Spot drill - how large should the pit be ?

woodchucker said:
From Guhring:

NC Spot Drills | Accurate holes for a secondary drilling operation​

NC spot drills are high precision drills designed specifically for creating an accurate hole location for a secondary drilling operation. NC spot drills due not have body clearance and are not designed to drill greater than the depth of the point angle. The 90º and 120º spot drills are primarily used to create an initial spot for 118º and 135º secondary drills respectively. The 142º point NC Spot drill, series 546, is designed to help center carbide drills with an angle of 140º.
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OK , my bad .... but it also say 140 degree drills require 142 degree spot drill. 135 degree spot drills should work according to their believe.
 
mikey said:
A twist drill cuts with the tip flutes on either side of the web so you only need to go deep enough to clear the web.
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I have heard about this but it makes me wonder why the angle of the spot drill tip matters.
 
I mostly grind my own spot drills out of very short drills or broken centers. So I grind them to match the drill point angle. Or change angle if needed.

Jimsehr
 
compact8 said:
I have heard about this but it makes me wonder why the angle of the spot drill tip matters.
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so I think it's because carbide is so brittle that just hitting the tips of the bit will break them.
I think that's what it says...
 
compact8 said:
I have heard about this but it makes me wonder why the angle of the spot drill tip matters.
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The angle of the spotting drill allows the drill to cut progressively deeper without chattering as the flutes engage. As more of the flute engages in the hole, this tends to stabilize the tip and the drill cuts smoothly. Try drilling just a pilot hole and follow with a drill and you'll get a lot of bounce so a spotter really does help.

The other thing you'll notice is that if you lower your speed and increase the feed pressure, the drill will cut faster, cooler and more accurately. A spotter really helps you to start cutting cleanly, especially when you drill like this at lower speeds and higher feed pressure.
 
mikey said:
The angle of the spotting drill allows the drill to cut progressively deeper without chattering as the flutes engage. As more of the flute engages in the hole, this tends to stabilize the tip and the drill cuts smoothly. Try drilling just a pilot hole and follow with a drill and you'll get a lot of bounce so a spotter really does help.

The other thing you'll notice is that if you lower your speed and increase the feed pressure, the drill will cut faster, cooler and more accurately. A spotter really helps you to start cutting cleanly, especially when you drill like this at lower speeds and higher feed pressure.
Click to expand...
I know this working principle of the spot drill but my question is if the divot made by the spot drill is just large enough to accomodate the web of the drill , the flute of the drill will be engaging with the flat surface of the workpiece instead of the conical surface of the divot so there will not be any stabilization effect.
 
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If the divot made by the spotter is large enough to span the web of the drill then only the flutes contact the surface of the divot, right? This is so because the angle of the hole made by the spotter is wider than the drill when the spotter is appropriately sized for the drill in use (120 deg spotter for 118 deg drills, etc). None of this is hard. Drill deep enough with the spotter to make a decent hole and follow with the drill. It will not hurt anything if the spotter hole is a little larger or a lot larger than the web of the main drill. A 1/4" diameter spotter will spot up a a 1/2" drill with no problems.
 
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