Metrology for the cabinetmaker

[Brian Holcombe]

Thanks gents, I appreciate the many comments on this thread. Really enjoyable to have a good discussion going.

[Brian Holcombe]

I wanted to touch on a few things, my background prior to woodworking was working at a machine shop. At the shop I was taught how to read mics, use snap gauges, dial bore gauge etc. After a while it becomes second nature and it's a speedy process. Putting calipers on a tenon doesn't take me very long at all. I have experience that stems from a machine shop so I tend to solve things in a machinist fashion when dealing with machine tools.

I had a few problems to solve that had really been creating annoying issues in my work, the most important of which was tooling height. For the Maka especially, the hollow chisel less so and the router much less so the height of the cutter is critical. I like to work off of a centerline for much of what I do and being on center is not the easiest thing to judge by knife lines. The Maka really brought this home because the tooling can't be easily brought up to the part and matched by eye.

The second was cutting squarely with both the table saw and chop saw. It becomes very easy to see wether or not a board is cut squarely when you can put an 18" grade A square on the corner and see plainly wether or not it's square (so long as your sides are flat). Cutting exact lengths is not quite as critical, but it does come up at times.

I build things like benches that have a stretcher spanning the distance and if the end boards are not exactly parallel to one another than the joints will gap at opposite sides. Actually it is that particular project which has caused me to ramp up the level of precision. There is no space to make up for a screw up, if I position something .010" off I will see a gap. If I cut a mortise oversize, I see it. It's a simple project, but it only looks good with very tight neatly made joinery. I cut the boards square then transfer them to the Bridgeport Knee mill for mortising. Very easy thing to do if your table saw cuts very squarely.

Precision tends to build on itself, if every tool is working within a tight tolerance then the work is fast and accurate, so I work toward getting everything within a narrow range and keep a close eye on things bettering them as I can afford to. Then when I work with CNC people like Jim Becker, it makes it easy because we're speaking the same language and I can provide very accurate parts for him to work off of.

When I worked mostly by hand I did not mind if every part was slightly different, but with many machines being relied upon my tolerance for variance is much much lower.

Certainly one begins to realize pretty quickly that wood provides a wider range of acceptable tolerance than say machining metal for a shrink fit, but the range can be understood and worked around effectively. I know when I run a large batch of tenons that I'm going to have a range of acceptable tolerance of .005". Fussiness can be avoided by understanding acceptable tolerance ranges and how to move the range to a point at which the parts work well, can be matched during final assembly or will simply have a snug fit/light fit that is within the tolerance of the glue being used.

The way I interpret something as being fussy is to mean that it is too time consuming. Certainly it is much easier to fuss over the first fit, then make the run and they all fit exactly as planned. It's much less fussy than messaging every mortise/tenon.

[Mike Cutler]

Brian

Nice article. Very well written.
Early in my career I worked in a mechanical standards lab, calibrating virtually every tool a machinist might ever need, so I also am a little biased to the need for quality measuring devices. I cannot imagine doing machine setups without quality measuring devices.
William Ng's Five cut method for squaring a fence is absolutely brilliant, but my large precision triangle has it square before the first test cut. Yeah, it cost a pretty penny, but now it doesn't take me a 1/2 hour to set my slider up any longer
It is true that the wood you cut today will be a different dimension tomorrow, but quality work,is still quality work.

[Jim Becker]

The bottom line for me is that ... nothing bad will ever come from having accurate measuring tools and methods, regardless of the material. The same goes for actually fitting components together based on the components themselves, rather than "measuring" and hoping something will fit on any given day, particularly with wood. The more accurate the setup of stationary tools is, the more consistent we can be, whether we are getting it right or making the same mistake over and over.

[Mark Bolton]

nothing bad will ever come from having accurate measuring tools and methods, regardless of the material.

That is until you come in contact with someone who becomes so obsessed with overly unnecessary accuracy, details, design, whatever, that they either never actually make anything because they are always working on those details, or they are never satisfied with anything they make because of obsessing with those details.

These conversations always remind me of a short stint having a full time pottery studio. There were several local potters in the area and we would all meet monthly to just talk about what we were working on. A couple in this group were very structured "artists" who had MFA's in ceramics, some others were individuals who had completed hugely in-depth full time 2 year programs in ceramics. Several of these people had simply educated themselves out of the ability to make a bowl, or a mug, the proportions were always wrong, clay body wasnt just so, glaze, on and on. They would never be happy, or furthermore profitable (they pursued this for a career path). Their pursuit of perceived perfection consumed all the time they had available to either enjoy their craft, or put food on their table which are the only two motivations to do any of this. Its either a hobby you want to enjoy, or a means to pay your bills. Of course your hobby may diverge into millionths level accuracy in the flatness of your cast iron table saw top, so I guess that works too.

Fussing over the details is great, until it isnt great anymore.

[andy bessette]

...my background prior to woodworking was working at a machine shop...

Prior to woodworking and boat building my background was design engineering. And only in my 70's did I finally pickup a couple machines I had long desired: an ancient knee mill and metal lathe to teach myself machining. Attached are photos of the first part I made (mostly on the mill) a boom end fitting for a sailboat. The metrology is still a work in progress.

[Mark Bolton]

That is some in-sane level manual machining for a lathe and knee mill. Super nice

[andy bessette]

Thanks Mark.

[Brian Holcombe]

Beautiful work, Andy!

[Mark Hennebury]

Nice work Andy.

[andy bessette]

Thanks fellas. Who says an old dog can't learn new tricks?

[Mark Bolton]

There seems to be an interesting sub-thread running through this thread _for me_ which may or may not be unique. I have been working with wood and construction for 30 years. I have an obsession with precision machining (you tube) though I will surely never be at that level, and also have a propensity to watch extensive youtube videos on sailing. Perhaps the problem is youtube, but I tend more to look to woodworking, machining, and sailing, as a common theme. Many of the sailing/cruising videos seem to be people with similar engineering/mechanical backgrounds. Perhaps its coincidence, perhaps its some goofy fascination with the mechanics of wind and fluid dynamics, perhaps its a notion for people of this mindset to want to get away.

Or perhaps its just a need for people who want to do something they are unwilling to pay market price for to figure out how to do it themselves. lol

[Brian Holcombe]

I've had jobs where one just shows up and does the work, mindlessly, and I can't suffer that. I need the wheels turning at times during the day to maintain my interest, exportation of various related subjects helps considerably.

I use for metal work also, so it's not just mental. Here are some nickel silver hinges with brass hinge pins. The pins are replaceable and precision and they're also custom made and exact.

It was entertaining for me to make this as accurately as I could possibly do on the bridgeport.

[Brian Holcombe]

I added these jaws to the vise for this particular job, they have pins which allow one to clamp small parts are precise typical angles such as 15,30,45. They are also able to used as stops and parallels and they're accurately parallel to the jaws.

[Mark Bolton]

And then comes the question with regards to if that were a paying job, and you didnt just make the hinges because your payment was the reward of making them, what would the material and shop labor cost be for a pair of hinges like that? My guess would be a full days work (if your fast) including drawing/sketching, plus materials. $300-400 a pair?