Need advice on a surfacing operation

[mzettl]

I need to surface a piece of bubinga that is about 6 ft. long, 4 ft. wide and 1 1/2 inches thick. The challenge is that my machine has only a 48" x 48" bed. I was thinking of making some type of frame for the piece to fit into, placing wedges under the high points for stability, securing the frame to the table, then surfacing a 48" square section. At that point, I could slide the frame a couple of feet and finish surfacing the the remaining portion. Once one side is flat, then I could do the other side without the frame.

Any thoughts on this technique? Also, I'm planning on using the 1 1/4" bit that I do the table surfacing with. Would there be a better option? I'm not really concerned about how long it takes. Accuracy is more important.

Any thoughts, recommendations, opinions, etc. would be greatly appreciated. Obviously, this is an expensive piece of wood, and I don't want to screw it up. I've got one shot at it.

[paco]

The pallet idea is what I would go with. If you cannot screw in then glue it with dime size spots strategically located.

Since you'll be moving the pallet for the second half, it may leave some marks depending on the pallet system. I mention it just in case you were hoping for a sand smooth finish.

You may want to give a shot at this surfacing routine (http://pacosarea.blogspot.com/2007/02/surfacing-along-axis-shopbot-routine.html). It go along the grain rather than offset pocketing...

[mzettl]

Paco,

I was hoping you might chime in on this. Your opinions are valuable, and always welcome. I hadn't thought of "spot welding" it with glue - that's a good suggestion. I also looked at the surfacing routine you referenced, and that really appeals to me as opposed to pocketing with its associated cross grain runs.

Thanks for your input.

[paco]

Thanks for the comments Matt! Glad I can help.

Make sure run the routine under preview mode to get used to it then maybe a few shots at air cut.

This routine will still cut one of your end in a cross grain fashion as part of it will be hanging off the machinable area but only over a lesser area (bit diameter) than from an offset pocket fashion. I generally set a longer length to make sure the bit make it's turn when it's not on the material and make the entry point off a bit... the routine over run the ends of the machining length.

You actually got me thinking that I could make it so that it do a gentle 3D move (ramp out) at each end to make it more sophisticated in cases like yours.

[jseiler]

If this type of work is something you will be doing more in the future, it may be worth making a high quality sliding fixture. One way to do this would be to machine a piece of mdf flat, screw down long lengths of aluminum extrusion to it (80/20 extrusion sells their surplus on ebay). they make slides that fit around the extrusion so that you can make a sliding table. this would ensure everything stays flat and eliminate bowing. The nice thing about t slot extrusion is that once you have some, you can use it for all kinds of things.

With a nice piece of wood like that, I'd probably glue a couple of pieces of mdf together into something approximately the same size as the real stuff for a practice run or two.

John

[dray]

I would surface as much as you can, slide it down then manually bring the bit to zero wedge one side flush with bit then manually take the bit along zero to the other side and wedge it as well then recheck both sides.

[mzettl]

Paco - A ramp move on each end would certainly be clever way to eliminate the cross grain moves. One of my concerns is that the slowed "turn around" within the material will cause some burning, and a ramp move would eliminate that.

John - I'm not sure how much of this kind of thing that I might be doing, but such a sliding fixture wouldn't be that difficult to make. It's a good idea, thanks.

Danny - That's an excellent idea which I will definitely put to use.

[paco]

Matt,

it doesn't burn when I use it but I do keep the RPM as slow as feasible and stepdown quite small... in hard woods. I now use it with a 2" CED. You can use any bit size with the routine. The smaller the CED the faster the RPM. A dull bit is more prone to burning too.

There is a ramp in for the entry point for non plunging bit but that's all.

[myxpykalix]

here is a idea i came up with after thinking about it. purple is T track, green is sliding rails to hold piece laterally preferably made from mdf, blueballs are those rollerbearings to slide piece.

7918

[ljdm]

I know it may sound childish, but couldn't resist - Blueballs? Makes me cringe. Another color choice. Ok, only lame attempt at humor this morning.

[ljdm]

Real question - what holds the piece down - won't it want to lift up possibly, or do the sliding rails apply enough pressure?

[waynelocke]

Surface as much of the middle area of one side as you can. Wedge the piece however to get it stable to surfacce. You will then have a flat surface to register from. Flip the slab over and use double faced tape or screws, if it is the bottom, to secure a piece of plywood or mdf to the surfaced area to raise it above the unsurfaced ends. Then flip it over again and surface the top in two setups. I have a little experience with large slabs of Bubinga and given the weight and strength of the slab, if the piece was clamped down or screwed, the 1 foot cantilevered ends should be no problem. You could extend the plywood beyond the sides to make tabs for screwing the slab.

[mzettl]

These are all good ideas. Thanks Jack and Wayne for your input. It took me a couple of readings to understand Wayne's technique, but actually it is very simple. I agree that the wood is strong enough that a one foot cantilever on each end would not pose any problem.

I guess what I need to decide is whether or not I will be doing this type of thing frequently enough to make a dedicated fixture for the operation. In that case, some type of sliding table arrangement would be ideal. If done infrequently, then Wayne's method would be easy and effective.

This board is just a terrific resource. I wish I had sufficient experience to help solve problems for others. Maybe someday.