Laravel 5 pivot table with 3 foreign keys

I have three models that are linked through a pivot table. Per order I would like to know which part(s) have been used on which machine(s). For example, an order > belongsToMany() > parts.

order

- id
- date

machine

- id

part

- id
- name

order_machine_part (intermediate/pivot table)

- order_id
- machine_id
- part_id

Four classes:

1. Order (extends Model)
2. Machine (extends Model)
3. Part (extends Model)
4. OrderMachinePart (extends Pivot)

When getting all parts for one order (Order::with('parts')->get()) I see the 'parts' relation on the Order model. This collection has different Part objects. The Part objects have a 'pivot' relation. The problem: how can I (eager) load the 'machine' relation for my pivot (OrderMachinePart)?

via Jeffrey