Gib straightening press

gbent · Jan 6, 2019

I had to straighten a gib for a cross slide. The press I use for shafts is way to big and clumsy for a gib. Gibs need to be straightened as closely as possible to minimize scraping. This gib was bad, with about 1/8" of bend on the big end. This set-up gave me excellent control and an easy way to measure the amount of deflection. The gib would deflect .115 with no permanent straightening, but .120 removed most of the bend. A little tweaking at different spots and different spreads of the jaws and it came out within a few thousands.

No tailstocks were harmed in the straightening of this gib. The force didn't even amount to moderate drilling pressure.

Lurk · Jan 7, 2019

Now thats a neat way of doing it - not sure Ive enough hands to hold everything while its started but I like the idea.

Ive used a G clamp over the 40mm thick steel saw table and a couple of MS blocks - the dti on a mad base helps. But I found the blocks position was the tricky bit to keep in position relative to the clamp and gib.

gcude · Jan 7, 2019

Thanks for sharing. I would have still been just using 123 blocks and C-clamps, if you hadn't shown this alternative.

LKeithR · Jan 7, 2019

The two keys to straightening shafts (or gibs as in this case) are:

1) Knowing where to bend which is the more difficult of the two since it's a "learned" thing and requires experience
and/or training to figure it out.

2) Measuring the amount of deflection accurately. I can't tell you the no. of times I have watched people straighten
a shaft in a press with no clue as to how far they are bending each time they apply pressure. As the OP points out the
difference between an elastic state and yield is often only a few thousandths of an inch and as such is beyond the
ability of the average person to see by eye.

I've always thought that this youtube video is a pretty good illustration of the use of a pull-down press and dual dial-
indicators to straighten a shaft...

Straighten shafts with pull down press - YouTube

Lurk · Jan 7, 2019

LKeithR said:
The two keys to straightening shafts (or gibs as in this case) are:

1) Knowing where to bend which is the more difficult of the two since it's a "learned" thing and requires experience
and/or training to figure it out.

2) Measuring the amount of deflection accurately. I can't tell you the no. of times I have watched people straighten
a shaft in a press with no clue as to how far they are bending each time they apply pressure. As the OP points out the
difference between an elastic state and yield is often only a few thousandths of an inch and as such is beyond the
ability of the average person to see by eye.

I've always thought that this youtube video is a pretty good illustration of the use of a pull-down press and dual dial-
indicators to straighten a shaft...

Straighten shafts with pull down press - YouTube

That was a great video - thanks. Is it the same Matt Maguire as on this forum ?
Mat

ballen · Jan 7, 2019

gbent said:
This set-up gave me excellent control and an easy way to measure the amount of deflection.

I have to straighten a gib in the coming weeks, and like this setup. As some people have pointed out it helps to have three hands for this approach. I was thinking of doing this in my vertical mill as follows:

-- support points: top of vise jaws
-- pressure applied by: nut riding on drill bit sitting in vertical spindle
-- motion control: raise/lower Z axis of mill
-- measurement method: use DRO on mill, monitor Z axis value

Am I overlooking some reason that this would not work?

Cheers,
Bruce

gbent · Jan 7, 2019

Actually its only a 2 hand set-up. Its easy to hold the gib and spacer in one hand. I thought about my vertical mill, but mine has no quill stop nut to pull against. I don't know about force available, but it should be enough. If you could pull the quill against a hard stop that was easily changeable I think it would work even faster as you have the mill table right there to use as a surface plate to check for straightness.

steve-l · Jan 7, 2019

I'm sorry, using a press to straighten a gib sounds really dangerous. These things are normally cast iron and very brittle. None of the methods described here would be my first choice. Another question that should be asked is "How did it get bent in the first place?" I would suspect any observed bend would be due to internal stresses as opposed to a physical force. In that light, I would attempt to do some heat straightening. It might be a bit tricky, but it would be infinitely safer.

Lurk · Jan 7, 2019

steve-l said:
I'm sorry, using a press to straighten a gib sounds really dangerous. These things are normally cast iron and very brittle. None of the methods described here would be my first choice. Another question that should be asked is "How did it get bent in the first place?" I would suspect any observed bend would be due to internal stresses as opposed to a physical force. In that light, I would attempt to do some heat straightening. It might be a bit tricky, but it would be infinitely safer.

| have only straightened steel gibs - but I can really see your point for cast iron ones.. horses for course applies I guess.

CBlair · Jan 7, 2019

Cast iron can break but it bends quite a lot, especially in thin forms like a gib. I wouldnt be worried about breaking a gib, just be a bit conservative and it shouldnt be an issue. The method shown appears very controlled and consistent and that is better than trying to bend it over your knee. Someone had a video from one of Richards classes where he showed how to straighten one. Cant remember which one it was though.

Charles

Peter. · Jan 7, 2019

Nicely done. Is that a cast iron gib? I hope so as it would give me the confidence to try it on cast iron gibs of my own. I would not have thought that cast iron would suffer being straightened in such a way.

P.S. Straightening a gib by G Bent - gotta be a joke in there somewhere....

CBlair · Jan 7, 2019

Found the video I mentioned, thought I remembered it was one of Richards overseas classes.

