G0704 Table Deflection

[Semus]

Hey guys,

This topic has probably been discussed to death, but I'm a bit worried about my table. I was cutting a 30lb aluminum block today that is 6" wide, 4" thick and 1' long. I tried to use a face mill on one side but noticed that in order to face off the entire length, the table would have to be moved beyond what I would consider a safe amount of travel to avoid deflection. I'm worried that the surface might be a bit convex when it was finished. I haven't taken any measurements as of yet, but was wondering if the amount of deflection I am getting with my table was too much.

With that 30lb block of aluminum set at the very end of the table, I am getting .013 deflection with the table all the way to one side. Maybe it's not called deflection? I can't think of any other word for it, so if I am incorrect, please correct me.

Is this a normal amount for this mill?

As a side note: My milling machine is bolted to the cement floor and does not move at all. There may be some slight flex in the stand that it comes with, which could account for a fair bit of it. I'm just not sure at this point. Tomorrow I am going to try the same experiment, but without the block. I'll use brute force on the top of the machine to see if I can get it to flex.

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[691175002]

0.013" of deflection is pretty bad but probably inevitable in a machine like the g0704 at maximum travels with weight on the table. There is just so much leverage that some bending is inevitable.

The exact error is probably mostly table sag, you might be able to reduce it by tightening the gibs but that will only get you so far before you need to lap or scrape.

A few passes on the gibs with some fine sandpaper might reduce the error by half if you are lucky.

It can be eliminated entirely by altering the geometry of the ways such that it slightly overcompensates for table sag, and thus cancels out when there is weight on the table. Doing so is a huge PITA though.

[Semus]

0.013" of deflection is pretty bad but probably inevitable in a machine like the g0704 at maximum travels with weight on the table. There is just so much leverage that some bending is inevitable.

The exact error is probably mostly table sag, you might be able to reduce it by tightening the gibs but that will only get you so far before you need to lap or scrape.

A few passes on the gibs with some fine sandpaper might reduce the error by half if you are lucky.

It can be eliminated entirely by altering the geometry of the ways such that it slightly overcompensates for table sag, and thus cancels out when there is weight on the table. Doing so is a huge PITA though.

That's what I was afraid of. I haven't messed with the gibs at all, and they are plenty tight. Right now, the table is even a bit hard to move side to side with the hand crank because of the tightness of the gibs. I don't want to over do it. I suppose I'll just have to live with it.

The only other alternative is to build a system of rollers on either side of the milling machine that will "catch" the table as it slides from side to side. Basically, the table would slide on these rollers that would be attached to some sort of stationary object on either side of the milling machine. I was thinking about using rubber rollers that are used underneath tables and such. However, that's a bit overkill just to eliminate deflection. Would be an interesting project, though.

[joecnc2006]

I do not think you will get the 0.013" error across the part as you will be cutting right under the head which will be a constant distance, sure the table sags at the end with weight but facing/milling the material under the head you will get just a little variance. nothing like 0.013", I would take a piece of aluminum 1' long maybe 3/4" thick for the test and mount it down centered in the bed, and with the weight you tested at the end of the table place it there at the end of the table to simulate the deflection go ahead and mill/face the 1' strip of aluminum and then measure the thickness, it may be well within your tolerances. Its pretty hard to deflect a 4" slab of aluminum with the table/slide but we do not know hoe much of the material is machined away. This would be an interesting experiment to see how much deviation in the thickness you have.

[Fastest1]

Where and how are you measuring this "deflection"?

[Semus]

Where and how are you measuring this "deflection"?

I used a dial indicator set off the table at the very end of the table where the aluminum block had been placed. I first moved the table all the way to one side and set the indicator to zero. Then, I placed the aluminum block on the table at that end. The indicator read -.013 after the block had been placed on the table at the very end.

I ran the same test with the block at the same end, but while moving the table from center to that side. The indicator read the same.

Is there some other way I should be testing it? I'm new to this stuff, so if you have any suggestions, I'm all for it.

[Fastest1]

No I don't know any approved way and was just verifying your technique.
Did you try the same thing on the opposite side and if you did were the results the same or close? Have you adjusted the gibs after this test and if so was there any change?
As stated above, the workpiece is directly under the spindle so deflection of this kind might be a nonissue unless you have a second vise or apparatus leveraging the table.

[Semus]

No I don't know any approved way and was just verifying your technique.
Did you try the same thing on the opposite side and if you did were the results the same or close? Have you adjusted the gibs after this test and if so was there any change?
As stated above, the workpiece is directly under the spindle so deflection of this kind might be a nonissue unless you have a second vise or apparatus leveraging the table.

You're quite right. It's really a non-issue unless I have some weird scenario going on that would require the work out that far. I always center my table and my work piece before I start to do any cuts, but deflection still may come into play (albeit not as extreme) when you have the head tilted 90 degrees and cutting the side of the work piece.

To answer your above questions, no, I didn't check the other side. Yes, I did adjust the gibs before and after the test with basically the same results. The gibs were already pretty tight when I started this experiment and the table took some force to move, which I don't think is recommended. The table should generally slide smoothly, and the handles really shouldn't be tough to rotate as far as I am aware. If I were to loosen the gibs, the results might be even worse.

I just wanted to know if this was excessive. I think what I am hearing is that it's not exactly ideal to have this much deflection, but that it shouldn't really be a factor in what I'm cutting as long as I take the proper precautions.

I appreciate all the feedback.