# Do center drills have runout ?



## compact8 (Oct 25, 2021)

I just checked three of my center drills with a dial indicator and all have big runout at the tip of magnitude 0.08 ~ 0.1 mm. Only the tip has this amount of runout.  The amount seen at the main body of the drill is normal.  Will it still do it's job ?


----------



## mmcmdl (Oct 25, 2021)

Center drills usually don't have runout , spindles or collets/chucks do . The farther away from your holding device , the runout will be magnified .


----------



## jwmelvin (Oct 25, 2021)

While a center drill may not have much warp or noncircularity, I think it’s appropriate to consider its runout when in a chuck, as you are doing. Dave’s point is that the observed runout likely doesn’t come from the center drill itself but rather the chuck or collet. If you see little runout at the drill near the chuck and more away from the chuck, the drill is not parallel with the chuck axis. I have a chuck with that problem, due to worn jaws. Not a lot you can do other than rebuild/replace the chuck. You can try inserting the drill fully into the chuck, as that can bridge the problematic area of a jaw.


----------



## mmcmdl (Oct 25, 2021)

/\/\/\/\ Well said !


----------



## Technical Ted (Oct 25, 2021)

Although I agree with the other posts before me (they most likely hit the nail right on the head), but if you have a cheap import tool, all bets are off IMO... Some import tooling is pretty rough in my experience. On the other hand, if you have a quality tool then I would say the way the tool is held in whatever you are using to measure it is the root cause. 

Ted


----------



## RJSakowski (Oct 25, 2021)

The proper way to test for runout of the tip would be in a V block.  Otherwise you have to account for spindle runout and chuck runout.  

Runout consists of two components, radial and angular. It is possible that a some point along the axis, they can cancel out giving a zero composite runout.  As you move further away from that point, measured runout will increase.


----------



## Nutfarmer (Oct 26, 2021)

Try using a collet to hold the center drill instead of  a drill chuck.


----------



## RJSakowski (Oct 26, 2021)

With a small center drill, the tip can flex slightly to start on an off center center punch.  The main body of the  center drill is much more rigid and will tend to follow the spindle axis.  On very small end mills (.010 or .015" tips),this will usually snap the tip of the end mill but a slightly larger end mill could result in a center hole which isn't concentric with the larger diameter.  If the larger diameter hole isn't deep enough to provide a guide for the following drill, it would have a tendency to start on the tip hole but as drilling progressed, the tendency to follow the spindle axis would become more pronounced and would result in drilling a hole on a slant.

When there is runout or spindle play, this tendency would become more pronounced as the tip is orbiting but once it makes contact, it will tend to start on the first contact point..  I have seen this behavior on a lathe where the drill is off center slightly and with small diameter carbide drills.  It isn't unusual to have drills wander on the lathe resulting in holes that exit off the spindle axis.


----------



## compact8 (Oct 26, 2021)

Thanks for all the replies. Just to avoid any potential misunderstanding on what is seen. I have taken this video demonstrating the different amount of runout at the tip and the main body of the center drill.   The diameter of the tip is 4mm, main body 10 mm.


----------



## akjeff (Oct 26, 2021)

As someone pointed out earlier, measuring the runout with the center drill in a chuck, collet, or any other tool holder, you're not going to be able to tell what is causing the runout. The spindle, the chuck/collet, the center drill, a little of all of the above? Put the center drill in a v block and eliminate the other variables.


----------



## compact8 (Oct 26, 2021)

akjeff said:


> As someone pointed out earlier, measuring the runout with the center drill in a chuck, collet, or any other tool holder, you're not going to be able to tell what is causing the runout. The spindle, the chuck/collet, the center drill, a little of all of the above? Put the center drill in a v block and eliminate the other variables.


My point is not so much about the absolute amount of runout but the difference between the main body of the center drill and the tip. All the mentioned contributing factors such as the spindle, arbor, collet, etc should affect the tip and the main body of the center drill equally right ?


----------



## akjeff (Oct 26, 2021)

There is no guarantee that the tool is being held dead straight by whatever is holding it, so the runout you measure is going to vary the closer to, or farther away from the point that the tool enters the tool holder. Again, put it in a v block and remove all doubt.


----------



## jwmelvin (Oct 26, 2021)

compact8 said:


> All the mentioned contributing factors such as the spindle, arbor, collet, etc should affect the tip and the main body of the center drill equally right ?



