Table Top Not Flat

[Chuck Keysor]

Thanks everyone,,,, here is a quick reply concerning the question of play in the Z axis. I often check for play in my X and Y axis, by pulling/pushing on the X car, and on the Y car, but I realized I had never done that on the Z/spindle.

So just now, I pulled/pushed on the X and Y cars, and could not perceive any movement. When I pulled up and down on the Z car, which is a bit more awkward, I was surprised to see movement (and expect I pushed harder on the X and Y cars). So I measured and found I was able to pull up .006" and down .014". How bad of a reading is that?

And a long time ago, when I first set up my machine, I had used my dial indicator on a trammel to square up my spindle. And while I don't recall the numbers, I did post them here and people told me they were good.

As to the spoil board delaminating, that may well be the case. I am pretty sure I used Tightbond glue and I put weights all over the top of the table. But who knows, that was maybe 7 or 8 years ago. I'll set up a cross beam with a dial indicator and see if I can detect any deflection over any areas of the table center.

Thanks,,,,,,,,,, later,,,,,,,,,, Chuck

[Joe Porter]

Chuck, I am just guessing here, but do you have a vacuum system? Could that be causing any pull down on the spoil board? ....joe

[Chuck Keysor]

Thanks Joe. I have a vacuum table, but no functional vacuum source......

I have gotten tied up, but hopefully tomorrow I will see if I can measure my table height, then add some big weights to the table, and see if the table deflects relative to my spindle height.

Chuck

[Michaelodunk]

I was having similar problems, I have one of the first PRS machines I added middle legs to to the rails and it helped a lot to fix the problem.

[EricSchimel]

I have (had) a 4 legged PRS that I upgraded to 6 because my extrusion had a bit of sag to it:

http://www.talkshopbot.com/forum/sho...-sagging/page2

It should be noted though that the rails also sagged so that even though the table wasn't "flat" it was still flat relative to the spindle as I used the spindle to surface the table. I always use a vacuum table so when I suck a sheet of plywood down it conformed to the table surface.

If you're not vacuuming your stuff down and you do have some sag (and there's no play in your Z) perhaps your plywood is rising up a little when you cut it and you don't realize it?

If I were you I'd surface your table, zero off of it and check the disance in a few spots throughout the table. Write them down and see if they change signifigantly.

You do say you have play when you pull up, and push down your Z. If you're numbers are correct you've got a combined .02" difference. That'd be enough to screw up VCarving a lot. If you've got a sagging table too that might also compound the problem.

I'm no super expert, but this is what I'd do:

Pull off your Z motor and check the pinion. This might be a good time to get a new pinion gear, they are a wear item. Make sure it's tight to the shaft. I'd also see if there's any play in your motor gear box. After that I'd check the Z carriage for any play before you put the motor back in. Then, put the motor back in (assuming you have a good pinion, or with a new one) and make sure it's good and tight. Check it for play. It should be solved.

Then I'd go about shoring up your table, resurface and check everything again.

[Chuck Keysor]

Thanks Eric and Michael. My table does have 6 legs, all standard Shopbot issued iron frame and legs. Yes, I had been surprised, as I would have guessed I would have had the table flat relative to the spindle.

I got tied up making some other non-shopbot things, but now I will go back down into my basement and see if I can measure any deflection of the table relative to the spindle per suggestions by Gary late last week.

As to the Z slop, I'll have to investigate of course. But as a clue, the last V-carving I made turned out fine AFTER I had shimmed up the low end of the plaque, so that when I zeroed my Z-height at one end of the 12" wide board, Z was still zero at the other end of the board! It was a thick shim, something like .030" for just a 12" wide board!

Thanks, Chuck

[Michaelodunk]

I have one of the first prs machines. I was having the same problems. The first thing I did was put middle legs under the rails I now have 6 legs instead of 4. The other thing was to replace the clamping system that I had made. The mistake I made was to make it out of plywood. Plywood is not flat so I replaced it with MDF which you can surface which you can't do with plywood.

