So I recently made a new spoilboard for my CNC shark pro HD. I made it out of hardwood and made it so there were T slots in the top. I surfaced it a while ago and then I noticed that it seem to be not planar anymore so I figured it must have been from the wood moving as a result of moisture etc, even though I keep my shop at a relatively constant humidity level. So I am just about to cut out a guitar body that is chambered and I thought I would flatten the surface again just to make sure. So I ran my planing tool path to take 1/32" of the whole top. I marked pencil lines along the top to make sure I was getting flat over the whole surface. Took 2 passes as there was a small area left with pencil marks. I went to put my work piece on the newly flattened spoil board and it rocked! So I got a straight edge and thought that the work piece had some movement but it was dead flat. Then I checked my spoilboard and my straight edge rocked on the top, only in the middle for some reason, at least 1/16". It looks like there is a 1/16" or more hump in the centre of the Y axis and about 2/3rds towards the end of the X axis. I'm kind of stumped here. How could this happen? I put a straight edge on the rails and they are straight. It is a quite pronounced hump not just angled. If anyone has any ideas it would be greatly appreciated. Thanks in advance.
Regards Peter.
Ps - I am using a 3/4" router bit with a 1/2" shaft. I do have a bit of striping, so I should adjust my routers alignment but I just can't see how I'm getting this hump.
Is the base supported entirely, or just at the corners. The weight of the gantry will cause the table to warp down if, the whole length of the Y Axis Guides are not supported below.
Just spit balling an idea here, but it may be more related to the material used for the spoil board than anything mechanical. Speaking strictly from my own experience (only some scientific data to back this up) real wood has inconsistent tensions related to the material itself. Although the machine bed may be flat, surfacing real wood changes the face surface tension in relation to variations in cell density. With that in mind, different expansion and contraction rates in the cell fibers can be observed from the "Top" of material than the bottom, causing a slight "cup" or "warp, which may be viewed as a slight rise of drop in overall height, as cell sizes vary throughout. This can happen almost immediately (although rare) after the material has been planed, thus resulting in your discrepancy in "height". Temperature, humidity, even air density (pressure) play a role in the stability of solid wood ( known by making wooden clocks myself), so this may be it. Before taking any drastic measures I would try using an engineered product like MDF as a spoil board. Surface that and see if your result changes with regards to your Z height. If it does, as they say, "Well there's your problem right there Verne".
"I'm not smart, I just remain on problems longer"
Albert Einstein
Making many BTU by experimentation. ...some days it gets too warm
Thanks for the tips guys. The base is fully supported on 3/4" ply on a very sturdy table. I'll check if there is deflection as the gantry travels along. I wouldn't be surprised if this could happen as the machine is not that sturdy and I have been disappointed with the problems with the T tracks not being level, the clamp being useless, and also the reported Z travel not being correct. I've done a lot to make this thing more useable and hope this isn't another one of those things that is "just the way it is"!
I can feel the physical dips in the cutting on the wood, so I don't think it is wood movement, but I could test that with a large mdf planning exercise. Is it possible that the support across the gantry is bowing down? I don't really think this is the case as the changes in the Z are quite severe over a short distance. The reason I got away from MDF in the first place was because the clamp was so horrible that it couldn't hold my router high enough to make the dust collector useful. I actually burned out a switch from the MDF dust from surfacing a spoilboard. Come to think of it I don't think I had the issue before with MDF, maybe it is the wood bulging after it is cut. I might make another spoilboard out of MDF as noted in that video now that I have the Dixie billet clamp. I did actually buy the Rockler T slot bit but my dad made me a hardwood T track spoilboard out of Oak. Thanks for all the help guys.
This probably is not the issue, but might be worth mentioning.
The way the shark is made, it is possible that the y axis bars are not straight. This would be due to the center support not being exactly the right height to support the center of the axis rods. If it is to tall or too short, it will pull the rod out of straight. Usually, with supported rods you will see all the supports fastening on to one plane. In the case of the shark it has the rod ends going into the front and back plates and the center support is fastened to the top of the base plate. In theory it works out OK if all pieces are machined and assembled exactly right. In practice I have see in off. In one case the center support pulled the rod out of being straight. I would lay a straight edge on the top of the bearing bars just to be sure.
It may be that you could be seeing movement in the hardwood after you machined it and it relieving internal pressures. especially if it is slotted.
If you repeat the surfacing cut, taking only a few thousandths off, will it still have the same hump?
Wish I had thought of this earlier in the day when it happened and took pics but the issue with cupping and warping happened today. I am a cabinet maker by trade and needed to rip down some Maple for filler from 1x6 to 1x1. Board was perfectly true, straight as an arrow before ripping (10' length). Sure as sam, once it was ripped, internal differences in tension caused them to warp more than an inch over the entire span. Kiln dried material with a moisture content less than 8 percent. Now to say this would happen that dramatically over the distance of a spoilboard would be ludicrous, but the difference we are describing multiplied over the distance of a spoilboard? definately.....Just saying....check the spoil.
"I'm not smart, I just remain on problems longer"
Albert Einstein
Making many BTU by experimentation. ...some days it gets too warm
Thanks guys. I haven't had time to figure it out, but I think it could be the internal stresses in the hardwood spoilboard. I will be making one of mdf soon and will then be able to test. That's a great idea about the angle under the bed to stiffen. It would be nice if the t-track bed was actually planar with the movement of the router. Thanks again.
So I got a straight edge and thought that the work piece had some movement but it was dead flat. Then I checked my spoilboard and my straight edge rocked on the top, only in the middle for some reason, at least 1/16". It looks like there is a 1/16" or more hump in the centre of the Y axis and about 2/3rds towards the end of the X axis. I'm kind of stumped here. How could this happen? I put a straight edge on the rails and they are straight. It is a quite pronounced hump not just angled. If anyone has any ideas it would be greatly appreciated. Thanks in advance.