Spoilboard not flat

[wardsa]

I have a Fein vac system under Lite MDF. I am getting a "dish" in my table. I first thought it was a deflection of the frame with the vac on, but I just surfaced again and I am getting .015-.020 lower in the middle of the table. This has been ongoing, but I typicaly cut sheet goods so I didn't worry much. I could always see that the cuts into my spoil board were deeper around the edges, but always thought it was deflection of the table. I put a piece of laminate on the table and turned the vac on, but didn't see a big difference so I kind of ruled out deflection. I'm doing more signs and I'm concerned with the finished product. Is this normal or do I have something going on? I have a PRT Alpha.
Thanks, Terry

[woodworx]

humidity will cause the mdf to expand and contract. Keep a barometer by your machine to see what is going on. It should be flat after surfacing though.

[wardsa]

It's not humidity. Not this time of the year in Michigan. I noticed this problem months ago and have changed the spoil board and surfaced many times since. I thought surfacing would take care of it too.
Terry

[blackhawk]

How are you measuring this deflection? Are you using a straight edge laid on the table or are you chucking a dial indicator into your router?

[wardsa]

Well, for deflection I put a piece of laminate on the table and turn the vac on. Then I put a straight edge (the same one, at the same location, I use without the laminate on just the spoil board)right across the middle of the table. I then measure with feeler gauges. When checking just the spoil board, I use a v-bit, go to my 0,0 corner and take the bit down until it just touchs the table. I then just move around with the arrow keys to different locations and measure with feeler gauges. I don't touch the Z.
Terry

[radone]

Terry
I to have had issues with this. Called Shopbot and they said .005"-.015" was good. Not for me. I now have mine 0 -.003" across an entire front half of a 5'* 8'. Across the back is out .010" low. Mine has also been consistant about this difference across the back with 4x planing of the table. This I have checked with my dial indicator. From my experience when a surface is planed regardless of how it is rotated in space it should be parallel to the plane the cutting head is moveing in. I will continue looking for answer.

[cnc_works]

I have a PRT96 and I know when I have the Z car in the middle of the gantry, I have a sag of at least .01 or so just from the weight. When it is out at the edges, no sag. Never has caused me any issues that I can see, though.

Donn

[tparr]

Terry; Sorry about butting in; I am interested in finding out where to get Lite MDF. I live in Brighton,MI and have not been able to find it.
Tom

[radone]

Tom
Toledo Plywood has it and the supply I have is effective in hold down.

[wardsa]

Hey Randy, how are you? I'm sure for what I do it will probably work fine. It's just one of those things that gets you scratching your head. It bothers me more that I can't figure out whats going on.
Donn, I see what you are saying about the gantry sagging, but it should also sag as I am surfacing the top too. I would think if the gantry was sagging I wouldn't notice a dip in the middle.
Tom, A couple of places. The first place I got it from was All Star Plywood, I think thats what it's call. It's in Detroit. I went there and picked it up, take your 45 with you, it's down by the Fairgrounds. You know what I mean if you've ever been down there. Last time I got it Toledo Plywood. I'm down there a couple times a month, so I just pick it up when I'm down there.

Any other ideas????
Terry

[cnc_works]

Terry, there is a small sag in the middle of my table, except it is not a sag, it is because of the sag in the carriage. The fact is the distance from the tip of the cutting tool stays the same distance from the spoilboard whether it is the edge where there is no sag or at the middle where it is at its worst so a straight edge would show a dip in the middle of the table.

Donn

[wardsa]

That's the thing Donn. The tip of the cutter is .020 higher off the table in the middle then it is at 0,0 even right after I surface the table. So a sag in the gantry would show the distance from tip to table the same all over.
Terry

[blackhawk]

Terry - How much are you taking off the table when you resurface? My only thought is that if you are taking a big cut, you could be pulling up the spoil board in the middle of your table. Maybe there is a little less rigidity right in the center and your cutter is lifting up the spoilboard as it cuts. Then after cutting, the spoilborad relaxes and makes a low spot. Just a theory?

You could try to zero your cutter in the lowest spot and then resurface the entire table by just the .020 that you are off. That way you shouldn't be cutting anything in the low spot.

[wardsa]

Brad, I'm taking off .030. I'm sure it's not lifting because I have a 4 zone vac system and I have silicone between the zones, so the spoil board is siliconed to the table.
Terry

[woodworx]

Unless you surface the board every time you decide to cut something you are always going to have a varying degree of spoil board fluctuation. It never will stay within .015 from one side to the next even over the course of 2 days due to swelling and contracting. If your gantry does sag in the center, it will always sag in the center which means even if you surface your bed, your z machining depth in the center will still be relative to the z depth on the outsides of the bed, every time.

