Centre drills vs Spot drills.

[yestertech]

I'm looking to improve accuracy when drilling holes for projects, usually ali or thin steel.
Having scoured some useful videos on the web, I'm confused as to which type of drill but would be best for drilling small holes or starting larger ones.
I would normally just use an ordinary HSS bit but they can wander and have a tendency to bend.
Any tips (sorry ! ) welcome....


Andy

[M0FYA Andy]

Are you using a proper drill-press?
How are you clamping your work down and positioning it correctly under the drill bit?

I use a centre drill bit, followed by a stubby bit if necessary to enlarge the hole.

Andy

[Station X]

Centre drills are for drilling centre holes in the end of shafts so that they can be mounted "between centres" on a lathe and shouldn't be used for anything else. They're very prone to snapping off if you're not careful and the remains are a to get out.

To start holes I either use a centre punch or a spotting drill.

I also remember being taught that all holes over 1/4" diameter must have pilot holes. The diameter of the pilot hole should be slightly larger than that of the larger drill's tip.

[Goldie99]

I'm far from a metal working expert, but anytime I need accuracy or want to avoid drills wandering, I just use a centre punch first and start with a small drill.

Alan

[Julesomega]

The machine shop where I used to work would oblige you by using a Rotobore for small accurate holes. I can't remember the smallest diameter though

[Julesomega]

My post above was not answering Andy's question. Two questions really
1) Starting the hole. Without a milling machine you'll have to punch accurately. Start by marking with a sharp scriber, maybe even give it a little tap, then you can locate a sharp centre punch for an accurate dot.
2) Drilling a reasonably circular hole in thin sheet requires supporting the work on a firm base of scrap material. Hardwood is usually fine, soft aluminium is even better. Or you can finish with a reamer of the required diameter

[David G4EBT]

Don't use normal HSS engineers drills on sheet materials or they'll snatch. One thick materials such as 5mm steel bar, angle iron yes, where you can clamp it down. Best to drill a small pilot hole first or the drill will wander.

For thin sheet materials such as aluminium, steel, ABS and certainly for acrylic sheet, there's only one show in town - a 'step drill' (sometimes called cone drills but while those are a similar cone shape to a step drill, they're without the steps).

For our fairly undemanding hobby purposes, you can get a set of three step drills for under £15.00 which will drill holes smoothly with no burrs and no snatching from 4mm to 32mm diameter. The point of the bit drills a 4mm diameter pilot hole, then you just carry on with however many steps you need to until you reach the desired diameter.

A couple of examples here:

4 to 32mm set of three £11.59 from a UK supplier:

https://www.ebay.co.uk/itm/3PCS-Step....c100005.m1851

Set of three, £11.99:

https://www.amazon.co.uk/Hiveseen-Me...NsaWNrPXRydWU=

Or for 'tool junkies' who don't like buying cheap, RS have a 4mm - 30mm one for £66.16 inc VAT:

https://uk.rs-online.com/web/p/speci...B&gclsrc=aw.ds

My own set is a budget one I've had for about four years and is still fine.

If a hole is just slightly too small but the next size would be a bit too big, a tapered reamer is useful.

A cheap one is quite adequate. This one goes up to 12mm, but larger ones are available:

https://www.amazon.co.uk/Rolson-3412.../dp/B00422ID9W

I don't have any connection with the above sellers - they're not recommendations - just examples for illustration.

There's a video of a step drill in use here:

https://www.youtube.com/watch?v=KBrjkqPpVvo

Below are three examples of home brew projects I've built with aluminium front panels and 2mm clear acrylic sheet, each with several holes of various sizes quickly and easily drilled with a step drill, leaving no burrs. If I'd used normal HSS drill bits it wouldn't have ended well, apart from the faff of having to keep changing the bits for larger sizes.

Hope that helps.

[chriswood1900]

Another vote for a centre punch and small drill. I use a Automatic Centre Punch for sheet alloy with a small drill of around 3mm and then go up in stages as required.

[emeritus]

If you need to drill a repeated hole configuration, such as for a flanged connector, a drilling jig can be made from a piece of wood such as an offcut of melamine-faced chipboard. Once one hole has been drilled, a bolt or spare drill bit plus a clamp can be used to positively locate the jig in position and ensure the subsequent holes are drilled correctly.

I like step drill too, I wish they did an "odd" sized set along with the evens!

[yestertech]

Originally Posted by M0FYA Andy

Are you using a proper drill-press?
How are you clamping your work down and positioning it correctly under the drill bit?

I use a centre drill bit, followed by a stubby bit if necessary to enlarge the hole.

Andy

To answer Yes, I have a floor mounted drill press ( thank goodness ) and usually drill with the work clamped into a cross vice. I use an auto centre punch after having scribed the hole centre. I should point out these are usually small holes for mounting screws etc. generally up to c.5mm.It always seems to me that despite all the care, an HSS bit of this size tends to flex and then errors creep in. The collective wisdom seems to suggest the use of a short and thick drill which won't, hence my interest in centre drills/spotting drills.

Andy

[Station X]

I've never found the need to do anything other than centre punch holes when drilling work in a drilling machine.

Mark out the position of the hole and make a very light punch mark to start with. Check whether it corresponds with the scribed lines and if necessary move the punch mark over by angling the punch. Once satisfied make a deep punch mark. With a fine fine point on the punch you can feel it drop into the intersection of two scribed lines.

Spotting drills come into their own when co-ordinate drilling on a milling machine or drilling machine fitted with an X-Y table. In this case I dispense with all marking out except perhaps for the first hole, using the hand wheel dials to position subsequent holes.

