What is the purpose of 90° spotting drills?

Mark Rand · Dec 1, 2021

I tend to use spotting drills before the main jobber or extended length drill to ensure proper centering. I'd recently noticed that my 1/4" and 1/2" HSS ones were starting to look a little tired, so I ordered a couple of carbide replacements. When they arrived today, I realised that I'd screwed up and got 90° ones instead of 120° ones.

My question is, what on earth are these used for other than for pointy plastic drills or as jerry rigged countersinks for woodscrews?

CITIZEN F16 · Dec 1, 2021

Someone should do a long study on whether the spot should be a larger or smaller angle than the drill. I often use a 90 degree spot for 118 and 135 degree drills, most everyone I worked with did the same. I have never seen any issues myself. It seems over time more people believe the spot angle should be greater than the drill.

gustafson · Dec 1, 2021

metric screws are 90 degrees
small chamfers

DanASM · Dec 1, 2021

I run Brown and Sharpe screw machines. 90* spot on everything, use 118* mostly. Sometimes 130*-135*. It leaves a 90* chamfer on the hole after it is drilled.

CITIZEN F16 · Dec 1, 2021

DanASM said:
I run Brown and Sharpe screw machines. 90* spot on everything, use 118* mostly. Sometimes 130*-135*. It leaves a 90* chamfer on the hole after it is drilled.

When was the last Brownie made? How many do you have?

mike44 · Dec 1, 2021

I am a home shop machinist , I am not sure if the angle of the spotting drill akes a difference. I use spotting drills to start a center hole and then switch to jobber drills. If a part needs center drilling I still use the spotting drill first.
This is the method I was taught by relatives that are machinists.
mike

DanASM · Dec 1, 2021

CITIZEN F16 said:
When was the last Brownie made? How many do you have?

Early 80's I believe the last ones were made, Ultramatics.

I just bought 2 more from a multi spindle screw machine shop at auction a few weeks back. Between my father and myself we have 20 machines that can run and make good parts all day. Another 10-14 in storage for parts or when space becomes available to run them.

I was always taught to use 90* on 118* drills. Spot drill has to have more of an angle than the drill following it. I am sure there is more of a science to it but I have had good luck with a 120* spot and a 118* drill. Was an accident, but I was able to make good parts.

There are youtube videos describing how spotting before drilling works. I feel like I have watched them before.

SeymourDumore · Dec 1, 2021

CITIZEN F16 said:
It seems over time more people believe the spot angle should be greater than the drill.

For some applications, Guhring strongly recommends the same or larger spot angle than drill, otherwise they just say to make the spot diameter smaller than the drill dia.
For me, it's 90deg spot for practically all holes. Small enough spot dia, just larger than the size of the split point ( hate chisel ).
If larger chamfer is needed for say a tapped hole, then I just come back after the drill for another, chamfer pass.

The only time I use a 135 deg spot is for small carbide drills, or really high L/D ratios.
Don't own any 120 degree spots ....

Murf · Dec 1, 2021

CITIZEN F16 said:
Someone should do a long study on whether the spot should be a larger or smaller angle than the drill. I often use a 90 degree spot for 118 and 135 degree drills, most everyone I worked with did the same. I have never seen any issues myself. It seems over time more people believe the spot angle should be greater than the drill.

Spot should be larger angle than drill.

Murf

Sent from my iPhone using Tapatalk

Murf · Dec 1, 2021

Murf said:
Spot should be larger angle than drill.

Murf

Sent from my iPhone using Tapatalk

According to Harvey Tool anyhow!

Murf

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CNC_Chip_Thin · Dec 1, 2021

Murf said:
According to Harvey Tool anyhow!

Murf

Sent from my iPhone using Tapatalk

Agreed, same or larger angle for a spot. Allows the tip of the drill to contact first and start the hole straight. If a shallower angle is used to spot the outside of the drill contact's first and will cause the drill to walk. Found this out the hard way.

90 degree would work for short holes where size and roundness of the hole aren't super critical but still not ideal.

90 degree tool's should be used for chamfering holes and deburring edges. Just my .02 cents

SeymourDumore · Dec 1, 2021

CNC_Chip_Thin said:
Agreed, same or larger angle for a spot. Allows the tip of the drill to contact first and start the hole straight. If a shallower angle is used to spot the outside of the drill contact's first and will cause the drill to walk. Found this out the hard way.

90 degree would work for short holes where size and roundness of the hole aren't super critical but still not ideal.

90 degree tool's should be used for chamfering holes and deburring edges. Just my .02 cents

Again, if you make the spot small enough, the edges of the drill will not be the ones contacting first.

Scruffy887 · Dec 1, 2021

Used 90 degree spot with a carbide drill bit after. Great choice for carbide drill salesman, not so good for me. Outer lips chipped fairly quickly.

CarbideBob · Dec 1, 2021

If the drill tip hits the inside surface first, why spot at all?
Oh wait.. the actual pointed tip has a much different angle than the lips on most common drill bits.
Rabbit hole.
The above drawings are right and very wrong on entrance and start as the contact tip not correctly shown. If only it was this simple.
Although the drawing absurd lets go with pic 3.
The drill is moving in Z and not static as shown. Drawn correctly the lips are not the same height at any time in the process. Tool in hand vs tool in use is super messy with drills.
One guy bites and is loaded before the other. We are now deflected off center at that half rev. until number two comes in and tires to fix it.
Now we have wobble.

Wat works great works and maybe we do not want to know why.
Bob

Gordon Heaton · Dec 1, 2021

CITIZEN F16 said:
. . .I often use a 90 degree spot for 118 and 135 degree drills. . .

Me too! Never had a chatter or wander doing that. In fact, in my experience they work just as well as a matching angle.

Mark Rand · Dec 2, 2021

Well, I've just gone and ordered some 120° ones to go with the 90° ones. I guess I'll have a choice now. :rolleyes5:

DanASM · Dec 2, 2021

barbter said:
Errrr.....then Guhring say this!
Perhaps they want to sell more drills through wearing out edges?
NC Spot Drills | Accurate holes for a secondary drilling operation - GUHRING

This is the way I have done it for 30+ years. Same Guhring drills that they recommend above are the ones we have been using.

Mark Rand · Dec 2, 2021

So some experts are saying that the chisel point can cause the drill to wander and others are saying that the flute edges can cause it to wander (or get chipped). Mayby it makes bugger all difference and it's the fact that a short, stiff spotting drill starts the hole in the right place for the longer, whippy drill to follow.

Ho hum.

It's amusing that we seem to have two almost identical threads on the go at the moment.

https://www.practicalmachinist.com/vb/general/center-drills-proper-use-397190/#post3849508

FredC · Dec 2, 2021

Gordon Heaton said:
Me too! Never had a chatter or wander doing that. In fact, in my experience they work just as well as a matching angle.

Never knew this was supposed to be wrong till about 15 years ago. So for 30 years before I was doing it wrong and did not know better. The last 15 I still do it wrong but know better!?! Using a 90* spot tool or a center drill with a 60* is still the way I get a chamfer and relatively good cencentric hole.
Makes you wonder about the people now saying the old way is wrong did they get 1% better concetricity by matching the angle or was 1 1/2%? Saving a position on a screw machine or turret lathe is still better than doing it "right" and coming back with another tool to make a chamfer.

Terry Keeley · Dec 2, 2021

Murf said:
Spot should be larger angle than drill.

Murf

Guhring says spot should be smaller for HSS and bigger for carbide.

http://media.guhring.com/documents/tech/SpotPilot/NCSpot.pdf

What is the purpose of 90° spotting drills?

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