Now I am going to risk it and ask the question. Why ? I have already said in other threads that I do not do this and not confident enough to do it accurately enough. But I think it's a topic that many would like to know how.
The question being :
What method / tools / how to trim an oversized DF rotor to fit a smaller shroud ?
Lets say for example you have a 55mm rotor and wish to fit to a 50mm shroud.
Over to you guys for all the ways / solutions ...
Last part : And how do you ensure you have balanced rotor after ? Not only in weight terms but also balanced thrust ?
I use a precision rule and a micro saw to cut each blade the same length. Balance can be done static by center balancing and then dynamic by attaching the impeller to a motor using a collet adapter.
If it is not balanced you will know but imbalance usually shows up in the static balance test and it will lean towards the heaviest blade. If the blades is properly cut, all blades the same length, and you don't do anything to alter the pitch of any blade, then the thrust should be balanced.
I have only done this twice with 70mm blades that I cut to use in a 64mm fan and I didn't need to do anything to them after cutting, balance was spot on. The result was disapointing and the cut down fans did not provide the thrust I was expecting so I ordered some GWS GWDFS001 6 blade impellers. These worked better than I expected and paired with a EMP 4500KV 300 watt motor they really put out.
I mount the rotor onto an aluminium sleeve and clamp it into my lathe. Using a sanding disk mounted in my dremmel - I first cut off the excess blade length then spin the chuck by hand while sanding the tips to conform to shape.
This is a very accurate way to trim rotors and I have been doing it this way for years now right down to making 20mm rotors.