. in.
87 divided by 144 = .6 sq. ft. (rounded off).
.6 x 6 amps per sq. ft. = 3.6 amps for 120 minutes for 1 mil.
3.6 amps is much easier to control than .48 amps.