Surfacing table, how often, how far?

[harryball]

How often do you guys resurface your table and how far do you go?

I've started allowing my bits to cut into the spoilboard ever so slightly. Everything I cut through leaves almost a ghost trace on the spoil board. Maybe I heard wrong or I'm just not as good but if I set it dead on and don't leave any marks the bottom side of my work piece never comes out clean. This means resurfacing once or twice a week.

Then, how thin do you let a 3/4" spoil board go? I'm down to just over 1/2" now. Am I to assume provided the screws are clear I can keep going as long as it lies there and makes a flat surface?

Robert

[mikejohn]

See if everything you need is here (http://www.talkshopbot.com/forum/show.cgi?tpc=7&post=33087#POST33087)

...........Mike

[jamesgilliam]

Robert, How important is it that the material is perfectly flat on the table? Reason I ask is we do puzzle rockers and cut through the material +.050 deeper to be sure of clean edges. I use 3/4" insulation foam that you can get at Lowes or most large building material stores. In the last 5 years I have only resurfaced my table twice, and gone through 3 sheets of the foam since we flip it and use both sides.

[mikejohn]

James and Jo-Anne
This is new to me, and I suspect new to many.
Can you explain in a little more detail the type of cutting you are doing?
I can see if everything is cut through, then a not-too-level table might work ok. What is your hold down system?

..........Mike

[jamesgilliam]

Mike, We do alot of parts for cabinet work and the rocking chairs. Mainly cutting deeper than the material thickness so the parts are "loose" at the end. For hold down I am using dry wall screws, clamps for full sheets, and anything else that is sitting around. The best part of using the foam is the chairs have some small parts and the heat from the tool slightly "melts" the foam and holds the part in place. We also do alot of v-carving for plaques and then we just screw the material right to the table and start cutting, usually laying out five or more plaques on the same board and cutting apart at the end with a saw.

[harryball]

First let me say I'm a Computer and Electrical Engineer... and an onion skin on the back of the work piece or marking on the spoil board is a failure to cut precise. I'm used to working with nanometer tolerances :-)

That said, the bat houses we produce have grooves cut about 1/32" of an inch deep over the entire interior surface. It's important that the table be flat enough that a 1/32" groove stay between 1/64" and 1/16" deep... that'd be +/- 1/32" tolerance.

When I cut through, leaving an onion skin slows things down a great deal but I don't want huge grooves running around the table breaking the vaccum.

I'm facing several challenges including such things as a standardized zeroing policy.

We have 50 or more bat houses to kit out in the next week. With 2 to 4 houses per sheet cutting times running 30 to 60 minutes per sheet no longer seem as good as they use too.

A good smooth vaccum sealing table top helps things along greatly.

I'm still a little stunned, to be honest, where this sudden flood of orders is coming from... stunned but not complaining. My math says my bot will be running non-stop for about 18 hours not including bit changes and sheet handling if I use my current files and settings.

I think it's time I kicked it up a notch to see what this thing can really do... I'll be posting questions later I'm sure... like is it practical to cut 1/2" ply in one pass with a 1/4" spiral bit at what speeds etc...

Robert

[bob_lofthouse]

Hi Robert,

I'm Cutting 1/2" ply in one pass with a 1/4" spiral (down spiral) 6ips @ 18500 RPM using alpha 3hp spindle.

When cutting through we usually go down 1mm further into the mdf spoilboard.

Yours
Robert

[harryball]

I don't see how you guys do it, I've been cutting about a week and the table needs to be resurfaced again. Ever so slight an amount but it needs it. The MDF has noticable traces all over it and a few are just deep enough to break the vaccum around the edges.

I've cut about 12 sheets since the last resurface. I changed my tactic to zero to the table bed today to see if that helps on cut throughs.

Robert, thanks for the info, I bumped my file up to 6ips at 18000RPM. I'm running an HSD 4HP single phase and that's my max RPM.

Robert

[beacon14]

I started with 3/8" MDF, actually more like 5/16" after surfacing, since that's what I had on hand, and I was concerned about being able to pull through anything thicker. It's now down to under 1/4", and I am starting to have problems with the edges curling up and, surprisingly, less holding power it seems.

It took me a while to figure out the mechanism, but I think I have a clearer picture of how spoilboard thickness affects holding power. It seems the thinner material allows more air to pass through anywhere that the spoilboard is not covered by a non-porous workpiece or masking. I was concerned about being able to suck air through a thicker piece, but if you think about it once the vacuum has been established there is very little air being pulled through - in fact the less the better. Essentially the system creates a vacuum inside the MDF spoilboard, causing anything touching the other side to be sucked against the spoilboard. It's when the platen gets thin, and can't hold the air back from the uncovered areas, that I start losing holding power.

I'm about to test the theory by starting with a piece of 1/2" MDF.

More on topic, I only surface when I am about to cut something where uniform depth of cut is important. Unfortunately that includes cutting full sheets, since in order to leave a consistant onion skin you need a flat table. Since I don't cut sheets every week, I have been re-surfacing only about once a month, but that frequency is likely to increase - another reason to start with a thicker piece. Unless I've made a toolpathing error, taking off .03 will remove all the cut marks and unevenness and leave a nice, clean flat surface.

[Ryan Patterson]

I resurface 3 to 4 times a week. I will get through 60 to 70 sheets before I need to surface the table. The more cuts into the spoil board the less holding power there is. It is possible to have a spoil board be to porous, like David has found. I get better holding using ¾” then ½”.

I don’t use ¼” cutters but I can cut ½” ply at 10” /sec at 17,000 rpm using a 3/8” cutter.

Ryan