Drilling holes

They say that the biggest fool can easily ask questions the wisest man cannot answer, so I feel no shame asking this...

What is the recommended technique for drilling holes? No, that's not really the question. What's the scientifically correct way to drill holes, and why?

It's often recommended that holes are drilled, using a succession of progressively larger diameter bits. The diameter increment is usually glossed over. OK, doing this reduces the load on the machine (and workpiece), but it has disadvantages. The major one is the difficulty of getting the 'next size' drill to centre, and not try to start to drill a pentagonal, or other non-round hole (for the well-understood reasons). If you ram the bit into the hole, to get it into cut without it dancing all over the place, the sudden load on the bit's corners can break them off.

I tend to drill in only two steps: first, a small pilot hole, around the same diameter as the final drill's web thickness, then the 'finished' size, but I have fairly grunty machines available.

So, what do you do, and why?

There's no shame in asking questions. A lot depends on the hole tolerence required; if its a clearance hole for a bolt then I would use the appropriate sized drill without a pilot.

For a more accurate drilled hole I would use the method you have described and for a reamed hole I would use the pilot drill followed by a drill some 3 to 5 thou under the reamer size.

I'm sure others will have other ideas.

Brian

The drill essentially has a spade/chisel tip form, it's the appropriate shape to facilitate cutting, much in the same way that other tools need to come to a sharp point or have the correct clearance angles in order to shift material.

The shape of the spiral flutes is merely to carry away as much swarf as possible without jamming in the hole, they could be straight if they wanted to but they spiral them because this shape allows more room.

You're right about using a bigger drill in a hole nearly as big; they don't cut a perfectly circular hole over another one, in order to do this properly, you need a reamer or a core drill, no i'm not talking about hole saws, these are 4 or more flute drills, which do not come to a sharp point, they are solely designed for opening up a smaller hole, they are quite rare and relatively expensive to buy.

I agree that so long as the pilot is a small hole, this can be rather useful for large diameter holes, However, it would be wise to make sure that you don't cut away the counter sink of the centre drill with your pilot drill, as this is what helps your final size drill to centralise properly as it begins the hole.

An important point for lathe drilling that I have put elsewhere, don't move the tailstock through a hole to attempt to drill a longer hole. Always drill from a single fixed position because otherwise the hole will start to go off centre.

Unfortunately this means you'll need to either change to a longer drill part-way through the drilling cycle or use a boring bar to finish. (if a hole longer than the travel of your quill is what you're looking for)

Michael W

Edited By Michael-w on 10/04/2017 10:49:38

Several (dare I say) ways to make a hole.

Pilot drills are only one way.

Better accuracy, on concentricity, might be to use a hardened guide to ensure the drill in question follows the required route. Another is to use a suitable milling cutter, after an original pilot.

The hole, depth, material, etc might need considering, as well as your required precision or accuracy.

I would say there is no one scientific way to make holes. This is engineering, after all!

'Drilled, punched, bored or countersunk' comes to mind. Thin materials might be better punched. Large holes bored and a few might need to be coutersunk.

For sheet metal, the tapered or stepped drills might be better for hole enlargement.

I quite often use my 'practool' for repetitive odd sized holes in various materials.

You can buy multi-fluted drills, at a cost, I suppose. Another way is the hole saw route. Maybe not quite as crude are the core type cutters - look up rotabroach. Not so useful for blind holes, though!

The reason for the helix on a twist drill is to present the correct rake angle to the work, drills come with a fast or slow helix for different materials, once apon a time you could get straight flute drills for brass work, these were some what more accurate than the flat spade drill.

Ian S C

A variety of views and opinions as usual. Here's mine.
The chisel point at the end of the drill cannot cut. Scientifically or engineeringally it ain't going to cut. It takes power and force to move that core material aside if it is still there. Every now and then someone comes on a forum who is an 'ex toolmaker' (must be ex because of being sacked for stupidity) moaning that their new hobby late must be rubbish because it won't let them run a 2 in drill (with it's 1/2 HP motor) like they could at work.

