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- Jul 1, 2012
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Today I had someone ask me how to scrape a gib on another forum. I want to share it with you too.
I will attach some photos I took while teaching a class. The pictures are from a Vertical or Z way of a VMC headstock. In the article I describe a Bridgeport Y Gib, but the principal is the same. You can also use this test method to set a gib. On a Bridgeport I set the gib at "lost motion" of the gib clearance on a rebuilt machine .0005" or .00025 per side for oil. On a used machine .001" lost motion. On a used Bridgeport knee they get tight on the ends, so you will have to keep that in mind when setting the gib. Rich
Q. His question was something like "How do you scrape a gib? What sides do you scrape first?
A. I am assuming you know how to scrape ??
Let's assume your scraping the Y Axis (saddle) of a Bridgeport. You have the top of the knee finished and the dovetails are scraped parallel to each other.
You have the saddle flats and non gib side scraped to the top of the knee. The first thing I would do would be to check where the back of the gib goes in the saddle. It's a non wear surface but the ends can get dinged up because someone used a punch to get it out or whatever. I would sit the saddle up on it's end and clamp it so it won't tip over. Blue up a dovetail straightedge and check the surface, be sure it hinges as described below. Look for dings or burrs where the gib screw counter-bore and other end, scrape, stone or file them off. On some machines you can cock the saddle and rub, that side against the dovetail so that surface is parallel from top to bottom too. Blue up the knee flats and dovetails. You have 3 surfaces finished, but the blue acts as a lube and keeps the finished ways from scratching up.
Then the gib. First thing you need to do is check to see if the gib is bent by laying it on a flat surface plate and sliding in a feeler gage under it. One can straighten gibs (in general) by hand pressing on a table edge, in a straightening press or with 2 blocks and a C-Clamp. It depends on the size of the gib. Once you have the non wear side straight you scrape it flat to 3 to 5 PPI / points per 1 inch, you cant slide in a .001" feeler gage under it anywhere and it hinges or pivots approximately 30% in from both ends Let's say the following is the gib ____x________x____, that is where it should pivot when flat. (rotation of points)
Some time when I have it laying on a surface I take a soft blow hammer and tap on it, listening for a solid noise when I tap on it. If it is not flat the gib will slap the plate and make a non solid noise.
Then look at the wear side and look for the unworn areas. Many times gibs have a ridge on the top side where it was above the matching way. If you can measure the ridge, that is great way for you to know how much wear it has. Then file or scrape off the ridge and scrape it flat. Some times it is worn so bad you will have to grind it flat using a sign mag chuck.
After your gib is flat to the 3 - 5 PPI and pivots the same as above. Then slide in the gib and feeler gage the big end when the gib is even with and located where it will be when new and the wiper plate screws on to cover it. After you have discovered how much shim you need, cut a shim the length and width of the gib or a little longer..I like to use Argus Color coded plastic shim. ( the shim is for testing and not a permanent shim) Then slide in the gib with the shim behind the gib and scrape the gib flat. You will be pushing the knee back and forth to transfer the blue to the gib. When you get it close put the gib screw in to hold the gib in place when pushing it back and forth. I get the taper correct before I glue on the Rulon wearstrip. (see below on getting the taper right)
You can save some frustrations by scraping or lowering the middle 2/3's of the gib wear side by approximately .001". Slide the gib in with blue on the opposite side and start to scrape it to match fit the blued way. You will need to check the taper so it is even on both end. I do this by using a mag base and an indicator. Mount the mag on the top of the knee and put the indicator on the saddle on big end.
Leave the gib loose. Then stand on the side of knee and push and pull on the addle and see what gap measures. It should be a minimum of .002" lost motion. Then pick up the mag base and move it to the small end. set the indicator with the same set up as the other end. be sure not to move the saddle or gib. Then push and pull the saddle and see how much gap you have. Then scrape the gib so it has the same gap on each end. After you have that good, then you will have to decide what type of shim you will apply to the wear side. I prefer Rulon 142. Hopefully that answers your question as I'm getting tired typing it. In a former post by Turbotadd he shows a gib holder he made. I believe it's on page one. You have to scroll down to the bottom of the page to see the page numbers.
