Ridges?

[applik]

Ok, I need some input. This is driving us nuts. We get a small ridge varying in depth from 1/32 to 1/8 deep around the edges of the carvings. Doesn't matter what size or type of wood. Even doors (though not quite as deep). We've tried putting cardstock under the zero plate to raise it just a hair. Doesn't help. If we adjust the zero plate thickness to a smidge more than it should be, then we will get a flat spot on the high point of the carvings. We securely anchor the project boards down on all sides (except doors just two sides). What am I missing? When I try using the actual outline in AC to do the toolpath; the bot jumps all over the place and leaves z height ridges where it jumps back and forth. We've been sanding these spots out but I've just about had enough of that. Any ideas on what we are doing wrong or is it some adjustment on the machine?
Shari

3833

[harold_weber]

When I first tried cutting 3D reliefs, I had similar problems caused by material warpage due to stress relief as the cutting progressed. If this is also the cause of your problem you can fix it by using vacuum to hold the blank flat on the table.

[paco]

If the ridge is continuous all around the perimeter and when you raise the Z enough to clear it you top up the relief than I tend to think there is something going on with the relief and the way it is recessed... like if the relief is at the same height than the perimeter of the recess. This is where you can see the ridge. Ideally, the relief should be a bit under the perimeter and the perimeter (highest point) set to zero.

If the ridge is not continuous around the perimeter than it might be related to wrapping or thickness of the material.

[Brady Watson]

Shari,
You may have overlooked how your material is setup. See attached pic below. The slider positions the relief in the material stock. The dark gray 'block' in the slider represents the relief. Be sure that you don't have a value in the top box that would cause the relief to be placed below the surface that you zero the Z axis to using the Zzero plate. In this example, I have set the top offset to .125", which means that the relief will be placed .125" below the top of the block. Normally if you have irregular material, it is sometimes helpful to go with a .01 - .03" top offset to ensure that you break thru the surface of the material across the entire relief.

Note the red material boundary lines and how the top one appears to be .125" above the gold relief zero plane.


3834


-B

[myxpykalix]

Shari,
I have been doing some big detailed carvings for blanket box lids and chairbacks and something similar has happened to me. What caused it was after you get to a certain point in doing a roughing pass you tend to take away a lot of material and that causes your material to curl up from the sides because of the relief of stress on the wood.
My vacumn would not hold that down. I took 2 2x4's notched them to hold material by squeezing material and then screwed mdf strips to hold material from curling upwards. The mdf is not illustrated in this pic but you get the idea. The next project i did i mounted the 2x4's on the long side. Then again this might not be your problem at all!

3835

[ljdm]

Jack - I cut a lot of white oak boards 12" wide 24" long, and if I cut a lot around a relief (only .2 - .3 deep) the board curls up on the cut side. I've dampened and clamped the board to get rid of it, but it curls back afer a while. Do you have that problem with your box lids? I use all kinn dried wood, first pass takes .2 off 1/4" end mill, then finish with 1/16" tapered ball mill.

[stevem]

"the board curls up on the cut side"

This also happens with plastics and metals. Have any of you tried taking material off the underside before beginning carving the relief? That helps alleviate the problem with other materials, although from the looks for Shari’s pic, I would guess that the issue is a parameter setting.

[ljdm]

Since I use rough lumber, I'm going to try and plane most of the material from the side I'm not carving, maybe by taking less off the carve side and relieving pressure from the other side might help.

[harold_weber]

A more satisfactory solution might be to find another lumber supplier. I spend a summer vacation from college working in a furniture factory, and never saw any warping issues. All lumber that arrived at this factory went through the factory's kilns and was "stress relieved" at the end of the drying cycle. This seems to be a step that many of today's kiln operators tend to ignore to save time. There are various "secret recipes" for doing this. See the link below for one of them.

http://www.cuecomponents.com/strema.html

You can also search the US Forest Products Research Laboratory's documents for other procedures.

Note that you will still run into the warping problem when you use materials like Corian (for lithophanes), and then you will need to use vacuum.

[lto]

Stress relief is important. It is also considered best practice to remove equal amounts from both sides of a board whenever possible. Harold is correct that the aim to shorten cycle times often triumphs over proper stress relief, another victim of the Walmart mentality of price first. You could search for "prong test" on the WoodWeb site to learn how to measure the stress in lumber.

[applik]

I want to thank everyone for their comments. The designs seem to be fine. Settings are where they should be. We use full length heavy duty hold downs and the edges are always down against the table. Since these ridges aren't equal depths across the doors or slabs, then I'm guessing from the feedback it's warping from the removal of material. Also it never seems to start until we are about 3 hours into a carving. Especially since the designs are a minimum of 24" x 36" and usually around .7 deep. We are in the process of joining our own wood slabs now so we'll be doing them 3" thick. That should help, right? But as for the customer's doors; I guess we'll just have to start making the carving very shallow. I don't necessarily like that look but I don't want to worry about any warpage either. Although we haven't had any complaints so far. If I V carve the back with a simple shallow design first, would that help? Is there a rule of thumb on carving depth in relation to wood thickness? For example: 1/4" carving depth for a 1" thick wood - 1/2" for 2" thick, etc.?
Thanks again guys for all the input,
Shari

[paco]

Shari,

are there any vacuum hold down involve?

[applik]

Paco,
I have not used a vacuum hold down yet. We have a pretty heavy duty gast vacuum pump but haven't gotten around to making the pucks, etc. Guess we better try it out.
Shari

[Gary Campbell]

Shari...
Are you able to do a relief cut on the back side of the material? A few concentric round corner rectangles that are close in size and depth to the carving might help. Similar to kerfs in the backs of stair treads or moldings that allow thickness, but reduce stresses caused by one face being cut differently than the other. This of course will only work if the rear is not seen by the customer.
Gary

[applik]

Gary,
I could try the rounded rectangles on a new project. Should they be done before the 3D or after? I'll also have to make the 3D less than half the depth of the board so the two won't meet in the middle. Maybe I'm carving too deep into the wood and not leaving enough thickness on the back. Lots of things to try now. One at a time and see what happens.
Looked at your site and your work is very nice.
thanks,
Shari

[Gary Campbell]

Shari...
Try one with the 3D relief at half the thickness of the stock and the relief cuts at a quarter... that should leave a quarter, and a little room for math error. I would cut the relief 1st, watch your vacuum mask! And thanks for the compliment, we try.
Gary

[Gary Campbell]

Shari..
Edit above to read... cut the BACK stress relief first while both sides are still flat.
Gary