Adding Values In Separate Columns Dependent On Text In Another Column?

I'm trying to make a database to track inventory or several items. Basically, I have four tables:

1) RawMaterialList - includes a list of all raw materials.
2) PartList - includes a list of all finished product using said raw materials.
3) RawMaterialRecieving - contains details from each packing slip of incoming raw materials.
4) ShipmentRecord - contains details of daily shipments.

Each of these tables is fed by a form of the same name. I should note at this point that I basically taught myself how to use Access and I imagine I'm in the dark about quite a few things it can do. I've made several databases over the last few years, but I'm stumped at this point.

Here's my problem. In the form RawMaterialReceiving, I have several fields aside from basic information:

1) Item - a list of of raw materials from table RawMaterialList
2) Description - also dependent on info entered into table RawMaterialList
3) Quantity

But, I have 12 of these instances.

Item1, Item2...Item12;
Description1, Description2...Description12;
Quantity1, Quantity2...Quantity12.

My problem is I want to add up the quantities of each raw material and I'm not sure how to go about that. Lets say on May 13, I received 15pcs of Part A and 20pcs of Part B. I enter this information as Item1 and Item2 respectively. On May 14, I received 30pcs of Part B. I enter this information under Item1. Now I want to add up all of Part B (50 pcs). But Part B has one value listed in the field Quantity1 and one value listed in the field Quantity2.

Example:

1st Entry:
May 13
Item1 = PartA Description1 = PartA's description Quantity1 = 15
Item2 = PartB Description2 = PartB's description Quantity2 = 20

2nd Entry:
May 14
Item1 = PartB Description1 = PartB's description Quantity1 = 30

How do I get it to add up Part B to get 50pcs?