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When we model a handle, it is preffered to have a curve on the one side, so that the fingers can hold it better... this is how we do that..
These are the steps....
Step 1
first of all we take a handle that we need to model the grip...
Step 2
then we go to the "right" plane (it depends on how you have modeled your part) and we create a spline to determine tha area that the grip will be...
Step 3
Next we split line it to all faces...
Step 4
then we delete the split line faces.... like this...
Step 5
Next we draw a sketch like how we want the grip to be...
Step 6
We create the surface using the loft surface feature (please note the constraints... tangent and normal to)
Step 7
this is the result...
Step 8
mirror surface...
Step 9
Knit surfaces (creating a solid body)
Step 10
This is tha final result...