I will attach some photos I took while teaching a class. The pictures are from a Vertical or Z way of a VMC headstock. In the article I describe a Bridgeport Y Gib, but the principal is the same. You can also use this test method to set a gib. On a Bridgeport I set the gib at "lost motion" of the gib clearance on a rebuilt machine .0005" or .00025 per side for oil. On a used machine .001" lost motion. On a used Bridgeport knee they get tight on the ends, so you will have to keep that in mind when setting the gib. Rich
Q. His question was something like "How do you scrape a gib? What sides do you scrape first?
A. I am assuming you know how to scrape ??
Let's assume your scraping the Y Axis (saddle) of a Bridgeport. You have the top of the knee finished and the dovetails are scraped parallel to each other.
You have the saddle flats and non gib side scraped to the top of the knee. The first thing I would do would be to check where the back of the gib goes in the saddle. It's a non wear surface but the ends can get dinged up because someone used a punch to get it out or whatever. I would sit the saddle up on it's end and clamp it so it won't tip over. Blue up a dovetail straightedge and check the surface, be sure it hinges as described below. Look for dings or burrs where the gib screw counter-bore and other end, scrape, stone or file them off. On some machines you can cock the saddle and rub, that side against the dovetail so that surface is parallel from top to bottom too. Blue up the knee flats and dovetails. You have 3 surfaces finished, but the blue acts as a lube and keeps the finished ways from scratching up.
Then the gib. First thing you need to do is check to see if the gib is bent by laying it on a flat surface plate and sliding in a feeler gage under it. One can straighten gibs (in general) by hand pressing on a table edge, in a straightening press or with 2 blocks and a C-Clamp. It depends on the size of the gib. Once you have the non wear side straight you scrape it flat to 3 to 5 PPI / points per 1 inch, you cant slide in a .001" feeler gage under it anywhere and it hinges or pivots approximately 30% in from both ends Let's say the following is the gib ____x________x____, that is where it should pivot when flat. (rotation of points)
Some time when I have it laying on a surface I take a soft blow hammer and tap on it, listening for a solid noise when I tap on it. If it is not flat the gib will slap the plate and make a non solid noise.
Then look at the wear side and look for the unworn areas. Many times gibs have a ridge on the top side where it was above the matching way. If you can measure the ridge, that is great way for you to know how much wear it has. Then file or scrape off the ridge and scrape it flat. Some times it is worn so bad you will have to grind it flat using a sign mag chuck.
After your gib is flat to the 3 - 5 PPI and pivots the same as above. Then slide in the gib and feeler gage the big end when the gib is even with and located where it will be when new and the wiper plate screws on to cover it. After you have discovered how much shim you need, cut a shim the length and width of the gib or a little longer..I like to use Argus Color coded plastic shim. ( the shim is for testing and not a permanent shim) Then slide in the gib with the shim behind the gib and scrape the gib flat. You will be pushing the knee back and forth to transfer the blue to the gib. When you get it close put the gib screw in to hold the gib in place when pushing it back and forth. I get the taper correct before I glue on the Rulon wearstrip. (see below on getting the taper right)
You can save some frustrations by scraping or lowering the middle 2/3's of the gib wear side by approximately .001". Slide the gib in with blue on the opposite side and start to scrape it to match fit the blued way. You will need to check the taper so it is even on both end. I do this by using a mag base and an indicator. Mount the mag on the top of the knee and put the indicator on the saddle on big end.
Leave the gib loose. Then stand on the side of knee and push and pull on the addle and see what gap measures. It should be a minimum of .002" lost motion. Then pick up the mag base and move it to the small end. set the indicator with the same set up as the other end. be sure not to move the saddle or gib. Then push and pull the saddle and see how much gap you have. Then scrape the gib so it has the same gap on each end. After you have that good, then you will have to decide what type of shim you will apply to the wear side. I prefer Rulon 142. Hopefully that answers your question as I'm getting tired typing it. In a former post by Turbotadd he shows a gib holder he made. I believe it's on page one. You have to scroll down to the bottom of the page to see the page numbers.
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