The 'words and music' in old MEs mostly mentions the use of a succession of drills as part of the standard writing style and to guide newcomers who might otherwise overstretch their machine. There is no sense with a small hobby machine in not to start with a pilot hole. The steps after that depend on the size and power available without stalling the motor or slipping the belts. Unnecessarily small steps will wear the tips of the drills and may also allow too deep a cut resulting in a snatch, drill spin in taper, or other problem so don't be timid but be prepared for whatever happens at breakthrough being worse for a bigger drill.
With thin material drilled onto hard wood as you can in the press not lathe is that the wood pushes back on the drill as you break through.

Nobody has mentioned split points yet as an aid to centring on a mark and not using a pilot. Still doesn't form a cutting edge so, in my view, a waste of time for a hobbyist who isn't in a tearing hurry.

Wandering drills do so often because the two cutting lips are not even, exactly even as when new, perhaps having been sharpened by the 'ex toolmaker' who thinks he can sharpen freehand as accurately as a machine.

If you can get them, pilot point drills work very well IMHO. They have an end formed a bit like a slot drill but with a short conventional drill point, with split edges, projecting at the centre. This starts the hole off and keeps the drill centred once the flat end edges start cutting. Their big advantage is being able to use a large diameter and therefore much more rigid drill from the start, either cutting to final size or for enlargement, with a clean break-through. This minimises the chances of the drill going off centre. Black and Decker used to sell them in sets and I've been able to find them through eBay. I also bought in a sale a complete set 2 to 10mm x 0.5 made by Elu. A Google search shows that DeWalt also supply them but hard to come by.

The problem of taking too large a bite with a big drill is that the feed per rev for the outside edge of the drill is too large for the edge near the centre. If you reduce the feed per rev to suit the inside diameter then the outside runs the risk of rubbing instead of cutting. So a technical reason for going up in stages that is not just to save overloading a small machine.

Martin C

During my time in the services I was always told to delegate the job ( or get someone else to do it) - seems infinitely the best way to me

Only comment is that the worn emery under the drill tip works if you want to avoid a 'triangular' hole when drilling out an undersize one.

Neil

What works for me is to use drills one third the size of the previous one, obviously depending on the torque available. As the original post said, basically the web is the problem, so the less 'scraped' or pushed out the way by the web the better. I frequenlty use 3mm or 1/8 as the first drill, but it does depend on the machine. About 1.5mm on my minilathe.

Bill

In industrial applications drill fixtures use guide bushes to guide two flute drills without pre drilling. What we as model engineers do wrong is use a two flute drill to open out a hole. Opening out with two flutes imparts sideways movement or chatter. Multi flute core drills three or four flute should be used. Also as said previously the available horsepower should be taken into consideration, with regard speeds and feed.

I meant to ask 'So, what do you do, and why?', wondering whether any consensus would reveal itself. Even without the intended emphasis, I'm glad that it seems to have generated some discussion. Thanks, everyone - can we keep it going?

I take Mr Stevenson's point, but I suppose I'm questioning the advice, probably repeated regularly, throughout Model Engineer's life, to drill with increasing diameter bits (in appropriate circumstances, material, etc.). As HOWARDT says, trying to open out a hole with a two-flute cutter can be hazardous, leading to the problems explained by Bazyle. Drill guides, or something in the toolpost, pushed against the drill, if drilling in the lathe, help, of course. It seems that lack of power, particularly in the days of treadles, might have fuelled the original advice. Is it still appropriate?

What I do and why.

Using 500watt powered lathe and vertical mill, Sieg. For upto 6 or 8mm straight drill in the mill, centre first only on the lathe. Drill chuck is 13mm so that is the maximum I can drill and power becomes a problem. On the lathe I can bore anything over 10mm. I have a rotary table for the mill so can use that and an end mill to interpolate a bore so long as its not too deep, beyond flute length. If the bore is too long I can mount the part on the lathe cross slide and use a boring bar in the chuck to bore out to size, the mill has no power feed so I can't use that. Material type doesn't make any difference to the basics, it just that you will use less power with softer materials but may need to use a different geometry drill and lubricant.

Drilling, reaming, tapping is a big subject and if money was no object we could all use the best tool for the job, but it isn't. So as a model engineer rather than a production engineer we use what we have to hand, bang a two flute drill, back off when we stall the spindle and hope we have a roundish hole when finished.