It helps to have a digital read out (DRO) which will enable you to do all sorts of things like drilling any number of holes on a pitch circle. I no longer use a rotary table for that. Just punch in the number of holes, PCD etc. and the DRO does everything for you apart from turning the handwheels.

[ChristianFletcher]

To be honest I have a lathe and just use centre drill for everything. The centre drill won’t know what type of machine it has been installed in. As other have said they can break but that only happens if your hand drilling. They work fine in a drill press and the milling machine. Run at top speed when centre drilling. My milling machine has a DRO god it’s a joy to do hole pattern now. As others have said the short shank drill work very well. But my best advice is buy quality drills from a reputable supplier.

You can see me being lazy here the long drill wanders slightly a stubby drill wouldn’t but this wasn’t a critical application https://youtu.be/3fFVMd66VIY see 47 minutes in

Unfortunate I am very lazy and it’s really held me back in life !

[Radio Wrangler]

Another vote for the automatic centre punch, but be careful who you lend it to. A boss at work asked to borrow a centre punch, so I handed him my nice original Eclipse automatic one. He'd never seen one before and didn't know about them. He hit it with a hammer! It survived!

If the drill you use next has a flat central chisel section in its grind, that is narrower than the diameter of the crater your punch left, then things should go well.

Look at almost any drill and you will see the chisel nose in the centre. This is a guide that the hole before using this drill needs to be a little larger in diameter than the length of the chisel.

Too big a previous hole and it will grab, even in thick material. Just a bit bigger than the chisel and the drill will work its way through. THis way you can look at the drill you have to start with, and the one you want to end with, and pick an intermediate sequence which will be good-mannered.

David

[Uncle Bulgaria]

I have a nice Eclipse automatic centre punch too. Bought a couple of years ago in a proper hardware shop in Stockbridge from a man in a brown tradesman's coat.

I'd second the centre-punch and drill, but make sure the drill has a 135 degree split point (the business end looks like it's been cut to a point rather than a ridge) which will do wonders with 'walking'. Here's a picture showing the split point on the right.

I've not tried cone/step drills but David makes them sound like the business. Unfortunately they're not sold here, but for everything else drill and tap/die related I can highly recommend Tracy Tools for those who want to avoid Amazon.

[Station X]

Originally Posted by Uncle Bulgaria

I'd second the centre-punch and drill, but make sure the drill has a 135 degree split point (the business end looks like it's been cut to a point rather than a ridge) which will do wonders with 'walking'. Here's a picture showing the split point on the right.

Do you have a source for these? I could grind my own, but have never found it necessary.

[ChristianFletcher]

Just a thumbs up for cone cutter and step cone they are the ideal tool for sheet materials and I have found they have zero wander. And because a picture paints a thousand words https://youtu.be/_wgZ3hcVK0o

This was an enclosure i machined for my friend George Christofi it’s an enclosure for a large variac transformer and I modelled all the equipment in 3D first. My old boss told me constantly I was a frustrated mechanical engineer

[Uncle Bulgaria]

Originally Posted by Station X

Do you have a source for these? I could grind my own, but have never found it necessary.

It's harder than it should be as few manufacturers mention it. I've got individual odd sizes from Tracy Tools as in my previous post, as they're in Torquay with knowledgeable operators you can ring them and get useful answers about their stock.

For the house build at the moment, which needs hundreds of holes through weathering steel, then a tapped 5mm hole behind, I've used the aforementioned Eclipse centre punch, then a combination of Tracy products and Screwfix for emergencies as they're down the road for click-and-collect. We had to buy a few different ones before finding the best, but are pleased with their cobalt offerings. I can't remember without finding the packet which brand we're on at the moment, but they are available. Best when things settle down to go to the shop and check the ends by eye.

[Sinewave]

Originally Posted by Station X

Centre drills are for drilling centre holes in the end of shafts so that they can be mounted "between centres" on a lathe and shouldn't be used for anything else. They're very prone to snapping off if you're not careful and the remains are a to get out.

To start holes I either use a centre punch or a spotting drill.

I also remember being taught that all holes over 1/4" diameter must have pilot holes. The diameter of the pilot hole should be slightly larger than that of the larger drill's tip.

I have used centre drills on a mill and a jig borer without any issues, but indeed wouldn't use on a hand held drill.

[David Simpson]

Loads of great drilling info. From a wee tap of a scriber, then a centre punch, then pilot drills - - & so on. A day or so ago I recommended folk who are new to radio chassis work to practice on a knackered old chassis. Removing components, de-soldering etc. Likewise - for drilling - practice on scrap pieces of aluminium, paxolin, bakelite, perspex, etc. Using wee clamps or a vice is essential.
For holes that are just not right for a particular wee bolt or whatever - I have some gently tapered reamers. For larger holes for grommets, switches, fuse holders, etc., I often use "Q max" cutters rather than thumping big drill bits. Also a decent drilling base is essential for a lot of sheet work, such as off-cuts of plywood or blockboard. A set of needle files is also essential for final sizing of holes for the smaller BA bolts. As the Bishop said to the Curate - working with big holes is jolly awkward. Using an engineer's compass to draw a circle - then a hellish amount of wee drilled holes need drilling all around the inside. Then all joined together with a needle file, then smoothed fine to the desired diameter with normal engineer's files.
RAF Apprentices for example, took between 2 & 3 years to reach a suitable high standard in cutting, bending, drilling, looming/lacing, soldering, & so on - - before they sat their final practical skill's exam. So an aspiring V/R enthusiast will need to spend hours & hours practicing for many many months before attempting major chassis repair work on an Eddystone, HP, Grundig, or whatever. Or attempt a homebrew project similar to any number of lovely well built items often pictured in threads by some great guys who I admire.

Regards, David