Rudimentary precision? an oxymoron?

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Forum topic by curliejones posted 04-04-2015 06:16 PM 1069 views 0 times favorited 9 replies Add to Favorites Watch
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145 posts in 1685 days

04-04-2015 06:16 PM

Topic tags/keywords: measuring straight edge square precision

I love nice tools! I’m not alone, considering the company I keep here on LJs. But one or more of you simply state that, the less I spend on Tool A, the more I can spend on other necessary tools B and C etc. Some or many of us have a budget, whether written as a fixed amount or imaginary, in that we can’t see spending $50 for a good tool if a $15 tool will do the job.

I have a building fairly completed and I want to again begin my woodworking journey, but with aforethought to a higher degree of precision and pleasure. I am thinking through lots of details as I set up shop and buy some hand tools toward “hybrid” woodworking.

Over the last couple of days I’ve reviewed some basic principles of measuring and layout and here are some Q & As along with some of the thoughts that prompted them.

Thought: I often see folks referring to using the “factory edge” of plywood as though the factory could be trusted to the end of time. Hmmmmm?
Thought: I see folks insist that one must have a precision straight edge and a 90-degree square before your first step.

Remember, I love nice tools but for those of you with some limit of funds, I’ll offer an idea or two. We all need a straight edge that we know is straight and a square that we know is accurate for 90 degrees. I tried the following this morning after buying a pair of brass stair gauges. I took an aluminum straight edge that lives in the shop, a yardstick 36” long, that cost less than $2. I carefully butted it to the edge of a nice piece of 1/2” cabinet plywood and drew a line with a 0.5mm lead pencil. I then flipped the yardstick and traced that same edge from the other side. Any variance in my line should represent twice the flaw or error from a straight line.

I’m happy – I have a true straight edge for my first point of reference. I then took two quick clamps and carefully clamped the straight edge onto the 2 ft x 4 ft plywood panel. Next I retrieved my aluminum framing square from it’s throne and attached my latest investment in precision, a $4 pair of brass stair gauges. Rather than buy an attachable “fence” for my framing square, I wanted to see if the stair gauges could work to a similar end. I tried two different methods for checking the 90-degree-ness (hey new word!) and used the stair gauges to provide a stop by attaching both gauges to the short leg of the framing square.

I indexed the two brass gauges to the plywood edge, drew a fine line, flipped the square and drew again. Pretty close! Hmmm. This time I indexed the stair gauges to the aluminum straight edge and drew a fine line. When I flipped the square, the lines were in perfect alignment.

What did I learn? If you have a straight edge, don’t worry that it did not cost a fortune; just check and be sure it is straight. Use that straight edge to prove whether or not your framing square is good enough for layout of panels. Lastly, regardless of how straight the edge of factory plywood, the slightest bump or divot can throw you off 1/32” over the 24” of travel. It’s better to reference from the smooth aluminum straight edge (the one you know is straight). Leave the stair gauges on the framing square once you have achieved nirvana! I fully expected to have to shim one or the other for this experiment, but that was not necessary.

Now I can go on to wishing for some other shiny useful object since my old stuff works just fine.

-- Like Guy Clark sez - "Sometimes I use my head, Sometimes I get a bigger hammer"

9 replies so far

View Richard H's profile

Richard H

483 posts in 1099 days

#1 posted 04-04-2015 06:38 PM

The one problem with cheaper measuring tools I find is that often it’s a crap shoot when you bring them home if they will be a good one or a bad one. I have a few framing squares from the big box store that are really good but I also have one that is pretty far off for furniture work although it’s fine for building decks. And a straight edge and framing square are pretty simple measuring tools. I have had pretty bad luck with combo squares from big box stores in general. Even if they are square at a specific point if you adjust the end at all you might throw it off by a lot. I have a Stanley combination square that is ok as long as I never extend the ruler part closer to about a inch from the end. If I try and butt it up all the way to the head it goes off by almost a 1/8”. These are problems that a good high quality tool at several times the price just won’t ever have with normal use. Now I could keep taking these things back and buying new ones or messing around with the castings trying to get it to work better or I could just live with the limitations the cheaper tool imposes on me. Or I could pay extra for a tool that works well right out of the box 99% of the time.

Having said that there is a bit of over obsession I think sometimes about precision in woodworking. Using highly precise measuring tools is a fairly new thing to woodworkers that older methods of work just didn’t worry about. Don’t get me wrong there are times where it’s important to have square and flat reference surfaces to make good joints but it’s easy to over obsess about it to. Is it really required for instance to have a table leg exactly square on all four sides when only 2 of those sides are used for joinery? The outside two sides only need to be close enough unless they are reference surfaces used by some machine during the milling work. Same with a table top compared to it’s edges or bottom. Only the bottom truly needs to be flat so it sits on the base, the top and edges only need to look good and be close enough to fool the eye.

