measuring into grooves

[Anonymous]

[me that he should measure between the sides

Jacob

Are you really sure about that? because your advice to him (ignoring his actual question) was to use a rod to build things so

that you don't need to measure or calculate anything

 

[Philly]

Paul
You can't beat actually measuring from the workpiece - Jacob's method can work but in the real world I find components have a way over being over/undersize. So measuring the gap makes sense.
Hope this helps
Philly

Depends which real world you live in :shock:
A 12 pane sash window would have about 35 wood components. A 6 panel door and frame about 15. A typical job might be a set of 5 windows and one door; 190 components in total, each of which has to be cut to length, planed to size, morticed, tenoned, rebated, moulded, joined, wedged, fitted, housed etc etc. The hardware too must be fitted, housed, morticed etc.
How to keep track of all these components and operations and minimise error and waste, maintain accuracy, organise so that for example all the mortices are cut in one sequence?
The answer - a rod (or several rods). There's nothing mysterious or difficult about it - it just means that all the measurements & calculations which you'd have to do anyway, you do at the beginning just once and record in one place.
If you find that you are having to measure something in the middle of the job, from the workpiece, it means you have already made a mistake, ("over/undersize") and are probably making another one!
It works just as well and is just as essential for a simple job such as a single coffee table & shelf from 4 pieces of mdf

cheers
Jacob
PS if you find that components are "over/undersize" you either replace them (and work out how to cut them accurately), or, you modify the rod accordingly and alter the design. Much easier to alter a few pencil lines than to work blindly around some unwanted and unrecorded design variations!

Well in Paul's "real world" he is making a coffee table. He mentions it in his original post? And I still stand by my advice - check the real measurement. Don't forget, wood has a habit of moving, so that "rod", while perfect if you were engineering, is not going to be as accurate as checking the actual workpiece.
I had a good laugh reading your postscript -"Much easier to alter a few pencil lines than to work blindly around some unwanted and unrecorded design variations!" Jacob, are you telling me that you never make an error? Most folk I know make mistakes in every project - and I'm sure the Pro's do too. These unwanted design variations are a huge part of the learning process, as well as a source of unusual and original design quirks. :lol:
Cheers
Philly

 

[engineer one]

quirks, philly, now i have to include quirks as well. :?

shucks i have just remembered that the sub atomic things are called quarks :lol:

however you are right, the very making of this table has demanded changes in the original spec, and subsequently some of the detail work too.

i guess i could spend the time making a model, but unless full size it is difficult to guarantee that it gives the correct impression

so for now, i will hope that it will come out almost as i thought it might :roll:

paul :wink:

 

[DomValente]

OoH, ooh, fight, fight. :twisted:

Dom

 

[Anonymous]

snip
Don't forget, wood has a habit of moving, so that "rod", while perfect if you were engineering, is not going to be as accurate as checking the actual workpiece.

The whole point of the rod system is to make work more accurate. I realise it's not obvious, but if you progress through a piece bit by bit measuring as you go then you may be building on previous errors and slowly departing from the original intention. The rod brings you back to base and maintains accuracy. It's absolutely essential if you are working to a predetermined measurement such as something which has to be fitted; window to masonry, drawer to chest, or merely repeating something, etc

snip
Jacob, are you telling me that you never make an error?

No I didn't say that and I've made plenty - but I know a really good way to minimise them - guess what it's called THE ROD

cheers
Jacob

 

[Anonymous]

The whole point of the rod system is to make work more accurate. I realise it's not obvious, but if you progress through a piece bit by bit measuring as you go then you may be building on previous errors
Jacob

Jacob, a every engineer knows, this is incorrect - it is drummed in from day one.

You ALWAYS measure from a datum point (for woodworking reference see David Charlesworth's posts and articles) so there are no incremental errors at all. In woodworking, your datum will be a face or side and all measurements are taken from that, not from previously measured points.

I do not disagree with you that a rod may be a useful tool, particularly for general carpentry, but I do strongly disagree with your point that it is more accurate than measurement (using the correct techniques).

One uses whatever works for yourself as both work equally well, but only one will determine the depth of a slot.

 

[engineer one]

surely the whole thing about rods is that you have to have experience with them to make them work, and in addition for the hobbyist, there is almost an imperative not to waste wood, so if we make an error, then we work round it whilst the pro may be unhappy but will pick up another bit of wood.

i would love to be perfect, but have to work within my skill levels and the mistakes i make. :?

paul :wink:

 

[Anonymous]

The whole point of the rod system is to make work more accurate. I realise it's not obvious, but if you progress through a piece bit by bit measuring as you go then you may be building on previous errors
Jacob

Jacob, a every engineer knows, this is incorrect - it is drummed in from day one.

