Short drill or Spot / Centre drill first

[Chris TickTockParticipant @christicktock]

Posted by SillyOldDuffer on 10/07/2020 15:28:43:

Posted by Chris TickTock on 10/07/2020 13:25:49:

Hi, it really is a revelation just how involved drilling a hole can be if you want it fairly precise.

Does anyone have a clear idea of whether a single point is a 4 facet drill which is a split point drill. The idea of this drill if my understanding is right is to replace the flat chisel part prone to wander on a standard drill with an additional 2 edges . Thus there are 4 edges which act as one cutting edge and if the drill is a stub and fully ground should make a precise hole. The included angle is 135 degrees which implies a 90 degree spot drill may be counter productive as the drill would foul on the walls.

But my main question is is a split point drill bit the same (or could be) that referred to a single point drill?

Chris

The 'single point' terminology has me confused, but twist drills are a confusing subject. I believe all twist drills are single point drills, as opposed to augers, saw toothed hole drills, and other configurations. Could be wrong.

Twist drills wander for several reasons, many of which are down to the operator. Allowing the flutes to choke with swarf, cutting too hard with a sharp drill, forcing a blunt drill, rpm too slow, or a poorly shaped drill can all cause trouble. Deep holes, more than 4 to 6x diameter, are more likely to wander than shallow ones. Hand drills always wander in my experience, pillar drills much less, and mills better again. A lathe is best of all because the work is rotating, hence prefer a lathe when straightness is important.

Drills tips are shaped to optimise performance for particular purposes. Rounded tips are preferred when breaking through sheet metal. Square tips for flat bottomed holes. Angles to suit Brass, fast metal removal, or whatever. As the flutes can also be optimised for different purposes there's a bewildering variety of drills to choose from.

As life is too short, I use general purpose 118° drills, sometimes started with a centre punch, more often with a centre drill. I'm happy to believe spot drills are better, just not tried them yet. What centre and spot drills have in common is being short and stiff, just what's needed to get a general purpose drill going in the right direction. General purpose drills are cheap to make, easy to resharpen, OK with hard and soft materials, fairly stiff, with decently wide flutes for ejecting swarf, and although inclined to grab and tear on exit, they aren't vicious. A good compromise drill, but in some circumstances it's worth sending for a specialist.

I think Chris is describing the type of drill where the chisel end is ground in either a sort of S shape or 'split point' to extend the drill's cutting edge closer to the centre, which improves the drill's self centring properties. They're more expensive and wear faster than general purpose drills.

135°drills are preferred for harder materials.

Most of time I find all that's needed is a decent accurately placed hole sufficient to allow an ordinary 118° drill to make a good start. Once going correctly it will do the job. This is provided I frequently clear swarf, lubricate, and take care to cut at a sensible speed and pressure. Overdoing it always causes a wander.

The 'decent accurately placed hole' can be made with a centre punch or a centre drill. I do most of my drilling with a mill and prefer a centre drill accurately aimed with a DRO. However, if the hole is non-critical, I often don't bother starting it – I just go straight at it with the drill. I think it works because a milling machine puts the drill accurately at a right angle to the firmly clamped work and is able to make its own all important first start. Curiously drilling in a lathe always seems to need a starting hole because the drill can be seen skidding about on its chisel before. engaging. I may be doing it wrong…

Dave

Edited By SillyOldDuffer on 10/07/2020 15:30:56

Dave I thought I was going bananas for a couple of minutes. I thought I had seen single point in relation to slit point and have just found out where. Yes very confusing. In some places they do refer to 4 facet drill bits as single point as the relief on all 4 facets must end in a single point. My advise to myself is forget that for now in terms of calling these split point drill bits as it does not help. Sharpening however (should I ever dare) it is I guess very important to take on board this.

Lots of good advice on the forum as always some conflicting but I am that much wiser to at lease experiment.

Chris

[Mick B1Participant @mickb1]

Centre drills aren't only short and stiff – the pilot and cone both have clearance to cut on the flute as well as the lip, so even if they run out a little in the mill or are a little off-centre in the lathe, the resulting machined surface has the best chance of being truly concentric to the centre of rotation. You have what *nearly * amounts to a bored feature.

Don't know about the spot drill, but the standard twist drill has a cylindrical land with no clearance so won't cut on the side. But you can (and I sometimes do) use the pilot of a centre drill as a tiny 2-flute endmill. Strictly it's a misuse, as the clearance is small, but it works if you're gentle.

Edited By Mick B1 on 10/07/2020 16:43:57

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