In our shop, we have a debate as to what type of tool is the most effective for centering a drill. Is it a spot drill or center drill? Since both of the choices are available in several different angles, what influence does the angle have on centering the drill or anything else such as tool life? We have had mixed results from a variety of these tools. Please help us choose the tool that will give us the best centering and life of a drill.
Shop Doc Forum » General Questions
Centering a drill
(12 posts) (11 voices)-
Posted 3 years ago #
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This is a very common issue and is easily explained. A center drill doesn’t do anything to center a drill. It’s made to leave a centering feature in a part so a live center or dead center can be located into its corresponding angle and be used in some sort of supporting application like a tail stock in a lathe.
A spot drill has a better chance of centering your drill, but only if the included angle of the spot drill is greater (blunter) than the drill’s included point angle. A great example of this is a 120° spot drill and a 118° drill point. The spot drill is 1° blunter on each side, allowing the drill’s point to reach the spot drill’s point before anything else comes into contact, such as the corners of the drill or the lip of the drill.
The problem with this is that many drills on the market today have drill points of 135°, 140° or even greater. So in order for this theory to work properly, a 145° spot drill needs to be used so the drill point makes contact first, thus centering the drill in the most effective manner.
A center drill actually has two incorrect angles to deal with. One angle is from the pilot, and the other angle is what the center drill is measured at or known by, such as a 90° center drill, which eventually leaves a 45° chamfer per side once the hole is drilled. Tool life suffers greatly when using a 90° center drill or a 90° spot drill with an incorrect angle. If you want a chamfered hole, chamfer it with a chamfer tool after the proper spot drill and drill are finished. By following these simple steps you will see drills lasting longer, and the holes will be more accurately located, straighter and rounder.
Jim Rowe
Mahar Tool Supply
Tooling Application, Medical AccountsPosted 3 years ago # -
I immediately knew the technical source could not be an experienced screw machine tooling engineer - most likely a CNC guy who has never had to get such tooling to perform at very high production levels.
Okay, let’s set the record straight for those who may have been misled. You want the spot drill angle to be less than the drill angle, so that the outer lips of the drill engage the conical zone left by the spot. A common application requiring a spot, drill, tap sequence on a screw machine, would typically be tooled with a standard 90 degree spot and drilled with the standard 118 degree tap drill. This is the right way to prepare a hole. The proposition of the Shop Doc article is simply WRONG.
The Malpractice Doc said to prep the hole with a point that would hit the center of the cut first. The problem with this proposition is two fold. #1 if you look closely at the bottom of any spot cut or any drilled hole for that matter, you will observe a flat bottom in the very center. This flat bottom will create a dance floor for a drill, if you allow the drill to hit this center first. #2 By not providing for the outer lip of the drill to engage the conical edge of the hole first, you do not give the drill the pull to center needed to draw a very flexible tool into the central axis of the part.
This same principle applies to following one drill after another and is well explained in an old engineering manual published by Index, “Calculating Cycle Times and Designing Cams†on page 34. “In work which is centred (their spelling), and in the drilling of stepped holes in successive operations, the succeeding drill must centre itself at its outside diameter in the conical portion left by the preceding drill. The centring drill must therefore have the smallest point angle, and the and smallest drill the largest.â€
Using this approach you can even get a multi-spindle with an inherent alignment error of .003 TIR to drill holes at .002 TIR. Unfortunately many of today’s CNC guys never learned what the seasoned screw machine guys know about how you getting tooling to really perform at the very highest productivity and quality levels.
Posted 3 years ago # -
I just faxed over a page from the Hanita catalog featuring the new 145 degree angle spot drill and their explanation of it's design and proposed success. This tool is about 2 years old that I know of and once I saw it and tried it the concept made me a believer from the too life increase and hole roundness results.
This is the concept/theory I have preached ever since. The only thing I have improved on is designing and selling a 3 flute version of the 145 degree spot drill which even makes a truer round spot feature with the blunter angle. This tool is paying big dividends for many customers. Hanita is owned by Kennametal so I would suspect this is good information to absorb.
From Machine Guru's comments he makes a few assumptions and also some remarks toward advancing with technology. I reviewed his website of the company his Father Founded. Quite impressive by all means. I commend him for running his family's business for the last 1/4 century.
