Thread: Very strange question about twist drills

Could be a combination of things. I would imagine that a pair of uneven cutting edges and too quick a down-feed to be the culprit. Another possibility is that the chisel edge on the 1/2" is too wide for the 7/32" hole, while means the cutting edges won't spot the hole properly upon drilling.

My advice would be to get your hands on a couple of countersinking bits for a reasonable price that won't have the same difficulties, mate.

http://www.sutton.com.au/IndustrialC...DeburringTools

I bought a single flute countersink. It will countersink wood, brass, aluminium, hard plastic and is very hard steel. It wasn't cheap but its very good. Can't think of the brand either. Will check it tomorrow and let you know.

The main reason that you're getting an out of round hole or in your case a countersink is that the material is not supporting the full cutting edge of the bit. You're basically following the flutes on the bits which causes the bit to jump and get slightly out of round, the thinner the material the more pronounce it is.

Most countersinkers are made with only one flute to stop the bit from jumping around in the hole

Machinery's Handbook lists about a dozen factors affecting accuracy of drilled holes. In this case, most likely uneven landing of the bit on the edge of the previous hole, and/or (very small) unsymmetrical grind of the point, initiates wobble. The speed of the bit, in combination with natural frequency of the bit as a cantilever beam, sets up a resonant condition so that irregularity propagates.

Such wobble was exploited in several square-hole and/or polygonal drills, exemplified by Watts' US patent no. 1,241,176 (1917), Harris 1,250,450 (1917), Powell 2,586,084 (1952), and a few others. Generally have a floating chuck of some sort.

I've gotten similar effect with multi-flute countersinks. As djstimber suggests, single-flute countersinks are the way to go.

Joe

Yeah this one is intruiging. A bit like the 3 sided holes you get when starting a hole in something like Al or any sheet metal. Seems like an even number of flutes produces a odd number of hole sides. I wonder if the reverse it true?

Anyway it seems like the single or zero flute countersinks do the job.

I'm sure someone will whip out a quadratic equation, or a matrix etc to explain. I bought a 3-cutter csk and it works fine - Festool also make a couple of very good alternatives, though pricey (as ever):

I do have a 3-fluted countersink. Somewhere. If it was an important hole, I'd likely try to find the thing or buy another one (out of reach is out of place). I'm going to look into those single fluted ones, though.

I guess that's right that both edges of the cutter aren't hitting. The box of drills this came out of was bought at a store that's commonly nicknamed "The Chinese Tool Store" around here. I think it was 29 or 32 bits in a set for something like $14 (US).

R. K.

Randy, I wouldn't blame the bits in the first instance. Even if the bits were perfect, any (even tiny) vibration combined with the drill runout and the fact that the work will not be perfectly flat will induce the effects you observe. Why a pentagon or a trigon (as in triangle) - I don't know.

Here's my take on it: because it's a pre-drilled hole, there's no centre support and you're cutting with the edges of the bit. Unless both cutting edges are machined exactly the same and the work-face is perfectly flat and perpendicular, then one edge will bite a bit deeper than t'tother and the torque of the drill will lever the bit off-centre, so the other edge will catch in the side of the hole and bite, causing the process to repeat over and over again. You'll probably find that the length of the sides of the pentagon or (whatever shape it forms) are a bit longer the width of the cutting edges...

[pauses to take a breath]

In effect, you're operating a powered Spirograph® machine.

The pentagonal bit I am not sure about.

The multi-sided holes and geometrically shaped countersinks are the end result of not enough rigidity for the job.

The cause of the lacking rigidity may be too fast a feed, too much speed, not heavy enough tool, tool flex, etc, etc. The list is almost endless.

What happens is that one edge will bite, for whatever reason. The tool will flex, then once the load is too great, it will spring and then bite again. Typically this results in a hole/csk that has one lobe more than the tool has cutting edges. And it usually happens with tooling that is 'perfect', as in all angles/edges are spot on.

