Gib Repair

Hello,

I have been retrofitting a Lagun CNC knee mill for a couple of months now, I am using the original servos with Viper servo drives and Centroids Acorn board the whole process has gone fairly smoothly there has been a steep learning curve but it has been fun. I am getting to the point where I was calibrating the steps per revolution and checking for backlash, I was finding .006-.007 of backlash in the Y axis and I thought I traced it to the thrust bearing so I bought new NSK TAC matched set thrust bearings for the X and Y axis (not cheap) and it helped but it was not the main problem. Come to find out that the knee is fairly worn in the middle if I loosen the gibs the backlash goes down to .0015 but with the gibs that loose I have visible movement of the saddle when rocked. The best compromise of gib tightness and saddle movement has been a .005 backlash with still some saddle slop. after looking closer at my Y axis gib I think some of the problem might be with it, it is in sorry shape it has a groove worn in the middle it was very bowed out in the middle and feels weird when tightening (it will get hard to tighten then loosen un then get hard again).

Could I have the Gib ground/machined down and have some turcite/rulon added to the wear side? My thought is that this would accomplish two things for me, the turcite would lessen the friction so I could keep the gib tighter without the accompanying backlash from sticking and the turcite/rulon would give it some compressing ability also allowing me to keep the gib tighter.

Sorry if this is long winded or poorly worded but I am just hoping for a way to save this project as I have quite a bit of time and money invested in it so far.

Thank you,

Shawn

What re-grinding/scraping have you done to the rest of the way surfaces?

None, the ways are hardened so I don't think I would be able to do it myself and I do not know where I could go to have them ground, the saddle is lined a turcite like material on the top and bottom so I assume all of the wear is in the dovetail with the gib and the corresponding dovetail on other side which is metal on metal (you know what they say about assuming though).

Shawn-
It's time to do some sleuthing/inspecting. Using a straightedge and a surface plate figure out how bent the gib is and how worn the mating surface is.
Gibs can be straightened pretty easily on a surface plate using shims and a simple indictor. If it's bent/curved you won't be able to get it adjusted properly.
Once the gib is straight/flat you may be able to adjust the machine well enough without any other messing around. Otherwise if the mating surface is hardened and worn search for a long thread in this forum about scraping a fadal using grinding.
Hahn Rossman

I was able to get most of the bend out of the gib but it did not help that much, I have been trying to find gridding services to just go ahead and have knee ground professionally. The one I have found that I was able to get price did not completely scare me away but they are too far to take myself and the shipping would prob almost double price. Does anyone know of good grinding operation around the southern IL area?

My machine has hardened ways and a synthetic type material on the top and bottom of the saddle, would the best option be to have the knee and saddle ground and I replace material on saddle and try and scrape it in myself. The saddle is much bigger than my bridgeport clone I would have to rig up some sort of hoist to scrape it in.

Thanks,

Shawn

It does not look like I took any pictures when the saddle was off but here are some of the top of the saddle. Sorry about the picture orientation I can't seem to figure out how to flip them.

That material looks like Linen grade Phenolic. If you check You Tube under Richard King Scraping..then check Norway class and I show how to straighten a Gib. Most people are afraid to bend it straight.

Here is a job I got paid to do and we installed and scraped new Phenolic. I can show photo's. Have you noticed the ones who are the "experts" never add photo's because they have never done it. If you fast forward to min 5:28 I demo how to straighten a gib. Richard King scraping class Norway August 2014 - YouTube

Richard King said:

That material looks like Linen grade Phenolic. If you check You Tube under Richard King Scraping..then check Norway class and I show how to straighten a Gib. Most people are afraid to bend it straight.

Here is a job I got paid to do and we installed and scraped new Phenolic. I can show photo's. Have you noticed the ones who are the "experts" never add photo's because they have never done it. If you fast forward to min 5:28 I demo how to straighten a gib. Richard King scraping class Norway August 2014 - YouTube

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Are you talking about straightening a gib OR just bending it somewhere else ?

Just askin'....

Orbital77 said:

Are you talking about straightening a gib OR just bending it somewhere else ?

Just askin'....

Click to expand...

Richard is talking about bending it flat enough to scrape in. Unless something catastrophic happened the gib is bowed, not kinked. Either way, you need to get it flat (trolls can argue how flat about how flat is flat) before you start scraping or milling etc depending on how worn it is. You could just mill/grind all the bow out of it (both sides) but that's a lot more work and just means you have even more material you need to add somewhere to make up for the loss.

Shawn-
Have you inspected the hardened surfaces yet? They may not be very worn if the linen phenolic did it's job. You can do a quick and dirty version with a good straightedge and a flashlight or feeler gauges.
Hahn Rossman

hahn_rossman said:

Richard is talking about bending it flat enough to scrape in. Unless something catastrophic happened the gib is bowed, not kinked. Either way, you need to get it flat (trolls can argue how flat about how flat is flat) before you start scraping or milling etc depending on how worn it is. You could just mill/grind all the bow out of it (both sides) but that's a lot more work and just means you have even more material you need to add somewhere to make up for the loss.

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That's not what I heard on the video but anyway, thank you very much for the explanations. We are re-doing geometry two M1000's including making new gibs with the integral locking and your explanation comes in right on time...

Again, thank you very much.

Richard King said:

That material looks like Linen grade Phenolic. If you check You Tube under Richard King Scraping..then check Norway class and I show how to straighten a Gib. Most people are afraid to bend it straight.

Here is a job I got paid to do and we installed and scraped new Phenolic. I can show photo's. Have you noticed the ones who are the "experts" never add photo's because they have never done it. If you fast forward to min 5:28 I demo how to straighten a gib. Richard King scraping class Norway August 2014 - YouTube

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out of curiosity, could the gib have been peened straight?

dian said:

out of curiosity, could the gib have been peened straight?

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Yes, that's often done. It's quick and effective. BUT - first you need to establish where EXACTLTY the bend is located. If you do not, you WILL end up with a wave shaped gib and that means for the same play you'll get a couple of times more specific pressure - NOT good for the machine. A problem with peening and precision is that it leaves stresses in the material. Things keep moving with time, load and TEMPERATURE.

Before straightening anything you need to figure out EXACTLY where it is bent - gibs are almost never bent exactly in the middle and because of their shape will never become perfectly straight by pushing into them there while resting the ends. Grinding or milling them needs attention too as they'll move under the magnet and tools.

The "feel" of a machine with perfect fitting gibs is very different from same with gibs being just so-so.