Spot Drilling: The First Step to Precision Drilling
Drilling an ultra-precise hole can be tough. Material behavior, surface irregularities, and drill point geometry can all be factors leading to inaccurate holes. A Spot Drill, if used properly, will eliminate the chance of drill walking and will help to ensure a more accurate final product.
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Choosing a Spot Drill
Ideally, the center of a carbide drill should always be the first point to contact your part. Therefore, a spotting drill should have a slightly larger point angle than that of your drill. Common drill point angles range from 118° to 140° and larger. Shallower drill angles are better suited to harder materials like steels due to increased engagement on the cutting edges. Aluminums can also benefit from these shallower angles through increased drill life. While these drills wear less and more evenly, they are more prone to walking, therefore creating a need for a proper high performance spot drill in a shallow angle to best match the chosen drill.
If a spotting drill with a smaller point angle than your drill is used, your drill may be damaged due to shock loading when the outer portion of its cutting surface contacts the workpiece before the center. Using a drill angle equal to the drill angle is also an acceptable situation. Figure 1 illustrates the desired effect. On the left, a drill is entering a previously drilled spot with a slightly larger angle than its point. On the right, a drill is approaching an area with an angle that is far too small for its point.
Marking Your Spot
A Spotting Drill’s purpose is to create a small divot to correctly locate the center of a drill when initiating a plunge. However, some machinists choose to use these tools for a different reason – using it to chamfer the top of drilled holes. By leaving a chamfer, screw heads sit flush with the part once inserted.
What Happens if I Use a Spot Drill with an Improper Angle?
Using a larger angle drill will allow the drill to find the correct location by guiding the tip of the drill to the center. If the outer diameter of a carbide drill were to contact the workpiece first, the tool could chip. This would damage the workpiece and result in a defective tool. If the two flutes of the drill were slightly different from one another, one could come into contact before the other. This could lead to an inaccurate hole, and even counteract the purpose of spot drilling in the first place.
Avoiding CNC Drill Walking With a Spotting Drill
Few CNC machining applications demand precision like drilling. The diameter hole size, hole depth, part location, and finish are all important and provide little recourse if not up to specifications. That said, accuracy is paramount – and nothing leads to inaccurate final parts faster than drill walking, or the inadvertent straying from a drill’s intended location during the machining process. So how does drill walking occur, and how can one prevent it?
To understand drill walking, think about the act of striking a nail with a hammer, into a piece of wood. Firm contact to a sharp nail into an appropriate wood surface can result in an accurate, straight impact. But if other variables come into play – an uneven surface, a dull nail, an improper impact – that nail could enter a material at an angle, at an inaccurate location, or not at all. With CNC Drilling, the drill is obviously a critical element to a successful operation – a sharp, unworn cutting tool – when used properly, will go a long way toward an efficient and accurate final part.
To mitigate any variables working against you, such as an uneven part surface or a slightly used drill, a simple way to avoid “walking” is to utilize a Spotting Drill. This tool is engineered to leave a divot on the face of the part for a drill to engage during the holemaking process, keeping it properly aligned to avoid a drill from slipping off course.
When Won’t a Spot Drill Work for My Application?
When drilling into an extremely irregular surface, such as the side of a cylinder or an inclined plane, this tool may not be sufficient to keep holes in the correct position. For these applications, flat bottom versions or Flat Bottom Counterbores may be needed to creating accurate features.
Hi, this is a good summary, thanks!.
Would you consider adding to this article consideration for the practice of spot drilling using a much smaller drill bit? I was taught that spotting with a smaller drill bit works just as well since the outer diameter of the final size drill will not engage first, it would be the core that would center and engage first.
You never want to use a smaller drill to spot with. The whole point of spotting is to put a spot for your drill to follow/start with that is in the exact position that you want. A smaller drill as a spotting tool will flex and walk and your actual drill will follow that point and be off location or cut on an angle. You need to spot with a rigid and/or stubby tool so that it transfers exact location.
I’ve seen it recommended to spot with an angle smaller than the drill tip angle when using a H.S.S. drill. I’ve also seen the same recommendation for soft materials as aluminum vs steel. I’ve had the best results by always spotting with an angle greater than the drill angle, soft or hard materials, HSS or Carbide drills. I think HSS is more forgiving for sure vs carbide. I’ve used plain old center drills in stainless steel as they are cheap and hold up well then drilled with HSS drills. Works fine but I cannot attest to the accuracy as at that place we really never checked parts on a CMM.
I dont feel this is correct. A drill tip magnified, you cant get a cutting edge. So if the tip touches first, for a split second it smears. I have spotted many 90 deg holes and never experienced “shock loading”. Thats why center drills are 60 deg. Lets the cutting edge of drill engage before the drill tip. Should you guys discontinue your 60 – 100 deg spot drills?
Hi Mike! Thank you for your comment and reaching out to us with some inquisitive questions.
When drilling, the chisel angle – the very tip of the point angle, comes in contact with the work piece. Because you have the two cutting lips overlapping here, you form a chisel angle where actual cutting is not happening. Here, you want enough thrust for the drill to penetrate the material and then the chisel angle starts to extrude the material. The longer the chisel angle, the more durable the tip is but you will need more thrust in order to penetrate the material. Bringing this into the context of spotting, we can see why if you just barely touch the material you will get more rubbing and less cutting. We feel like you bring up a great tip here, no pun intended, that when spotting you want to take enough of an axial depth in order to go past the chisel angle and actually start cutting the chip.
