Bas-relief Huge learning cure

[Gocarter]

I have spent a great deal of time and energy in trying to get this project to end up the way that I want it to. I am MORE OR LESS happy with it now; I may cut it again, now that I think I know what I am doing. It sure took me a while to get here!

I got this model, and my first cut was the cherry one, that you can see a few pictures down. It actually looks a little better in the photo than it does in real life. The bas-releif is very shallow. I then changed some of the parameters in Aspire and I brought the model to 180% of it's original and cut it; that is the maple version. That looks stretched, and it significantly thickened the model.

I then took the model to 140% of it's original, and that looked much better, but the base was still very thick. I then used the fade feature to bring the base of the project to a more reasonable thickness and that worked very nicely.

I really liked the effect of the ash with the brown stain on it, so after success with the small 17 x 23 " piece, I thought I would try a 50 x 39". So I got some 12/4 ash, and glued up a great big blank, and threw it on the CNC. While I like to consider myself an experienced woodworker, I was unprepared for the amount that a 12/4 slab of ash will MOVE when you remove large amounts of wood from one side of the block only. It cupped in the direction of carving, MORE THAN 3/4 of an inch, actually lifting my hold downs right off my CNC bed. $500 worth of ash in the fire place! It took me a while to calm down and decide to try it again. But, ,I did, and I did figure out how to keep the blank flat as I cut it.

I also spent about 10 hours with the modeling tools "improving'" the model, or making it more "cuttable". Some of the detail was lost on my test cuts, and I found that if I used the modeling tools I could increase the depth of cuts here and there and get more contrast. I think it paid off.

That is where I am at with this project for now. I think I may cut the whole thing again in ash, now that I know what I am doing. I'll let this sit for a while as is though. Time for a break from this for now.
Gordon

[dwilli9013]

No matter how you word it that is some pretty incredible work. I am more favorable of the last one though.
Thanks for sharing .

[MTSO]

Gordon....Mate... that is massive!. What a cool project, you must have a big machine!
How long did it take to machine?.

Regards

Brian

[Gocarter]

Gordon....Mate... that is massive!. What a cool project, you must have a big machine!
How long did it take to machine?.

Regards

Brian

Roughing was 7 hours with a 1/2 end mill, finish was 9h15 min with a 1/2" ball end mill.

[Leo]

Awesome work.

Yeah wood moves, and it is very powerful when it does move. I call it stress relieving.

In the metal industry there is a stress relieving process whereas we will rough machine, but leave material, send the metal into a specific heat process to allow the metal to relieve the stresses.

In wood - we can do something also.

Lets get the bas relief into the "middle" of the material, so it is balanced and so that we are not removing a lot of material out of one side.

We can pocket out the back, then flip over and do the relief - BUT - leave a 1/2 inch or so.
You said the material bowed 3/4. If you remove a pocket in the back and leave 1/2 inch in the front - let's hope that is good.
At least it is balanced to some degree.

Take it off the clamps and raise it up and let is sit on blocks - stickers.
Let is set for a while - days - weeks - I don't know how long.
Out of the sun - maybe with a very slow fan very slow air movement.

Let is "stress" relieve.

Then flatten the back perimeter - flatten the front perimeter
Maybe let it stress relieve a bit longer - days

Pocket out the back to clean it up
Stress relieve some more??

Not go back and finish the BAS-Relief

I have not tried this, but based on knowledge of metal machining, having the same issue of my own, reading other peoples experiences, knowing how turners prepare bowl blanks, years of working with wood, understanding stresses - this sounds like a reasonable process - at least to me - and is in my plans to try it someday.

[Creation in Wood]

A good project

[Rcnewcomb]

and I did figure out how to keep the blank flat as I cut it

What was your solution?

If there is a difference in relative humidity between your shop air and the wood then the wood will cup, bow, twist, etc. when large amounts of material are removed.

[TReischl]

Heck Randy, it will even bow, cup, twist and generally warp when the humidity matches!

Evidently the outer fibers of wood build up stresses, once they are cut, time to for the wood to move.

The article I read said it is especially true for kiln dried lumber. They refer to it as "case hardening". And since those folks make up the rules they run a test by cutting a U shape and measure how much the wood moves. Like all things, a certain amount is acceptable over a certain distance.

Here is an interesting factoid! I have this old 1946 Delta lathe. It still has its original pine boards. The boards are about 10 inches wide and 1.5 thick. Both of them have a series of saw slots cut on the bottom about half way through but not end to end completely. Obviously factory done.

[Gocarter]

and I did figure out how to keep the blank flat as I cut it

What was your solution?

If there is a difference in relative humidity between your shop air and the wood then the wood will cup, bow, twist, etc. when large amounts of material are removed.

In cutting the piece, I had been cutting it on face grain, the most pleasing face of a board, especially in hardwood. But the movement happened as I removed material from one face and not the opposing face. So I wondered one day about making up a blank of edge grain material. The wood won't protest (too much) if you remove material from the edge; you can remove it with relative impunity. You can get rather significant differences in colour of the boards, but since I was using a darker gel stain (after a sealer) I was not overly concerned with this.

The prep time for a block this size was considerable, as there was a lot of jointing and planning to get the stock the way I wanted it. I ended up with 33 boards glued together. But the block is stable. At least so far.

[Leo]

Nice solution.

Quarter sawn wood is precisely that - edge grain. Yes, that is known to be more stable.

This also makes me think. END grain, like a true butcher block. That would even be another magnitude of stability