BTW, a drill bit uses a 118* point, a screw has an 82* angle, they don't match very well.


A single point countersink is both heavy/rigid and is not symmetrical/balanced, eliminating the chance of lobed countersinks. The multi-flute ones will do it every time, unless you set the drill press to depth and slowly get to that depth. You should get a neat csk without too much trouble, but it can be hit and miss.

What I do is buy cheap countersinks, then grind off one cutting edge. The bit is no longer balanced cutter wise, and cuts a perfect countersink every time no matter what.

Skewy, I think you have part of the answer. If it was all the answer then you should get variation with drill speed.

One of the guys at work (and Schtoo too) reckons its related to twist drills behaving like a coiled spring and he could be right - see bolded section below

The research on this appears to have been done just a couple of years ago. In a pair of papers in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING by Gong YP, Lin C, Ehmann KF, they investigate the Dynamics of initial penetration in drilling. Unfortunately my on-line access does not extend to this journal, but maybe somebody else can download them?

Here are the Abstracts of the articles for those that want to get really confused.

Abstract Paper 1: This two-part paper is aimed at developing a theoretical and numerical simulation basis for initial penetration phenomena that profoundly influence hole tolerances and shape. In Part 1, dynamic force models are developed followed by models of the drill's dynamic behavior in Part 2. Next, these models are combined and used to predict initial penetration behavior and hole shape. A comparison of simulated and experimental results concludes Part 2. In this part, by considering the effects of drill grinding errors and drill deflections, dynamic cutting chip thickness models,are developed which, in combination with workpiece surface inclination effects, allow the formulation of expressions for the dynamic chip thickness and cutting chip cross-sectional area. By using these quantities to replace their static counterparts, static drilling force models are extended to facilitate the prediction of dynamic cutting forces. Separate thrust, torque, and radial,force models for the major cutting edges, secondary cutting edge, and for the indentation zone are formulated. The effects of drill installation errors on the radial cutting forces acting on the chisel edge and the major cutting edges are also included.

Abstract paper 2: This part of the paper is aimed at the development of models for the drill tip's transverse and angular motions, the definition of models for establishing the drilled hole's profile and, by combining these results with the dynamic force models of Part 1, the formulation of the complete model for drill skidding and wandering. An experimental verification of the models concludes the paper For the development of the drill motion models the drill is simplified as a pretwisted beam subjected to a compressive axial load and radial forces acting on its tip. The governing equations are developed using Hamilton's principle. Subsequently, the weak form of the governing equation is formulated to facilitate their solution by the finite element method. The corresponding boundary conditions for the motion model are also defined for three drilling phase, i.e., drill skidding, drill wandering and stabilized drilling. Based on the drill tip's wandering locus and drill rotation, a mathematical model for describing the drilled hole's profile is developed.

As usual in these "buy me on line now" journals you get no real idea of the results from these abstracts.

Aha! Hamilton's principle! Why didn't we all think of that

Seriously, this problem seems to be more extensive with sheet metal. Pilot holes are often used in solid metal. Even there, you can get initial wobble until the bit has enough metal to dig into and run more or less true. In sheet metal, you never get enough metal. Very slow advancement, just touching at first in fact, can help a little.

Joe

Originally Posted by TassieKiwi

I'm sure someone will whip out a quadratic equation, or a matrix etc to explain. I bought a 3-cutter csk and it works fine - Festool also make a couple of very good alternatives, though pricey (as ever):

I'm convinced you only post here so you brag (cr@p on endlessly) about your Festool stuff. Mind you, if you got it, flaunt it.

Originally Posted by Shedhand

I'm convinced you only post here so you brag (cr@p on endlessly) about your Festool stuff. Mind you, if you got it, flaunt it.

Mcjing sell the same style of "chamfering cutter" for around $10 - $15. These are sometimes called "zero flute" countersinks.

They also have a neat looking 5pc chamfering cutter set covering 3 - 28 mm 82° for $55. Must get me some of those with my next order.