Drill running off course

Hi, there are many reasons for drills to have a tendency to run off and a few have been suggested, but if your drill is not quite sharpen concentrically, i.e. one of the cutting edges is slightly longer then the other and/or one of the edges has a different angle, then it will tend to crab and run off. small drills seem to be the worst for this as they bend easier.

As far as using centre drills to spot, I've no problem with this and have been doing it for years where I wish the hole to be as true as I can get it. I've had this little chap for at least 15 years and have spotted countless number of holes and used it in the lathe for drilling centres.

Only yesterday I spotted 20 holes before drilling each one with a No. 42 drill. If I break one end today, it's still earnt its keep.

Regards Nick.

P.S. I have two centre drills that are even smaller than this one and have spotted with them on a few occasions.

Edited By Nicholas Farr on 02/10/2019 08:29:25

Very many thanks to Ian Johnson and Vic for the links.

(My mistake was searching for "spotting" rather than "spot" drills)

I'm very much a beginner and have used Centre Drills in the past (with too many breakages).

How does one decide what size Spot Drill to use, please?

Very many thanks for tolerating my questions

MC

Yes Rennie tools are a good company, best to go direct I think rather than through shopping channels.

As for correct size spot drill, don't worry about it too much. For my milling machines I have a 4mm spot drill permanently mounted in a holder which I use before every hole I drill. A 4mm spot drill will leave a small spot or a big spot, depending how deep you go. It's quite a versatile size. I do have a 3mm and a 6mm but rarely use them.

For my lathe work I have a number 2 centre drill permanently mounted in a holder, which I use for everything, saves messing about changing tools if I machine with a rotating centre in the tail stock.

And after all the years machining at home, and in industry, if I had £1 for every centre drill I broke I would have ..... er......about £5. Must be lucky or something.

Ian

There's no such thing as a stupid question. They remind me how much I don't know!

The problem here is that twist drills tend to wander and are particularly likely to do so when starting. Various reasons for this, perhaps the main ones being:

• Having deep spirals means twist drills are bendy. So if a hole starts 'off', the drill tends to follow the bad direction rather than correct it.
• The geometry of an ordinary twist drill point means it can't cut at the very tip. Just before the drill starts cutting there's a tendency for the point to skate over the surface. As a result the hole might start off-centre and at an angle. Once off-course the drill tends to follow the false start.

One way of starting a hole accurately is with a centre pop. Although the dent made with a centre punch is small it's usually enough to start a twist drill correctly. Getting a centre-popped dent exactly where needed, straight and deep enough requires a certain amount of skill, as does aligning the dent with the drill and applying appropriate pressure. It's the sort of thing old-hands think is easy but beginners struggle with. The method is most appropriate for a drill-press or hand-drill.

A centre drill provides a rigid way of starting a hole accurately. They have a stubby body, no flutes, and short stiff cutting section. The hole they make is deep compared with a centre-pop and twist drills follow them well. Alignment is good because the centre drill and twist drill are both set from the same chuck. Main disadvantage is small centre drills are easily broken. Handle with care!

I'm not sure there's an ideal size for a starting hole provided it's big enough to stop the blunt tip of the twist drill from skating about. Using a large diameter centre-drill seems to be the answer but I've had the twist drill jam into big holes. A starting hole rather less than ⅓ the diameter of the main drill seems about right. (Do others agree?)

Although I use centre-drills all the time for starting holes, it seems that's not what they were designed for! I've not tried a spot drill yet, but they certainly tick a lot of boxes - stiff construction and a strong tip designed for starting rather than deep drilling.

The other important aspect of this is learning the ropes. Drill speed and the in-feed pressure depend on the material and the diameter of the drill. Small drills need to be spun fast, usually too fast for home equipment! In my case it took a while to find the right pressure. Too gentle rubs the cutting edge and blunts the drill. I've found tools last much longer when they cut rather than rub! Too much pressure risks opening the flutes & cutting oversize, stressing the machine and work-holding, damaging the cutting edges and breaking the drill. Jerky pressure is bad. It's important to get a good straight start and keep the drill at right-angles to the work. Usually best to lubricate - I always use a cutting fluid - and make sure swarf is ejected.

Dave

My preferred size spot drill when spotting a hole to be tapped is the same diameter as the thread OD, so for M3 I use a 3mm spot drill, this is taken 0.50mm below the surface to save de-burring after threading but also provides location for the screw when engaging the hole.

Emgee

SOD said "A starting hole rather less than ⅓ the diameter of the main drill seems about right. (Do others agree?)"

No. Or yes - it depends on the length of the chisel point relative to the diameter...

When a larger drill is fed into an existing smaller diameter hole, what tends to happen is that one cutting edge 'grabs', stops moving, and acts as a pivot, around which the drill orbits - for part of a rotation. Then, the 'pivot' lets go, and the other (or the same) cutting edge grabs again, and the cycle repeats. Eventually, things tend to settle down, but lobed holes are often produced in thin sheet, where the 'settling down' can't happen, because the side of the drill may never engage properly with the side of the hole. This is perhaps the most obvious manifestation of this phenomenon, but 'orbiting' drill bits can be observed when drilling from the lathe tailstock, as they engage an existing hole in the workpiece. In this case, it may be helpful to employ the very old trick of starting the (follow-up) drilling by holding or restraining the free end of the drill, using a bit of metal in the toolpost. This temporarily deflects the point of the drill, which then acts as a boring tool, with only the rearmost cutting edge initially engaging. (Try to set the drill bit's cutting edges horizontally).

This phenomenon varies with the drill stiffness and the relative diameters of the starting hole and the following drill. I suggest the best size of a starting hole is the same diameter as the length of the non-cutting chisel edge of the following drill bit. This, of course, assumes the machine is powerful and rigid enough. The often-advocated idea of using a series of bits of increasing diameter is just asking for trouble.

120 degree, four-facet spotting drills make ideal starting dimples and four-facet drill bits (no chisel point) make drilling so much more civilised, often allowing drilling without the need for any centre-pop or spotting drill dimples.

Having said that, millions of holes have been drilled successfully using all sorts of techniques, many of which are passionately held to be the best...

Edited By Kiwi Bloke on 02/10/2019 11:27:05

Edited By Kiwi Bloke on 02/10/2019 11:28:36

I was taught to drill large holes in this way and believe that I've NOT had any trouble.

What are the views and experiences of other subscribers?

MC

As a lot of hobby users won't have machines that can drive a large drill I would say that is really asking for trouble by overloading and over heating that may pop a motor, board or strip gears. Better to work up through the sizes keeping the load down and often allowing a faster speed which keeps the motor in it's happy band and cooling fan working.

As for drills snatching or producing lobed holes in thin work get a step drill or two. The near zero helix stops them snatching and the previous diameter pilots the next one up the drill so they won't wander helped by being short and stiff. Or if doing a lot of sheet work the get sheet metal drills.

There you go Neil, next months subject can be step drills.

As for sizes of spot drills as I said early on 1/8" and 3/16" have done me for the last 7-8 years, it is only when you want to use them as a combined CSK that you need a size to suit the screw head rather than the hole. All my commonly used drills have gradually been replaced with split point ones so don't have such a large "flat" on the end and in many cases can just be used without any punching or spotting, infact I seldom mark out now as the DRO can do a better job of it.

Edited By JasonB on 02/10/2019 13:30:45

Edited By JasonB on 02/10/2019 13:33:37