Drill sharpening. Sharpening the jumper of the drill. Part 3 of 3 parts

Shows how to grind the jumper of the drill.
This is the 3rd part of the video on my channel about sharpening the drill. The first video was about the angle at the top of the drill, the second one was about sharpening the back surface.
“SHARPENING THE DRILL. Angle at the top. Part 1 of 3 parts ":
• ЗАТОЧКА СВЕРЛА. Угол п...
“SHARPENING THE DRILL. The MOST IMPORTANT thing when sharpening a drill. Part 2 of 3 parts ":
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I must say right away that undercutting the jumper is not a mandatory operation, but a desirable one.
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On new drills with factory sharpening, the jumper is most often not undercut.
Jumper undercut improves drilling, reduces feed force. But undermining the jumper reduces the strength of the drill.
When sharpening, the jumper is made approximately 0.2 of the length of the cutting edge. For example, a drill with a diameter of 12 mm has a cutting edge length of approximately 6 mm. So the jumper should be 1.2 mm. At the factory drill, the jumper was 2.6 mm. So it's better to sharpen it.
There are many forms of jumpers and ways to undermine them. There is only one goal - to reduce the length of the jumper. The jumper point must be made symmetrical. It is sharpened from the side of the grooves.
On a conventional straight profile grinding wheel, it is difficult to perform high-quality undercutting of the jumper on small drills for the following reasons:
- usually on abrasive wheels, the edge of the circle has a large radius.
- undercutting of the jumper is made almost blindly.
- some types of sharpening require high precision. This can only be done on machines or with the help of complex fixtures.
3:29 This video shows how I sharpen the jumper of the drill in several ways.
1. The simplest, but it can only be used for large drills. It is done on the edge of a conventional abrasive wheel. Along the peripheral and side surfaces of the abrasive wheel.
2. Undercut with access to the front surface of the drill. In some cases, this is even good, because not all drills have ground flutes, i.e., a ground front surface.
But at the same time, the front angle decreases, and this must be borne in mind.
With these two methods, the grinding process is poorly visible.
3. Grinding with angle grinders. Large drills can be ground with a grinding wheel, and small drills with a cut-off wheel. I use an angle grinder with a speed controller. I grind at the smallest rpm. The grinding process is clearly visible.
4. Grinding with a rotary grinder. I grind at medium speeds. The process is clearly visible.
5. Undermining the jumper with a diamond file. The grinding process is clearly visible. Needle files of various profiles can be used depending on the shape of the point.
I was asked to show how small drills are sharpened.
Sharpening small drills (1..2 mm) is no different. Only the jumper is not undermined. The problem is that it's hard to see.
After sharpening, I immediately do a test drilling.
If the drill does not drill well, it means that the required back angle is not maintained (because it is hard to see), and the drill has to be reground.
13:12 It is shown how two identical drills work, differing only in that one drill has a jumper undercut. It can be seen that both drills work well. But with a sharpened jumper, the drill works better. I repeat once again that the undercut of the jumper is desirable, but not required.