My career has taken me thru screw machine shops, large job shops, captive production facilities, medical machine shops and all the while running Manual machines and CNC Machines alike. I have been lucky enough to run machines being a member of the workforce and also running the shops being a member of the management. My career has now taken me into the technical side of tooling for a Tool Supply company where I enjoy teaching machinists on a daily basis. Learning more about the technology that is being used in the world not just in my own shop is very exciting and when I see new things I try them before bashing them without having the facts. But I have found that there are basically 2 type of people in the shops today. That is the people you can't tell enough too and the ones you can't tell anything to. The latter being because they simply know it all and don't need to hear anything else about how they make their living.
Just as Machine Guru worded it himself "explained in an old engineering manual published by Index" old meaning like WWII era I'm guessing. If we based everything upon theories from 1945 we would still be using bias ply tires. We would still be looking at a Radio Dial and listening to Bing Crosby. We would be waiting for a 100 Sq Ft machine called a Computer to take over simple tasks like adding several hundred lines of numbers together for us. In reality tooling and it's theories have changed too. We now not only use Radial tires but have tires that you can run even if they are flat. Not only do we listen to music on the radio but we see music videos on our Cell Phones. The Personal Computer is simply amazing and has opened a new world to everyone that will accept it. For those that don't there are still calculators, note pads and homing pigeons to crunch numbers and relay info to others for us.
Posted 3 years ago # -
both theories can easily be tested using an engine lathe a bar of brass or steel,a test drill of 5/32 dia & 118 deg point & 2 spot drills 1 ground at 45 deg & one at 135 deg.i have done this test myself & found that the 45 deg spot will center the drill better every time.
Posted 2 years ago # -
Both theories are valid.
If you are using WWII machines then yes a std. spot and 118deg point HSS drill is the way to go.
If you move up to the 90's and are using a screw machine that is tight/ saddles are on center within .0005" then you get to use carbide. Then you want to use spotting theory 2.
It all is in the type of equipment you have to use.
I just had this same issue by acquiring a job that had always run with HSCO drills and spots. I was told that carbide was tried but the corners of the drills worn off or the drill would brake. The tool life with the HSCO tooling was very low for both the spot and the drill.
We tested a STD carbide drill, and spot with the same series drill just twice the dia. for rigidity. Spotting depth only to the dia of the finish drill or smaller. Tool life is now 5 times what the HSCO was. This is a long running job so the savings is in the ten of thousands per year, not including uptime. The drill we use for spotting lasts for months now instead of days.
again you always need to test and use the best method for your application.
Posted 2 years ago # -
I dunno, I just use a stubby drill, hand ground or machine ground, with a split point, that is rigid and will center itself to go as deep as necessary then when followed with a longer drill the drill will follow the hole as a drill guide. Deeper holes can be followed with a succession of longer drills.
I have drilled accurate (ok +/- 1/16" tir for 9/16) holes over 24" deep with this method.
The method scales down to .020 1" deep with .005 tir if carefully applied.
TomPosted 2 years ago # -
I think spotdrill is by far a better option
Posted 2 years ago # -
I have always used center drills for this. I never center for drills 1/2" and larger. For the best start with a small drill, choke the size drill up close (1/8"?) and start the hole, then follow it with a size drill long enough to finish the hole.
Regards, Ray in FLAPosted 2 years ago # -
The problem with using a lathe to test this is that the part is spinning, which will tend to center the drill by force to a small degree.
The deciding factor for you may be cost. Center drills a cheap, spot drills not so.
I have experienced low cutting edge life when using a spotting tool that is of less an angle than the hole drill due to the chattering (it's there even if you don't hear it).Jim a.k.a. "Noah", post a link for the tool you mentioned if you would. I'm always interested in looking at improved tooling. I use a raft of different Hanita endmills and can attest to their quality.
Rgds,
JohnPosted 2 years ago # -
Through my own experance and research, you can use center drills or simular combination tools with HSS drills and yield good results. With carbide tooling you always want the center of the carbide drill to touch first or the whole tip should contact at the same time. If the outside corners touch first your drill life will be poor due to chipping. A spot or pilot drill should have an included point angle equal to or larger than the drill following it. To minimize the affect of the flat spot that
Machine Gurutalked about use a spot drill with a thinner web than the jobber drill and a point angle no more than five degrees larger than the jobber drill. And the biggest secrets with carbide is rigidity and runout!! Most tool manufactures will agree with all of this.Happy machining.
Posted 2 years ago # -
Again this makes good sense out of a very precarious topic!
Posted 2 years ago #
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