Mike

[Mayo]

Chuck, when you resurfaced did you remove material from the entire surface of the table top?
Or were there any areas which were still untouched by your resurfacing bit?
Does your Z have springs to assist it like mine does? At one point I had to increase tension on those springs to try and reduce some of the play in the Z but I don't think it ever eliminated it.

[Chuck Keysor]

Hello Gary C. I measured the deflection in my table top, at 5 locations.

I went to the middle of the y dimension, as logically, the deflection should be greatest there. And two of my test points, were right over the big angle iron that supports the table between pairs of legs. With my 60 x 144 table, that put my points of deflection measurement at: (4,30), (22,30), (40,30), (58,30) and (77,30).

I set the spindle with a 60 degree V-bit at each of the above listed points, then used C2 to zero the bit. And I checked it with a feeler gauge. Then, I climbed up on top of my machine's bed, and crouched down as close as I could to the spindle, and used feeler gauges to measure how far the table had sagged. (I weigh only 140lbs, and I am guessing my center of gravity was about 18" away from the tip of the V-bit.)

At (4,30) (over a leg cross support): Deflection = .003"
At (22,30): Deflection = .011"
At (40,30): Deflection = .007"
At (58,30): Deflection = .009"
At (77,30) (over a leg cross support): Deflection = .004"

As added notes: I have a three layer vacuum table top. The bottom two layers are single 5'x12'x 3/4" Medex (or equal brand, I forgot if the supplier had that exact brand or not). The top layer is made out of Ultralite, but that was not available in a single big sheet. So I actually cut up 4x8 sheets to get 6 equal sized panels, which I glued on top of plenum layer.

There is one small part of a joint where the top layer has slightly delaminated, as the height of that region of the table top is .015" low on either side of the joint, but for one inch on either side of the joint, the table height is exactly zero (relative to the (0,0) reference). But when I push on that area, I can't see any movement.

So, there is some deflection, but at least I can't see how that could account for the low areas that are low by as much as .080" relative to (0,0).

Does this suggest anything?

Thanks, Chuck (PS: I attempted to map out my entire table top, to show the patterns of where and how much, my table top is low. That was laborious, and a somewhat interpretive process, which is why it took me so long to reply to your question about deflection. I'll put those measurements in the next post.)

[Chuck Keysor]

Concerning my table top not being flat, I have attempted to map out the topology of my table top. I hope that by looking at my following notes, and then the figure with my measurements, that someone can figure out some pattern that may help to figure out why my table top is not very flat. (It is bad enough that it messes up V-carvings.)

I did not use my spindle for this measurement, as I don't know if the spindle itself is part of the problem or not. So I have two very good levels, one is 6 feet long, and the other is 8 feet long. I bought these when I had to assemble my Shopbot.

I started with the 8 foot level, with one end located on 0,0 and moved along the X axis. As the level spanned the distance from 0,0 to the 74,0 point and read level, and still read level when spanning from 74,0 to 146,0 this seemed acceptable to use, instead of the ideal 148" long level.

I used feeler gauges to establish how big the gap was between the bottom of the level and the top of the table. Looking at the figure, along the X axis, the table never went above zero, and never went lower than -.030" and I made my measurements in ranges, never trying to measure say .013". Anything between .020" and .015" I called .020". Anything between .015" and .010" I called .015".

Then I used the 6' level to measure dips in the table in the Y direction. In the attached image, you can see where I placed the 6' level: at x=0, x=24, x=49, x=77, x=98 and x=145

The readings shown going in the Y direction are relative to the height at (0,0). So if you look at (49,30) the shown reading of 50, means that relative to (0,0) the table is low by .050"

All of the numbers (except for the 5 deflection figures) that are not on the outside edges of the table, are the depth of the table below 0 shown in thousandths of an inch.

I had hoped to color code this to make bands of equal height more obvious, and possibly aiding in someone figuring out what may be wrong. But I gave up as this was beginning to look imponderably complicated........ Thank you, Chuck