|Neil Wyatt||06/10/2021 09:51:12|
18809 forum posts
Another thread has wandered from this question, for unsurprising reasons.
Can I ask more specifically, if you were marking out a circle out small (about 0.4mm) holes and then drilling them by hand, how accurately could you place them?
|Dave Halford||06/10/2021 10:06:58|
|1820 forum posts|
That's an eye sight dependant / age question.
My daughter at 18 could be spot on easily, at 40 she could not as glass lenses had not been invented, or had they?
Edited By Dave Halford on 06/10/2021 10:09:07
|Neil Wyatt||06/10/2021 10:37:26|
18809 forum posts
I'm assuming a skilled worker with no physical impediments.
So what is 'spot on'?
|larry phelan 1||06/10/2021 10:37:46|
|1119 forum posts|
I seem to remember Sparey commented on this in his book.
|Mick B1||06/10/2021 10:54:16|
|2047 forum posts|
Also fatigue-dependent. On some recent work I wanted to reach limits like Neil's suggestions and *think* I succeeded, but there were only 4 holes to do, and bigger ones at that. And if I'd failed, the job could stand me opening the holes up a bit. I don't think I could have remained that accurate for more than perhaps 8 or 10 at a time, without a long coffee break between sessions. When I was younger, I might have done twice as many at a time, but eyes get tired at any age.
I was using a 3.5x magnifier - perhaps comparable with the Nimrud crystal from ancient Assyria held in the British Museum.
Then there's the question of how do we know the actual accuracy we've achieved, without spending more time and effort measuring results than we did on doing the job. Perhaps some of us might be prepared to invest that sort of effort, but in most cases I'd find it hard to justify.
Edited By Mick B1 on 06/10/2021 10:59:16
|Michael Gilligan||06/10/2021 11:19:52|
19323 forum posts
Then put-aside the measurements for a while, and consider the hole diameter as a ‘unit’
Would your idealised worker be satisfied with an placement error of +/- 50% ?
… The ‘skilled worker’ will find a way to improve upon that. [*]
[*] __ Quoting from the reference to John Bird, on the other thread:
Bird prepared an 8-foot scale accurate with a vernier to 1/1000 of an inch. Few of his contemporaries could have made such a rule, and this was just to set the openings of his compasses.
Edited By Michael Gilligan on 06/10/2021 11:38:25
|Bill Davies 2||06/10/2021 12:21:05|
|247 forum posts|
I make no claim for my ability to achieve such accuracy, but my instructor taught us to mark out perpendicular lines, pick up intersections and lightly mark with a sharp prick punch (60° centre punch), then using an eyeglass, 'nudging' the position until properly located, then apply a heavier tap. We used a small centre drill and opened up and reamed in the usual way. He claimed, with care, 0.005".
I think Chapman's first volume of Workshop Technology also claims five thou. I've put my copies somewhere safe, and can't find them. There is a set of all three on Ebay for £27.99.
Machine Tool Operation ( [US textbooks] Burghardt, etc., 1953, vol 1) claims it is possible to set external calipers 'easily' to within 0.002-3" against an engineer's rules. In those days, they were accurately ruled and deeply etched.
By my time, vernier height gauges were universally used, and we had mills and jig boring machines for accurate work.
21649 forum posts
A lot will depend on how you are marking out, if its a steel rule and scriber I doubt you will place the lines as well as if you were using a height gauge. Number and spacing of holes around the pcd would also affect things eg 4 holes easy to place 27 less so.
Then what about punching, some seem to go straight in with a centre punch, I would possibly use a dot punch first and inspect the mark and pull it over if needed but more often that not I use my Veritas optical punch which gives me a more accurate result.
And what about hand drilling are we using a pin chuck, archamedies drill, egg whisk hand drill or handheld cordless drill. A non ferrous material will also be easier to drill than steel so less chance of wandering. Then their is quality of drill bit.
All comes down to what the individual has available to them and their ability to use it.
Edited By JasonB on 06/10/2021 13:02:41
|Michael Gilligan||06/10/2021 13:39:06|
19323 forum posts
Sorry … I am not at liberty to share my downloaded copy; but you may have access, or be able to find this paper elsewhere:
“Ancient machine tools for the construction of the Antikythera Mechanism parts”
It’s interesting reading, but not a definitive answer to Neil’s question.
