G0704 Z-axis gib tightening issue

Hey guys,

I have been reading on the net that some G0704 owners are having an issue tightening the lower gib tensioner for the Z-axis. The problem is that when it's tightened, the spindle will deflect. If the top tensioner is tightened by itself, you don't get deflection. I am having this same exact issue and I cannot seem to find any solutions.

Has anyone come up with one yet? Are there some things I can try? Should I just remove the lower tensioner completely and forget about it since the top one seems to tighten it by itself? I was thinking that this might be the best way to go.

Any suggestions?

I just trammed mine with bouth screws tight. It has worked for me.

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Originally Posted by kd4gij

I just trammed mine with bouth screws tight. It has worked for me.

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I solved the problem by removing the bottom tightener completely. I now use that bottom hole for the bolt that holds the bracket in place for my DRO. Worked perfectly.

The top gib tightener works fine for just keeping the head tight, so I didn't see the need for both of them.

I have the same issue, and haven't found a better solution than yours.

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Originally Posted by Semus

I solved the problem by removing the bottom tightener completely. I now use that bottom hole for the bolt that holds the bracket in place for my DRO. Worked perfectly.

The top gib tightener works fine for just keeping the head tight, so I didn't see the need for both of them.

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I doubt you accomplished much. There are 2 screws for a reason, to position the gib at the correct location. The gib is tapered on 2 planes as is the Z carriage. Your gib is loosening on the way up and then sliding into place as the head moves down. Your gib is most likely not flat nor matching the carriage or the ways. Blue it and see.

Kevin, Why wouldnt you elaborate on what that "solution" was?

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Originally Posted by Fastest1

I doubt you accomplished much. There are 2 screws for a reason, to position the gib at the correct location. The gib is tapered on 2 planes as is the Z carriage. Your gib is loosening on the way up and then sliding into place as the head moves down. Your gib is most likely not flat nor matching the carriage or the ways. Blue it and see.

Kevin, Why wouldnt you elaborate on what that "solution" was?

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I think you may have misunderstood which screws we are talking about. I didn't remove the screws that hold the gib in place. I don't think that would be a good idea.

I'm talking about the two screws that tighten the gib on the "side" of the head. They have little "handles" attached to them so that you can grab them with your hand and turn them. Most of the time, those "handles" are loose. However, if I tighten the top one, the head doesn't move at all. So I didn't see any reason to keep the bottom one, since that was the one causing deflection.

I've already taken the gib out and checked it. There were a couple of high spots, but nothing that I felt would cause any major issues. I gave it a good lapping and re-installed it. The head moves up and down very smoothly, as it always did. But, that was never really the problem. The problem was the deflection that was happening when I tightened the bottom "handle" on the side of the head.

I hope that clears some things up. If not, I'm open to other suggestions.

The carriage locks? I see, yes they are not needed (either one) in a CNC conversion.

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Originally Posted by Fastest1

The carriage locks? I see, yes they are not needed (either one) in a CNC conversion.

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Carriage locks, yes. Thank you for giving me the proper name. I couldn't think of the name to save my life!

I don't have a CNC converted mill. It's still a manual G0704.

My solution was the same as the OP, I just don't use the lower lock. I said "than yours" meaning the solution, such as it is, that the OP had already described.

Take care,
Kevin

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Originally Posted by Semus

I've already taken the gib out and checked it. There were a couple of high spots, but nothing that I felt would cause any major issues. I gave it a good lapping and re-installed it. The head moves up and down very smoothly, as it always did. But, that was never really the problem. The problem was the deflection that was happening when I tightened the bottom "handle" on the side of the head.

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I did the same. I'm thinking the ways on the head are not very parallel. The next time I have the head off I'll measure top to bottom. There must be a taper.

Take care
Kevin

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Originally Posted by Kevinj308

I did the same. I'm thinking the ways on the head are not very parallel. The next time I have the head off I'll measure top to bottom. There must be a taper.

Take care
Kevin

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Yes there will be taper on the head. The taper will oppose the taper of the gibs on 2 planes. This allows the gib to be tightened yet keeping 2 sides of it parallel to the ways of the Z.

Hi Semus,

As you have no doubt seen, I posted about the same problem recently. My current work around has been to do what you have done also, which is only tighten the top gib lock on the z. If I do that I see probably less than 0.001" deflection. Using just this method I have been able to set it up to somewhat in tram and it is "just" workable. If I try to tighten the bottom one in any way at all it all goes to hell. My travel isn't smooth up and down but binds constantly.

However, I'm not sure what sort of solution this will be when it comes to taking a heavy cut. The reason the head is deflecting with that lower Z gib lock is because of poor fit - either poor fit of the gib, the tapered dovetail in the head that the gib mates to, the dovetail on the column or a combination of all three. I'm not sure if living with that poor fit will cause any deflection issues when using it to take a cut but I suspect it might seem ok while stationary then deflect when under side load. People say that once you CNC it you wont need the gib locks so it wont be a problem, but in my opinion that is not true - because then you are relying on a good fit of the gib to control the slop in the head which is where we have the problem. My gib fits the column so poorly that without using the gib lock you would just have a sloppy heap of mess.

I've measured my columns dovetail width and its not parallel but is narrower in the centre than at the ends, like a banana - I thought my problem was just a warped or undersize gib and I was going to scrape and shim it. But then I realised the deflection in the curve is 0.19mm (or 0.007") narrower in middle. Hence my gib is loose in middle and tight at the ends of travel. Luckily for the most part I tend to use roughly the centre area so I've been working with a happy medium at the moment but its not a solution. If I CNC it, it will just bind at the top of travel and be sloppy in the middle. So, yes, my gib is undersized but its undersized because the dovetail varies in width - it fits the top of the column but when in the middle of travel it is undersize and becomes loose.

I've been talking with the people I bought the mill off. They don't believe that the column dovetail is curved, so I'm about to take the column and head off tonight to take pictures of how I am measuring it and show the values (I'm using dowel pins in the dovetail to measure the width between the angled dovetail surfaces). I suspect I'll get no where with these people as they are just denying any problem. As an example I have a Sieg X3 also and the column dove tail doesn't vary by any more than 0.01mm (less than 0.0005") over the entire length.

Out of interest I measured the outside of the column, not the machined surfaces just over the paint and all, just the total width of the whole thing and interestingly my whole column bows in the middle by nearly the exact amount that I measure on the dovetail so in my mind that suggests to me that post machining of the dovetail the stresses relieved in the cast iron have caused the middle of the column to bow in - which it can because on the G0704 the centre of the dovetail is clear for the Z-axis nut to travel in versus the X3 which is hollow at the back of the column but solid on the front. My guess is that seeing how often this problem is coming up that this is really starting to be a common problem with the design and manufacture of this mill. Maybe because of the sheer demand for these mills they are rushing them through production too fast and the castings are warping.

Sure, a lot of people might be happy with theirs but mine is a dog.

The mentioning that the carriage locks arent needed if CNC'd doesnt mean the problem can be ignored. It just means they arent useable in this configuration. You could install a series of set screws that press on the back of the gib strip to possibly help keep the gap consistent and improve overall performance and lessen deflection.

To those of us in the hobby end, the deflection is most likely the issue that causes bad surface and profile finishes. These have to be finished by hand or scrapped. Either way not a solution for a production machine. We have to design around the tolerances we can achieve.

The objects I am trying to mate together are the inside how to get fiberglass out of skin of a bearing wall and an extruded cylinder with small tolerance so it will fit snugly .