That was my point about parallelism. In my experience, a drill chuck especially can have issues in that regard, as a single worn jaw will let the tool tilt relative to the chuck axis. Trying in a v-block would not have that issue and would be interesting in this case.


----------



## Superburban (Oct 26, 2021)

I could not watch the video on this tablet.

Are you accounting for the relief ground into the cutting surface?


----------



## RJSakowski (Oct 26, 2021)

Superburban said:


> I could not watch the video on this tablet.
> 
> Are you accounting for the relief ground into the cutting surface?


I thought about that too.  But you have to look at the difference between the readings at each cutting edge.  It looks to be about .1mm as opposed to the .02 mm on the shank.


----------



## compact8 (Oct 26, 2021)

Okay ....  here is the checking result of the recommended Vee block method. I hope I have done it right because I have not used Vee blocks for such purpose before.


----------



## woodchucker (Oct 26, 2021)

so what you are showing is not very helpful.
All drill points have relief behind the cutting point.
The tip is what sets the hole in this case.

is your tip wandering.

for what you are showing ; you would only need to show 2 points to us , right on the cutting edge. showing the rotation its normal for the needle to wander ...


----------



## compact8 (Oct 26, 2021)

woodchucker said:


> .... need to show 2 points to us , right on the cutting edge....


I have thought about that but I am afraid I don't know how to check the runout right on the cutting edge.


----------



## brino (Oct 26, 2021)

woodchucker said:


> so what you are showing is not very helpful.
> All drill points have relief behind the cutting point.
> The tip is what sets the hole in this case.
> 
> ...



Hey Jeff, 

I think it's valid.
Showing the full, slow rotation lets us see the maximum points.
How would you easily "clock" the cutting edge directly under the indicator tip....not easy.

As @RJSakowski stated earlier it's the difference between the two edge maximum readings and we can see that from the video.

The only thing I would do differently is to rotate the centre drill the other direction for two reasons:
i) not presenting the sharp cutting edge to the dial-indicator tip; that is making the ball climb up the larger sharp step, and

ii) it would be easier to sneak up of the maximum reading as you'd be slowly climbing the relief.

Just my thoughts......

-brino


----------



## woodchucker (Oct 26, 2021)

brino said:


> Hey Jeff,
> 
> I think it's valid.
> Showing the full, slow rotation lets us see the maximum points.
> ...


yep, good idea rotating it the other way, but I would simply find the high points..
And still, the tip is the most important aspect. The tip is what sets where it is going initially.
 a D bit and it only has one cutting edge... while not a great drill, have you watched clickspring? he drills perfect holes with a dbit.


----------



## Bi11Hudson (Oct 26, 2021)

It may be "off base" here, but way back I was given to understand that a center drill held in the tailstock against rotating work would "self center". This from an old timer in the plant machine shop who seemed to me to be on point. The theory I have held for many years is that the finished center cavity may be slightly oversized but true to the center of rotation. This is based on the rigidity of a short drill cutting to the side as the work rotates. With an oversized center hole, a center device will find and hold the center of rotation to get true work. 

Sorry, I don't know the terminology for some of the points I'm trying to make. Hence there are too many "centers" here to make sense. Perhaps I don't have my caffeine level up to snuff yet. I don't do enough precision work to half, or less, thou to know if the thought is valid. At the level I usually work, it seems to pay off. The theory as explained to me makes sense and I don't worry about concentricity of my center drills. I will concede that most of my work involves small drills. My large size is only 1/4 inch / 6mm diameter for thr body. I have larger center drills but they are so seldom used that I don't consider them. Like the letter size drills, they aren't allowed to get rusty, but could easily do so.

.


----------



## Superburban (Oct 26, 2021)

I take it this is like a regular drill ground with the point not on center. It will drill a bigger hole. Since a center drill is not normally used for precision, they may be within their designed tolerances. The biggest issue I see is one side will wear, and take more of the cutting then the other. Used on a lathe, as long as the tail stock is centered, the resulting hole will still be centered. Only have a bigger hole.


----------



## RJSakowski (Oct 26, 2021)

Your second video confirms what you showed in the first video: namely that the tip has a .1mm runout with respect to the shank of the drill. I would toss it as it serves no useful purpose.  This could very well be the cause of your slanted holes.  Try again with a good center drill and see if it makes a difference.  Let us know what happens.


----------



## jwmelvin (Oct 27, 2021)

+1 
Thanks for showing your testing. Do you have other center drills? Do you have some split-point drills? This makes me want to check some of mine to see how they fare.


----------

