Table Surfacing problems

[john_r]

Anyone have this problem? Or better yet, a solution. I have squared the Z axis. I tried squaring the bit on a flat machinist plate. I have set it up 3 different times all look like it should cut perfect. But I still have small lines that seem to taper up towards the center of my spoil board.

Thanks in advance,
John
3771
3772

[fleinbach]

John

It appears you have not gotten the Z axis truly square to the table. Once truly square the surface will be extremly flat. The main problem you are having if I read your question properly is that you used the bit alone to determine squareness to the table. This is a poor way to get square, you need to extend the surface area out several inches from the Z axis to get the best results. There are several ways to do this but I will explain one of them here. Get a 2 to 3" bolt 1/4 or 1/2 depending on the chuck size of your router. Cut off the head so it will fit in the chuck. Now drill a hole in a 1/8 X 1 X 6" steel bar. You could acctualy extend more then 6" to make it easier to determine small changes in adjustment. Using 2 nuts chuck this rig into you router. Now you are ready to make your adjustments in both the X and Y plain. Line the plate up with the X or Y axis. You could measure from the end of the steel plate to the table surface but this allows for more error. I prefer using a block of MDF. Place the block on the table and move the Z axis down until it just touches. When getting close to the block slow down the Z movement so you don't ram into it. Now rotate the plate 180 degrees and check height to block again. Make your adjustment until the steel is touching the block equily. Now do this again in the other axis. Your spoil board will look much better after this.

[mikejohn]

John
Don't switch on the router

.........Mike

[weslambe]

Another thing that it could be is the x rails to some degree. Stick a level on top of each rail and look under it for light. If you have hills or valleys down the length of the rail you will get the marks that you are getting in some places and not in others.

Thanks for the tip Frank. I like your way better than mine. I used an old surfacing bit and cut the teeth off of it so that the bottom was flat then I glued a thick piece of 12" x 12" lexan to it. Then I put it back in the router and lowered it to just touch the surface.

I like your way though.

[gerald_d]

From your photo it is difficult to figure out what you are really seeing - it could just be lighting? What do you see if you lay a steel rule over the table - small ripples, or a big table-wide error? Or no error?

A photo of our table would also look like that, but it would only be due to lighting and cutting direction - like the patterns in the grass of a fancy playing field that is mowed from different directions.

Search this Forum for the word "trammel".

[fleinbach]

Mike, Good point

Wes, I like your lexan idea. I just happen to have steel on hand but you could substitute lexan if its thick enough. Mainly you don't want anything that flex's to much.

Gerald is correct. Even a perfectly adjusted Z axis will allow you to visualy see the path of the bit. This is due to the behavior of light reflection. All cuts bend fibers in MDf, wood, etc. to some degree. Light reflection will show the path the bit took since the fibers are bent 180 degrees on opisite sides of the axis. But John said "seem to taper up" and from the photo they appear pretty pronounced.

[gerald_d]

I have a nagging worry that John has a creeping z axis, but I don't want to scare him until he has put a steel rule across the table.

[mikejohn]

Gerald
A genuine question.
What's a creeping z axis?
..........Mike

[billp]

Another thing that sometimes makes those 'ridge lines'look exagerated is when you do your surfacing passes in TWO directions ( i.e, left to right, then right to left...on the X axis) Instead of using the CR command I wrote a simple file that has ALL of my passes go in the same direction, with a fast (5IPS) jog speed on the returns between passes (Alpha...) This eliminates some of the differences in cut quality based on the fibers in the table , as well as the rotation of the router bit.
Since I have a large (2") morticing bit I use when surfacing the table, I can do this pretty quickly ( 20 minutes) as I take light passes (.05"), and it's not a big deal to do this even when I set up a temporary spoilboard of Trupan for cutting out/through multiple nested parts where I don't want to keep cutting into my "real" table top.

[gerald_d]

Mike, sometimes an axis does not hold its position, maybe due to electrical interference. It has been most commonly reported for the z-axis.

[mikejohn]

Gerald
Is there a straightforward and simple cure?
.........Mike

[toe_jam]

lol, that's how my table first looked. it looks like it's all the Z axis. Also make sure your Y rail is resting on the bottom V of the lower bearings.

[weslambe]

The simple cure for electrical interference is to ground your z car and make sure that all of your wires going to the motors are not lying right next to each other on the black plastic bridge. In a former life (pre shopbot) I was pretty big into the computer world (cisco engineer, programmer etc..)and the problem there was called crosstalk. Impulses from one wire are picked up by the other wire by attenuation I believe. Either twist the cables around one another or place them an inch or two apart. The twisting works on the same principal as telephone or network wire. It all has a tpi value (twists per inch) that all but eliminates crosstalk and attenuation.

wes

[gerald_d]

Guys, I am still interested to find out what John Roberts is really seeing, and trying to show in his photos. Then we can go over all the old discussions of how to get a ShopBot to behave itself.

