Protractor

[dickm]

Tried a search on the forum but can't find an answer. Discovered my 65 year old school protractor and equally ancient memory of geometry wasn't up to accurately setting the angles for an octagonal frame. Can anyone recommend an accurate but affordable tool for setting a measured angle. I've got a small M and W engineering protractor, but really need something a bit bigger

 

[worn thumbs]

You probably already own everything necessary.I find a good rule and a straight edge,when combined with a calculator with trigonometry functions is more than adequate.For accurate engineering work a sine bar comes into play and all you need to do is scale up the numbers to suit your job.

 

[NazNomad]

... if a sine bar doesn't work, resort to a wine bar. :-D

 

[custard]

Couldn't be easier. Which is why the octagon has been such a staple for generations of furniture makers. One instrument (a compass), just one setting, and you're golden.

Set out a perfect square, find the centre of the square, set your compass to the distance from the centre to a corner. That's the clever stuff over! After that you just reposition the compass point to each of the four corners of the square and strike off the points on the square that then give you the corners of the octagon,

http://dwise1.net/trivia/octagon.html

 

[Eric The Viking]

What will kids do these days, who haven't been taught simple geometry?

The only proper home-made hand tool I've ever really fussed over is a large-scale pair of compasses. Neat by my standards (rough by Mr. Maddex's et al), they are so very useful and easy.

Want better accuracy? Simply make bigger arcs!

E.

PS: I had to calculate the mitre angles for an octagonal roof (bird table) a while back. For that I did give up and use a spreadsheet! Horses for courses...

 

[Racers]

These are good http://www.trenddirectuk.com/dar-200.html or an equivalent. but as custard said a compass works.

One of the best tips is to glue up in two half's then trim the ends to fit.

Pete

 

[Eric The Viking]

These are good http://www.trenddirectuk.com/dar-200.html or an equivalent. but as custard said a compass works.

One of the best tips is to glue up in two half's then trim the ends to fit.

Pete

Ditto on the two halves thing...

... but... I had one of those digital angle gauges, and it wasn't correct. I didn't realise until long after I bought it (didn't actually get to use it for a long while), and it was too late to return it.

I now have a Gem Red one (oddly, on the desk next to the keyboard at the moment), which seems to be fine, as I am able to measure it. That said, the pivot isn't central to the arms, so it still fails a 180 test (for practical purposes).

I'm sure they all come out of the same factory, just with differing pass-fail quality standards. I dismantled the faulty one and discovered the electronic "protractor" inside (which it uses to count pulses) was mounted visibly off-centre. I strongly recommend testing these things carefully before relying on them (and the electro-magnetic "bevel box" levels for sticking onto tablesaw blades).

In the case of the hinged angle gauge things, mark out 30, 60, 90, 45 degrees with a compass and check all round the circle (any error should reverse at some point). They also rely on the parallel nature of the two edges of each arm, and the two arms being of identical thickness - also worth measuring as accurately as you can, as any differences in the two arms will affect accuracy in daily use (e.g. you mustn't try to zero at 180, because there's a parallel offset).

It's useful, but the design of the things (from whomsoever you buy them, and whichever brand they are) is deeply flawed, and I certainly don't trust mine - I use it more as a mitre gauge than a measuring tool.

E.

PS: It's similar to the introduction of digital meters in electronics years ago - just because the display has a decimal point it doesn't mean the instrument is reading correctly. I have three DMMs and two moving coil meters. One DMM is lab quality (HP3478A, which I have reason to trust), one moving coil meter is an AVO 8. Every single one of them reads differently, and has errors in different ways, and crucially none of the DMMs (bar the lab one) are adjustable.

 

[bugbear]

The traditional way is to represent your angle as a ratio (or, equivalently "rise and run") and use a roofing square.

BugBear

 

[custard]

Re-reading the OP's post made me think a bit more about this, and it occurred that it illustrates one of the differences between theoretical cabinet making and how the craft is actually practised in the real world.

Googling the subject shows lots of examples of people laying out an octagon direct onto the workpiece. But in professional workshops you'll hardly ever see the laying out of an octagon done on the workpiece itself. The default method is to use the rod or a piece of MDF, ply, hardboard, or even a sheet of newspaper for the laying out and then to transfer the octagonal corner marks onto the workpiece. This brings several advantages,

-no risk of the pin of a compass marking the workpiece
-you're not trying to balance the pin of a compass on the very apex of the corner of the workpiece
-if you're making a table top, especially with hand tools, you'll be unlikely to hit the rod dimensions precisely, forming a perfect square will often leave you out from the rod by half a mill. You can then reflect this onto the drawn square and still make a perfect octagon
-if you're making an octagonal frame then by drawing it out you have everything on the same plane, and can lift the angles and more importantly the lengths direct from your plan.