You ALWAYS measure from a datum point (for woodworking reference see David Charlesworth's posts and articles) so there are no incremental errors at all. In woodworking, your datum will be a face or side and all measurements are taken from that, not from previously measured points.

I do not disagree with you that a rod may be a useful tool, but I do strongly disagree with your point that it is more accurate than measurement (using the correct techniques).

One uses whatever works for yourself as both work equally well, but only one will determine the depth of a slot.

As I've said before - various engineers use the same (similar) system for the same reason, and boat builders, sail makers etc and many other manufacturs I guess.
Structural engineers lay out steel work on chalk lines on floors - full sized drawings, and take measurements from that. Though I guess it's more computerised nowadays. Boat builders loft out with full sized drawings the same.
Google "boat building lofting out" and you will see examples
Some make 3d models full size to check for accuracy and take off measurements. I've seen ships engines modelled like this.
It's more or less universal but woodworkers seem to have lost the knack.
The woodworkers datum is the face/edge mark, which is an essential part of the rod system.

cheers
Jacob
PS the point is if you "measured" everything on a big job as I described earlier it would be extremely tedious to keep having to do it, and error would be bound to creep in. And you'd have to keep checking, recalculating. All those little back of envelope notes! Infact quite impossible. Hence the rod - you measure/calculate once only, or not at all; because the measurement/calculation is done for you by the graphic process.
e.g. in a big job you might have say 20 rails, meeting rails, cills, all the same length all with same spacing of mortices/tenons etc. Are you going to make 20 measurements, and then 100 sub measurements for the mortices etc? Not bloody likely - once is enough! On the rod, and marks taken off.

 

[bg]

Thank you Mr grimsdale for reminding us about using a rod. Of course it is something I know I know of, but its not something I've remembered about when I've actually been making something. Well I hope I will remember not to spare the rod for my next making effort.

And, as I'm just about to make a few saw benches so as to put a frame on them to facilitate cutting sheet material at a respectable hight, your design of a simple saw bench could be just the ticket. But then I look closely at those compound angles and see its not simple at all. Those legs slant back as well as outward. I would guss the interesting bit is fitting the legs to the top as you have to grove the top out in two angles. Still trying to work out how to set my CSMS to achieve it. Its going to be an interesting excerise, nerver thought a saw bench would be so challenging.

 

[mr]

Praps I'm just stoopid but as far as I can see a rod is the only way to do it. From the outset of my life in wood mangling it just seemed obvious to have a bit of stick with marks on it at the relevant places. That way all components are the same size every time without fail and without thought.
Mike

 

[Anonymous]

Thank you Mr grimsdale for reminding us about using a rod. Of course it is something I know I know of, but its not something I've remembered about when I've actually been making something. Well I hope I will remember not to spare the rod for my next making effort.

And, as I'm just about to make a few saw benches so as to put a frame on them to facilitate cutting sheet material at a respectable hight, your design of a simple saw bench could be just the ticket. But then I look closely at those compound angles and see its not simple at all. Those legs slant back as well as outward. I would guss the interesting bit is fitting the legs to the top as you have to grove the top out in two angles. Still trying to work out how to set my CSMS to achieve it. Its going to be an interesting excerise, nerver thought a saw bench would be so challenging.

Hi BG
Yes you've got it , well nearly. To get the legs to align at right angles in the housing, or to make a rectangular footprint, the cross section has to be a rhomboid.
Conversely - if you use a rectangular section for the legs then the footprint becomes rhomboidal, and the housing angles adjusted accordingly.
Doing it the first way is neater and easier. To work out the dimensions and bevels (the dihedral angles) you can do it graphically with a full size drawing with projections etc (just fits a piece of A1) which becomes a 2 dimensional rod -you lay pieces on and take marks off.
Or you do it with trigonometry, or roofing tables/squares etc. Infact it's the classic basic exercise in roof cutting.

