Accurate drilling

[GrahamRounce]

Hi - I needed to drill exactly centrally into one side of a square steel bar. I did it using a piece of pinboard, but it made me wonder more generally.

I usually make a different jig for everything like that that I need to do - but it seems clumsy, and rarely works that well.

Am I right in thinking that there must be a whole field devoted to this, with many standard instruments?

What is that field called? I want to google and find out what accuracy-enhancers are available - I'm sure I could use more than a few of them.
Thank-you again,

 

[Vulcan]

You don’t say what size bar but if it’s not too big then you can measure it with calipers, use layout fluid on the bar then scribe from both sides with the calipers set to half the width. You can then use an optical centre punch and then follow up with a spotting drill.

https://www.axminstertools.com/axminster-optical-centre-punch-100244

https://www.tracytools.com/product/8-mm-spotting-drills/

Edit: Although I do have some layout fluid I haven’t used it for years. I prefer permanent jumbo markers.

https://cultpens.com/products/pilot...BoQw24UpXIdHko6rfmuF2IQeaPasCPljY_TTEWs&gQT=1

 

[Ollie78]

Normally to find the centre you can use a simple jig with 2 pins and a guide hole directly between them. If both pins are touching then the drill is in the centre. I have a star M drill guide that works this way.
Not great at the end of things because you need to register both pins.

 

[Vulcan]

Another way I’ve used to mark stuff out is with a scribing block. I’ve got a small granite surface table but the cast iron table of my bandsaw is normally good enough.

https://www.axminstertools.com/axminster-scriber-block-220091

 

[ajs]

The standard way of doing that in an engineering shop would be plonk it on a vertical mill, touch up to one of the sides, ideally the reference edge, with a spinning cutter (first with a bit of damp paper on the edge, then edging up until the cutter "polishes" the edge) then index over on the dials to the cutter radius and whatever offset is needed.

Centre finding marking out would only be used as a visual indication the hole was about to go in clearly the wrong place.

 

[Vulcan]

The standard way of doing that in an engineering shop would be plonk it on a vertical mill, touch up to one of the sides, ideally the reference edge, with a spinning cutter (first with a bit of damp paper on the edge, then edging up until the cutter "polishes" the edge) then index over on the dials to the cutter radius and whatever offset is needed.

Centre finding marking out would only be used as a visual indication the hole was about to go in clearly the wrong place.

I assumed the OP doesn’t have an engineering shop or I doubt he’d be asking?
I have a vertical mill and use an edge finder (never a cutter) and use the dials for locating a point on the workpiece. For most of my stuff though it’s much quicker to use a caliper as a couple of thou either way isn’t normally an issue.
Self centering vices can be handy for quickly putting a hole in the centre of a bar as well.

 

[okeydokey]

A simple approach would be to use a compass such as used in Schools that has a sharp point on one end a pencil on the other - a few lines at various adjustments would swiftly find the centre.
This sort of thing - would surely be good enough for most practical purposes
https://www.ryman.co.uk/helix-oxford-compass

 

[CliveB]

If you are using a pillar drill, you could just use a cross table or x-y table together with an edge finder to touch off both sides to find the exact centre.

 

[RonnieI]

A way I often use to drill a hole across a round bar is to put a drill bit in my drill press, lay a steel rule on top of the bar and trap it between bit and workpiece, put a little magnetic spirit level on the rule and move the x-y on the vice to get the rule level. Works pretty well for a quick and dirty approach. If it needs to be more accurate I'll put it in a vertical slide on the lathe.

 

[GrahamRounce]

(@ Vulcan: 6mm - the smaller the more difficult!)
I don't have a well-equipped workshop, unfort. Just a bench drill.
The reason I'm asking is that I can't get good results with identifying the centre, by whatever means, then making an indent. The indent invariably moves, and/or after initially drilling with 1mm, a larger (3mm) hole drifts too.

I'd have thought that prior to hyper-accurate factory machinery, there must have been an array of standard, accurate templates, guides etc for this and many other purposes. How else did they do it in the 19th century?

I have an old box of little machined cylinders of exact length (they can be "wrung"), which I was hoping were only the tip of the iceberg of the craftsman's tools. Specifically, in my current case, something that clasps an object and has holes of exact diameter and guaranteed accurate placing...?

Btw, I've come across the word "metrology", which alas applies only to measuring, but surely there must be a similar term for applying measurements, eg in some of the incredibly intricate mechanisms made pre-computers?

 

[ChaiLatte]

I don't have a well-equipped workshop, unfort. Just a bench drill.

OK, now you have actually described the size of problem and the equipment, we can suggest solutions specific to your case rather than some of the necessarily blue skies thinking above (although how trapping a ruler on the face of a SQUARE bar finds its centre is rather too blue skies).

Put a 6mm drill bit (but 6mm diameter plain round bar is better) into the drill chuck.

Clamp a straight piece of material to the table of the drill press such that its midpoint touches the rod in the chuck. That is your fence, set exactly 3mm from the centre of rotation.

It is exactly the edge finder method described above by Vulcan but the edge you are finding is that of the fence, not the item to be drilled.

If you had a 20mm wide bar that you wanted a hole on its centreline, use the same 6mm rod and put a 7mm spacer between its edge and the fence. Or make a 20mm diameter dowel with a 6mm hole in its centre and use that.

Do not drill with a standard 1mm drill: use a lathe centre drill or a spotting drill to locate the centre of the hole and then drill with the finished size. A standard length twist drill does not self-centre well on an unmarked surface. You need something short and stiff.

In the days of yore, the machinist would likely have one accurate measuring tool - a micrometer. That, in conjunction with a lathe, means it is easy to make round, concentric things of known diameters. After that, the grey matter is all you need. There are many old metalworking books on the Internet Archive for free download. A catch-all term to find relevant ones might be 'workshop practice'. Once you find one, browsing the contents pages of others will tell you whether they will be worth your time to read further.

 

[Vulcan]

A way I often use to drill a hole across a round bar is to put a drill bit in my drill press, lay a steel rule on top of the bar and trap it between bit and workpiece, put a little magnetic spirit level on the rule and move the x-y on the vice to get the rule level. Works pretty well for a quick and dirty approach. If it needs to be more accurate I'll put it in a vertical slide on the lathe.

Yes, I use that method quite a lot. It’s surprisingly accurate if you take care. I have a variety of turned pointed pins I use rather than a drill bit, but a Spotting Bit works as well.