Spot drill - how large should the pit be ?

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compact8

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I have read somewhere that the pit made by the spot drill should be just large enough to accomodate the web at the tip of the drill bit but my intuition is that bigger should be better as the chance for the drill bit to grab and wander is less. I am talking about 120 degree spot drill used in conjunction with 118 degree drill bits. What's you guys' understanding on this ?
 
I'm no expert but I find that just about any divot will guide a twist drill. The only time I'm fussy about the depth of the spot drill is if I'm gonna tap the hole. I like to have the divot about 1 /16" bigger than the hole diameter. This eliminates a step later on as the chamfer is already there.
 
compact8 said:
I am talking about 120 degree spot drill used in conjunction with 118 degree drill bits. What's you guys' understanding on this ?
Click to expand...
As long as the spotting drills angle is larger than the drill bits you can run it in deep enough for a chamfer thus saving an operation . I won't say we never use a 90 degree spot drill for a 45 degree chamfer on a rigid set-up , but , this is not the norm . The drill bit can wonder and also break in this case , but we do it anyway .
 
I have had a lot of trouble finding a definitive source.
One manufacturer will say less angle, one will say more.
For instance
https://www.guhring.com/BrowseProducts/Products/NC-Spot-Drills Less is better they say
https://www.harveyperformance.com/in-the-loupe/choosing-spot-drill/ more is better they say
http://ohiodrill.com/spot_amp_center Only offer 90 and 118
I used to be able to find the Cleveland web site recommendation for it, which went into a lot of detail, but I can't find it now.

given that different manufacturers recommend different things, I would say it's still a matter of preference. I think it's a religious argument.
 
I'm with you Jeff , that's why I said we use whatever saves time hence the 90 degree spotting drills . Most people use a 60 degree center drill and couldn't care less ! :grin:
 
compact8 said:
I have read somewhere that the pit made by the spot drill should be just large enough to accomodate the web at the tip of the drill bit but my intuition is that bigger should be better as the chance for the drill bit to grab and wander is less. I am talking about 120 degree spot drill used in conjunction with 118 degree drill bits. What's you guys' understanding on this ?
Click to expand...

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. The larger the drill, the larger the web and the deeper you need to go.

A center drill like we use to properly drill for live centers on the lathe has a 120 deg starter tip and can be used as a spotter for smaller drills. The 60 deg angle, while often used for starting drills, is not the proper use for spotting. Doesn't stop anyone from doing it, though.
 
Thanks Mikey. I have learned something today.
Have a good day
Ray
 
woodchucker said:
One manufacturer will say less angle, one will say more.
For instance
https://www.guhring.com/BrowseProducts/Products/NC-Spot-Drills Less is better they say
https://www.harveyperformance.com/in-the-loupe/choosing-spot-drill/ more is better they say
http://ohiodrill.com/spot_amp_center Only offer 90 and 118
Click to expand...
Cant see that from the given links. The first 2 links said that the angle of the spot drill should always be larger than that of the drill. The 3rd link is not accessible.
 
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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