Center drill question

[cazclocker]

I'm getting set to buy a set of 5 center drills (drill & countersink combination). All the drill bits I own are 135 degree split point. But the center drills I see are almost universally 60 degree included taper. That means I can start a hole with a center drill with no problem - but when I follow up with my drill bit, the first thing my drill bit will cut is going to be the outer circumference of the starter hole. To get the best concentricity, shouldn't the split point of my drill bit be the first thing to begin cutting the workpiece? What am I missing?

 

[Liljoebrshooter]

Center drills are for using on a lathe to use a live or dead center on not for using as a guide for drilling a hole.
Spotting drills are for starting a precise hole for drilling .

Joe

 

[Nutfarmer]

Use a spotting bit that matches the angle of the drill bit. A center drill is for drilling centers on the lathe for turning between centers. A lot of us will use a center drill for starting a drill just because that is what we have not because it’s the best way. Each their own. It all works.

 

[mmcmdl]

Joe sums it up correctly above . And nutfarmer is correct also . We all use center drills to spot holes because it's what we have . It doesn't cause issues when drilling .

 

[cazclocker]

Center drills are for using on a lathe to use a live or dead center on not for using as a guide for drilling a hole.
Spotting drills are for starting a precise hole for drilling .

Joe

Ah, okay. My mistake - glad I asked. My intention is to drill a series of 12 precisely-placed holes, about 1/4" diameter, in 1/4" steel plate of unknown composition. Thank you!

 

[mmcmdl]

Ah, okay. My mistake - glad I asked.

Not a mistake cazclocker , either will work fine . One big difference is when running the CNCs with a 1000 holes that need to be chamfered for tapping or whatever . The spotting drills don't have the pilot of the center drill , thus the Z depth may be cut by 3/4s saving loads of time . Bottom line , use what you have .

 

[jmkasunich]

My two cents: if you care about hole location, get and use spotting drills. You only need one or two (one for little holes, maybe 1/8" or so, and one for bigger holes, 1/4" or so). Can be purpose made spotting drills (named and sold as such) or just screw-machine-length drill bits. The key factor is that they are short and stiff so they don't go wandering off. Once you've spotted the hole in the correct location then a longer more flexible drill can be used to finish it.

 

[cazclocker]

Not a mistake cazclocker , either will work fine . One big difference is when running the CNCs with a 1000 holes that need to be chamfered for tapping or whatever . The spotting drills don't have the pilot of the center drill , thus the Z depth may be cut by 3/4s saving loads of time . Bottom line , use what you have .

Thank you for that. I should have mentioned that I'll be doing this project on a benchtop drill press, not a milling machine. Sadly, I sold my 5-piece set of center drills along with my milling machine when we moved to be close to family. Bottom line, at this point I don't have either center drills OR spotting drills!

My two cents: if you care about hole location, get and use spotting drills. You only need one or two (one for little holes, maybe 1/8" or so, and one for bigger holes, 1/4" or so). Can be purpose made spotting drills (named and sold as such) or just screw-machine-length drill bits. The key factor is that they are short and stiff so they don't go wandering off. Once you've spotted the hole in the correct location then a longer more flexible drill can be used to finish it.

I just looked at MSC and McMaster-Carr, and I see that MSC has a good variety of drill point angles - I see 140 degrees, which seems appropriate for my collection of 135 degree cobalt & HSS drill bits (all screw machine length).
Given my drill bit point angle, does a spotting drill point angle of 140 degrees seem appropriate to you guys? MSC also carries 130 degree point angle spotting drills, but 140 seems more logical to me.

 

[jmkasunich]

Thank you for that. I should have mentioned that I'll be doing this project on a benchtop drill press, not a milling machine. Sadly, I sold my 5-piece set of center drills along with my milling machine when we moved to be close to family. Bottom line, at this point I don't have either center drills OR spotting drills!

If you are doing the work on a drill press, how are you doing the layout? You don't have leadscrews and dials....

If you are scribing lines and centerpunching, you do NOT want spotting drills (or center drills).

Think of it this way: on a mill, you use the dials to accurately put your target spot directly under the spindle. Then you use a spotting drill to make a dimple is accurately centered under the spindle before following it with a more flexible drill bit that hopefully follows the dimple. But with a drill press, your layout and center punch mark is the target. So a flexible drill that will find and follow the center punch mark is better than a stiff spotting drill that will follow the spindle centerline and ignore the center punch mark.

 

[mattthemuppet2]

one way that you can use center drills to spot a hole is to use a follow up drill that is the same diameter as the tip of the center drill. Essentially you're creating a piloted hole for a pilot, then you can follow up with your larger drill.

I also second using screw machine length drills - if you need any, I have lots That's all I've used, though I do have spotting drills now. As Dave ^ wrote, they're handy for adding a chamfer to the hole first, so that you don't need to go back and do it after drilling the hole.