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Tolerances: when is something good enough?

Blog by Rob Brown
Strong as an Ox

Every type of trade has a different level of accuracy that’s needed for the work being done.

For example, a carpenter can be off by 1/4” or more in many situations and not have the finished result effected in a negative way. On the other hand, if a machinist was off by 1/8″ in most situations the results would be completely inadequate.

I’m not saying one is more skilled or important than the other, just that tolerances are specific to the trade. On top of that, tolerances are also not set in stone. Just because a carpenter can get away with being 1/4” from time to time, doesn’t mean they always can. Same with a machinist. Although there are very strict industry-wide tolerances that generally need to be adhered to, once in a while, while working on a unique job, strict adherence to tolerances isn’t needed.

Tolerances for 2x4 shelving

Some woodworking tasks need extreme tolerances, while others just need to be “close enough”. The trick is learning when the “close enough” approach should be taken. This mainly comes with experience, but it’s also specific to the person. Every one of us has a slightly different tipping point for going between “this needs to be perfect” and “this is close enough for this job”.

I find I live more on the extremes. I start with “this needs to be perfect” and stay there for a long while. Usually too long, to be honest. But once I switch over to “close enough” mode, I’m fully committed to the rough end of the spectrum. Gaps and mis-matched joints are completely acceptable, as long as the final project will function as needed.

Dealing with other people’s work

We’re in the midst of changing around a lot of storage in our home. This means everything from nicely finished wall units to rough and tumble 2×4 shelving is being constructed. We had some 2×4 shelving in one room in the basement, but we wanted to move it to another room, also in the basement. The size of the two areas were virtually identical, so I thought I’d just take it apart, move it to the other room and put it back together. I eventually did exactly that, but the process was interesting.

These 2×4 shelving units were made by someone else and have been in place for about a decade or so. In my mind, they looked a bit overbuilt for what they were, but that’s better that being underbuilt. Once I started to take the unit down it was obvious I was right. Loads of screws and extra 2×4 supports made breakdown slow and tedious. Slowly but surely I was winning the battle.

It was once I was able to remove the 8’ long x 16” wide x 5/8” thick plywood shelving material that I realized just how overbuilt it was. Each piece of plywood was (very thoroughly) fixed to the 2×4 frame with about three dozen 1-1/2” x #8 screws. My first thought was how about a half dozen screws would have sufficed at keeping the shelving from shifting, and gravity (and the contents on each shelf) would have done a great job at keeping the shelving down.

Although it’s hard to complain when something is made too strong, my mind did go to how much faster and easier this would have been to build in the first place if they just used a regular amount of hardward and 2×4 material. It was also hard to not think about how much easier my disassembly job would have been if different tolerances were used during construction.

Those darn shelf supports

The thing that really drove this home was when the plywood was off and I was removing the supports that stretched between the front and rear rails. First off, there were too many supports. The supports could be omitted altogether, as far as I was concerned. With only a 13” gap between the front and back rails, 5/8” thick plywood could surely support whatever contents were placed on the shelf. This shelving unit wasn’t made to store an anvil collection, after all. Large plastic bins of blankets, camping goods and the like wouldn’t be overly heavy.

Aside from the fact that they were there in the first place, their location was quite surprising. The location of each of these supports were marked accurately with a pencil mark at even distances across the 8’ span, then the supports were aligned with the pencil lines and carefully screwed in place. Yikes. If I was helping the previous owner build this shelving unit, I would have surely left at the end of the day having thought I’d wasted half of my time.

If I was going to add supports, I wouldn’t have marked their location, but just adding them between the front and back rails and quickly screwed them in place. But that’s just me. Thinking about this a bit closer, I likely would have marked the first section for a few supports, then realized that level of accuracy isn’t needed. The second section would have received randomly spaced supports. The third section would have received none at all, as I would have decided these supports weren’t needed and they were just slowing me down. I really am equal parts my accountant father, who meticulously planned and undertook each project with great care, and my down to earth mother, who was perfectly fine with a job that was rough around the edges.