[jhicks]

Terry, I can appreciate how this would make one scratch your head. Unfortunately I can not think of a single thing that would create a mechanical machine condition where you surface the table, then go back to the center where you just surfaced it and are .020" above that surface only in the center.??????
Have you returned with the same bit in and sent Z to zero or the -.030 or is this just with your level?
How long is the level? In order to see this .020 dip, I would assume the ends of the level are on the spoil board and the gap grows from 0 to .020 in the middle. So I guess what I'm asking is, Does this gap start at the ends of a 4 foot level and gradually increase as it progresses towards the middle? Is it the same when you lay out the level on both X and Y?
Does it coincide with any areas of your Zones that do not get vacuum? Like a gap between zones?
Or with your Vac ports under the spoil board?
This is very strange given that the vacs are on, the bit goes to the -.030 to surface, then something magically changes to develop a lower spot???
Are you surfacing on a offset pattern going around the perimeter of the rectangle from inside to outside or raster by going back and forth on the X?
One thing to consider MIGHT be IF you are using an offset cut from inside to out AND the vac is really not pulling the sheet down fully in the middle, (maybe its between zones, maybe more flexible silicone in the center for some reason, and the Z plunges in the center but also pushes the sheet down further, starts cutting, and as it extends to the outside where the sheet is fully seated on the base board the spoil board has nowhere to go (meaning its fully seated).
Very Strange and defies logic but I think I saw another post with similar concerns up to .062" variation over a sign being area cleared so somethings up.
Have you checked the spoil board after surfacing to see if the spoil board raises up any with or without vac on in the center AND the outer perimiter?
Might try physically pushing the center or edge areas down with your hand to see if they are truly seated with the vac on or if there is some flex in the center or outside edges.
Finally its conceiveable its just the opposite of what you think it is. Maybe the center is being pulled down to the base board but the outer edges are lifting up.
This could definately happen IF the edge vacuum was weak (suction being lost around the edges) of IF you only surfaced one side of the spoil board it will cup. Both MDF and Trupan will cup if only machining one side.
Have you surfaced both sides? Are the edges of the spoil board sealed in any way?
Are the outer edges of your base and plenum surface boards sealed?
The more I think about it the more I tend to suspect the center is really down but the outer edges are up due to weak edge vacuum and/or one side surfacing causing the material to CUP.
When you turn the vac off do the outside edges lift?
Hope one of these thoughts gets you closer to a solution.
keep us posted

[rb99]

Maybe when you cut the up pressure on the Z flexes the Y sag UP and then when the pressure of cutting is gone, it drops back down. Or maybe it just waggles a bit up and down the whole time you are cutting...

RB

[myxpykalix]

I had the same problem and I pulled up my spoilboard and I saw tons of these things running around causing havoc under there. They were making a noise like....rarry, rarry, rarry

6791

[wardsa]

Jerry, Thank you for all the great questions and suggestions. The more I get into this the more frustated I get! I'm almost possitive I don't have table deflection. Almost!...One thing I didn't make clear and I'm not sure if it would matter, when I surface my spoil board I do not have my vac on and then when I check for flatness I too do not have my vac on. So I surface and check in the same conditions. I have not checked the center with my surfacing bit. Here's something I did just try...I took a .25 endmill bit, took it to the center of my table, 48,24 and zeroed the bit at that point. I pluged my router into a wall socket so I could turn it off and on manually. I raised the bit, took it to 0,0,0 and then manually made a box around my table ,then ran an X from corner to corner. What I ended up with was a box .016 around the perimeter of the table. The X on the other hand went from .016 in the corner to fading to 0, back to .016 in the other corner. Both legs of the X were this way. It is "dishing" perfectly in the center.
Jack, Thanks, but I spray for those!
Justin, I understand what you are saying, but right now I have 24%RH in my shop and I checked for flatness within minute of surfacing.
Still Lost!!!!
Terry

[myxpykalix]

Thinking outloud...I didn't go back and reread every post but if your table crossmembers were in any way sagging in the center and your spoilboard was attached...but first time you surface that would eliminate that issue.

Have you taken a level or long straightedge and checked your X and Y rails? It seems to me that your issue is in the relationship of the router to the surface. If it is a repeatble problem with the center of the table only then it shouldn't be a problem with the Z axis but must lie somewhere in the X or Y.

Still thinking outloud here...can you do a table surfacing routine and tell if your "dish" in the center is longer in the X axis or the Y?

One way to test to see exactly the perimeter of where your problem lies within the table might be to take a chalkline and run a line every one inch in the X and same with the Y. When finished your table will look like a grid with 1" squares.
Take and surface the table a few thousandths and the grid that remains will be a clear picture of where within the table the problem lies.

