How to Machine Trapezoidal 3D Chair Feet

      Suggestions for how a simple three-dimensional shape can be fabricated in quantity from wood. February 1, 2015

Question (WOODWEB Member) :
I've been making these poplar feet by hand, using a horizontal belt sanding machine set at 15 degrees tilt. I attach a wood handle to the top of each foot, otherwise it's quite dangerous to do, holding the piece against a stop up close to the belt. If the picture doesn't quite show it, the top of the foot is an 'apple' shape, about 5" across and 2-1/2" tall.

I'm in need of enough of these now that I think it would make more sense to have a CNC machine involved. I contacted a local CNC machine shop and sent examples of the feet to them to look at. This is what I received for a response: "I can't come up with a practical approach to carving these on the CNC. The best I can do is about $20/each. That is for cutting labor only." It's not the price, necessarily, that has me concerned, it's that I suspect I will not get a good finished product from this shop. I asked if he knew of other shops here in Maine that could make these, perhaps with a more advanced machine, and here's what he said: "I sent the file to the few people I knew in the industry that I thought could help and they came back with similar machine times."

I'm not getting much information from this person, so I think I need to move on to a different shop. First I wanted to ask here about the difficulty involved in cutting these feet, and if I can expect to get a finished product from a CNC machine. Any feedback and/or ideas on which direction I should go with this would be great.


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Forum Responses
(CNC Forum)
From contributor R:
A five axis router with a long bit will make a final cleaning pass around the foot to make a clean cut. The roughing cuts will be the time hog on the run. If you would bandsaw them to within an 1/8" it could be run in two passes. It will require some minor fixturing, but it could be held down with a single bolt through the center. I can't quite make out the transition at the floor. If that is just broken with sandpaper, no problem. If a machined radius, more CNC work.



From Contributor O:
If you are in Maine, locate one of the many turning mills there and ask them for pricing. These can be done on a faceplate auto lathe faster and more reliably than a CNC. They will also have the capability to sand them for you, so you get a finished product. They may even have a NC controlled auto lathe that can do these. Pricing will be quantity dependent - the more you buy, the less the unit price, with that first one being expensive.


From contributor T:
I am not sure what you mean by "apple" shape, from the picture it looks like a 3D trapezoid? If most people are quoting this with a 5 axis, I think you are going to find similar pricing. If the "apple" geometry is not to complex, that part can be either fixtured or fixtured with angle ground tooling and done on a 3 or 4 axis machine. Most shops would weigh the volume over the cost to grind tools or buy custom shaped tools.

From Contributor W
Member

You could do this on a small table top or any 3axis machine by doing a 3d file ballnose upside down.


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From Contributor D:
Even faster, use a 15 degree bit that has a flute length of 2.75 inches.


From the original questioner:
Thank you everyone for the great suggestions. I will contact a turning shop here in Maine and see what they can do. The 15 degree long-flute bit sounds like a winner to me, but I don't want to specify, just want a nice finished product back, so I'll let whoever I send the job to figure it out. Nice picture, and pretty accurate. The only difference is, looking at the top of the foot it is a trapezoid more or less. I call it an apple shape because it more or less looks like the outline of an apple (sideways) (no stem). No radius on the top or bottom edges, we just cut those by sanding. It would be nice to have the holes drilled too. There are three mounting holes all the way through and another hole in the center to install a threaded insert. It currently takes me about half an hour per foot in my shop. It's an easy half hour, meaning the wood is delivered to me and I simply go right to work making them, and six hours later I have a dozen feet sitting there all sanded, drilled and ready for paint.

One of the things I'm mindful of is that with CNC work, I still will probably have to do some work to these pieces after I get them back, like finish sanding, possibly drilling, etc. Also, there is the delivery and pickup (or shipping), correspondence back and forth, unexpected issues. For me there is a burden with doing this that is not there when I simply take some wood and go make feet. That's why I said it's easy work. It's just work and in the end it's a half hour a piece. At my current shop rate of $60 per hour, with materials the cost is $31 per foot. High yes, but my life is simple with this method! I guess what I'm saying is I think CNC or turning is a good option for these feet, but changing to a different process has the potential to be problematic, still time-consuming, and in the end not save any money or perhaps even cost more in terms of time and actual cost. The attached picture is of a different foot that is the same shape but a bit smaller around, and 3-1/4" tall.


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From contributor R:
Sure these can be run on any 3 axis upside down, but he is looking for a fast run time (low cost). So multiple roughing passes and close step over with the bits for smooth result. It will take more sanding after a 3 axis step over cut compared to wrap around cutting with 5 Axis.

From Contributor Z

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You could try using a new CNC fitted with the lathe option. Then cut them apart on a table saw.


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