I am on a steep learning curve here but making some progress. I want to cnc machine timber to make hollow oar shafts by machining pockets out of the inside of the oar shaft, which is made in two halves split down the centreline of the oar so the inside timber can be milled out.
There are two things which are making this difficult. One is that the shafts taper with a very slow linear taper in both width and depth, less than 1 degree. It is hard to stop Sketchup for Web thinking I mean not tapered at all.
The second thing is that the places where solid oar transitions into thin 10mm walls need to be gradual to avoid stress concentrations so they are arcs of circles. These are then extruded upwards to make a solid shape, only they are potent sources of short edges and holes which the Solid Inspector (I picture an unsmiling fat man in a top hat like the Fat Controller) frowns on.
If I then slope the top surface down to make the depth taper, the place where the pocket sides meet the top surface is full of holes again. Frustrating, because then the model gets rejected by the software making the G code.
I tried scaling the shaft up to eliminate small geometry errors which is why the uploaded model is 40 metres long, but to no avail…
First thing I would do is change the Length Uniuts to meters and set display precision as high as it will go. Then turn off Length Snapping because that will just make your work more difficult.
Second, I would model the oar shaft as a solid and then model the volumes of the spaces you want to remove. Make sure the oar shaft half and the volumes for the areas to remove are solids.Then use Subtract from the Solid Tools to subtract those volumes from the shaft.
I will try setting to metres and turning up the precision. Why metres?
I’ve been doing the solid modelling by drawing a base plane which is a long tapering 4 sided shape. Then I make a semicircle on the wide end and draw a slot for the thole pin, and extrude this surface upwards for 28mm.
This is the basic block. Then I need to taper it in depth. I have tried two ways to do this. One is to tilt the top plane till it gets to a reference line on the narrow end.
The other way is to draw a long plate separate from the model, tilt it by the right amount, place it on top of the model and subtract it. That works but causes a lot of leaks where the curves meet the sloping plane.
I will try making the pockets by subtracting solids.
The component is half of an oar shaft which is hollowed out for lightness and balance. The handle and blade are added later to make an oar 4.5m long for a 4 oared boat called a St Ayles Skiff.
This is not some theoretical solid, I have already made one by hand held router and it’s a great oar, I’m going for another row with my friends in an hour. I want to make more for my rowing club using a cnc router.
Here is a photo of two oar halves and another of the finished oar:
If you model as if millimeters are meters you can avoid issues with tiny faces creating leaks (holes in the surfaces and you can use larger numbers of segments in arcs and circles.
Now I understand the kind of oar you are making. When I first saw it I was wonder why not a birdsmouth loom but that wouldn’t work so well for this style.
I understand from your model and description how you are approaching this. I’ll suggest a different option if I may.
I started by modeling the loom so its centerline is on the red axis (I added a red guideline for this image.) Even though you probably won’t taper the real blank on both sides, modeling it as I did makes it easier to add the other features since it allows you to work on axis. Initially I extruded the loom slight more than the maximum thickness and made it a component ensure it’s solid. Then I created a “cutter” in the shape of a long wedge and used Subtract to cut the top off the loom in a taper. Imagine the volume of space a cutting tool would pass through to make that taper.
After that I modeled cutter volumes for the rest of the features. There’s a cylinder (green) for the hole at the end, an oval extruded all the way through the loom (blue) for the thole pin. and the tapered ovals (pink) extruded partially though the loom. Then Subtract to cut the features. I used Trim so I wouldn’t lose my cutters for this example. I drew the cutter volumes so they are aligned with the axes instead of aligned with the taper assuming the spindle will remain perpendicular to the bed. It’s only about 0.3° so it probably doesn’t matter. I didn’t check the angle before modeling the cutters. I also radiused the edges of the pink cutters to create a rounded edge in the bottom to avoid stress risers (6mm dia. core box bit) but maybe that’s not needed.
This is very kind of you Dave.
I did start a model using metres as you suggested, but I inadvertently defeated the object by reducing the number of metres to make the same dimension. I made the oar 3.995m long instead of 3995mm. I now see that what you meant was draw an oar 3995 metres long, which is some oar!
I will start again and avoid the taper problem by drawing it along the red axis. The reason I had done it with the red axis along one of the tapered edges was because that is how it fits into the stock when it is in cnc manufacture. But I can rotate it slightly when I get to fitting it onto the stock.
The drawings look nice, it gives me hope that I can make a leak free model!
The stock is a plank of Laminated Veneer Lumber 45x300x4000mm with 15 veneers 3mm thick glued together with all the grain running along the plank. This stuff is factory made in 12m long billets which I get cut to 4m less the kerf. There is a lot of waste in what I’m doing as the material is not available thinner than 45mm but even so the material cost for one oar is only about £35.
Birdsmouth hollow shafts are elegant but a rectagular hollow shaft is actually lighter for the same strength and stiffness, given that the main stress is horizontal.
I’m in Ullapool, Scotland, but my son in law who has the CNC machine is in Dunbar.
These are pretty long oars already! Actually 4 halves nest together on a plank so that is pretty good. But the stock is nearly twice as thick as it needs to be. The main point is, it will not need skilled work to form the thin webs precisely, which means it will be affordable by the community clubs which row these boats. I’m assuming you are in the USA? There are about a dozen there.
My son lives in Ashland Wisconsin and he has the same problem so he skates.
Can I ask you how you made the pocket cutter tool or tools, and how you made sure the floor thickness was 5.5mm? When I drew the pockets I was using a 10mm offset from the tapering sides, but it would be harder to draw the cutter off the tapered body.
I’ve tried drawing the top ovals on top of the already tapered top surface, but I think the bottom surface will be sloping like the top if I extrude the top oval downwards into the body.
Ashland isn’t really all that far away but usually their ice is harder or at least thicker than ours here.
I drew the oval for the pockets on the horizontal bottom face of the oar and then extruded them up beyond the tapered top face with Push/Pull. Then I pushed the bottom face of the extruded oval up to leave the floor thickness. Quickie example.
Yes it does now tilt. I should perhaps have extruded the pockets downwards while the top was still horizontal. The last time I did that, when I used a wedge to subtract from the top of the model, the oval slot and pockets generated a hideous amount of errors, but the Dave method of modelling at x1000 may cure that.
I used Trim because I wanted to keep the cutters for show. If you don’t want to keep the cuters when they’ve done their job Subtract would do the same and remove them.
Well I’m happy to say I got my 4.5m oar shaft half design leak free today using your techniques. so thank you very much! I had to repair some leaks around the double wall below the pocket floor where the extrusion had a sort of folded double wall which in the end I deleted.
I tried with the other oar I wanted to model, the 4.0m oar, and I tried a variation. I put the rounded end of the handle and the pin slot into the ground plane and then extruded them up 5.5mm (to x1000 scale). Then on top of this layer which represents the floor of the pockets I drew the outline of the pockets and made them polygons to make sure they did not leak. With the extruder I raised the rest of the surface up to 28mm, the max height, and tested for leaks, of which there were a few.
Then I selected the top surface and rotated it around the thick end so the thin end went down to a mark I had made on the end face. This more or less worked but I’m still chasing leaks. So your method is currently more successful than mine! The 4m oar shaft has about 400mm cut off the end unlike the 4.5m one.
Yes, and when Solid Inspector tries to fix the problem it takes out lines which expose more faces. I will try again with your approach, but it puzzles me why my approach does not work. It seems the algorithm can’t handle the interface between the side surface of an extruded shape and a tilted surface which the extrusion passes through. Almost as though the lack of orthogonality is upsetting it.
you can at least nest the two halves side by side, right?