Maybe from there you can narrow your search to those areas for clues.

[ljdm]

Couple of questions........... is the table(plenum, etc) dished as well? Is there any dish effect in the y direction, or just x,0 - x,96?
If the plenum, etc are dished, the spoilboard will follow that shape. Also, you really should run the vac while surfacing, and then when checking results. The vac will stop any movement in the spoilboard. Have you measured the thickness of the spoilboard after surfacing - is it thicker at the ends?

[wardsa]

I have surfaced and replaced the spoil board many times and still have the same results. This is not a new problem, it's just one that I now have time to work on(thanks to the economy)! The dishing is in both directions. It's like if I poured water around the entire perimeter of my table, it would all go to the center.
Terry

[ljdm]

My comments still might apply. If you dont use vac hold down, the spoilboard might be curling all around. Releasing tension on one surface of the board could cause that, also, moisture is allowed to enter the board from one side more than the other. Thats why I was asking if thickness of the board was the same all over - middle,edges same? Since you have the time (unfortunately, same as I do, since I'm typing, not cutting)try using vac while surfacing and checking results. Gotta use vac while doing it.My 1-1/2 cents.

[ljdm]

Meant to ask - if not using the vac, how are you holding the spoilboard during surfacing? Silicone,etc? If it is held by other means than vac, never mind my previous reply.

[Brady Watson]

Terry,
Here are a few suggestions:

1) Make sure that the base/support board is bolted down tightly. (Let's say it is)

2) Be SURE that the spoilboard is GLUED down correctly to the support board. NO SCREWS! Glue.

3) Determine if your gantry is sagging in the center. This is particularly true of 5' wide PRT Alpha/4G style gantries (NOT PRS tools). A few lengths of B7 grade threaded rod running thru both sides of the gantry AL extrusion will allow you to adjust sag and true the gantry up straight. This is not such a problem on 4' tools, but 5' tools show more sag in the center. Using a dial indicator on the metal supports (after you have removed the spoilboard) as you move the Y will give you decent readings.

-B

[daski]

If you surface the table, then turn the router off and check for your "sag", the only part of the eqaution that has changed is the running router and therein lies your deflection. It may be vertical movement inside the router when the bit is spinning. It appears that is the only thing that has changed. It's all science so try to limit the things that change between the two experimental conditions. It is analogous to measuring your saw blade for runnout when it is not spinning. You have to measure runnout when the blade is cutting for the answer to be accurate and of value.

[drodda]

Is this a condition of sag in the center of the spoil board or swell out at the edges. Whenever I sirface my spoilboard the edges tend to fuz or swell almost instantly. Are you sure you are not reversing the actual issue you are noticing. Perhaps you are having the same issue I noticed a while back. I surface my table every two days or so to keep this to a minimum. Easy way to check this it to run your surface table routine again instantly after your first one runs and notice if you see marks around the edge that fade to nothing as you get closer to the center. Leave the Zero set the same for both runs.

I've got every possible wrong combination with my machine, 12" Z and 5 ft wide prt alpha. However I have learned to realize that I can measure far tighter than my wood tollerances. So I have learned to live with this by placing my sheet in the center whenever possible to avoid the edges that have raised. Here in Iowa the temp is -25 degrees and the humidity is -25% this winter so we are having more fun than should be allowed with movement of materials and clear coat issues also.

-D

[ffaelan]

I know I have been having similar problems with my surface since I started using this bot. I'm starting to wonder if the whole support table is set up incorrectly. I wasn't with the company when it was set up. Here's what I'm looking at and maybe someone will have a better understanding of whats wrong. PRS Standard, Support table is 1 sheet of 3/4 MDF bolted to Bot, then a second piece of 3/4 MDF screwed to that, then a third layer. I have heard about gluing the spoil board, but it seems to me replacing it would be a real chore, or do you just clear it down with the bot and start over. It is not a vacuum base set up, and we use manual hold downs, ie: clamps, screws, tape. This is also Los Angeles Ca, and we it comes to temp and humidity, well it varies, hourly. Any thoughts would be great.

This is a great forum.
ff

[billp]

MDF absorbs any moisture available and will swell up on you unless it is totally sealed.....

[bstern]

I have had a similar problem. It drove me crazy for a while. My problem turned out to be that the vacuum plenum had come loose from the support board under it. Evidently when I was gluing it down, the gorilla glue had already dried on half the table before I could get the plenum in place.
It seemed that when I surfaced even with the vac on, the spoil board lifted slightly in the middle. Thus it cut more in the middle. Once I put a full sheet on, it held tight against the table.