In the below example you are making bread, but have to order flour from your supplier.
If you have raised a sales order for bread. When you run MRP and click Raise Purchase Orders it will only take into account the supplier lead time as no works order exists.
For it to take into account the supplier lead time and the MA lead time, you will need to have the Works Order raised before running MRP for the purchase order. You can do this in three ways
Raise a manual works order
Go to Production
Click on Works Order Enquiry
Press F4
Choose the Part, Quantity and Date for the new works order
Raise a 'back to back' Works Order
This is a works order that you have raised from within the Sales Order, you do this by:
On the sales order, press Create Order
Press Create Works Order
Confirm everything is correct and press Create
Use MRP
You can actually MRP to create Works Order and Purchase Orders. The only downside is using MRP to create Works Orders will not create them linked to Sales Orders as with the above step. The best practice with MRP is to do a 1st MRP run, and raise the works orders from that. Once the Works Orders have been raised then you would do a 2nd MRP run. This will then detect the demand of the works orders you previously created.