Praps I'm just stoopid but as far as I can see a rod is the only way to do it. From the outset of my life in wood mangling it just seemed obvious to have a bit of stick with marks on it at the relevant places. That way all components are the same size every time without fail and without thought.
Mike

No it's the others who are stoopid :lol: or just plain ignorant! What you describe is the basic one dimensional rod , but at a more complicated level it becomes a powerful graphical calculator and setting out device. Can move into three dimensions with pattern making etc.

cheers
Jacob
PS when I say "device" this doesn't mean you can find one in a gadget catalogue, covered in brass knobs :lol: . I realise this may cause some dismay, especially to those who see the solution to every woodworking problem as the purchase of another gadget!

 

[Anonymous]

Jacob

You still don't get it and seem to have a very closed mind - a rod is not the best solution in all cases, just because you find it useful making windows!

Most members make small items of furniture, not windows with 100+ components and they make each piece to fit as they go along due to inaccuracies in the making, or developing skills.
Sure a professional joiner who makes batches will find a rod useful, but mostly, we make one-offs.

It must just be me, but I find simple trigonomety (especially with a calculator) and the use of a ruler quite easy and plenty accurate. I often wonder about the claims of 'full size drawings' being more accurate too, as a bit of maths will accuratley dimension every component (10 decimal places on my cheap calculator); surely no-one in their right mind would take dimesions off a drawing? (probably made with a ruler in the first place) - made easier with CAD over the past 25+ years.

 

[Anonymous]

Jacob

You still don't get it.
snip
surely no-one in their right mind would take dimesions off a drawing? (probably made with a ruler in the first place)
snip

Well you'd better tell all the millions of different sorts of fabricators, who have been using the rod with variations, for thousands of years, that they've all been doing it wrong :lol:
Timber framers do it - layout on floor etc. Greek temple builders did it - full size drawings have been found faintly scratched on floors showing the layout of complicated bits, column entasis etc.
The really surprising thing is that modern woodworkers don't seem to know this simple and essential technique.
First experience of the rod system for some would be model aircraft making - the Keilcraft sort with balsa and tissue. You don't measure all those spars and bits; you lay them on to the drawing and cut or mark. You then go on to pin and glue on top of the drawing itself.
If you tried to measure or calculate all those tiny components it'd be utterly impossible.
You obviously never made model aircraft Tony. :roll:

cheers
Jacob

 

[Anonymous]

snip
as a bit of maths will accuratley dimension every component (10 decimal places on my cheap calculator);
snip

No that's where you have got it wrong. This is a key detail but hard to grasp if it's unfamiliar. A calculator does not "accurately dimension every component (10 decimal places )" .
It does it precisely to 10 dec places, but this is not the same as accurately; it could be out by 3 inches at the same time. The rod is about accuracy, the degree of precision involved is what you bring to it yourself, with the means available to you.
This basic misunderstanding probably accounts for the obsession with precision which features such a lot on this group e.g. recent thread about the precision of engineering squares.
Its about confusing precision and accuracy. The first does not give you the second.
Believe it or not - you could make a very accurate saw horse (as above), with precisely fitting joints and precise angles, if you had no measuring apparatus available at all other than a piece of knotted string! You'd also need a straight edge and a pencil.

cheers
Jacob

 

[Alf]

measure between the sides and add on 2x the depth of the groove/housing.

Finally !!

Dom

Erm, 'scuse me, but given that we're all busily being told off for calculating the measurements for our projects (regardless of whether we actually know of, appreciate and use a full size drawing or not...) surely having pinch rods clamped at the correct distance to measure off onto the work is to be preferred over unnecessary mathmatical addition wherein error can creep?

Jacob, I think most of us do actually get the use of a rod and we don't actually need to be told we're stupid or ignorant for still arguing with you. What you've failed to grasp is we're not arguing with you on the use of the rod per se, but on its effectiveness:

a) in this case, where there is no rod and the project is already started
b) in cases where, frankly, what we intend to build doesn't always turn out that way
c) given that the customer (SWMBO, TPTBs) is quite likely to change the spec after the project is started...
d) when one is paying for the materials oneself instead of a customer, and the tendency tend to work round any problems rather than enjoying the Utopian ideal of scraping that piece and starting again.

Added to which I still can't fathom why anyone would deliberately resist measuring the actual project if it was there, available, and not a significant issue to do so.

Cheers, Alf

 

[mr]

Added to which I still can't fathom why anyone would deliberately resist measuring the actual project if it was there, available, and not a significant issue to do so.

