Your machine is too complex to be treated as an 'assembly' in QB - so we wont discuss that.
Self-produced inventory items have to be manually added into inventory - both quantity & value. Quantity is easy - value is the key question. Since you've been doing this so long I expect you already have a full schedule of needed parts per unit, and a good estimate of assembly time. Put this into excel and update the prices of components based on your most recent purchases. If you employ anyone to do assembly then also include their wage rate and the estimated assembly time. Include your own time if you pay yourself wages from the business. Now you have of estimate of cost per unit.
Also use this same sheet and list the quantities of parts on hand per item extended and tallied to produce an estimate of parts inventory on hand.
There are a couple of different ways of doing the actual accounting. In you case I suggest that all the component parts & labour be categorised into 'Cost of Goods' type accounts. Then add 2 other Cost of Goods accounts called 'Parts used in production' and 'parts inventory change'.
The machine is an 'inventory item' and that needs 3 settings for inventory, cost and revenue. Inventory goes to "finished goods inventory' asset, cost points to a COG account "cost of machines sold", sales points to sales revenue "machines sold"
All other parts are non-inventory type items that point to CoGS account.
After assembly make the 'inventory adjustment' to add 1 machine into stock, at the cost tally, and the 'adjustment' account is 'Part Used in Production'.
When selling the machine, create an invoice or sales receipt with 1 "Machine" item
At the end of the year/month update the schedule of parts on hand and set that value in an "Parts Inventory" asset, with the offset going to 'parts inventory change'.
Thats all of it.
You may get other advice about putting the item purchases into WIP assets - but doing it this way ensures that if your cost per machine production estimate is wrong - you will still end up with the right income tally - eventually.
