exemple :

purchase : Raw steel diameter 25 mm long 5 m ans purchased 10 € per kg

                  This part of steel weight 10 kg.  Tien thé total cost is 100 €

Manufacturing : We used this to make several parts and we manage wharehouse in mm.

                   for part A we used 15 MM. ,  for part B we used 65 mm

Wharehouse : 1 MM for 100/ 5000   >>>  1 MM for 0,02 €

First solution is create  New category with uom1 = kg   and uom2 = MM  with coef = 1/2000

But if we have another Raw but in Wood the coef change !!

I think the best is create new class with columns : product_id , purchase_unit , product_unit , coef.

But the most is new class with columns :

product_id

unit.   = purchase_unit if buy_ok = 1   Or = sales_unit if sales_ok = 1. 

product_unit

coef

I try this .....but no good résul for the moment 

You can think about buying this product in tons, (as you explained). then define a workcenter in the supplier company to cut these plates into units, and continue you manufacturing process.
The question is why you insist to use dimensions to each unit?

Create a New category. Then create a Ton and Unit UOMS in it and give a coefficient. This will solve your problem.

If you have many suppliers from whom you buy in Tons, then call the new UOMS in new category: Tons of X, Unit of X, in order to aviud duplicate UOMS.