Depth of sled

[badbert]

I built my first sled with changeable 1/2, 3/4, 1, and 1 1/2 inch rails. This way i could change rails, based on the thickness of material. Per an earlier discussion, I thought to try one "deep" sled to work with all thicknesses. I built a new sled 14.5" x 48" x 3/4" Birch plywood. I then mad 2" rails and 2" x 4" end boards. It does measure without tape from end to end, so I did make a step forward. The problem is, it doesn't work! I loaded a 12" x 12" x 1" board. It measures the sled just fine. It then asks for a bit. I load the bit and it calibrates fine. It touches the bit plate and then asks for jog to touch. I jog the bit over the center of the board, and I can physically move the Z truck to make the bit touch the board. I press enter. The bit comes to the key pad side and homes, then it goes back to the center of board... but it will not lower the bit low enough to touch the board. after a few tries, it returns to asking me to load bit. And it is stuck in this loop. I added two 1/2" sheets into the center to raise the board up 1" and it works perfectly.
Has anyone else been able to get this working? Is the there a magic number that we can make the sled deeper than the wood?
I secure the board using masking tape on the board and the sled and then use hot glue between the layers of tape. It is very secure. So I am not worried about it coming loose.

[fwharris]

Might be totally off base but, it might that the difference in your rail to top of board is more the 0.80" (the depth of the carving bit...

[badbert]

The .08 must be a limit in travel below the rollers?

The 1/8th bit will not touch the wood either. And it has a 1" depth of cut. And the board is only 1" below the rails.

There must be some logic used to dictate the height depth relationship. I can physically move the z truck to touch the wood, but it will not do it when calibrating. So maybe if I measure the distance from the sled bottom to the lowest point of the bit, and subtract that from my rails we can determine the maximum depth that a sled will work. If my logic is flawed, please tell me so. My thought is that table top CNCs do not have rollers holding the material in place. If we can make a sled that will work up to 2" thick, we can then add a sacrificial 1/4" Luan board. And never have to replace the sled. I like the fact that there is a cutout pattern left in the sled. So If I want to recarve, It will do it exactly. I screwed up the finish on a very nice piece of maple, It was way to intricate to sand down or strip. I glued/taped it back down to the sled, in the exact spot it came off of. I lowered the depths of the carve .1". It stripped the board (with raised text and detailed carvings) like it had just been carved! But I do not like having to replace the 3/4" birch after a couple of weeks. And the interchangeable rails idea is a pain every time you change wood.

[badbert]

Latest configuration= 1 1/2" rails and end boards. One 1/4 sheet of Luan. Now there is only 1/4 between the rollers and the board. The bit registers, the bit touches the board during jog to touch. But when it measures thickness, it asks me to enter the thickness. I enter 1 and press enter. It just keeps asking for the thickness...

[bergerud]

I think the machine simply expects to find the wood surface just under the rollers. Any significant deviation from this is considered an error. I remember trying to trick the machine to carve deeper than 0.8 by touching deeper into a first carve to do a second carve. It did not work for any significant difference in depth.

[liquidguitars]

For my money I make the sled rails the same thickness as the wood i use and keep it flush to the top as a rule, for a mulit use jig i add the dead wood to pad out. if my wood is thicker let say by about 0.125 than the rails I use the jog to index. so if i have a guitar body thats 1.75" thinck that will be the rails thickness and i always keep the carving flush, if i want to carve a .75" thick top i add dead wood on the base of the sled and flush the part, i never seem to have issues after years of carving with the exception of the CW moving off x,y a 1/16" at times : bug?