Help with imperial measurements

[Screwdriver]

A real chippy, working on a 10m boat would have used their pencil gripped securely between finger and thumb , to mark lines approximately half-way from each side of the timber - and then halved the space between the lines.


Are you really referring to sugar in your tea? If so, think again.

It's really multiple measurements and rounding errors. Anyway, I thought chippies made cabinets and joinery, not boats. There is something about transferring plans to a lofting floor and thence to a boat. I better go find it. Think I did post on it here, relating to Leo and Tally Ho off YouTube...

Haha. I missed a zero, should have been 0.006kg.

 

[caprinut]

Try putting 9/16th into a CNC without checking in a calculator what is in thous. I see a cock-up coming
Then you might want to know what that is in metric for next stage of the job, it's all time.

Bert

 

[Screwdriver]

Try putting 9/16th into a CNC without checking in a calculator what is in thous. I see a cock-up coming
Then you might want to know what that is in metric for next stage of the job, it's all time.

Bert

Yes. It's a case of horses for courses. Sometimes you want metric, sometimes you need microns, sometimes you just want to use a ruler and strike against a mark.

While watching Tally Ho and Acorn to Arabella, I do wonder if it would even be possible to produce the woodworking from CNC. It would be some fearsomely complex model...

 

[mtt.tr]

IKR but it's the rounding errors that get you when you need to produce a long line of similar measurements. Plus even if you're working to say 1/4", you will need to line up your chisel/knife to a line. Imperial rulers have all the fractionals, do the math for mm and it's easy to slip up and the nearest mm could easily be plus or minus 1mm of error when what you really want is for the end result to be flush.

I read somewhere that your fingertips will easily detect 1-2 thou" step in a smooth surface.

As someone who cut his teeth cabinet making (for my sin's) we worked everything out in mm, used to do some really detail reproduction / repairs often requiring tapers.

Actually nearly lost parts cutting curved along laminated parts to a taper.

Boss was more worried that I had potentially ruined 3 days work

 

[bricol]

Personally I don't have a problem with mm and inches but do remember the c*ck ups when using 1st and 3rd angle projection. Had a good few laughs when a fabricator sends prototypes up to the Drawing Room made in the wrong projection.

I did once tell a supplier he was looking at a third angle drawing and as he was used to first angle that was why he couldn't work it out, when in my first engineering design office in London back in 1991. In reality of course, I'd made a total horlicks of the drawing . . . Still, I got a free trip to Southend on Sea from London to sort it out . . .


Just to add - I reckon todays super duper 3D CAD systems have a lot to answer for not understanding projections - they can be set up to give whatever projection you wish, and give you the 2D views of a 3D object with no thought at all. When you had to use a pencil/pen, or even a 2D system, you have to think, work out what went where, what line was visible where, what one was in hidden detail.

Which also meant you could look at a 2D drawing and see a 3D object.

A fabricator ought to be able to work out what is what, especially as a drawing frame should state, and show, what projection is used. I have to admit I couldn't tell you which was which name, but a glance at the projection symbol, and I can see what the object is from the 2D views. I've had to work in both, both here and in the US.

I have been bemused when someone new at a supplier calls me to ask for a 3D model of long standing components that only exist as 2D drawings - they seem unable to work out what they are looking at without the 3D CAD to spin around - even more bemused when its a simple part - a simple rectangular block with a chamfer down one side sticks in my mind as one instance.

 

[bricol]

There is some merit in using fractional inches for boat building (for example) due to the complexity of multiple similar items gradually tapering or whatever. Something like half this half that, double the other etc.

With the metric system you inevitably end up with fractions of a mm, so you have to either go to the nearest mm or keep track of the rounding errors because at the other end of your boat they're all going to add up!

Example say you want 4 1/4" inch from two planks, that's 2 1/8" each.

The equivalent might be 105mm or 52.5mm each. But now you want to mark the middle and you need to find 26.25mm. I mean really? You're going to build a 10m long boat to 10 microns??

The two obvious issues are 1. you need a calculator and 2. you'll struggle to find a ruler marked in 0.1 of a mm let alone 0.01 without resorting to the sort of metrology we all know and love.

I'm sure there are examples where mm can be said to be superior to fractional inches but I stumbled on a (much better) explanation of why inches were so handy which I could dig out if pressed.

In reality though . . . how accurate do you mark out your 2.125" ? And does that allowance cock up the last plank anyway? just apply the same allowance, tolerance, to your metric measurement. I can quite happily measure, and mark out (cutting however . . . ) to +/- 0.05mm with a rule and even a tape - much to quite a few others surprise.

But if you'd told me you needed 4-1/2" from two planks . . . you'd get two planks, each 4-1/4" shorter (within whatever tolerance you specified - and if you didn't, you'd have to accept whatever I supplied . . )

 

[Pete.]

How do you mark out to 0.05mm when the tip of even an ultra-fine line marker is twice that thickness?

 

[Screwdriver]

How do you mark out to 0.05mm when the tip of even an ultra-fine line marker is twice that thickness?

You can't. Even if you had a ruler with markings (!) you'd struggle to line up with it.

In the world of boat building, they will transfer from a plan to a sheet of paper to a piece of wood with multiple repetitious workpieces. Like decking for example or the framing where you might have a zillion very similar items to make or keep track of. Rather than try to wrestle the (quite interesting) reasoning from my frazzled brain, here's the post: #64

Here's what it says:

Where imperial does still make perfect sense is in wooden boat construction. Imperial is king. They actually use something a bit like base 16 to count or measure planking. A stock plank in a certain section might be something like 4/3-1 and they use a plank scale to do it (no not that sort of Planck).

