I often find myself needing to mix small quantities of epoxy resin, less than 1/2oz. With quantities that small it can be difficult to gauge the correct mix of resin and hardener. If the mix is incorrect the resin may not reach full strength or fail to cure completely.
To overcome this problem I have pressed my magnetic prop balancer into service as a precision set of scales. The accompanying sketch shows the details. Nothing is critical, just keep everything as light as possible. The cross piece can be 3mm lite-ply or hard balsa and should be a good fit on the balance shaft. On mine the cross piece is about 200mm long. Adjust the wire pan supports so that the pan sits horizontal with the pivot point above the pan centre, and give yourself some clearance above the bench, 10mm should be sufficient.
click for larger image
To mix the resins I use a small pot cut from the bottom of a plastic aspirin pot or small yoghurt pot. Make sure the plastic is compatible with the resin system. A small quantity of tin foil would also be suitable.
Before adding the resin etc. place the pot on one pan and balance with blue tack before adding your measuring weight. I use 1p pieces as medium weights (they are circa 1/8oz) and M5 or M4 nuts for small quantities. If you want to be really fussy use the same item for both resin and hardener, balancing between with more blue tack.
Note that if you need to mix unequal quantities a second balance point can be added as shown. The quantities will be mixed in the ratio of pivot point positions e.g. 5:1 as shown. Again before adding resin balance the pot plus hardener with blue tack, with the measuring weight off the pan.
My prop balancer is the magnetic type and hence will only cope with a total suspended weight of about 3oz; I suspect that other mechanical types would serve a similar roll and cope with larger masses. With mine I can easily see the difference of ‘one M3 nut’ in weight; as I have found that circa 22 M3 nuts equals 1/4oz this equates to a precision of 0.01oz.
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