Historical Accuracy

I have been building my workbench for some time now and as with all long projects, I get inspiration for changes and adaptations along the way.  I’d love to be able to say that I design a project from start to finish on paper or in sketch-up before I ever put saw to wood, but I just don’t work that way.  I have a pretty good concept of where I’m going when I start building, but only have specific designs for the part of the project that I’m working on at the time.  I often find that seeing the completed 1st part of a project, gives me inspiration in the design of the second step.  This sometimes changes or amends where I’m going.

So, now that the base of my workbench is complete, I was contemplating what part of the project to tackle next.  I was looking and some of the various benches in the plates in Andre Roubo’s L’Art Du Menuisier, when I came across this:

Gas Spring installation discovered in a Plate 11 workbench. From Andre Roubo’s “L’Art Du Menuisier.”

Gas springs!!!  Brilliant, why didn’t I think of this???  The frame and panel lid on my workbench base would be soooo much easier to open with gas springs.  Plus it will stay open when needed without some other latch mechanism being required.

I’m not sure if Andre Roubo ordered his gas springs from McMaster-Carr when these benches were made in 1789, in fact I’m not sure if McMaster-Carr has been around that long.  But I digress…  In any case, the McMaster-Carr website was exactly where I headed in search of historically accurate gas springs.

That’s were I hit a wall.  Holy crap!  I never realized just how complicated these are. There are thousands of variations, throw length, pounds of force, mounting options, etc., etc..  At this point I did a lot of online reading to try and calculate which springs I would need.  This reading lead me to believe that I was going to need a full size drawing if I was ever going to figure this out.    If Roubo had been truly conscientious, he could have at least engraved a part number somewhere at the bottom of plate 11.

The lid is 27″ and I drew it (and the stretcher height) full size, but all the important stuff is on the left side of this drawing. (Sorry for the crappy photo)

I drew a cross-section of the lid and stretchers at full size.

I found some mounting brackets and gas springs that I thought might work and used the dimensions of these parts to lay out the drawing.

I drew out all of the important measurements.

I drew the mounting bracket that attaches to the lid and then made a duplicate on a post-it note.  I used a yard stick as my lid and attached the post-it note in the correct position. By “opening” the yardstick lid, I could then measure from the mounting bracket on the stretcher to the mounting bracket on the lid to see if the gas spring would have enough throw.

The lid will only be able to open to 47° because at that point, the front edge of the lid will be touching the underside of the benchtop.  This is the main reason that I want to add the gas springs, they should hold the lid open, even at an angle.

I measured the length of the gas spring both closed and open.

The last variable was the pounds of force needed to lift and hold the lid open.  My frame and panel lid weighs 21.7 lbs.  However, in my drawings the springs will not be exerting this force at the center of gravity (COG) of the lid, but rather somewhere in between the COG and the hinge.  This increases the amount of force necessary.  I found conflicting information online about how to calculate this force, but I ended up determining that it will take 33 lbs of force at the bracket point drawn above.  Let’s be honest, this is a best guess.  Since there are two gas springs, I should need 16.5 lbs of force from each. Everything I read online said that you should round up to the next nearest size when ordering these, so I went with two 20 lb springs.  I hope it doesn’t prove to much.

The parts that I ordered were:

Part number 4138T543 – Gas Spring with Ball-Joint End Fitting, 20 lb Force, 12.2″ Extended Length, 3.94″ Stroke

And:

Part number 9512K95 – Ball Mounting Hardware, Zinc-Plated-Steel, 90 Degree Angle Inverted Ball Bracket.

I’ll let you know how the installation goes.

Oh… and a big thanks to Andre Roubo.  Your gas spring idea… Genius!!!

– Jonathan White