Need help leveling a CNC Router table

[Scalesoar]

All,

I have a 96x48 ShopBot PRT Alpha (servo) CNC router that needs to be extremely level. I have been using a digital level that measures to 0.01 degrees (SPI-Tronic Pro 3600) and have leveled the machine to within 0.03 degrees across 32 points in the XY plane. My concern is that this will not be accurate enough for my CNC mold making needs.

I have heard numerous CNC folks mention the term "laser leveled" but I am not familiar with this process or the equipment needed to get a perfectly level table at all points in the XY plane. Can anyone assist in this laser leveling technique (methods, equipment etc) or suggest perhaps a more accurate leveling process beyond the digital level?

Thanks in advance,

Tony

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[atifeh]

Hello,

I had some experience with laser levels which you might find useful in:
http://www.cnczone.com/forums/showthread.php?t=14051

I personally did not get a feeling of dependability/reliability with it therefore I only use it for primary alignments and leveling. I use ordinary measurement methods.

My tools for leveling which might give you the idea of how I work are: a 16x40x2000 mm carbon steel ruler which I have planed it to 0.015 mm, a 400 mm grade 2 accuracy machinist square, a 0.02 mm precision liquid level, feeler gauges form 0.02 to 1 mm, a precise hand made blob and a 1000 mm ordinary liquid level. With these tools + patience I work within 0.02-0.04 mm.

Nader

[MarkT]

Tony,

I have always used a precision spirit type level to level numerous machine types ( lathes, mills, routers) You can research these on the web in various degrees of accuracy. It is common on CNC machinery to have the sensitivity to level to within ten thousandths or millions of an inch over a specified distance ( usually 12" , 300mm, etc.) Leveling is important, just as important is eliminating any "twist" that may be in the bed.
The tolerances and accuracy you state you want to achieve, you also need to calibrate the perpendicularity of the router spindle to the table once it is level. This is done mounting a precision test indicator in the spindle and "sweeping" a flat plate mounted to the table. You must tram the plate flat with your indicator first, then "sweep" it rotationally with the indicator. This will insure your router is perpendicular to the table.
Best of Luck!
MarkT
www.cnccustomservices.com

[Scalesoar]

Mark/Atifeh, thanks for the guidance.

The digital level I have measures down to 0.01 degrees and I used 36 specific points across the XY plane which gives me a grid at 12" spacing.

0.01 degrees at 12" provides an indication to 0.002". Since I have only achieved +/- 0.03 degrees to date, that implies only an accuracy to 0.012" which is not real good.

The Shopbot is considered a low cost router and therefore designed with that in mind. It is a simple bolt-together table with no alignment pegs in any component and therefore becomes a little bit of a crap shoot to level.

Now some may say that an accuracy of 0.012" is good enough for RC models but I suspect, like many, I am a perfectionist and it doesn't feel good to me.

My mind tells me that a digital or spirit level provides me a datapoint or set of points across the table but it does not provide datapoints with regard to one reference point - say the zero/zero point. Since this is my first time attempting to level a machine, is there any advise you can provide that allows me to use such a device to measure the change between the reference point and any other point on the table or is this completely the wrong tool for that job?

The scanning laser level, I assume, can be placed at such a reference point and scanned across a table presumably in both the X and Y directions. A second piece of equipment again, presumably, could be placed at various points on the table and a delta indication taken. Does such a tool set exist? Am I on the right track?

Mark, regarding perpendicularity, I am not sure I fully follow your "instructions". Is there any way you could possibly expand on your comments?

Again, thanks for your responses.

Tony

[mxtras]

Another method would be to level it the best you can, then place a somewhat sacrificial piece of material in place (MDF or even aluminum) then take a skim cut with the spindle after the machine is leveled to the best of your ability. The skim cut will put the surface of the table parallel with your axii - it's a good way to cut your deviations by a large margin if you can live with having a sacraficial board/material on your table/working surface. Just a thought - I don't know your machine or your hold down requirements.

As far as making it perpendicular, search for "tramming" - you will find lots of threads about tramming a Bridgeport spindle and this procedure is what Mark is refering to.

Scanning lasers are a reality. You haven't lost your mind! You may find one that can be rented or you might make some calls to local alignment professionals - see what they use and you will likely see why there are not that many hobby machininst that have such equipment. The typical commercial systems start around $30K.

Scott

[Scalesoar]

Scott - thanks.

The leveling I have done to date has been on the X and Y rails of the CNC frame and not the table itself. The level has been attached to the Y carriage that's mounted to the X rails and that's where the leveling has been done - both in the X-X direction and Y-Y direction (a total of 64 measurements). I have even stripped the table down to its bare frame and started from scratch because I was not satisfied with my first attempt!

Once the FedEx damaged parts of my recently aquired dust extraction system are delivered, the sacrifical MDF (already installed) will be skimmed as you note. One of the first "jobs" the shopbot folks indicate as necessary. FedEx damaged the 4" vacuum tube and the output filter fine dust collection drum so once I get those, I'll be good to skim the table - just don't want to do it without collecting the dust! That will take out the 0.060 to 0.070" of undulations on the bed material I have already measured with respect to the spindle. That will at least allow the table surface to follow the leveling I have performed on the X and Y carriages.

Point taken on the laser system!!!! Guess I need to find some local machine shops to see if they would rent such a device! The fact that they exist is a good thing and I apreciate the guidance with that.

My apologies for all the questions but, if I fail to find such a device for rent, is there some other method available to measure points on a table for level and twist using alternate methods and a single reference point?

