How flat is flat............. ?

[Matthew N. Masail]

:confused: yep.... that face, that's where I am. I set out to tune up my combination square, I used sandpaper on a flat piece of glass, then lapping grit because the sandpaper was too short. I checked it against my new Veritas aluminum straight edge held up to a light. when it was very flat (couldn't get it perfect - went back and forth from almost, to not very flat) I used finer sand paper to try and fine tune it and polish. after a long time not getting it perfect, as before, I decided to try a used my bandsaw table which also checked out flat, this got it a little better, but still not perfect.
so out of curiosity I checked my new Veritas precision square, and it was more out of flat then the rule I've been working on! and so is the lapped apron plane... which works wonderfully with whisper thin shavings... so now I'm confused... am I doing something wrong? is everything except the straight edge out of flat ( I have 2, both read pretty much the same), or am I trying to get it to too tight a tolerance.. ? I was under the impression that it wasn't too hard to get something flat... but I'm having a real hard time with it.

[David Weaver]

Is the apron plane one of the ones that gets a rotary lap? if it's cloudy gray on the bottom with no perceptible machining lines, there's no way that it's out of flat.

I'm going to guess your rule that you're sanding looks high in the middle and falls off at the edges?

[Matthew N. Masail]

the apron plane doesn't show many machine marks, but It's not gray like to router plane or little victor. the rule dips more in the middle or at random spots. I've been using single strokes not pressing on the edges.

[James Owen]

As long as your combination square is flat to within a couple of thousandths of an inch over the length of the rule, it's more than adequate for woodworking.

If you really need .0001" or better tolerances, then take a look at the Starrett or the Brown & Sharpe combination squares. Both are exceptionally good quality, but be prepared to have your wallet lightened significantly: a 12" Starrett starts at about $80; a 12" Brown & Sharpe starts at about $110.

*****

Getting stuff truly flat is very difficult, very labor intensive, and very expensive: it's one reason why surface plates, for example, cost so much. It's also one of the reasons why many woodworkers use sprung joints for edge glueing: it's a lot easier to deliberately put a controlled, slight amount of concavity into the edge of a board than it is to get it totally "flat." But getting back to your combination square: it might help to keep in mind that our ancestors made exquisite and ornate furniture, using wooden squares that weren't necessarily all that flat by modern standards, so I wouldn't get too worried about whether your combination square is "perfectly flat"; practically speaking, all you really need is "flat enough".

[Tony Shea]

Getting the rule part of the square flat is really only part of the battle with tuning the combination square. You need to get both edges very parallel in order for you to get a square square. These can be very tricky to tune up and could result in more work than it is worth.

[Sean Richards]

First point for doing something like this drawfiling is a far superior method. Second point is that despite what you might read on forums such as this you don't need anything perfectly flat, straight or otherwise to make beautiful furniture.

[george wilson]

Is the rule on the comb. square HARDENED? If so,no draw filing. Combination squares are not the most accurate of squares. Many here seem to use them as primary squares,though. They get out of square from the blades getting slid. The ones with hardened blades AND stocks are the best. If you really want a more accurate square,get a solid steel 6" or 8" Starrett that is not adjustable. Any adjustable square will eventually get out of whack for real precision work. Bridge City addressed the problem by inserting hardened steel pins into the track of their combo square's stock,but the best combo is still the all hardened one.

I think every one should try to get their own little set of master squares and straight edges together. My ultimate squares are 3 granite squares. I have Starrett and Brown and Sharpe squares from 2" to 2 feet long,and a 30" granite straight edge,plus other scraped camel back straight edges,and granite surface plate. But then,I'm a tool pig anyway,and a machinist,too. AND I didn't pay 10 cents on the dollar for most of them.

Most of my life I made do with steel Lufkin rules,with BIG,FAT lines in them(how did I ever accurately measure sixteenths with them?) And,I made wooden squares to use when in the instrument shop. Most instruments are a collection of curves,though(except harpsichords.:))

[Matthew N. Masail]

Thanks guys... I'm glad to have my struggle validated as normal by you. everyone talks about lapping planes and such, I got the impression it should be easy. also David Fink talks about the combination square in his book about plane making (great book) and says it's fairly easy. there should be some warning about this, it can drive someone who doesn't know better crazy! I was wondering if it's me. funny thing though, wood tests perfectly flat.... compression?
I don't specifically want a perfect square, I just want what I need. my most interest at the moment is plane making, and classical guitars , which are the reason this all began for me. Thanks again... ! I'm finding that flattening the back of a plane iron isn't as simple as implied either.

[Sean Richards]

Thanks guys... I'm glad to have my struggle validated as normal by you. everyone talks about lapping planes and such

I think you will find it is mostly people on internet forums who talk about lapping plane soles etc ...

[Matthew N. Masail]

I see your point...

[Prashun Patel]

Each person has to determine how flat (and square) is good enough for them. I can usually flatten my planes about 95%. There's always a pesky area that I can't get to. So after a little time, I just chuck up the blade. If it performs as does yr Apron, then it's good enough for me.

As for the square, try using it to tune yr mitergauge on the tsaw. Cut a four sided rectangle from a scrap and use the square to confirm that all corners are 'tight without light'. If that works, then does anything else matter?

[Dave Lehnert]

Getting the rule part of the square flat is really only part of the battle with tuning the combination square. You need to get both edges very parallel in order for you to get a square square. These can be very tricky to tune up and could result in more work than it is worth.


+1 You may be getting the ruler flat but my guess it is going to be out of square worse than it may have been.

[Michael Peet]

I think every one should try to get their own little set of master squares and straight edges together.

Quality squares (both combo and machinist) and straightedges were among the first items I acquired when getting into this. I am guilty as charged with regards to using the combo squares for most day-to-day work, however. The machinist squares live in their boxes and are used for reference only. The combo squares are just too handy though, as long as they stay true.

I would chuck that square, chalk it up as a lesson (there are lots of spendy ones in this hobby, seemingly), and get a new one.

Mike

[george wilson]

Chris Vesper makes very accurate squares. 2 from him arrived today. Very careful work,made to mate against high precision standards such as cylinder squares.

[Sean Richards]

Chris Vesper makes very accurate squares. 2 from him arrived today. Very careful work,made to mate against high precision standards such as cylinder squares.

Thankfully as I don't (and can't and even if I could I wouldn't) work to 20 micron accuracy when woodworking I can get by with something quite a bit cheaper ...