What kind of builder are you? Do you plan and execute with precision and accuracy in mind or do you bang it together as quickly as possible? Or are you like me, who starts with accuracy and quickly gets frustrated with yourself for wasting so much time?

Strong as an Ox

This is the corner joint of the 2x4 shelving I spent too long taking apart the other week. My approach would have been to rotate the leg to the front face of the front rail and secure the lap joint with screws. Then the additional vertical 2x4 support wouldn’t have been needed. The side rail could have then been extended across the leg and screwed in place. I’m just being picky though.

Strong as an Ox

Too Accurate

This is the shelving unit with the plywood removed. Supports run front-to-back and are accurately marked. If I added them at all, their location wouldn’t have been marked.

Too Accurate
Published:
Last modified: April 16, 2024

Rob Brown - [email protected]

Rob is a studio furniture maker and the editor at Canadian Woodworking & Home Improvement. Instagram at @RobBrownTeaches

8 Comments

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  2. I’m in agreement with u. I consider myself a minimalist never using 2 screws where one will do. I hate it when I am taking something apart and there’s a thousand and one nails holding something together
    John

  3. My work bench is made almost exactly like that except my top was dense particle board. I have no fear of stacking any kind of weight on it because of the jack and king construction. I like order, so I would use a pattern not random. I have shelves in my garage built the same way. not quite so many screws though.

  4. I’m with you. As a newbie woodworker with an unfinished basement to play in and a desire to play with wood, the learning curve has been steep. One thing that I have learned (thanks, Steve Ramsey!) is that a ‘shop project’ – like garage shelving, carts for rolling woodworking machines around on, and the like – definitely does not require that level of precision. I am happy with square corners and ‘strong enough for the job.’ That said, adding some trim to plywood edges protects against splinters and, if I’m going to add some – which I do because I hate splinters and there are enough from other sources – it may as well look nice. During my first addition of trim (to a drill press cabinet) I learned that mitred corners are a big pain to get just right, so have opted for butt joints on subsequent builds, learning along the way that the front trim should cover the ends of the side pieces ~sigh~ Whether the trim is fashioned from 1×2 poplar or a ripped down 2×4, getting the fit right, then cleaning it up by plane and/or router, is very satisfying when it comes out looking good. On the over=built front, I once helped a friend remove an old worn out kitchen floor. We found that there was very little wood visible to us, as nail heads covered almost the entire area. It was clear that (a) there must have been a squeak and (b) nails must have been cheap. Our blistered hands and aching backs were testament to the hours it too to un-build someone else’s poor decision. Cool article!

  5. It isn’t overbuilt, its “I don’t have a feel for the strength” of 2 x4’s and I don’t want it to fail, and the marks for bracing is just needed to tap the brace at 90 deg.

  6. Hi Rob,
    I mostly disagree with your comments/approaches to woodworking / life. I’ve always believe and have raised my children with the idea that we should always try to do our best at what ever we do. If we don’t, we might not be able to when it’s important or we will say it’s good enough when our customers say’s it not, etc. So yes it might have been a little over built, but he doesn’t have to worry about it deteriorating before it is no longer needed. However, if I made a small error in cutting something, I would ignore it, as long as it didn’t affect the desired stability/safety of the structure.

  7. Looks like that shelf was built by someone who put strength and quality workmanship ahead of everything else. Screws – my favorite fasteners. I try to use them for everything for the simple fact that if you have to, you can take it apart and I have taken aparts things that I built over 20 years ago. Everything from deck screws, to drywall screws to standard wood screws. Deck screws, I use indoors for anything involving 2 by material. For out doors, I either use stainless steel screws or revert to hot dip galvanized nails as my experience tells me that the coating they put on deck screws does not protect for long and then rust takes over. Drywall screws are a cheap and effective fastener that will work on many projects. Although they are not as strong as the other two, they are usually plenty strong for most purposes.

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