Spotting Drill bit

Anyone have a rule of thumb for spotting drill bits? like use a 90 degree for drill bits under 120 degrees and ???? ... It seems that there are several angles for these and am trying to eliminate my own corn-fusion ...... and cannot find a clear referance for
thanks
Pj

Your spot drill angle should be more than the point angle of you drill bit. For 118 degree drills use a 120 degree spot drill. For 135 degree drill use a 140 degree spot drill.

The drill should start cutting in the centre not on some outer part of the cutting edge.

A lot of folks just use a 90 degree spot drill for everything and I shudder when
I hear that grinding screech as the cutting edge first catches the outer rim of the spot drill hole.

This is one reason that it is not recommended to spot drill when using a carbide drill; carbide is too britttle to stand up to that initial grab on the outer part of its cutting edge; it will chip very easily. Unless of course you use a spot drill with the correct angle.

In fact, thinking about it a 90 degree spot drill is of no use at all.

Regards,

Mike.

We use a 30 degree (120 included angle) spot most of the time.
If you have the cash, invest in a SECO minimaster for spot drilling and chamfering.
It has both 30 and 45 degree indexable inserts, as well as 1/2", 3/4" milling inserts.
It's really a great tool.
We have one for each mill in our shop.
Check them out at SECOtools.com (minimaster)

MikeJB said:

Your spot drill angle should be more than the point angle of you drill bit. For 118 degree drills use a 120 degree spot drill. For 135 degree drill use a 140 degree spot drill.Regards,

The drill should start cutting in the centre not on some outer part of the cutting edge.

A lot of folks just use a 90 degree spot drill for everything and I shudder when
I hear that grinding screech as the cutting edge first catches the outer rim of the spot drill hole.

This is one reason that it is not recommended to spot drill when using a carbide drill; carbide is too britttle to stand up to that initial grab on the outer part of its cutting edge; it will chip very easily. Unless of course you use a spot drill with the correct angle.

In fact, thinking about it a 90 degree spot drill is of no use at all.

Regards,

Mike.

Click to expand...

I pretty much disagree with your evaluation. *Regards

In the Moore Special Tool Company Book "Precision Hole Location" they like to have the spot drill angle narrower than the drill angle. This allows the drill to start a "counter bore" in the cone which acts like a drill bushing to guide the drill.

If you are going to rely on the spot drill's "little bitty" spot point, where the following drill touches, to capture the following drill and guide it's accurate location, you may have a dissapointment.

In support of your comment of chatter throwing off the following drill's location, I agree.

The trick is to have the following drill's RPM and feed rate in the correct proportion so chatter doesn't develop. In other words, "Take a cut."

The other way is use no spotting drill. However this requires well ground drills which are running concentric to the spindle; and drilling a perpendiciular smooth surface.

I have been doing this for years without difficulty.

I hope this helps.

Regards,

Stan-

Are there 140* spot drills? I think I've only seen 90 & 120 and always heard it as 90 for 118 drills and 120 for 135 drills. I use the 90* because it's easy to do the math in my head to leave the right size chamfer. I actually use the 90* mill-drills and also use it for chamfers, milling, etc. Not the purist method but it works.

Any of the tooling guys want to chime in on what is the textbook answer?

Not to jump on any band wagons but I agree, the spot should be of a smaller angle than the drill. Reason being if its bigger (spot cone) then the chisel end of the drill bit will be riding on the inside of the cone, it will be first in contact. Looking at the chisel end you dont want it to be the first to determine the path, thats why you use the spot, among other reasons, the chisel will rock back and forth in the cone of the center drill cone until it breaks past the cone. No, you want the cone of the drill to contact the cone of the center drilled hole. JR

To continue the theme here are two quotes:

From Wikipedia:

'While the above is a common use of center drills, it is a technically-incorrect practice and should not be considered for production use. The correct tool to start a traditionally-drilled hole (a hole drilled by a high-speed steel (HSS) twist drill) is a spotting drill, or a spot drill, as they are referred to in the U.S. The included angle of the spotting drill should be the same as, or greater than, the conventional drill bit so that the drill bit will then start without undue stress on the drill's corners, which would cause premature failure of the drill and a loss of hole quality.

Most modern solid-carbide drills should not be used in conjunction with a spot drill or a center drill. They are specifically designed to start their own hole. Usually, spot drilling will cause premature failure of the carbide drill and a certain loss of hole quality. If it is deemed necessary to chamfer a hole with a spot or center drill when a carbide drill is used, it is best practice to do so after the hole is drilled.'

From Cutting Tool Engineering Aug. 1999:

'The point angle of the spot drill plays an important role in hole accuracy. A spot drill with a 90° point is convenient because most holes require a 45° chamfer, which means you can spot and chamfer simultaneously. However, this point angle may not be the best choice.