[mikejohn]

Gerald
Above I asked "Is there a straightforward and simple cure?"
If you 'search' the forum for 'electrical interference' you get a huge amount about grounding, more about running wires close together, more about outside interference (mobile phones, CB radios) and more obscure possibilities.
I'm afraid this is one problem where old discussions don't necessarily point to a 'straightforward and simple cure.
...........Mike

[gerald_d]

It will hijack this thread if we start that discussion over here now - I think that John_R's question (re the pattern after surfacing) hasn't been answered yet, simply because the question is not clear (imho). However, this is probably a good time for you to start a thread, seeing that you will soon have to put on your electrician's cap, leather gloves, rubber shoes and leakproof underpants.

[ron brown]

"Then we can go over all the old discussions of how to get a ShopBot to behave itself."

I want mine to stay have too.

Ron

[Brady Watson]

John,
Inspect 2 parallel passes, IE the Y ones or the X ones. Which way are the ridges sloped?

If the ridges are sloped in the X passes and not the Y, then the router is not sitting square to the table when viewing it in the Y direction. By looking at the ridges carefully, you can see which way you need to tweak the router to get it square. Just the opposite is true if you are getting ridges ONLY in the Y direction. If you inspect the parallel passes, they should BOTH be sloped the SAME way.

If you are getting ridges in BOTH the X & Y, then I would be inclined to say that you are either out of square in both the X and Y directions on the router...ONLY if you see angled scalloping. If you see flat steps with no angled scallops, then you may have a "Z-drop" condition as described above...I doubt it.

Any of the conditions above may also be a result of the wheels not contacting the rails, the carriage is tweaked, or simply is not tightened down enough.

Another thing not mentioned is how much you are taking off in each pass when surfacing the table. Taking off too much in one bite will cause the gantry to lift or surf along the table...don't think that is the issue, but I have seen that happen before on a Benchtop machine.

Hope that helps!
-Brady

[gerald_d]

This sentence of John's post "I still have small lines that seem to taper up towards the center of my spoil board." read together with his photos which show that he is "circling" around his table, working to a finish line in the center, has me confused.

If it was an angle adjust problem, then he won't get "tapering up towards the center of the spoilboard". With a wrong angle, the ridges will be in the same direction right across the table, and the center of the spoilboard will not be relevant. That is why I am reading that there is something different about the center of the spoilboard, than about the outer edges - as if the whole center area is raised up?

Cut too heavy, cutter blunt, gantry flex, lifting rollers, or a creeping z-motor. Or nothing wrong at all - only light patterns......

[gerald_d]

But, looking closely at his photos, do I see a ridge where his cutter is making the 90 degree direction changes? Almost as if the cutter lifts slightly when the direction change happens? If it is ridge, then it is pointing to the center of the table........*reads his sentence again*

Where are the thread starters when you need them??!!

[fleinbach]

Looking as close as I can at such a small picture I see what looks like ridges in both X and Y planes. It apprears the Z axis is out of adjustment in both X and Y planes. When everyting is adjusted properly the lap marks can be seen due to light angle but they don't have much contrast. The contrast in these pictures indicate a ridge that you could most likely feel.

I'm sure we'll know as soon as John reappears.

[Brady Watson]

Gerald,
John didn't indicate that the ridge tapers where happening in BOTH the X and Y directions and if the ridge pattern was akin to a trapazoidal pyramid (I think that's what you are suggesting?)...If this was the case, then something is seriously wrong!!!....

I doubt the ridges are in a trap. pyramid pattern. Ridges may be apparent in X AND Y directions, but the angle in which the ridges slope have to be identical in slope (when viewed from the same place in parallel) on parallel passes. Otherwise, the router is moving in a circular fashion, which would ONLY occour if the Z-axis mounts were seriously loose, and the router was lagging behind much like a tangential knife would.

-Brady

[sawkerf]

J. Have you been snooping around my shop? I could swear that is my spoilboard you're showing. I have the same problem and it is definately the Z axis slightly "out of wack". I know this because I once had my Z perfectly "in wack", but I ran it into the edge of my spoilboard while trying to suface it at the depth of .5 instead of .05 (those decimal places get real important when using a CNC machine). I just haven't had time to re-adjust it. The "bolt and bar" method described above will fix you up.Kip

[davidallen]

I use 1/4" copper tubing about 3' long bent into a Z shape. mounted in the router and turned by hand it gives a very large radius to square with. whether it's tubing, bolts, or bar stock it's the same concept.