That drawing is good as far as it goes, as far as was intended by the author/illustrator. BUT the left side of the drawing is NOT a good representation of what happens when you enlarge a pilot hole. It shows an existing hole or dimple that is LARGER than the drill bit being used and that causes the outer corners of the drill bit to make first contact. A pilot hole will be SMALLER than that final size bit. What that drawing is showing is the use of a SPOTTING bit that makes a dimple that is larger than the final hole. Many machinists in commercial shops like to do it that way because the larger diameter dimple will also serve as a chamfer on the final hole: NO deburring step is needed after drilling to final size and that saves TIME. Time is money in a commercial shop. This can be considered an advanced skill. This is also why that side of the drawing shows a 90° angle: chamfers are often specified at 90°. And spotting drill bits are frequently found with 90° tip angles.
So, in addition to the two cases shown in the drawing, there is ALSO the case of a small pilot hole followed by the full size bit. Using a small pilot hole will do two things for you.
First, if you are at least somewhat careful/skillful in drilling it, it will be located with an accuracy that is smaller than the radius of that pilot drill. So, a 1/16" pilot drill will, in all probability, be centered on your mark within 1/32" of the intended location (punch mark) and likely much better than even that. The pilot hole's diameter should be about the size of the center web of the final, full size bit you will use. The pilot drill should also be a short, screw-machine length bit to best prevent wandering off course.
The second thing that a small pilot hole will do is center the full sized drill well, REGARDLESS OF IT'S POINT ANGLE. 90°, 118°, 135°, or whatever angle, they will all center well on the pilot hole. You can do the drawing, but the edges of the small pilot hole will contact the edges of the full sized bit near it's center, not at the outer corners. And since this full size bit will follow the initial, pilot hole, your hole is still located withing that radius of the pilot bit that you used. So enlarging a pilot hole is more like the right side of the drawing where the initial contact/centering is at the center, than the left side where contact/centering is at the outer edges.
This does not translate to sheet metal work, but for most holes it is the most accurate way for drilling holes by hand.
If you want an accurate hole in sheet metal I would again use a small sized pilot bit and follow it with a step bit. You will get an accurately located hole and a nice, round one; no lobes. A small diameter pilot bit (1/16") will probably have it's full diameter inside the hole before it breaks through the rear side so it acts more like it would in a thicker piece of stock: it is right
at home in sheet metal. And step drill bits excel at keeping the hole round. If you are enlarging an existing hole in sheet metal, the step bit will still do an excellent job, with one small caution. If the original hole is not the same size as one of the steps on a step drill, then there is a possibility that the center of the enlarged hole may wander by an amount up to half the step size. If you need the best accuracy, a taper reamer can be used to enlarge the hole to the size of one of the steps on the step drill and then use the step drill.
If you are going to use a spotting drill bit which makes a large dimple as per the second side of the drawing, then yes, it is best to have a spotting bit with a 135° angle or larger.
Thank you to all of you for the explanations. Saving up money is always very welcome. I thought it was a common practice when it comes to enlarging.
In another order of things, I would like to ask you -if not abuse- about pilot holes.
Researching about it I found machinists placing an emphasis on the relation between "spot angle" and "drill angle" because of vibrations, etcetera. A photo paints a thousand words:
View attachment 284525
According to the pic the initial contact is quite critical, however, the case placed on the left is similar (not equal) to the one that we face every time we enlarge a pilot hole.
Does anyone know if this pernicious effect is present while enlarging pilot holes? I am trying to find a proper method to drill precision located holes and I am not sure whether or not this is the way to go.
Thank you!