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View Full Version : How to grind a homebrew spotting drill???



torker
02-28-2008, 01:30 AM
Hey guys!
I believe there was someone here awhile back who made their own spot drills from broken center drills.
Could any of you post a pic of one you've done?
I can fire up my sooper dooper end mill sharpener(hide your eyes Russ) and grind something up I'm sure...but I need to see how or what you changed in the process.
Thanks!
Russ

Paul Alciatore
02-28-2008, 01:42 AM
I didn't see that post, but a spotting drill is just a short drill. Same point geometry (shape) as a standard drill. It does not need to penetrate very far so the straight flutes of a center drill will be just fine. Grind the tip off and sharpen like any drill.

Fasttrack
02-28-2008, 01:53 AM
I think most spotting drills actually have a different geometry. The ones i've seen generally either have 60*, 82*, 90* or 120* included angle for different bits/applications

I think the 120* point is used to start 135* bits but i could be wrong...

winchman
02-28-2008, 03:40 AM
Info from McMaster-Carr catalog that might help:

"About Point Angle

Drills with larger point angles (flatter points) work well for hard, tough materials; drills with smaller point angles (sharper points) are better for soft, nonferrous metals."

"Cobalt Steel CNC Starting (Spotting) Twist Drill Bits

Chisel point prevents walking, while a short flute and overall length provide precise spotting prior to drilling. Spot drilling ensures more accurate holes, especially if the next drill bit to be used is a jobbers' length or longer. Spot drill bits handle higher feed rates than combined drill bits and countersinks (often incorrectly used as spot drill bits). To spot and chamfer simultaneously, use a 90 angle spotting drill bit one size larger than the twist drill bit that will follow.
Drills with a 120 angle provide a spot with the same angle as the twist drill bit to be used, resulting in a perfect seat. Using a 120 point versus a 90 also allows you to drill a shallower hole and attain a given diameter hole quickly. The shank diameter is the same as the drill bit size." Page 2383

Roger

John Stevenson
02-28-2008, 04:18 AM
Buy stub drills and then you don't need to spot and swop drills. Do it all with one provided you aren't going 2" deep.

.

DR
02-28-2008, 04:50 AM
A spotting drill has a very thin web. Much thinner than a standard type drill, stub or jobber length.

Center drills have thin webs also. So by just totally removing the small diameter tip and grinding the cutting edge to the desired angle you can make an acceptable spotting drill.

A spotting drill is typically fairly long, like jobber drill length, stiffer though since the fluted area is short. You can't get that much length in common center drills. Long center drills are available, but probably cost as much as a spotting drill.

We use 90 degree spotting drills to both spot the holes and create the lead in chamfer at once.

torker
02-28-2008, 07:14 AM
Yabut...when you grind the end off...or in my case if you go through the box of broken ones that are already that way ;D there is a "bald spot"... do you just grind the flute part deeper on the same angles or would you change that?
You can't just start grinding it as a regular bit...the "bald spot" is still there.
I want to use my machine if I can as it's going to do this far better than I can by hand with less than perfect eyesight.
Thanks!
Russ