Making a bench - CNC help

Tags: #<Tag:0x00007fa49b6d7cb0> #<Tag:0x00007fa49b6d7b20>
#1

I’ve started a new project which will be a two seater indoor bench. For shaping the seat I like the idea of doing some of the bulk removal on the big CNC and am hoping I might be able to get some advice / help.

The bench will be made similarly to this chair I made. The only real change is that I’m using a solid slab rather than laminating the seat and it’ll have stretchers to support the legs:

image
image.jpg520×691 119 KB

For the chair I carved out the seat by hand using a scorp, travisher and card scraper. However, the slab I’m using for the bench seat has some very gnarly grain which put up a real fight whilst I was flattening it. Given this, I’m wondering about doing the lion’s share on the cnc and then finishing by hand. This has the added bonus of learning a bit about the CNC process, this would be my plan should I ever make a set of chairs for repeatability and ease. For reference, here’s the slab complete with pretty, but unruly grain. It is English Walnut and will need some bowties and/or epoxy to stabilise some cracks:

image
image.jpg918×691 382 KB

Questions for CNC folks out there:

  • To do this I’d need to model the slab and the shape of the impression I want to carve out. I only have access to a Mac. What free software could I use to do this? I’ve not done any CAD since uni so something easy to pick up!
  • I’m not inducted on either CNC nor have any experience, is anybody interested in this project and up for helping?
#2

Yes to the first point. You can use fusion 360 on macs.

I won’t be able to help you this side of Christmas, definitely in January if you can wait.
@cnctechs?

#3

I’ve done a bit of 3d carving with CNC using stl files in vCarve. You could also model in other 3D software other than CAD, e.g. Blender, Meshmixer etc. if you have an iPad with pencil, Nomad Sculpt is great for 3D modelling (£14.99). Anything that can be used to create and sculpt a 3d file that can be converted to an stl might be used. The easiest might be Meshmixer - certainly easier than Fusion 360 for sculpting.

You did a lovely job with the chair. You could consider making a physical model using an easy to carve material, such as extruded polystyrene, or low density fibreboard, and then digitise that. We don’t yet have a 3D scanner or 3D digitising probe for the CNC (yet…), but, though not ideal, photogrammetry could be good enough.

In any case, consider gathering scrap mdf to do test CNC carving on. It could be a good idea to start with some tests at ¼ or ½ scale to see how they turn out and refine the stl model before trying full size test carvings.

What sort of size would it be?

BTW, for stabilising cracks, consider Paraloid B72. It’s what professional conservators use for that purpose.

ETA, photogrammetry might be used to model the current slab, unless you can get access to a 3d scanner. Try RealityScan (IoS and Android).

#4

Does have a tendency to turn white on contact with water and if your wood has moisture content it may be a problem .

Glass cast as a resin works , or cactus juice but the latter requires vacuum and needs heat to cure.

Bow ties in the same or contrasting wood or brass can give a nice detail , but some hate them!

#5

I can potentially help before Christmas. E.g next week or after. I’ll DM you my number and we’ll discuss.

#6

Interesting…. I’ve not encountered that and B72 is used with e.g. conservation of outdoor historic monuments. It’s always good to test first, and B72 is reversible, which can be a key advantage, and why so often used in conservation. If want to be highly water resistant (‘superhydrophobic’), then adding fumed silica nanoparticles to the B72 is recommended - see link below. Since B72 is a resin, that might be akin to ‘glass cast as resin’, which I’m not familiar with. Silica / SiO2 mixed in with the resin makes it stronger and less flexible too.

@lewiss, I’d be really interested in what specific products you’d recommend for that

ETA, I tried this with a wetted testing stick which was slightly damp when I applied b72 with an acetone solvent, no SiO2. There’s no whitening with that. (See below - about 2cm on the end was treated with b72). A search turned up that there are studies showing that if there are fairly significant amounts of water mixed with the solvent, then get a white deposit, especially if ethanol is used as a solvent. That becomes an issue when the b72 is drying rather than after it is dried, with the water interfering with the bonding as the solvent evaporates. It seems that can be dealt with by applying acetone to re-dissolve the b72.

I tried wetting the area treated with b72 after it had dried for 10 mins. No issue - see 2nd pic below with drop of water on the area.

.

IMG_1063
IMG_1063.jpeg3711×885 840 KB

IMG_1064
IMG_1064.jpeg3310×1361 1.1 MB

#7

Thanks all for your replies.

@dannz @lewisss, when I bought the wood I also bought some ‘z-poxy 30 min resin and hardener’. I was planning to use this unless there is a good reason not to? I’m then torn as to whether I do bow ties as well, I dont mind the look, but there are a few cracks close together and I think it could look messy with multiple bow ties. I’ll perhaps mock it up then decide.

@Christian, thanks for the offer! I’ll drop you a message. I need to work out how to model it first so I’m not really ready to cut yet, but perhaps we can discuss this too!

#8

@lewisss knows better than me on the resin and bow ties. I’d expect that a suitable resin would be more than strong enough and no bow ties needed, but of course be best to see the slab. Take account of resin with the CNC if you treat it before CNC work. The wood will be harder, so check speeds and feeds with CNC techs. Also some resin melts at lowish temperature, which might be an issue. Check the datasheet. I’d also check if it can be removed with acetone etc. in case you find you need to clean sticky resin off the endmill. Otherwise might be best to apply the resin after the CNC work. Looking forward to seeing it!

#9

Happy to help with the model part in fusion, too.

#10

I’ll throw another crack sealing solution onto the mix, though it’s probably more divisive than bow ties. You can use a bunch of overlapping plugs:

IMG_7118
IMG_7118.jpeg3024×4032 3.57 MB

#11

1702032371723668037137657928134
1702032371723668037137657928134.jpg3072×4080 2.47 MB

I’ve committed to trying the epoxy. It’s a lot thicker than I had expected so is sinking in very slowly and will likely need a second round. Fingers crossed