Have a look at this chap

20 minutes in he tells you why imperial still "works". I did understand it briefly but tbh, the precise reasoning now escapes me. I think it was because you could give a plank a "name" like 4/3-1 rather than a literal dimension which would likely do your head in during both the calculation and the build.

 

[MetalMonkey]

I think there is merit in being conversant in both systems. Certainly not a disadvantage to be familiar with both - I can't understand people that steadfastly refuse to use one or the other.

The imperial system was largely based around convenient measurements that fitted around humans - and they are still often convenient for that reason, especially for building and construction.
The metric system makes so much more sense for anything scientific, and for most everything engineering.

Just use whichever system is most appropriate for the task IMO, and check what you are working in beforehand and don't assume anything!
Biggest issue I have now is seeing the smaller divisions of either of them on a tape measure or scale...

 

[8ob]

I can’t read any more of this twaddle. I am off down the road for a pint.It’s only two miles down the road

Bob

 

[tigdlo]

Both exist so learned both. One is divided completely arbitrarily and the other makes sense.

 

[Screwdriver]

I never use fractional inches except in conversation, half an inch here, couple of feet there I only reach for metres when things are far enough away. Even then, if I was going to measure it, I'd be using mm.

Something I have learned to my cost though is never to mix metric and imperial in an engineering project. I have to convert into imperial if anything goes anywhere near the lathe for example. I have tried mixing it up and sure enough, I got mixed up!

 

[jerrytug]

Both exist so learned both. One is divided completely arbitrarily and the other makes sense.

But which is which?

 

[bricol]

What does cause a moment of uncertainty when you have an American torque wrench in hand . . . Lancia workshop manuals with tightening torques in deca-newton metres . . . where's my calculator . . .?

 

[bricol]

But which is which?

Depends which rule/tape you picked up . . .

I stopped my desk tape measure going walkabout in the office by swapping it for one that was free from Harbor Freight . . . seems only having inches on a tape makes it un-useable by most . . .

 

[stevethegrease]

The thing about Imperial is that you can use ( certainly in engineering) fractions of an inch and / or measurement in thou's . Some one versed in imperial would understand a statement like " An inch and seven sixteenth's minus five minus ten" to mean' The dimension specified is within the range 1.427" to 1.432". ' ( everything has a tolerance) . And so on . It makes sense to me. The Americans don't even use " The Imperial system" just their own unique take on an inch based system , this is not helped by them taking the wrong sized bucket so they have ended up with a gallon of 4 litres.

 

[mtt.tr]

I did once tell a supplier he was looking at a third angle drawing and as he was used to first angle that was why he couldn't work it out, when in my first engineering design office in London back in 1991. In reality of course, I'd made a total horlicks of the drawing . . . Still, I got a free trip to Southend on Sea from London to sort it out . . .


Just to add - I reckon todays super duper 3D CAD systems have a lot to answer for not understanding projections - they can be set up to give whatever projection you wish, and give you the 2D views of a 3D object with no thought at all. When you had to use a pencil/pen, or even a 2D system, you have to think, work out what went where, what line was visible where, what one was in hidden detail.

Which also meant you could look at a 2D drawing and see a 3D object.

A fabricator ought to be able to work out what is what, especially as a drawing frame should state, and show, what projection is used. I have to admit I couldn't tell you which was which name, but a glance at the projection symbol, and I can see what the object is from the 2D views. I've had to work in both, both here and in the US.

I have been bemused when someone new at a supplier calls me to ask for a 3D model of long standing components that only exist as 2D drawings - they seem unable to work out what they are looking at without the 3D CAD to spin around - even more bemused when its a simple part - a simple rectangular block with a chamfer down one side sticks in my mind as one instance.

This is probably to simplify cam processing they don't want to be redrawing it in 3d for cam.

Yeah it's simple but they can save time

 

[mtt.tr]

How do you mark out to 0.05mm when the tip of even an ultra-fine line marker is twice that thickness?

Yes I thought was rather impressive

 

[Screwdriver]

Yes I thought was rather impressive

So if you were tiling a plane in mm (or decking out a boat) you'd probably round it off to the nearest mm.
I refer the honourable gentleman to the reply I made some moments ago.

As for "naming" a component identical to many others but with the wiggle room required to fit, does anybody else follow RRBuildings on Youtube? He makes very impressive post frame buildings.

Nothing is ever perfect so they might want an eight foot post with a 4 inch rebate and (typically) +1/8" to level to the not quite perfect concrete. He shouts down to the chap by the saw, "add an eighth to that one and just shy of a sixteenth to the other" etc. That's a human sized unit with its own name rather than a series of numbers.

Sounds about the same as "add 3mm to one and just shy of 2mm to the other one" but in the real world, thats more like a list of numbers than the name of a thing. The "thing" is mm and the measurement is just the number of those things. I don't want to overegg it but there is definitely a place for human friendly imperial measurements which can be more easily remembered or spoken than constant lists of numbers.

I think the problem is that the world never really made a firm commitment to wipe out imperial and go all in for metric. Sheets are still 8x4 (or 2440x1220), lumber is still 4x2 or a 6x2 and that is true even if they sell you a 100x50 or a 150x50. If I was on site and asked a lad to fetch me "three four by twos and two six by twos" I might be more confident than asking for "three hundred by fifty and two hundred by one fifty's". Even writing it down is a chore but the chore is the same when the numbers are spoken....

 

[owas]

I just wish the worker I've been lumped with to cut my steel could read a tape