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The approach to structure your BOMs is always a nuanced answer. It really depends on what you wish to achieve with your ERPNext implementation.
In your case, as you make those wooden parts for a chair, do you immediately make the chair (or any other piece of furniture)? Or do you try and optimize the block of wood to minimize wastage, even if it means that you make parts that you don’t have to make into furniture just yet?
If you make the furniture item as soon as you cut the pieces, you could just make the furniture item and flush out the wood (in Cubic Centimeter/meter/feet) for the furniture item you make. That will show the left over wood, even if it cannot exactly tell you the dimension of the left over wood.
If instead you cut the block of wood into sheets and then make furniture parts out of it and you store both the wooden sheets and the furniture parts for future orders, you could set up items for the wooden sheets, make the wooden sheets from the block of wood. And then consume up quantities of the Wooden Sheet in (Sq Centimeter/Meter/Feet as you’d have setup separate items for sheets of different thicknesses) to make the furniture parts and then consume up the furniture parts to make the furniture. If you notice that means 3 different manufacturing entries.
Another simple (but drastic option) is to have BOMs only in Cubic Centimeter/Meter/Feet AND not use the BOM at all to make manufacturing entries. In fact there is no manufacturing entries. You just issue the Block of Wood into WIP, as as you make furniture items, you use Stock Entry purpose Material Receipt and get the Furniture item into your Finished Goods store on ERPNext. When the shopfloor requires another block of wood, you issue it from the Raw Material store into WIP. That way you have a correct count of the Blocks of wood in your Raw Material store. You also have a correct count of the Finished Goods items as the shopfloor is making the Stock Entry purpose Material Receipt into the Finished Goods store just as these furniture items are rolling out of your shopfloor. Now you build a small custom report on ERPNext to compare the items you have made, and using the BOMs that you have setup (consumption of material is in Cubic CM/Mtr/Feet) you try to make sense of what is being issued to what is being made and estimate whether your costing is okay and whether the shopfloor is doing a good job of optimizing the wood that has been issued.
Since in your business wastage (due to end bits, etc.) cannot be avoided, just focusing on BOMs may lead you to ignore the cost of wastage. So that custom report that compares what you have produced to what you have consumed is anyway necessary, even if you decide to make manufacturing entries off BOM) to be on top of things.
I don’t mean to mess you up, but just because ERPNext has a ton of features does not mean that you need to use all of them. The shopfloor can only handle a certain amount of complexity. So the focus of every ERP implementation is to see: 1. What the shopfloor can manage (in addition to them churning out quality products that you want your customers to come back for more) in terms of ERPNext entries, 2. What metrics helps you drive your business optimally. These two considerations are the biggest (and perhaps the only) inputs into how you structure your Item Master, Your BOMs and your Stock/Manufacturing entries appropriately.
Just implementing a detailed BOM based implementation of ERPNext may make you realize many months or quarters later, that your ERP(Next) implementation isn’t really serving you well.
Hope this helps.