When you use the value: Vol
binding knockout won't convert your input to integer just stores the user entered data as a string.
So you need to parse it to integer before calculating the total:
$.each(self.Parts(), function() { total += (parseInt(this.Vol()) ) })
A fixed fiddle.
Now because you are doing the parsing you should not forget to deal with when case the user enters a non numeric input.
However you can easily write an custom binding handler which abstracts away the parsing.
Or you can do with using ko.extenders
. The documentation has an example as well here.