Jobbers-length drills usually have a 118° point. Using a spot drill with a 90° point causes the 118° jobbers drill to initially contact the workpiece at its outer margins (Figure 1a). If the jobbers drill is not sharpened perfectly, one of it flutes will touch the workpiece first. The drill will deflect away from this flute and could eventually begin to walk. This improper contact can also cause the drill to flutter back and forth slightly until it is fully engaged in the workpiece. Fluttering may cause chipping of the drill point when cutting extremely hard or tough materials.

It’s usually better to spot the workpiece with a drill that has a 120° point. Since a spot drill has a thin web, using one with a wider angle will force the 118° jobbers drill to make contact at the outer part of its chisel edge (Figure 1b). This will minimize the chance of an inaccurate hole being drilled. Additionally, choosing a spot drill with a 120° point instead of a 90° point allows you to drill a shallower hole and attain a given diameter more quickly.

This is the first time I have looked up this subject; my earlier comments were based on my own experience.

Regards,

Mike.

Ok thanks for the class on cornfusion.....
Use a spotting bit with larger included angle than the drill bit so the bit starts in the cone and not the upper edge. 135* bit requied a 142* spotting bit, 82* bit required a 90* spotting . so this would lead one to think about split points as the lead point would be well down in the cone and start first before the edge hits the upper part of the cone ? Only if the lead point is long enuf to engage the deeper part of the cone first ....

I find this funny, I have owned a screw machine shop since 1972, now also CNC & have always used 90 deg spot drills & I've ran millions of parts & they work fine. Most parts require a 45 deg chamfer so why worry about two different spot drills or going back to chamfer the hole at 45 deg. In my CNC mills I use a 90 deg spot for drilling & to chamfer edges 1 tool does all.

You don't want the drill to try to center on the flat the spot drill leaves you want the cone to help the drill center.

Maybe I'm just doing this wrong, but I'm still running parts since 1972.

I guess we could take it to an extreme...

If a drill's point is ground off center, not on purpose. Drilling a hole with it will yield an over size hole, because the "crankshafting" drill point will act as the center of rotation.

I hope we can all agree to that. *s

Taking the same drill and drilling a hole which has been pilot hole drilled first will drill the hole on size, because the point isn't influencing the cutting process. Of course the drill entering the pilot shouldn't chatter.

If we agree to that, then starting in a cone should build a "bushing" which is the drill size.

Years back I used to build aluminum molds. One half would have two 1/4 (.2502") dowel pins the other half would have two .251" holes. If I center drilled and followed with a jobber drill and drilled fast enough not to chatter I could slide the two halves together, pins in holes. If the drill chattered, The holes would be off location.

This has been my experience.

Regards to you all,

Stan-

Ok, so to start/spot and chamfer ,use a spotting bit w/same angle but one size larger dia. If not needing a chamfer then the spotting drill should have more angle ( included) than the bit drilling the hole, ie. the drill bit at 135*s then the spotting bit at 140*s , and again: jobber bit at 118*s and the spotting bit at 120*s, and again the split point at 118*'s and/or 135*s if using a 140*s spotting bit. If using a 120*s/ 90* spotting bit. the drill bit angle should be less than the spotting bit angle . If using a 140*s spotting bit , use/drill with anything under 140*s. So now if one wanted to use just one angle for spotting bit ( no chamfering ) , all the hole drilling bits should have less included angle than the spotting bit. If one has many different angle bits in one's shop, the best thing to do is get a spotting bit with the largest included angle or toss ALL the bits out and retool with ones with less angle. I like that idea , OR ........... regrind all the bits to just under the spotting bit angle until they need replacement.
Howzat sound?

I respectfully disagree. I believe that the spotting drill SHOULD have less of an angle than the drill. The drill should not start on the point, or the corners, but somewhere in between. If it starts at the point, then there is a chance of skipping around at the bottom of the spot drill flat, causing inaccuracies. If it starts at the corners, you run the risk of damaging the corners of the drill. Just my $0.02

I'd like to see an engineer from Guhring, Titex or Cleveland weigh in for what might be an educated response.

Hmmm, so mid way down the cone the follow drill's chisel, or lip ? should engage the cone wall , counter bore the cone to hole size then engage the edge and the rest of the chisel once the counter bore has enuf cut to hold the bit in place.......one would have to have the SB and follow bit ground off 1*or ???? to force engagement at the correct depth. Having t&d make matched sets would be the way to go , especally CNC ops . so much for thumb rule ...... According to ICS and ENCO the way to go is more angle on the sb than on the follow drill as the concern was for the edge of the follow bit , not the chisel skating around. ( no chamfer), so I'm gonna order a 120*sb and jobber drill 118* and try em on mild steel,navy bronze,and oak(face grain ) and see wha happens.
I have a 90* sb and I'll try that but no unground bits in the size ( 135*s)to try on the 90* or the 120* sb , so I'll get a brand new shinny one and giver a go .