Next, using the term “shock loading” may not be the best description on what is happening when you contact somewhere on the point angle first. Shock loading typically comes from interrupted cuts so as long as the point angle is symmetrical and located perfectly, that should not be happening. What we want to get across here is that this is not the ideal way of spotting because of how the drill will now wear unevenly along the point angle and may lead to chipping. For example, if the drill always contacts half way up the point angle, you will start to see more wear there than what is happening close to the chisel angle. Historically, and with softer materials, this was less of a concern but with the advent of tougher and high temp alloys, we would recommend staying away from this. A flatter point angle on the spotting drill will help relive extreme stress on the lip of a drill in these materials. Ultimately, it is up to the machinist to weigh the pros and cons of any operation and we just want our readers to understand more on what is going on.
Finally, as mentioned before, everyone has their way of doing things. We have heard that our customer use spotting drills as a combination tool and leave a chamfer at the top of the hole with them. This is why we offer such a large range of included angles to help with ideal spotting situations to help saving room in your tool carousel. We hope you find this information useful and if you are having any more questions or any challenges with our line, from smearing to angle selection or anything between, please contact our experienced tech team at 800-645-5609 or email at [email protected].
Using a carbide drill in a CNC milling machine would be a poor choice. If you are peck drilling the carbide is going to take a beating. If the material is too hard I would use a carbide endmill in a High Speed milling machine. If possible I would use a undersized HSS drill when the material is soft……say prior to heat treat. Then finish the holes in a High Speed Milling machine with a 4 flute CEM.
Carbide drills are excellent in cnc machines. most manual machines can’t even spin a carbide drill to it’s proper sfm. You can drill without pecks on a cnc
I have to disagree here. We use carbide drills as small as 1.6mm to drill Inconel and titanium with great results in both tool life and accuracy, pecking is an absolute must as any “stringy” swarf build up in the flutes will snap your drills in no time. HSS drills are better suited to Softer materials up to 316 stainless steels.
A 60 degree spot drill is 1 degree wider than a 59 degree drill, so the drill’s point would touch first.
Scratch that, I was thinking of 120 degree spots
I disagree with the statement: “Thats why center drills are 60 deg. Lets the cutting edge of drill engage before the drill tip.”
The main reason “center drills” are 60 degrees is so that you can prepare the end faces of lathe workpieces that need tailstock support. The most common tailstock centers are 60 degrees (included), hence the most common center drills are 60 degrees. Sounds like you are using a combined “center” drill as a spot drill.
Excellent way of telling, and nice paragraph to get data about my presentation topic, which i
am going to deliver in college.
If I had to drill into some concrete, I’d for sure make a spot drill. That way, I can be sure that my larger drill would be in the same place. However, I would have to go out and purchase all these tools as I don’t have any in my possession.
It’s good to know that you need to have the proper spot angle when drilling concrete. My brother wants to drill some concrete that he has on his property, and he wants to make sure he goes about doing it properly. I’ll pass this information along to his so that he can properly drill his concrete.
this is a good summary, thanks!.
I’m surprised to see no mention of
135° split point cobalt drills which I’ve used a lot with no spot drill necessary!
I’ve also worked in Aerospace and NASA related hardware that mostly used tighter tolerances on hole location and sizing that required a chamfer or break all edges
call out on the print. One company that I worked at was primarily making hydraulic, oil, and fuel filtration systems for many of our commercial airline industry and military fighting jets. We always utilized a 90° spot drill and most often followed with a 118° or a 135° dŕill and these surely had tight tolerances that were easily kept…
Multi billion dollar company that made filtration for the F18’s all the way through the B2 Bomber that had a price of $250,000.00 for each one and it requires 8 total…
That’s really nice post. I appreciate your skills. Thanks for sharing.
I appreciate what you said about using the center of the carbide drill. I need to hire a crew to drill a hole in the backyard. I want it wide enough for a pool.
How do you choose the right diameter for a spot drill?
Let’s say I’m cutting 1/4-20 UNC 2A internal threads and targeting a 0.201″Ø hole. I’m using a 0.2009″Ø drill (130° angle) and a 0.201″Ø reamer. I’d go with a 140° spot drill but what diameter?
Hello Brad,
Great question! We recommend using a spot drill with the same diameter as the drill you are using.
Thanks Guy! The problem with using a spot diameter the same as the drill diameter is that Harvey doesn’t make a spot diameter in 0.2009″ (I doubt anybody does). There’s 0.187 and then 0.25. If I go with the smaller diameter then I fear the point angle will wear unevenly (“shock load”). If I go with the bigger diameter then, well, I’m making a bigger hole than I want.
Hey Brad,
In the circumstances like these, we suggest using a larger spot drill but do not drill fully to the OD of the spot drill.
🤦🏻♂️ Seems obvious now. Thanks.
Last question: what’s the difference between your two types of spotting drills: on-center (type I) vs ahead-of-center (type II)?
On-center Spot Drills design reduces walking and minimizes the flat at the bottom of the spot. These tools are ideally suited for starting smaller-diameter drills and shallow spots. Ahead-of-center Spot Drills design improves tip strength. These tools are ideally suited for larger-diameter drills and tougher materials.
In the article, you mentioned the importance of matching the spot drill angle to the drill angle for optimal results. How does the material being drilled influence the choice of spot drill angle, especially when working with composite materials or layered materials with varying hardness?
This is a good summary..