Do I need a straight edge?

[Jacob]

What you do is cant the spirit level on edge.

That would only work if the face is ground accurately, since the arris you're using is the intersection of 2 planes (edge and face); they'd both need to be accurate for the arris to be accurate.

I don't think the makers of spirit levels certify the accuracy of the face, although it may be good enough for joinery work.

BugBear

Stabila are well made and good enough for setting a planer table. Whatever it is you don't need a certificate - you can squint down the edge if you are not sure it's straight. In the end it's done by eye anyway (unless you are going fiddle about with feeler gauges but that's an engineers approach) which means your DIY edge is going to be accurate enough.

 

[No skills]

Have we found an alternative to the sharpening thread?

Put a light behind the level if it's that important.