View Bill White's profile

Bill White

4405 posts in 3379 days

#2 posted 04-04-2015 08:16 PM

And just to think that the pyramids were made without any known exquisite devices. How they could have possibly done that?
I guess that my point might be how did any woodworker ever achieve the degree of quality evidenced in our cherished antiques without the ability to measure to within the nearest micron?
I’m with Richard on this subject. How much is enough?
We’re not sending objects into space to target another moving object. I still can’t fathom that technology. I’m still not that good shooting clay targets with a shotgun. :)


View Gerald Thompson's profile

Gerald Thompson

804 posts in 1653 days

#3 posted 04-04-2015 08:22 PM

The enemy of damned good is just a little bit better.

-- Jerry

View Jerry's profile


1709 posts in 1067 days

#4 posted 04-04-2015 09:52 PM

I think that precision is very important when you are making boxes of any kind, whether they be jewelry boxes, or cabinet carcasses or drawers. When you have to make 6 sides fit, then precision becomes absolutely necessary. More forgiving, however, are the examples Richard H cites for unnecessary precision, and his statements absolutely have merit. It just depends on what you are doing. If you are making box joints, you have to be within .003 or they just won’t fit. As far as the pyramids go, I’m pretty sure ancient aliens did that stuff ;-)

You hardly ever find a carpenter’s square that is square, but it’s easy to fix, there’s a video that shows how right here.

View on YouTube

-- There are good ships and there are wood ships, the ships that sail the sea, but the best ships are friendships and may they always be.

View Ghidrah's profile


667 posts in 641 days

#5 posted 04-05-2015 02:51 AM

The pyramids were built with precision tools, string, plumb bobs and water levels. It’s hard to find or make a tool lore precise than gravity.

-- I meant to do that!

View curliejones's profile


145 posts in 1685 days

#6 posted 04-05-2015 11:48 AM

Greetings! I appreciate the link on how to “fix” or adjust the framing square. I do find it necessary, though, to use a known straight edge, preferably a metal one, rather than the factory sawn edge of plywood or composite board. I got different results with the brass stair gauges once I used a known metal straight edge for reference, albeit 1/32”+. Using the short leg of a framing square along the reference edge and flipping it puts the two outside points 32” apart. My results were repeatable and 1/32” to 1/16” off. When I used the metal edge that I had “proofed” as described above. My lines were exactly overlapping. Could have been operator error? Maybe, but the metal edge made it easier to feel the stair gauge stops.

Like Richard says above, it is a crap shoot with inexpensive measuring tools, but in my case I found a couple that are dead-on. Before you newbies out there go banging on your framing square, just be sure it needs it. On the other hand, before you blow your monthly allowance, check carefully and be sure you need to.

If you have a couple of measuring tools that are capable of precise measurements, treat them nicely and don’t drop them. In the labs where I worked we had reference materials, weights, etc. that were only brought out to check for explicit accuracy at the beginning of a project, the results were dated and recorded. I suppose this practice seems stringent, but yet a good idea to protect the expensive or inexpensive precision instruments.

-- Like Guy Clark sez - "Sometimes I use my head, Sometimes I get a bigger hammer"

View MrRon's profile


3891 posts in 2662 days

#7 posted 04-06-2015 04:36 PM

I do my woodworking to machinist precision. Why? you ask. Because I can. I don’t always do so for rough projects, but in the model building area, precision is very important. Making dovetail or box joints requires a pretty high degree of precision if you make them by hand. I have metal working machines and all the precision tools common to machinists. For my woodworking, I took a cheaper combination square that was out by a 1/32” in 10” and machined it to more precise dimensions. That is the one I use out in the shop, not my expensive Starrett square. That I keep only for reference and for metalworking.

View ksSlim's profile


1203 posts in 2308 days

#8 posted 04-06-2015 04:57 PM

Agree with Curlie.
As a retired metrologist, keep the most precision tools as a reference.
In my experience, a Starrett combo square, used in a mfgr., assembly environment, can be used or 30 yrs and still pass certification.

Bottom line, use the best you can afford. Double check the cheap tools.
Good lumber/wood isn’t cheap.

-- Sawdust and shavings are therapeutic

View MrRon's profile


3891 posts in 2662 days

#9 posted 04-07-2015 04:36 PM

Just a thought; I bought a CHEAP 1” micrometer years ago. It was made in the USA by General tools. The cost was around $9. It doesn’t compare with my Starrett or B&S mics, but I can toss this mic on the bench and not worry about ruining it. If it is off a couple of thousands, no big deal. I also use a cheap HF digital caliper; again no comparison to my Mitutoyo caliper, but precise enough for woodworking. With these cheap “precision” tools, you can achieve pretty good woodworking precision.

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