Edit: __ This page is hosted by academia.edu
… so you may not be comfortable visiting it:
Edited By Michael Gilligan on 06/10/2021 13:55:57
|Ian P||06/10/2021 14:28:29|
2537 forum posts
I'll stick my neck out and say think I could do 0.2mm if I was careful and took my time (on something like brass), would be far less confidant about 0.1mm but I can imagine that a craftsman with unlimited time and a suitable magnifying glass could achieve that by continuously monitoring the start of the hole and tweaking (most probably) a bow driven or archimedes type drill.
|Rod Renshaw||06/10/2021 18:46:12|
|347 forum posts|
The late Tubal Cain had something to say about this type of thing in his article on co-ordinate drilling in his book "Simple Workshop Devices." The holes were a little bigger than those Neil writes about but TC does list some actual results of experiments to see what accuracy was possible by different means.
|pgk pgk||06/10/2021 21:42:49|
|2366 forum posts|
If one needed accuracy and had a lot to do then the obvious answer is to cheat - fix your hand-drill at the radius with some sort of vertical arm on a simple hinge. if the sheet you drill through is thin and the arm is long enough then any curvature becomes insignificant. Fix your plate to be drilled on a large enough circular support with witness marks on it's circumference and again the error is down to how large that disc is - a spare wagon wheel?
|Jon Lawes||06/10/2021 21:59:40|
700 forum posts
I can't drill with that accuracy using a DRO on the mill...
|pgk pgk||08/10/2021 06:42:06|
|2366 forum posts|
An interesting article postulating that ancient craftsmen may have been myopic with natural 'magnification'.
Also consider that with magnification there are some folk capable of such good hand-eye coordination that they can engrave the lord's prayer on a pin-head. I wonder ho accurate such folk can be even without a lens?
|Martin Connelly||08/10/2021 08:31:30|
1938 forum posts
When I was at work one of the fitters was very myopic. If he wanted to look closely at something his glasses went up on his head and he held the object about 50mm from his eye to look at it. Maybe in an earlier time he would have ended up as one of these craftsmen doing detailed work by hand.
On a different scale of accurate hole drilling, we had a large portable Asquith radial drill that was a bit worn. When it was being used to drill large holes accurately positioned the initial centre pop had four additional centre pops around it on a PCD equal to the hole diameter. As the point of the drill was creating the initial cone shaped hole the process was halted and the position was compared to the outside centre pops. Any off-centre positioning was corrected with a die grinder and a bit more was removed with the drill and the process repeated until the drill was correctly following the desired hole centreline. The resulting hole had the half pops around the edge evidencing the good positioning of the drilled hole. Magnetic based drills eventually pushed the Asquith to the side-lines and it was eventually disposed of.
|pgk pgk||08/10/2021 09:05:34|
|2366 forum posts|
The other thing we forget is patience. Many ancient construction projects took centuries from concept to completion. Even in later history a daunting project like a cathedral took more than 250 years to build with a huge team and often double that. It hardly seems unreasonable to image an orrery being designed and built over a few years of fiddling and faffing - a bit like any of my projects...
Imagine a few apprentices tasked with the grunt work of sourcing and preparing the blanks, perhaps practising on wood or stetched canvas panels coated with resins? Making patterns on a cheaper easier worked substance for later transfer? Unless there is a source of documentation then most archaeology is speculation often extrapolated from prior speculation rather than fact.
|Mike Poole||08/10/2021 09:16:48|
3099 forum posts
To hit 0.1mm it would a lucky shot for me, to consistently get that would need much careful working. Would using a pillar drill count as by hand? I had the luxury of short sight from my early teens but ageing has robbed me of this and I now miss it. Drilling 0.4mm by hand would soon have a pile of broken drills if I was in charge of the drill. The marking out will need to be very good if it is not going to contribute its own factor into the final hole position. If you are taking the job on Neil then good luck and rather you than me.
|Bill Pudney||08/10/2021 09:52:00|
|570 forum posts|
During my apprenticeship we had to make loft plates, typically 1/8" al.alloy, with an accurate photographic "picture" of what was required, printed on the sheet. They were usually developments of folded/pressed structural members, some were small 6" x 6", some were long and with a complex shape, the biggest I saw was about 72" x 12". The outline had lines from memory 0.006" or 0.010" wide, holes were indicated by crossed lines, with a circle appropriately sized for the drill bush destined for that hole, usually either 3/32" or 1/8" bore, with an outside diameter of something like 5/16" or 3/8". The bushes were very accurate, I think that they were sintered. The outline had to be filed to (ideally) half the line thickness, and the drill bush ideally would show half the line thickness all the way round. This is a long winded way of saying that, in my yoof I regularly achieved +/-0.003" or +/-0.005".
|Neil Wyatt||08/10/2021 10:08:30|
18809 forum posts
That's particularly useful, thanks Bill.
|Neil Wyatt||08/10/2021 10:08:59|
18809 forum posts
And so is this, from another Bill, thanks.
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