Metalcutter,

A drill sharpened with one lip longer than the other must cut off center whether started on the surface, in a spot drill cone, or following a pilot drill. In the case of the surface start, the point will trace a circle on the part until it is forced in to the work at some point on that circle, after which it will drill both off center and oversize. In either the pilot drill case or the spot drill cone, pressure on the unequal lips will force the drill to bend so that the lips center the point on the hole or spot drill cone, after which the drill will cut on location but oversize.

I recently discussed the effect of a new but incorrectly sharpened drill on following a pilot hole on this thread. My pride is still smarting.
http://www.practicalmachinist.com/vb/showthread.php?t=159488

Well, I am glad that is settled. I'm afraid to use any spot drill that is not EXACTLY the same angle as the following one. But, I have been using them all along and they worked. ???

I never thought of doing the spotting and the chamfer at the same time. I always spot to a smaller diameter than the hole size and chamfer with a countersink after drilling. But, I guess doing both at once would save time.

I too would welcome an informed, professional opinion. Not to say that all of the above are not informed professionals. Total confusion here.

Stu Miller said:

Metalcutter,

A drill sharpened with one lip longer than the other must cut off center whether started on the surface, in a spot drill cone, or following a pilot drill. In the case of the surface start, the point will trace a circle on the part until it is forced in to the work at some point on that circle, after which it will drill both off center and oversize. In either the pilot drill case or the spot drill cone, pressure on the unequal lips will force the drill to bend so that the lips center the point on the hole or spot drill cone, after which the drill will cut on location but oversize.

I recently discussed the effect of a new but incorrectly sharpened drill on following a pilot hole on this thread. My pride is still smarting.
http://www.practicalmachinist.com/vb/showthread.php?t=159488

Click to expand...

All I know is, every time I opened up an existing hole with a larger drill, the hole size was the size of the last drill.

Regards,

Stan-

Is this authoritative enough? from Ingersoll Catalogue:

http://pdf.directindustry.com/pdf/ingersoll-cutting-tools/hole-making-catalog/17869-11078-_93.html

I guess this must qualify as an informed professional opinion.

Regards,

Mike.

MikeJB said:

Is this authoritative enough? from Ingersoll Catalogue:

http://pdf.directindustry.com/pdf/ingersoll-cutting-tools/hole-making-catalog/17869-11078-_93.html

I guess this must qualify as an informed professional opinion.

Regards,

Mike.

Click to expand...

Well, it IS an opinion.


Actually.. I looked back in "Pecision Hole Location" to find my information. I was WRONG! I had the wrong book. But it was in "Tool Design." So I'm including the information here.

BUT! Not in concert with "Tool Design" Written in 1973 by Donaldson. Lecain & Goold
where they state: The included angle of the center drill should be narrower than the included angle of the drill and the center or spotting drill hole should be larger than the following drill for the best centering action.

pp 868

So your authority can fight with my authority. *Smile "Do it however you want!"

Best regards,

Stan-

What a can of worms eh? It is like trying to do a job and having to fix the tool first.....ship yard work is... er was like that. I am glad we all got something out of this, I also had never thought of using the sb to start and chamfer the hole , but then again I do very little holes needing a chamfer and the wood working taper drill thingy has a chamfer on the pilot cut for wood screws and really never paid attention to it. neat idea.A design engr friend says that on the bits used for these ops that the chisel would/should be very narrow and that the cone will force it to center, and is better for the edge of the chisel to engage the cone and start a pilot hole untill the edge of the follow bit engages , with ideal being they engage at nearly the same time and the important thing is the sb has to be dead on if it is a precision hole, soft metals present a different approach and I see that Stan/Metalcutter had that one down pat.Sometimes we do unorthodox things to accomplish the objective and sometimes they just do not make sense to some but ya know when they work .... it becomes the rule when it should be the exception. Kinda like Doberman Dave. So the rule of thumb could be follow bit < angle than the sb unless you chamfer then should be = * but larger dia.
huh,works for me.
thanks guys!!!

EPAIII said:

Well, I am glad that is settled. I'm afraid to use any spot drill that is not EXACTLY the same angle as the following one. But, I have been using them all along and they worked. ???

I never thought of doing the spotting and the chamfer at the same time. I always spot to a smaller diameter than the hole size and chamfer with a countersink after drilling. But, I guess doing both at once would save time.

I too would welcome an informed, professional opinion. Not to say that all of the above are not informed professionals. Total confusion here.

Click to expand...

Yeaah,,,Me too, I use a 3/8 spot drill and follow with 5/8 twist drill and every thing goes where I want, WTF am I doing wrong??? Now I'm afraid to drill any more holes!! because I have more angle on the spot drill than I have on the follow drill, Sometimes I think you can have too much theory and not enough chips!!!