I'm think something like the construction of an internal frame suspended over the table but under the carriages and making that perfectly level and then matching the carriages to that. I don't have access to such a frame but maybe I need to invest in something like this to be more confident of level and no twist.

Anyway, thanks for your advise and I will definately search for tramming for the Z.

Tony

[ViperTX]

I seriously doubt that a laser level would do a better job then a machinist level....be it digital or bubble.....for less then $40K.......the problem with laser beams is that they spread out the further they get away from the collimator lens....

So, unless you're spend a bundle on the laser optics the machinist level is still the best way to go.

[MarkT]

I am sorry I did not answer in great detail. Tramming can be a finicky process and varies from machine to machine. Review the link below to get the basic nuts and bolts.
http://www.gadgetbuilder.com/MillTram.html
Hope this Helps.
MarkT

[Scalesoar]

Mark - Got it! Believe it or not, something I did actually learn about 30 years ago in England during my machine shop apprenticeship. Not sure the Brits call it tramming but I do recal having to do this!!!! Age creeps up on one very fast!

[Scalesoar]

Mark, it also occurs to me that the X and Y planes need to be perfect before attempting tramming. If they are not, then the results of the tramming exercise will vary where it's done. I think you already implied this but it makes me even more determined to find a good method of leveling this machine.

Here is a pic of the machine I have - as I said, not one of your high-end, machined level systems!!

[mxtras]

Level your axii - not the table surface.

You only have four level control points, right? If it's level on the ends and not in the middle, it is bowed and there's not a whole lot you can do unless you are into installing jacking screws or reworking the machine.

Level the y across at one end, then level down one X side, then run the gantry to the other end and level Y by adjusting the untouched corner.....does that make sense? Re-check the whole thing and make sure all the feet have weight - if not, your frame might have a twist. Then skim the table surface, tram the head on a flat plate resting on the table surface and then skim again to remove any steps and re-check your tram (which should be any different unless it was WAY off before).

That's how I suggest attacking such a task.

Scott

[Scalesoar]

Scott - got it.

That's the approach I have used!

I also put some mind power to the problem last night and came up with a method of creating the reference location using an off-table scanning laser - similar to the lasers used and generally available for leveling shelves and the like. A second laser pointer (focusable) would then be mounted on the Y carriage, first in the Y direction and then in the X direction. With the reference line leveled and positioned correctly on a wall or vertical reference board mounted to my floor, the point laser will be fixed to the carriage and the carriage moved from Y=0 to Y=48. If the point laser deviates from the scanning laser on the wall/board (either up or down) during the traverse from 0 to 48, I will have a reference location to adjust from. Combine this with the leveling of the rails I have already done should yield fine adjustments to the rails that I can do with shims (the rails are bolted to the aluminum extrusions and can therefore be shimmed).

The point laser will be fixed to the carriage using a generic telescope laser attachment bracket (actually two of them - one in Y and the other in X).

Total cost of system around $350 and I think it provides the reference I need (scanning laser leveled to the planet(!!)) and moving references on the carriages. I have located the focusable point laser (~$70) and the mounts ($49 each). The only thing left to find is a reletively accurate self-leveling scanning laser I can mount to a platform elevated from my workshop floor. The Black and Decker 360 degree, tripod mounted self leveling laser looks to be a fairly good option and only $170.

Fortunately, I already have about 8 points on my table that are level per the digital level (0.00 degrees in both directions). These will offer good points to set up the scanning and point lasers. A 0.00 measurement yields a level measurement of 0.00104" worst case at a specific point. So this is a good starting point for the other references.

Tony

[routalot]

I think there might be a touch too much technology obscuring a simple solution.Flat is one thing and level is another.As has been mentioned in an earlier post,it is the axes that need to be flat and the table structure needs to be solid enough that deflection under the weight of the parts being machined is taken out of consideration.If these criteria are met,how will a tilt of a fraction of a degree spoil the part being machined?For a quite usable solution,you can use a basic water level.If the axes are flat and ideally the Z-axis perpendicular,the light skim referred to will give the result you seek.You might also try zeroing a dial gauge under the centre of the gantry and moving the motor across the full width.There may be no deflection or there may be more deflection due to the weight of the machinery than your precision level has found in the table.It moves on a bit if you use spiral flute cutters for machining and still hope for flatness.There is a lot of anguish in making the leap from very good accuracy to perfection.Good luck with the project

[Scalesoar]

Fully understand your point! My view of this is that a 2.5D profiling part, good enough should be "good enough". In my mind however, a 3D part that needs to be accurate in both form and twist to within 0.001 or there abouts, needs a little extra attention.

The project is aircraft flying surfaces. In particular, very long and skinny aircraft wings (8 to 10 feet long sections, 10 to 15 inches chords). Twist in a wing has some very significant negative impacts on the flying characterists of a high aspect ratio wing - the types I intend to machine, so, flat and twist-free is fairly important. Since I only have a 8 foot table and intend to make up to 10 to 11 foot single piece panels, indexing is also a requirement. Having twist in a table WITh indexing will not allow me to move a part that is partly machined and then start the second step of machining with accurate, step-free results unless the machine is perfectly level and twist free.

It's not so much that I demand it but the part itself demands this for accuracy.

Anyway, I fully understand that the axii need to be flat, level, twist-free and perpendicular. The table skim is simply the last step to align the bed with the axii.

The tips provided by all of you have really helped and I have ordered the leveling equipment to further refine the table. Thanks again.

Tony