What's new
What's new

Vertical cnc with a crowned table, how to proceed

lowCountryCamo

Stainless
Joined
Jan 1, 2012
Location
Savannah, Georgia, USA
I began this in the cnc section just to see what advice I would get. I got some good, and Mr King also commented. However, I should have just started here to begin with. I have leveled the machine, and using an indicator in the spindle I am getting a consistent .004 high in the center with the ends both at 0. This is a year 2000 Taiwanese machine with low hours. The table is 20" by 40" and on rails. With the level on the table, moving from end to end, the bubble does not move. My best granite straight edge, only 18", also shows a crown using the spin test. This is the inaccuracy I see in the mill. The way forward might be different than would be for a way machine table as there are no dovetails underneath to deal with.

I have read Conelly's book and have scraped 2 machines. This is alot to remove though. I was advised to either mill the table or remove and grind. I do know that Fadal and others have factory programs to mill the table when needed. I also know this would big mistake on a bridgport, as the dovetails would then not be co planar with the table. I might concider milling the bulk, and scraping to finish. Or do nothing. Any advice?

20150706_005430.jpg20150706_005519.jpg
 
Here's a sideways thought. Do you have any interest in mounting a fixture plate on the table? If you do, you could just shim the fixture plate as needed to be exactly level and avoid the need to cut the table. Fixture plates are a big plus anyway.

Otherwise if I needed to fix it, I think I'd mill as much as I could and scrape the rest to the best of my ability.
 
Hey there,

I'm still having a hard time believing your table is actually out... If you told us that it was low in the middle I could accept that more easily. One could chalk it up to wear. High on the other hand... How does a table go high? I still think you have something else going on. It just seems too uniform in the ends in relation to each other and where it goes high. Put me in the camp that thinks you should inspect underneath it before cutting anything.
 
I share Zahnard's scepticism.

Looking at the table, the four bolt pattern and the lower lip, suggest that the rolling element is one third from each end. from what els you have said, I hazard a guess that the rails are bent being low in the center, and perhaps the table is crowned a thou or so.

Place the granite straight edge at the far end and level top before clamping it. Then run the indicator over the top with a long extension arm. Repeat on both sides, if there is little change in the readings then the table is crowned, if one or both have large discrepancies then the rails are curved or you have to dig deeper.
 
sooo .... how is the lower slide suported? are the ways fully suported?? The first thing I would check is the condition of the T slots, if there really beat up and and have been compressed from lots of use and over tightening it can bend the table up in the middle. Remove the burs in the T slots and recheck.
Cheers Don
 
Hor Bore makers over here finish machined the tables on a vertical boring machine. They machined new tables hollow 0.0005" to 0.001".

The idea of a sacrificial upper sub table makes a lot of sense to me.

Regards Tyrone.
 
Before you go and cut the table there are a couple more things I would look at. Pull your way covers back and break the ball nut lose from the slide and re-tighten. While you are at it I would break the bearing trucks lose on your x-axis and re-tighten them just to make sure there is no stress in the table top. Those are the 5 points of contact with the table top. You should have a positive side to one set of bearing trucks with pusher wedges but I would leave those untouched.

Reading your posts I know you have a good understanding of machines and will take care of common sense details like chips etc before breaking anything lose. I'm like the others...trying to grasp how it is .004" high in the middle. Daryl

One more thing, when tightening the trucks use a torque wrench and tighten in steps and in sequence. Maybe a Fadal rep has a procedure for tightening? I might be able to find one for another brand made in Taiwan.
 
Last edited:
I will do all these things first. The last thing I want to do right now is mill an scrape my table. Hard for me to believe also as my 60 year old Bridgeport was out no more than .002.
 
After re-calibrating my level .0005"/10" I went through a dozen iterations to get the center of the table level. Then I used the rear 2 column screws to bring z in using a .0005 indicator and granite square that has an old calibration tag. I was surprised to find the column out .010" front to back and how easily it was brought back in. So xy are level to 1/2 division and z is even better. I then leveled my granite held in the vise and set the indicator up. Moving x back and forth just a hint of movement on the .0005 needle. I had to do this in two sets as my granite is only 18" So I was again surprised to see so little deviation in the rails.

Also, by moving the level parallel to x, across the table(with table still) the bubbles shows a rise and fall from end to center to end. This correlates what I find by moving table across x, with indicator in spindle touching the table. The way covers are off and I see no obvious problems underneath.

Is is possible the table has warped over the years? I don't know what else to think. What a pain in the ass, as I have a project on hold.

20150709_125337.jpg20150709_125312.jpg20150709_122530.jpg20150709_122457.jpg20150709_122449.jpg
 
Just mill it flat by the amount indicated.
It will then be planar, according to your measurements wrt the spindle.

1. You have nothing to lose.
At 0.004" == 0.1 mm thinner, only, the table wont be less rigid or strong.

2. You can always add a subtable, later (or now).

You can always add a rigid, flat, highly accurate steel/cast iron subtable, thats already been precision ground flat.
Mount with epoxy meant for this like hysol = loctite 3478.

Will carry over 2 million kg, or 10 million lbs, on that surface size.
Mouting with industrial adhesive means it floats, and is thus "flat", until it sets when its effectively one piece.
Dont clamp it, clamps will distort it.