Good luck

 

[Jelly]

What will kids do these days, who haven't been taught simple geometry?

Trig!

The only proper home-made hand tool I've ever really fussed over is a large-scale pair of compasses. Neat by my standards (rough by Mr. Maddex's et al), they are so very useful and easy.

Want better accuracy? Simply make bigger arcs!

The one thing I resent about my otherwise excellent schooling was that the graphical methods of calculating things which are useful in real life, were glossed over for the numeric and algebraic methods more useful in pure maths. This is no more evident than in Geometry and Trigonometry...

What's even more annoying, is that if you look at the graphical methods with your pure maths head on, the relationships between functions, and why formulas are what they are all click into place, then you remember it because you understand, not because the man at the front told you to recall it!

 

[dickm]

Thanks, guys, all useful stuff. Was going to try the "two halves" method, but in the end got the bits good enough for a single glue-up. Like others have said, setting up the 67.5 degree angle using compass was standard teaching back in the 50s, so no trouble with that. The difficulty was then transferring this angle accurately to the sliding bevel and to the mitre gauge on the saw. Which was where a really accurate protractor would have been useful, but it sounds as if the digital ones are a bit of a gamble. Had forgotten about sine bars - there's one in the engineering tool chest but have only ever used it once for real.
If I was doing this more often, it would be worth making a set of templates for the the common angles. Maybe when I retire........... oh, darn, I am retired!

 

[Racers]

I have a couple of these http://www.pullingers.com/p/926433/250m ... 0wodNRYJ2g

But this one looks better.

http://www.theofficesuppliessupermarket ... Gwod25kImw

Cheaper

https://www.graphicsdirect.co.uk/linex- ... GwodWJICgg

Pete

 

[Eric The Viking]

Wow, Pullingers -- still going. My first fountain pen came from them, almost fifty years ago.

 

[Jelly]

If I was doing this more often, it would be worth making a set of templates for the the common angles. Maybe when I retire........... oh, darn, I am retired!

They're handy to have about, I have 22.5°, 45° & 90° reference blocks, and am building up a collection of Ply templates for setting out complex shapes.

 

[monkeybiter]

If I was doing this more often, it would be worth making a set of templates for the the common angles. Maybe when I retire........... oh, darn, I am retired!

They're handy to have about, I have 22.5°, 45° & 90° reference blocks, and am building up a collection of Ply templates for setting out complex shapes.

Added to my 'to do ' list.

 

[CHJ]

For simple layouts I often just look up Octagon or whatever Polygon and print it out as a stick on pattern.
For smaller items it's often more than good enough to cut or finish to the line.

 

[Billy Flitch]

I find this whole thread fascinating Custards method with a compass on sheet material or table tops is so simple that its gob smacking and in the link he gives I find it some how strange that the guys farther had worked in the ship yards.
I know where Jelly is coming from when he talks about the competition between graphical and pure maths, functions and maths, Geometry and trigonometry.
It wasn't till I was an Apprentice that the world of Geometry and trigonometry really opened up for me and I enjoyed every minute of it. Once a week a Shipwright Loftsman would come to the Collage To teach us the methods of laying a craft off and explain how to understand the scriveboards. I was ahead of the other guys at that because I had been working with the Loftsman all week. Through the week he would explain to me just how you must do it as a loftsman and because he was a Shipwright loftsman he was also a Carpenter and then he would explain how a Carpenter would do it.
Sometimes the saving in time or effort was mind blowing. It was then that I started to understand that there was no competition between Geometry and Trigonometry in fact they complement each other. As a Carpenter you can get along with knowing one and not the other but it becomes all so easy if you can understand both.

Later the old hands in the spar shed taught me how to lay out masts, spars, oars. All with a spar gauge all so simple, just as Bugbear says with ratios the setting is 7: 10: 7, I`LL post a link saves me typing it all. the beauty of the gauge is that very few mast or spars are parallel but the gauge moves on both sides and keep the ratio and gives you the octagon that you want is it spot on, no I believe it is 7 hundredths of what ever measurement you use out, big deal.
My eyes will see a 1/16 but my hands will feel a 32nd thats close enough for me. Scroll down the page and you will see the gauge
http://www.pettigrews.org.uk/lm/page030a.htm

 

[Billy Flitch]

I thought I would show you where I last used the gauge on a chisel handle this type of handle is often called a London

patent but I prefare it when the handle is tapered it just sits so better in the hand.

and on the bench my spar gauge