IF TWO TYPE OF GOODS (PARTS) HAVING TAX RATE 18%(9%+9%) EACH ASSEMBLED TO FORM NEW PRODUCT (ATTA CHAKKI) HAVING DIFFERENT TAX RATE IN SCHEDULE. WHAT WILL THE CORRECT GST RATE APPLICABLE TO NEW PRODUCT (ATTA CHAKKI).

Reply— GST rate on Pawan Chakki that is Air Based Atta Chakki is 5% and it is covered under chapter 84. 
As stated in the query, GST on inputs used in making the product (Atta chakki) is having 18% tax rate which is more than tax on output supply, hence, it will covered under inverted duty structure. 

As per section 54(3) of CGST Act, 2017, A registered person can claim a refund of unutilized ITC on account of Inverted Duty Structure at the end of any tax period where the credit has accumulated on account of rate of tax on inputs being higher than the rate of tax on output supplies.