Manufacture metal pieces

[Begus]

Hello,
I'm sure everyone is already tired of all the beginner questions, but I didn't find an answer to this one.

I've designed my first CNC mill, which is supposed to be mainly out of metal.
I have tried to use mostly off-the-shelf components such as aluminium profiles and square aluminium pieces, but there are some custom shapes.
As of yet, I do not own a mill, just the most basic tools such as a grinder and a drill press.
Wouldn't it be sufficient to just cut these custom pieces out with a grinder, make them as flat and accurate as possible (with my tools), assemble it and machine a plywood or MDF bed flat to then machine these pieces with this temporary setup?
As I understand it, the imperfections of the machine are relative to the workpiece/bed, so if you were to machine the bed itself flat, the pieces should be accurate enough.
Am I correct in my assumption, and what other possibilities would I have if I were wrong (without spending thousands on an actual mill just to manufacture parts for a CNC mill)?

[ZASto]

Before machining your bed, you should tram your spindle.
Spindle that is not tramed will make a stairway (not to heaven :nevreness:) on your bed during flattening.

[Begus]

Before machining your bed, you should tram your spindle.
Spindle that is not tramed will make a stairway (not to heaven :nevreness:) on your bed during flattening.

Thanks for the advice!
Is there anything else that could go wrong with my plan?

[dazp1976]

Before machining your bed, you should tram your spindle.
Spindle that is not tramed will make a stairway (not to heaven :nevreness:) on your bed during flattening.

Which would you prefer to have mounted on the table for best tramming results?.
A sheet of glass or thick perspex?.
Or just stick your needle on the table itself?.

[JAZZCNC]

Nearly impossible to answer because it depends on the part and the accuracy you are aiming for and how good a job you make of the first attempt.

For a milling machine then you'll want fairly good accuracy and you may find it cheaper and less hassle to have the main important parts made for you and important surfaces machined flat and parallel etc.

This will save you time and money long term as you will only do it one time rather than spending weeks or months making failed attempts as you chase accuracy.
Often trying to save money in the wrong places costs money long-term and becomes a false economy.

[Voicecoil]

One thing that can help you sometimes is tooling plate. This has reasonable accuracy in both thickness, flatness and the two sides being parallel. But there will likely be some parts where you need surfaces accurately at 90 degrees etc., and there as Jazz said it may be better in the long run to get them machined for you.

[Begus]

Thank you for your answers!
I guess I will look around for someone with a mill, as I would rather do it myself than spend an enormous amount of money to have the parts machined for me.