Cheers, Alf

I don't mean to be contentious but I would say that I deliberately resist measurement wherever possible particularly fine measurement. The problem is that my eyesight is so poor (praps this counts as a significant issue) that I find it hard to see the numbers on whatever measuring device. It's much easier for me to have another bit of wood (for example) to hand to compare something to. For example the edge of a raised panel needs to fit into an offcut of the groove prepared for it. Like so;

As I type I have a small block sat on my desk that I know to be 19 mm thick. I use this for setting cutting gauges for thicknessing stock and so on (and yes a lot of my projects use 19mm thick stock) . Anyway Im fast getting a name as a complete Luddite so I'll shut it now (he says busily tying knots in a bit of string - must try that out).
Cheers Mike

 

[Anonymous]

measure between the sides and add on 2x the depth of the groove/housing.

Finally !!

Dom

Erm, 'scuse me, but given that we're all busily being told off for calculating the measurements for our projects (regardless of whether we actually know of, appreciate and use a full size drawing or not...) surely having pinch rods clamped at the correct distance to measure off onto the work is to be preferred over unnecessary mathmatical addition wherein error can creep?

Jacob, I think most of us do actually get the use of a rod and we don't actually need to be told we're stupid or ignorant for still arguing with you. What you've failed to grasp is we're not arguing with you on the use of the rod per se, but on its effectiveness:

a) in this case, where there is no rod and the project is already started
b) in cases where, frankly, what we intend to build doesn't always turn out that way
c) given that the customer (SWMBO, TPTBs) is quite likely to change the spec after the project is started...
d) when one is paying for the materials oneself instead of a customer, and the tendency tend to work round any problems rather than enjoying the Utopian ideal of scraping that piece and starting again.

Added to which I still can't fathom why anyone would deliberately resist measuring the actual project if it was there, available, and not a significant issue to do so.

Cheers, Alf

Risk of going round in circles here. It's a bit of a hobby horse of mine. Mind you you should see this precise obby os i've just made with nowt but a piece of string and some mdf scraps!
BTW I'm not mad enough to argue against using a tape measure (it's not like honing jigs :lol: ), it's just how you do it - procedure etc.
And IMHO the obsession with precision on this group is really a misguided search for accuracy. It's not a snark its a boojum :shock:

cheers
Jacob

 

[Alf]

Mike, sorry, should have been more exact. When I say "measure" I meant with pinch rods, not a tape or rule. That way you have a set length you can mark from and no need for good eyesight. I'm all for eliminating numbered measurements in favour of stops, blocks, the mating piece etc.

Cheers, Alf

 

[StevieB]

Erm, tis a really bad idea to use a bit of string on the grounds that a bit of string stretches.

Back in the days of undergraduate biology we had a lecturer involved in measuring the lengths of pigs intestines (don't ask why) and since they are difficult to lay out flat (even when you take it out of the pig) he used a bit of string to measure them. Got some fantastically significant results from his two groups - mostly because the string stretched as he used it repeatedly.....

Accuracy and precision - semantics to my mind. I can cut a piece of timber precisely an inch short or I can cut it accurately to the mark I made which was an inch short. Doesn't matter what I call it - its still short.

Steve - now thankfully far removed from the distant days of pig intestines.

 

[engineer one]

gawd and i thought i could be contentious asking about other things :?

jacob you are absolutely right about the benefits of a rod.
HOWEVER. it depends on being taught or shown how to use one,
which we now have a better idea of. it also depends on what you are
making.

a lot of people got back into woodworking through using mdf, which can in principal be very accurate in both cutting and usage because it really does not move.

"real wood" MOVES and has knots and other failings. some of which you only find after cutting/planing and thicknessing. this is important to remember and understand.

as an amateur, you don't have much spare wood, so you work round the problems, which immediately makes a story stick kind of redundant. :roll:
TO THE AMATEUR.

once you have some experience, you can then move on to rods/story sticks and because of your skill will find them both useful and accurate.

there is another point, which is that my table has kind of grown organically from trying to accomodate the top and some other ideas to the somewhat simpler and i think cleaner and more attractive proposition now. that has meant changing things as i go, which is why i need to measure the grooves after making.

if you are copying something, or making it to fit a specific hole, then rods are vital because as we know tapes have movement in them. however, if you use pinch rods, you can still have errors if you mark the intersection incorrectly, or do not tighten them properly.

my request is not for complex 10 decimal accuracy, it is to combine the movement of the wood with non movement of mdf in a way that will not at some time in the future blow the joints and make the table fall apart :?
but it is for precision. which may not be the same thing.

nice to see mike back too.

paul :wink: