# What is causing the elliptical shape around the marked hole?

## Background

So I am looking at this piece titled "Ring" and I cant figure out what I am missing in the drawing that cause the elliptical shape at the two hole entrances on the right side of the drawing and the half ellipse in in the section cut.

Close up of oval in right portion of drawing:

Oval shape in section?

## What I have tried

When I modelled it I wound up getting this:

Since this is not my field of expertise, the only thing I could think of something like a grinding wheel taking a rounded chunk out at each hole.

## Question

Aside from scaling the drawing, does anyone have ideas what is causing the oval shape, and why it would be there?

# Update

Isometric views as requested:

• Probably because the hole is drilled perpendicular to one surface but that second surface is not perpendicular. Commented Jun 23, 2022 at 6:50
• @SolarMike I agree that what you are describing would generate an elliptical shape. Why would you wind up with a circle in the middle of the ellipse? I had actually considered that for the sloped holes, but the ellipse was in the wrong direction. Commented Jun 23, 2022 at 6:54
• What happens if you have two circles that intersect at an angle? then consider at which angle you view it from? Commented Jun 23, 2022 at 7:09
• @SolarMike The inner holes that are put in at an angle, show the path of the hole which does not allow for left right boring angles which would generate the ellipse Commented Jun 23, 2022 at 7:17
• Why would you need "boring angles" ? Commented Jun 23, 2022 at 7:18

Very rough image:

A cylinder, or hole through an object when viewed on the axis looks like a circle, or rectangle when perpendicular to the axis.

But at an angle to the axis then the ends of the cylinder show a "curved" end - due to the viewing angle.

The effect of your hole going through curved surfaces shows the cuts as the dotted lines you are concerned about.

for some examples.

• Totally agree. As the cylinder axis is rotated away from the viewing angle, the width of the ellipse is equal to the diameter of the circle. The other sides of the ellipse will shrink (in the direction of the tilt). Basically the ellipse can never be larger than the cylinder's circle. The ovals from the inner ring got larger in the wrong direction. For the other ring the view is straight down the axis of the hole, so there should be no oval at all. At least that is what is going through my head Commented Jun 23, 2022 at 8:27

The hole is through a cylindrical surface running perpindicular to the axis of the hole. The arc diameter of the curved surface is likely to be greater than the hole diameter making the elliptical shape in plan view.

The hole is then chamfered with an angular bit (usually 45° bit) to remove sharp edges so countersunk bolts may be used for a flush fit.

Update: Having had agood lok at it from my computer (rather than my phone...) I can see now that the holes look tapered with different start diameter to the finishing diameter

• I don't think you've got this right. The upper hole in the second image is perpendicular to the view so we're looking straight through it. Can you do a 3D drawing of how a 45° chamfer would give the ellipses and what the straight and curve are beside the 8 mm diameter in the cross-section view? The question has me puzzled. Commented Jul 10, 2022 at 20:15
• I agree that an angled drill hole/cut on both sides would produce an oval outline in plan and the D shape in section. As for countersunk bolt, are you saying this chamfered area is creating a shoulder for the bolt to rest on? Since its an 8 mm dia through hole, I am not sure how a counter sunk bolt would work here. I need to find some damned assembly drawings! Commented Jul 11, 2022 at 8:54
• Drilling into an angled surface (as the edges of the hole are), will leave a ridge profile at the down-hill edges of the hole. Like this /| |\. When the ridges break off in drilling, or are ground down, you get an oval in plan (you already have an oval in cutaway). I don't think a clean 45 degree chamfer would have the same effect. Commented Apr 7, 2023 at 0:07
• Drilling through a curved surface will always produce an elliptical section view perpendicular to hole axis. Commented Apr 11, 2023 at 13:07