# Choosing The Right Centre Drill Size.



## Tozguy

A brief search didn't turn up any useful info on my dilemma. I have a set of centre drills and when it comes time to use one it is never obvious to me which size to use for the job at hand. Besides the intuitive tendency to use as large as possible are there any guidelines for sizing the countersink based on certain parameters?


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## Wreck™Wreck

Just large enough that the tool will not hit the center when turning the part.


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## Andy Rafferty

I guess it depends on the diameter of the work and mostly if the remaining end is included in the finished part. If it is I try to end up with the best  proportioned size to the size of the shaft that gives the 60 degree land area 1/8 th of the finished diameter. Otherwise I use the largest I can to be able to use the live center with the least amount of inward pressure. I too have no legal answer so sorry if I wasted your time. I just like what I end up with using my guide.


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## Charles Spencer

When using them on a drill press as a starter drill I like to use one large enough to countersink the hole at the same time.  

For the lathe, here are relevant illustrations from South Bend's "How to Run a Lathe":


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## Wreck™Wreck

There is no "legal" answer, whatever you mean by that.
Drill centers at a size that will not interfere with the work to be done that is all, insert tooling is almost always physically larger then HSS due to the insert + the holder therefore a live center body will often get in the way of the tool. 35° Diamond insert tools are useful here as the angles allow working close to the center itself.

There is only one way to define right or wrong in this game, one way works and one does not, books and charts covering general best machining practices are just that, general guides, do whatever works regardless of what the book says as you are for the most part writing your own book as you go.

As an example, a 5" diameter X 80" long steel cam, it has 2 centers in each end, one at the actual center and one offset by .375" in the same plane on each end. Rather small centers were required so that they did not overlap, it works fine.


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## RJSakowski

Here is what I practice.

If you are using a center drill for its intended purpose, (i.e. providing a bearing for turning on centers) then I would say smaller is better if you plan to use a dead center.  The smaller bearing surface has less friction due to the smaller area and lower sfm.  It does have to be large enough to support the  forces generated in the turning process without distortion
.
If you are using a live center, then friction is not an issue but as WreckWreck stated, it becomes more difficult to work in close if the center drilled hole is large.

A helpful exercise would be to observe the center drilled holes in turned shafts such as motors, axles, drive shafts, etc.  It will give you a visual as to what is practiced.

If using the center drill for spotting a drilled hole, I usually go smaller there as well with a hole large enough to seat the central web on the drill.  I try not to drill so deep as to create a vertical wall as the following drill can bite into the sharp edge and deflect slightly, defeating the purpose of spotting the hole.

Bob


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## Splat

I was just mulling this over because I've got to do some between centers work soon. I don't know where I got this but I saved it and will provide it here. IIRC, isn't this info also in the HTRaL by South Bend? Thanks.

PS, just saw thread's from 2015. DOH!!


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