Tapered Gibs

[Richard King]

The way I scrape the gib is different then Nick. First thing I do is file the ends as they usually have a burr or raised from when the the gib bolt is over tightened. Then I check the gib to see if it is flat by laying the back unworn side on a surface plate, they are usually high in the middle because it wears on the other side. I measure it with a feeler gage It's the same thing when you mill or grind a thin piece of steel on one side it bends. After I discover how much it's bent I flatten the gib in a few different ways, I hope Tadd or Bill one of the other students who took a class can supply a picture here as I don't have one.

But I bend it by normally by laying the gib back side up on 2 wood blocks on the ends and press down with my hand or use a c-clamp taking a small amount at a time. I sometimes will set a mag base behind it and put a 1" travel plunger indicator on the gib and tight the clamp then loosen it until It's straight. You will be surprised as to how much you can bend the gib and it won't break. In the classes I shown how I can bend a Bridgeport gib a 1/4" just to show how they don't break.

With that said on some of these small Chinese machines be careful bending them as some of the iron is very brittle. Then I blue up the back of the gib and scrape it so it has 2 - 5 PPI and 60% coverage POP. One thing you have to do is to check the surface the back side of the gib rests on. That surface is sometimes high in the middle and the ends have burrs. This should also be tested with a blued straight edge and be sure it is flat by checking the airy points or 30% hinge from both ends. Be sure to pull the gib up off the bottom of the dovetail or the hinge will look like it's on the ends.

I don't hammer in the gib like Nick does, I put the gib in and slide the side back and forth and check the taper so both ends are the same using a mag base in indicator as seen in the picture below. Tou have to have some slop in the test, say .003" on the tight end. So if you push pull the part on the one end and you get .004" and move the indicator to the other and and do the push pull and you get .006" the gibs taper is off .002" and needs to be stepped scraped. When it gets close I being finished I put the gib screws in to hold it. I seldom buy a new gib because it slides in to far as the gibs you buy will have to be scraped.

I just saw where a company I do work for (now) bought some new gibs for a centerless grinder because the old gibs were out of adjustment expecting them to fit. The taper was off .015" and the work head wiggled and the machine crashed. So don't assume the new gib will work. My cure for a worn gib is to glue on a Phenolic shim on the back side or glue on Rulon on the front side and scrape it. Scrape it and when I am done I relieve the middle 40% of the gib approx .0005 low", so when the gib wears it wears flatter. Using the Rulon makes the slide tighter and it slides like butter. I would say 90% of all machine built today has either Rulon, Turcite, some other brand of Teflon / bronze filled wear strip on them. I have no clue of the percentage machines coming out of China with it. From the hand full of ones I have seen they are cast iron and in rough shape.
I need to scoot, working on a grinder this week More later. If any one else can add to this please join it. Rich

 

[Erik Brewster]

After I discover how much it's bent I flatten the gib in a few different ways

One thing I learned as a new scraper: Take the extra few minutes to (re)bend the gib as straight as you can. Often gibs are small and can be scraped quickly and easily. I fell for this and scraped too much material off a gib (to compensate for 0.01" or so of bend) and now it sits very deep. If I had spent a few more minutes straightening it, I would have had to scrape much less. The few minutes scraping wasn't a big deal, but the resulting fit of the gib would have been better. Remember that a bend shows on both sides of the gib, so you scrape off twice as much material as indicated by the bend measurement...

I straighten gibs on an arbor press with a magnetic attached dial indicator keeping track of how the bending is progressing. It works well, though anything will do, so long as you can measure your work and control the amount of bend you put it.

I'm really amazed at how much I took away from Rich's class. It really got me to the point where I can scrape well enough to be confident I will make the machine better, and not worse. I can't say that about many classes...

 

[Richard King]

Here are some pics of the tapered longitudinal gibs on my Gorton 9J.

The saddle on these 9J's are very wide (42" with a 48" table) and support the long table very well. Much longer than those on a Bridgeport for instance. Even these long supports don't entirely prevent table sag on the ends as evidenced by the worn away scraping on the ends. This table sag can be much worse on machines with a shorter saddle.

(Rich Here) Many old machine designers did not consider how the machine would get oil. You can see that here they only have gravity feed cup oilers feeding the 2 straight oil grooves. I can't see the bottom of the table and it may have some sort of oil groove in it. If you think about it where does the dirt get into the ways here? From the ends and when the table runs off the end of the saddle gravity of the unsupported table will make end wear too. Some say the table doesn't bend, But I have seen tables bent and saddles worn in my career. I also believe the stretching of the tee-slots has an affect of the table bending too. A combination of both.

The scraping and last cut square flaking on this machine is I'm guessing .002" deep or it was that way when it was new. Some machine builders did not 1/2 moon, they cut square or rectangle shaped cuts. So with that said I would guess the top of the saddle is worn at least 003" low on the ends. If you look at the bottom of the saddle and how narrow the top of the knee is, I would assume the knee top and it's dovetails are worn alot too. especially on the side dovetails. are also worn. The table must be heavy. If you have a granite table you could flip the saddle over and scrape it flat, then you would need a triangle shaped or a camelback straight-edge with 45 deg. angle on it and scrape dovetails. Many times when we scrape saddle ways that have a long table we scrape the top of the saddle concave so as the table travels out it curves up to compensate for the sag, sort of counters the sag. hard to explain..
I would as I said before relieve the middle 40% .0005" to .001" so as the outside ends wear the saddle gets flatter, if it is flat when you are done it wears and the middle gets high again. I also say the low area acts like a lake of oil as oil will lay there and when you move the table it oils the way right away. I would also grind or mill in some more oil groves the total length of the ways. Now the ways are getting stick slip or its as if you are ringing gage blocks together. No room for oil and it will start to wear fast and gall.

Lets say you need the machine now and don't have time to rebuild it. You could scrape in new oil pockets or 1/2 moon flake it, cut in more oil groves, clean the oilers and holes and if you want to really improve it, add a BiJur style lube pump and metering units. The machine wil still be not as accurate as it was when new, but would work if your tolerances weren't that tight.








Richard, what do you think of the condition of the scraping here?

Please pardon the deplorable condition of my temporary shop.

It says my message is to short so I am adding this down here. The bulk of what I said is above the 1st picture. Rich