You could test this, by laying a medium surface plate on the mill table, say about 400x600 mm 18x20" or s.
If it indicates to better than 0.001" from the spindle, this subtable is a valid solution.

And, the subtable can be taken off with heat, later, without any damage to anything.
At 200C approx most glues come off.
 
It's possible that the table had distorted but not very likely. I'd give the odds at about 20 to 1 or more. The sacrificial sub table is the way to go. For me milling the existing cast iron machine table is an absolute last resort. Once it's been milled you can't return to where you were. Unless you've got a " putting on tool ".

Regards Tyrone.
 
In my experience with Fadals with boxways,over time the machine will end up cutting stock thinner in the center of the table than
towards the ends,which is opposite to what Bridgeports and the one Haas I worked on did.

I believe that repeated overtightening of the vices/T-slot nuts leads to peening of the table and eventually
it bends.I made myself extra large T-slot nuts that somewhat help with this.
My Fadal table was flat within about 8 tenths except for one corner which was .002" low.
I remachined it with a 3.5" facemill,the area that I wasn't able to reach i filed/stoned.

In your case,first I would make sure that the y-axis bearings aren't allowing the whole table to rock.
If the table is actually crowned and the x-axis linear ways are not ,the x bearings trucks would be arranged on an arc,
so you would ideally remachine the top of the table and the bearing supports on the bottom of the table.
 
if you are sure machine is leveled and axis travel is in straight lines and perpendicular to one another just
.
mark up table or blue it and mill .0005" with a sharp inserts facemill and just use handle mode. i am not sure what the big deal is. you should see high spots milled off .0005" at a time. so first few passes the center of high spots will clean with face mill. and as you do more passes in depth it will show more of table getting flatter.
.
usually only .001" is needed every 5 or so years to flatten a worn table and yes after 50 years it will have to have .010" removed. usually there is at least .020" extra stock on table surface for resurfacing
.
bottom line is a worn table is often no longer flat and sometimes castings warp a bit over time. i see this often mill one side and opposite side is no longer flat and requires a .001" recut
.
.004" seem a bit much for a table to have warped high in center. sometimes a manufacturer allows some error for when weight is on table in center it will bow flatter and if nothing on table it will be high in the center a little. .004" is a bit high but .001" i would not think unusual. i often am asked to allow error in a direction where normal wear or part weight will make error get less. that is leave a little extra material where wear is expected.
 
High on the other hand... How does a table go high?

I have -zero- experience with vmc's.
However, this was a common problem with planers and is well addressed in the literature.
All the clamping gradually (combined at some times with dropped tools, and gradual dinging of the top surface from the work) peens the underside of the T-slots and the top table surface, and the tables eventually hump up in the middle, can be quite a bit, over time.

smt
 
Before milling or scraping the top buy a carbide T - Slot cutter and cut the T-Slots as this will relieve the stress or peen-ing of the slots. A friend of mine, Axel Fors of For's Machinery Rebuilding in Salt Lake City mills all his milling machine T-slots before he does the top as he has told me that will usually remove the curve of the table top. A.W. Fors Machine Shop
 
Last edited:
Have you considered peening the bottom of the table straight? Whenever I cannot get a table sent out and machined I rough scrape the top, or if the machine is capable I fly cut .005 off the top before disassembly, this allows a majority of the stress to relax, then peen it as straight as I can before scraping. I use an air hammer with controlled medium throttle, guiding the punch in my left hand with a slightly rounded end on the punch. You can lay down very even consistent rows of peen marks. You'll want to run them long wise end for end in the same direction as the table travels, don't peen across its width or you may cup the table. If your machine has webbing underneath, peen the tops of the Web, not down in the bottom, the idea is to stretch the underside to match the top side. The tops of the webbing is further from the opposing table top than the bottoms of the webs thus it has more effect. It's a very controlled method and the results are instant, plus the air hammer helps stress relieve as you go. I recently rebuilt a 74' bridgeport with 42" table which was crowned .017, the previous owner had pressed cast iron plugs in the table top to fill the drill marks without knowing the press fit plugs were stretching the top. I was able to peen it within .002 before scraping. I have also rebuilt several of the larger k&t mills with 96" and 106" tables, some bowed as bad as .050, and peening brought them back in nicely. Just one method for you to consider, wich if done correctly may allow you to leave the top untouched.
Best of luck!
Chris German
 
If you break loose the bolts for the X trucks, I think you will find that the table will move all over (in terms of flatness).

Like a said in the previous post, the way the machine is built, the table is machined as a casting to whatever spec the manufacturer determines. The X axis ways are most likely shimmed to be flat or set in moglace. They set the table on the trucks with a shim between the table and each truck. They torque it down and check the flatness. The shims are then ground to pull the table flat.

It sounds like someone goofed on this step when the machine was built. The other option is that one of the trucks is bad and it is letting that corner or section of the table ride up or down.

Have you contacted the manufacturer? What do they say?

Machining the table would be the very last act of desperation for me. If you are ambitious, pull the whole table off and see what the hell is going on. I don't think you could make it any worse.
 








 
Back
Top