The case for CNC cut frames

There was a lot of effort required to program the frame geometry into the software that drives the CNC router. It could be done quick and dirty, or it could be programmed. Quick and dirty means describing each frame one by one, figuring each of the dimensions from the offsets provided in the plans. To program it means accepting each of the offset values as a parameter, and using that parameter abstractly, like in algebra, and making the calculations around those parameters as though they were programming variables. I’m a programmer, so there was not a choice. It took time, but once done, all frame dimensions were entered into the CNC software as parameters in the order they appear on the plan offsets. Loading the dimensions in this way eliminated the risk of a data entry error causing a mistake.

It was a lot of effort, but I reckon that effort payed off in a number of different ways:

  1. time to actually cut the frames
  2. making sure the frames (and thus) hulls were identical
  3. locating the stringer notches on each frame
  4. frame numbering for reference
  5. accurate guidelines layed out for waterline, centerline, sheer and upper chine heights
  6. accurate access cutouts saved time in setting up on the strongback
  7. accurate access cutouts saved time in ensuring the two hulls were parallel and level when setting out the bridgedeck

Before I elaborate on each of those points, there were additional benefits too, in having calculated the angle of each side of each frame. When it came to cutting the bevels for the ring around the frames, knowing the angle meant that I could simply dial in the cut on the bevel saw and get a perfect join each time.

When it was time to start fitting out the hulls, knowing the angle of the sides at the location of the frames made it simple to cut the angles on the bevel saw for the furniture and fitout.

1. Time saved cutting the frames:
As the frames are cut with a router rather than a saw, there were no torn edges on the cuts. There was no transposing from the table of offsets into measurements on the sheets of ply. All of that went into the programming of the software. Simply load a sheet of ply into the machine, and press the button.

2. Identical Hulls:
As both sets of frames were cut perfectly with a machine, there is no risk that one hull might be different from the other.

3. Locating the Stringer notches:
Stringer notches were cut into the plywood frames, but had to be recut into the frame “ring” after it was glued and secured. But that required no further measurements. Simply pickup the jigsaw and cut out the notch.

4. Frame Numbering:
Each frame was numbered, in the middle above the sheer line. This made it easy when working with others that were unfamiliar with the hull.

5. Guidelines:
Accurate guidelines were marked into each frame for the critical levels. This made is easy check accuracy of the structure.

They guidelines were the waterline, the centreline and the upper chine line. These reference points have proved useful at all stages of construction.

6. Accurate Access Cutouts on the Strongback:
Having all the cutouts accurately removed from the frames provided a quick way of checking the vertical accuracy of each frame, and the longitudinal positioning.

The usual method for checking accuracy is a string line down the centerline of the strongback and to plumb bob off that.

As well, I could string line through the middle of the frames.

7. Accurate Cutouts for Setout of the Hulls:
Accurate cutouts made it easy to measure the accuracy of how I layed out the two hulls before joining with frames and bridgedeck.

Rather than having to measure from the hull centerlines, I could measure directly off the cutout edges. The tape measure could easily be handled by one person to measure the distance between offsets on the two hulls.

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2 Responses to The case for CNC cut frames

  1. Lee says:

    Did you program the CNC yourself? I’m considering building a Sarah and would love to have the frames cut perfectly. Do the DXF files come with the plans?

    • Chris says:

      I did have to program the CNC myself.

      The plans are not provided in any digital format. I had to seek permission from the designer to be able to transcribe the offsets into a digital format.

      The CNC that I have access to is a Biesse machine, so it is programmed in their propriety software, which uses BPP files for data. The owner of this CNC machine is our local timber merchant and joiner, and he has been invaluable to me throughout this project, in terms of supplying timber, materials and advice. It would be possible for him to supply your with plywood already cut with the frames, and repackaged for transport. Transport of the repackaged cut sheets would be no different than the whole sheets. Permission would have to be arranged with the designer, but to start with precut frames seems like a perfect way to commence a new build.

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