Thread: Combination squares fitted with 600mm blades?

Hello JC,
in theory, if the combination square is square at 300mm it will also be square at 600mm.
Of course any tiny imperfection that is hard to see at 300mm will be greatly magnified at 600mm.
Personally, I find combination squares fine for construction work, but are too inaccurate for cabinetmaking. I wouldn't add a 600mm ruler to mine because I know its limitations. If I want better accuracy in a longer square I use my builders square. It is one piece steel and much better for me.
Hope this helps
SG

Originally Posted by Scribbly Gum

Hello JC,
Of course any tiny imperfection that is hard to see at 300mm will be greatly magnified at 600mm.
Personally, I find combination squares fine for construction work, but are too inaccurate for cabinetmaking.

Hi and thanks, but is that the case even with the better combination squares such as Starrett?

Originally Posted by johncee1945

Hi and thanks, but is that the case even with the better combination squares such as Starrett?

Hmm, can't say.
I haven't got a Starrett, only a Stanley and a Rabone.
Anything that has moving parts and a fixing mechanism will be prone to some wear, and hence some inaccuracy I would think.
Others might have had excellent results with a 600mm adjustable, - it would be interesting to hear some feedback.
Cheers
SG

Starrett has a reputation of being one of the finest toolmakers in the world (I have a few of their tools and I think this reputation is well deserved) and even if just a fraction of the accuracy of their Master series (squareness to 1 in 60,000) follows through to the rest of their tools, I would trust it.

Having said that, they're also much more expensive than a Stanley or the like and a good one-piece framing square should be perfectly fine for a fraction of the price unless you want to work to fraction-of-a-mm tolerances.

Originally Posted by johncee1945

Hi all I am thinking of purchasing a 600mm (or 24 inch) blade for my combination square. I understand that they retain squareness with a 300mm blade, but I am not sure about 600mm.
My question being, are they accurate for marking out work - out to 600mm?

I bought an 18" blade to go in my Starrett square, but very rarely use it.
I find the 300mm very useful and square, but not as useful as the 150mm combo square.
I also have a LV 4" machinist's square, which is also very useful and accurate.

If I had to state a preference, it would be
150mm combo
100mm machinists
300mm combo
150mm machinists
-- it's frequently useful to have two squares with the rule set to the same projection
300mm combo fitted with an 18" rule

The problem with accuracy isn't in the square itself but in the size of the head of the square. On a narrow piece a bit of deviation wouldn't matter, but when you are laying out a line almost two feet long you need a longer reference for the head to counter any deficiencies that may be present in the stock. There may be a bit of snipe on the edge of a board or a slight bow which would throw the long edge off making it out of square. If the material being squared has a perfectly straight clean edge (more typical of metal working) than this wouldn't be a problem. The framing square with its long legs makes for a more accurate layout as it spans minor deviations in the edge being squared from.
Mike

I bought a drafting 'T Square' for my longer requirements, and have found that it works very well. Quite an inexpensive item from officeworks.

theres a very high chance 600mm will b out of square plus it looks funny.

Originally Posted by chrisb691

I bought a drafting 'T Square' for my longer requirements, and have found that it works very well. Quite an inexpensive item from officeworks.

Hi all and thanks. After the points raised here I will not go with the combo square and 600mm blade. It would mean laying out about $140 for the Starrett 600mm blade which would not be 90 degree accuracy!
Instead I have some spare, reasonably solid, aluminum u-shaped channel which I can make into a T-square of sorts. Anyway, it will be a good long square for layout work and I can add a stick down tape. The tricky part getting it square but I have a good 400mm engineers square to set it up. So thank you all kindly.

I believe that square makers are only required to garrentee their accuracy on the inside of the blade....I know this was the case when Fine Woodworking did a review and visited square manufatcures across the USA, including Stanley and Bridge City Tool works. At the Time Stanely's premium "framing" squares were tuned by hand using a wooden mallet and a wooded anvil. Infact their premium square was aluminium as it had much better memory than the steel....

cheers,
Serg

Have a look at the aluminium Gyprock "T" squares [branded Gyprock Wallboard 10432] - mine is an original Gyprock branded one and it is and always has been spot on - size is 560 x 1200. Over the years I have seen some unbranded ones but couldn't comment on them. I also have a shorter aluminium version [unbranded] which I purchased from a drafting shop which is also dead accurate - size is 300 x 610. Both have a ledge which goes against the sheet/timber for accurate alignment rather than like a flat builder's square.

Regards,
Bob