How do I make a right angle block

[homebrewed]

Did you not mean the face is then very close to parallel to the spindle - or do I misunderstand?

Yes, sorry, the (first machined) face being DTI'd is parallel to the spindle's axis of motion after the alignment is done. Once it's machined the second side will be perpendicular to the spindle axis. Apologies for the confusion.

Descriptions I've read about making a machinist's cube include the use of a DTI to guide the scraping steps (assuming you don't already have a machinist's cube to use as a reference). So, milling or scraping, ultimately you will be limited by the DTI. For something that uses "L" stock, flex due to machining forces is going to cause problems w/respect to flatness. That's where scraping would come into play.

 

[graham-xrf]

Getting the mill trammed to move up/down at right angles to the table (DTI against a machinist's square) is a convenience. Theoretically not necessary if you shim up the workpiece to end up parallel to the spindle, but the flat surface you can make reaches only as far as a fly cutter can cut.

I think, tramming the mill, and having it do the best it can, then moving to a surface plate, spotting the first flat, then using a cylinder square (made on a lathe), to get the corner references on the second surface exact. Then scrape the second surface, continually checking with the cylinder square until you arrive at two scraped surfaces exactly orthogonal. At-least, that's where my mind is taking me. I have to keep referring to Connelly's book, but I am sure there are experienced folk here who know better ways with less hassle.

 

[Flyinfool]

Yes you defiantly want to weld in some strong gussets before any machining, and make sure your setups are fully supporting the surface you are machining.