CNC cut stringer notches

I'm putting together a design using the stringer frame method and am wanting to get as many parts CNC cut as possible. The stringers mostly cross the bulkheads at a small angle - how do people deal with this in a basic CNC setup? Do you find a machine which will do appropriate beveling and model with full bevels in the notches, or just cut parallel edges and fill/file until the stringer runs nicely? Or do you just accept a bit of unfairness and force them in to parallel slots?

 

Could you show us an sketch of the problem you're describing?.
On the other hand, many cnc machines have what is called "2 and a half axes." That is, they can cut the plate at an angle of chamfer.

 

Hi Tansl, it's something like the picture here. The stringers cross the frames at an angle, ie they are not perpendicular to the frames (in which case I could cut them without beveling)

Andy

 

Im still not 100% clear what you're trying to ask or show.

Stringers must be perpendicular with the shell plate they are attached to, not the frames they pass through. If that helps.

 

I mean the stringers (longitudinals) , in coming together at the bow for example, cross the transverse frames at an angle other than 90 degrees (perpendicular, which could only be the case in a hull with all sections identical like a cylinder or barge). So therefore the notches in the frames to accept the stringers must also have an angle cut in them to accommodate the run of the stringer. So I'm wondering if CNC machines do this angleor if it is done by hand during building? Does that make sense?

 

No, i refer you to my comment above.

For some reason you're trying to 'force' the stringers to remain perpendicular to the frames, not sure why, is what you appear to be saying?

As noted above, stringer must remain perpendicular to the shell. The frame is irrelevant in this context. For ref see below:

 

If using a 3 axis machine, cut the notch to the control edge of the frame/bulkhead/mold. You can go back and trim to fit or you can anticipate the frame/bulkhead/mold thickness and take this into consideration when the notch is cut, permitting the stringer to fall into it and edge set against the properly distanced corner. This means the frame/bulkhead/mold will also need it's edges milled to the taper or edge set the control edge, just like the notch. Of course, a 5 axis machine solves this issue.

Another solution is to still cut the notches to the control edge, but have them stand proud of the frame/bulkhead/mold, so these don't need to also have an edge set or bevel applied. A lot depends on the build type and if the stringers will remain in the boat or are just to permit the planking/plating to lie fair.

 

If your cutting with a 2 1/2 axis machine (X and Y with limited Z) the frames (bulkheads) will need bevelling and the notches. It actually does not take long to plane off the excess on the frames and chisel (maybe cut and file, depending on profile) the notches. Just make sure you get the 'parts' cut to the larger side....

The stringers will then sit nicely in their housings, but assuming a straight flute round cutter you may need to radius the stringers to match. Worth noting is that if there is a lot of bend on a wooden stringer you may find laminating will significantly reduce the stress(es) in it. There can be a lot of difference in length between the upper and lower parts of a single stringer as well as the more obvious side bend.

Worth bearing in mind to have some Datum reference somewhere on the frame/bulkhead for getting the setup right....

 

Thanks guys, that makes sense. I think I'll get the stringer notches cut so that the area of the notch common to front and back faces of the frames has been removed, then cut/file the rest by hand to get the stringer in snug. Unless as PAR says I can find a 5 axis machine.

 

The 5-axis machines are not used in shipbuilding, at least for cutting elements of the structure. No need to do chamfers in any floor or bulkhead. I hope the attached drawings help you understand how, as my facility to explain things in English is very poor .
The simplest is to cut the floors or bulkheads as shown in Figure 1. But, and this is very important, then you must attach the reinforcement element which traverses. This can be done by welding an small plate as shown in Figure 2. What is done, it is much cheaper, it is to cut the floor or bulkhead as shown in Figure 3.
Classification Societies have clear indications of how to make these holes, dimensions, radius, etc. and many other constructional details.
It is also very important, as indicated Ad Hoc, making reinforcements perpendicular to the plate on which they are based. If not, the reinforcement loses its effectiveness.

 

Well maybe shipbuilding needs to catch up with yacht building, as 5 axis machines are cutting perfect kits, with beveled molds/frames/bulkheads all the time.

I also think he's talking about wooden construction, which is approached differently than metal.

Attached is the way I setup a cut file. The top image requires the mold get beveled after it's cut, but provides a healthy notch to hold a stringer, which can also be backfilled with epoxy to provide a solid landing. The lower image is the way I build, by insuring the depth of the notch, permits the stringer to land flush on the short side of the mold (opposite of the control edge). This arrangement accommodates no beveling of the mold edge. I find this more convenient, simply because there's less work. It does mean the stringers alone, control the boat's shape and if much torque is applied with the planking, the bottom of the notch may need to be back filled for additional support.

The way the boat is built also plays a big role too. If the stringers stay in the boat, there has to be accommodation for the molds to be removed, while the stringers remain. I usually use the wedge approuch TANSL shows, which also permits you to "tune up" the stringer runs, which on a lapstrake or multi chine build is critical, if you want fair, clean plank runs. If the stringers just hold the boat's shape and they and the molds are removed after the hull shell is completed, then I prefer the inset notch.

Lastly, while doing a lapstrake recently, I made no provision for a stringer notch, instead just a mark. Stringers were "lined off" directly on the mold edges and screwed to the molds, once satisfied (they'd be removed later, with the molds). What this permitted was templating and cutting of the individual strakes with impeccable precision. Because the stringers stood completely proud of the mold, a sheet goods tracing of the stringers could be made. This template was roughly transferred to the planking stock. The planking stock was hung to witness marks and a flush trim router used, with its bearing following the stringer, cutting the planking exactly along the edges of the stringers. The stringers were sized to the depth of the lap, which made things easy.

 

Very differently

 

Hi,

In CAD systems for ex. Nupas if we have an not perpendicular intersection of stringers with frames - CAD just transform shape of cutouts for stringers. In this case you don't need any beveling. If you want to make it more precise you can use beveling, but I am just afraid it will be very difficult bevels programming in your case. We use some cutting with bevels for steel parts, but very often it is fixed beveling angle for edge preparasjon to welding.

 

Again, this is a wooden CNC cut file, not a metal one.

 

Alexanov, I'm not sure I understood correctly what you say. So I'll tell you what I have understood and you chasten me if I'm wrong. Apparently your CAM program automatically decides whether to make a cut chamfered and, in a case like figure, cutting red would be at an angle of chamfer that your software decides what is.