Archive for the ‘Drive train’ Category

Trimming the keychest

Thursday, October 28th, 2010

I put in the wheel for the first time since gluing on the back. I needed to check the fore-aft keychest measurement, because I thought I had the side pieces slightly too long, so that they extended past where the sides of the body attached to the head block.

The instructions say the center of the 7th key has to be exactly 6 15/16″ from the bridge. And the bridge has to be about 7/16″ from the wheel. So, checking all that with the pieces in place, I found that the sides were in fact too long by 1/8″ to 3/16″.


Chopstick serving as 7th key to measure proper spacing from the bridge

In the process, though, I discovered something else a little more bothersome — I had the whole keychest way too high. When putting a straightedge on the angle of the wheel, it rested right on the keyshafts! [At least it appeared I had cut the bottom of the sides at the right angle.] The best I could tell, the sides and end pieces needed to be cut down on the bottom by about 7/32″ to have the chanter strings the right distance above the keys. That’s almost a quarter of an inch. How’d I get so far off?

  1. When cutting the bottom edges of the keychest side pieces, I had overcompensated for the curvature of the soundboard. There is hardly any curvature at all to deal with 1 1/2″ either side of the centerline, even though I had doubled the curve amount from that specified. I knew I had to sand the keychest bottom to fit the soundboard curvature, but  I had about 1/8″ extra there — too much to sand.
  2. The wheel seems to sit lower in the body than on the plans. This is not a problem with the plans, but is related to some adjusting I did when making the braces in the body. I had added some 1/8″ thick wood to the tops of the large braces before rounding them. The reason I gave April 15th for this was…

You can see an 1/8″ thick strip of wood has been glued on top of the brace. The short brace is 4″ high, but the two larger braces were 1/8″ shorter. This modification to the larger ones makes all three 4″ high, which is the height of the body. This also makes the threaded hole for the bearing closer to the same height of the shaft hole in the end of the body. I could have sanded down more from the sides of the body and the top of the short brace, but decided to try this.

In hindsight, perhaps I should have made the small brace shorter and sanded down the entire top of the body more. Anyway, this obviously effects the wheel height, lowering it by whatever was left after rounding of that 1/8″ strip. You can see the strips I’m talking about here.

So, this accounts for why I was almost 1/4″ off on the first cutting of these pieces, despite all my attempts to accurately measure the plans. Jerry was good about letting me come over and re-do all these tedious cuts last night. Everything went well, thanks to his careful expertise and excellent tools.

Unfortunately, this necessary shortening messed up the nice double rabbet joint I had on the bottom of the keychest front. Now the base is thinner and weaker than the original piece. I will probably end up adding a second strip on top of the base to reinforce the joint.


The front piece after re-trimming, beside original piece

This front piece is now the same height as the original — what a coincidence! I was wondering why it was so much higher before. Hopefully, that’s a positive sign I’m more on the right track, now.

The good news is, if you’re making one of these kits and sticking to the playbook, you won’t have any of these self-inflicted problems. The keychest will be all put together, cut the right size, and everything!

Wheel hole

Monday, July 5th, 2010

Cutting soundholes is one thing, cutting the hole for the wheel is something else. You can’t cut it and then find out later that it doesn’t match the shape of the wheel, or is too big! So, I measured the wheel carefully and drew it in CorelDRAW.


Wheel cross section made with CorelDRAW

Then I cut this pattern in a piece of scrap veneer to check the fit. The first time I had miscalculated, because I made the pattern the diameter of the wheel, but the wheel center is below the level of the soundboard, so the hole is actually shorter by a very small amount, about 1/32″ or so on each end. I made three test cuts using veneer, before I decided I was ready to risk the soundboard.


Test of my wheel hole design using scrap veneer

[The veneer used had a strong two-tone look. What you see is all one piece of wood.] The plans call for 3/32″ gap between the wheel and the edge of the soundboard. All the tapers and bevels and rounded corners were a challenge to draw.


Getting ready to cut wheel hole after threading blade in drilled hole

The instructions say to cut the soundholes and wheel hole after the soundboard is glued on the body, but my scroll saw will not cut something that high, and I didn’t see how I was going to do as good a job doing things in that order. I wanted to cut these while the soundboard was still flat. So, I spent a bunch of time carefully aligning the location of the wheel hole with horizontal and vertical reference lines, in addition to measuring the distance from the wheel to the back of the body.


Looks good – now if only I can get the top glued on right

The hole needs some final cleanup, but I wasn’t going to do any of that until the soundboard was glued in place, and I could compensate for any “issues.”

The engine

Saturday, May 15th, 2010

I couldn’t resist putting together the drive train for a brief test: bearings, shaft, wheel, crank, knob. Spins real smooth and quiet!


See, it’s not a guitar!

The nose bearing is adjusted now —screwed into the brace the proper amount— for the preliminary alignment of the wheel. This will be finalized once the soundboard is glued on and the hole is cut for the wheel.


3D view of the “motor”