Richard King scraping class Norway August 214 - YouTube

Charles

M.B. Naegle · Jan 7, 2019

steve-l said:
I'm sorry, using a press to straighten a gib sounds really dangerous. These things are normally cast iron and very brittle. None of the methods described here would be my first choice. Another question that should be asked is "How did it get bent in the first place?" I would suspect any observed bend would be due to internal stresses as opposed to a physical force. In that light, I would attempt to do some heat straightening. It might be a bit tricky, but it would be infinitely safer.

Why it's bent? IFAIK, they get over torqued trying to compensate for wear, and sometimes will gradually bend to match a worn surface.

I believe they are often made from ductile iron, with is still fragile though still has a range of flexibility. To heat it to a temperature where it would be soft enough to move, I'd be afraid of actually inducing cracks when it cools off, same as what you have to work around when welding cast iron.

ballen · Jan 7, 2019

Thanks for the quick answer.

gbent said:
I thought about my vertical mill, but mine has no quill stop nut to pull against. I don't know about force available, but it should be enough. If you could pull the quill against a hard stop that was easily changeable I think it would work.

I was thinking about moving the Z axis, not moving the quill. This has two advantages. First, there is plenty of leverage in cranking Z compared to the quill. And second, there is an accurate DRO on Z, so no need for the dial indicator to keep track of how far you have pushed the gib.

Am I missing something?

dgfoster · Jan 7, 2019

steve-l said:
I'm sorry, using a press to straighten a gib sounds really dangerous. These things are normally cast iron and very brittle. None of the methods described here would be my first choice. Another question that should be asked is "How did it get bent in the first place?" I would suspect any observed bend would be due to internal stresses as opposed to a physical force. In that light, I would attempt to do some heat straightening. It might be a bit tricky, but it would be infinitely safer.

Lurk said:
| have only straightened steel gibs - but I can really see your point for cast iron ones.. horses for course applies I guess.

Nice technique posted by the OP. Looks like a good method to remember.

For those uneasy with bending their gib, peening offers a really simple, quick, and controlled alternative. It has been discussed here in the past a couple times. Here is a link: https://www.practicalmachinist.com/...ted-ci-warping-when-sawed-282453/#post2258824

Since posting about this a few years ago, I have had the opportunity to use this technique on a variety of items to either straighten or intentionally curve them. Works well and it is possible to localize the bend induced very accurately. By listening to the sound a gib makes as it is hammered you can actually hear by the change in its ringing (actually ringing less) when it is lying flat on the stone. FWIW. Just for grins invest ten minutes and try it on a piece of scrap iron or steel a time or two to see how it works in practice. Chances are you will like it. If you don't, you're only out ten minutes.

Denis

Spinit · Jan 8, 2019

gbent said:
I had to straighten a gib for a cross slide. The press I use for shafts is way to big and clumsy for a gib. Gibs need to be straightened as closely as possible to minimize scraping. This gib was bad, with about 1/8" of bend on the big end. This set-up gave me excellent control and an easy way to measure the amount of deflection. The gib would deflect .115 with no permanent straightening, but .120 removed most of the bend. A little tweaking at different spots and different spreads of the jaws and it came out within a few thousands.

No tailstocks were harmed in the straightening of this gib. The force didn't even amount to moderate drilling pressure.

View attachment 246284

That is smart. I would normally not try something like that as I would be concerned about the strain on the tail stock. When you need something done you tend to use what you have. I have used blocks to space up something to straighten before and used a Bridgeport to straighten. I locked the spindle after retracting it all the way up then raised Z. I mostly just use a nice scale to see how I do. For gibs the indicator gives a good control.

ballen · Feb 28, 2019

I straightened a gib yesterday using this method. It is 500mm (20") long, cast iron, tapers from about 11mm (7/16") to 6mm (1/4") thick. I used the Z-axis of my mill and the DRO to keep track. I wasn't sure how much bend was needed and have never done this before, so I started with 0mm in the center and worked up in increments of 0.1mm (0.004") hinging the gib on a surface plate after each round. It took about 20 minutes for me to work up to 5.4mm (about 3/16") where it went beautifully flat. Before that it was hinging in the middle and rattled at either end when tapped. Now it hinges at 30% in from either side and no rattle anywhere.

An interesting thing I noticed. As I started to get close to the right amount of deformation, when I put the gib on the surface plate it would hinge away from the center for a few seconds. But then it would relax back into the original shape.

This is a great setup for bending. Precise control over the amount of deformation, no sweat, no fuss, plenty of leverage from the Z handwheel. You can't eliminate the fear that it might just snap, but at least you are rid of any other worries.

Richard King · Feb 28, 2019

Most machine tools have cast iron gibs and I have been straightening them for years. I use a Bridgeport Mill Z axis gib during the classes and I support it like Ballen does.. (he learned my technique in last years Austria class) I purposely show the class you can bend it 1/4" and it does not break as many think, people are afraid to bend gibs. I have even bent Green Machine gibs when they look like they would break. I like Glens Idea. Oh in all my years Ihave never broken a gib doing it my method.

Gib straightening press

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