Gibb screws X axiz

[FFlores63]

Hello. I recently purchased a preowned year old PM25MV mill. Everything is like new with hardly any wear. The one thing that I have a question about is regarding the x axis gibb screws. The screw on the left side is screwed in to the point that you have to look deep inside the hole to see the head and the head of the screw on the right side sticks so far that you can see the complete head of the screw stick out of the hole. There is no slop or weirdness about the travel of the table on the x axis. My question is, does the settings of gibb screws sound normal or ?

 

[Just for fun]

If it's not too loose and not overly tight, it's probably right. The X axis on my mill PM935 is kinda like that, I don't think it is as severe as yours. I will have to look at it again but I think mine is the other way, the right screw is in further.

Tim

 

[benmychree]

From your description, we do not know which screw is at the big or small end of the gib, thus we don't know whether or not you are running out of adjustment or just beginning to use up adjustment. If the screw that is all the way in is on the big end of the gib, meaning that you have used up nearly all of the adjustment, things can be evened up by installing a thin shim behind the gib to get the adjustment screws back where they belong.

 

[dabear3428]

could be very good or very bad, the screws work by pushing the gibs along the dove tail with the gib getting thicker as it goes, so for your lightly used (almost new mill) you are at the start of the adjustment range (most likely) if you are at the other end (most likely not) then you just need to add shims to the thickness of the gibs.

 

[B2]

The screw on the left side is screwed in to the point that you have to look deep inside the hole to see the head and the head of the screw on the right side sticks so far that you can see the complete head of the screw stick out of the hole. There is no slop or weirdness about the travel of the table on the x axis. My question is, does the settings of gibb screws sound normal or ?

I will assume that the gib is too thin. That is the thick end of the gib is deep inside the hole. This means that the gib is a bit too thin for the job. If it is the other way around then the gib is a bit too thick. If it is too thick then you may need to make it thinner. If it is too thin then we can add a shim. Let's assume it is too thin.

As far as positioning is concerned, it could be set just right or it could be loose or over tightened. To determine this if you loosen the end that has the screw backed out so you will then be able to determine if you can push the gib in any farther by turning the deep screw head in a bit (clockwise). It maybe already bottomed out? The screw should be firm but easy to turn. Anyway, there are a number of ways people have found to adjust the gib to the best position. I prefer to measure the backlash, then tighten the gib a bit more. If the backlash increases then you may already be too tight. Loosen it and measure again and again until the backlash does not increase any more. Each time, between backlash measurements, you must lock the gib via the screw that is out so far already. Other wise the gib will slide around as the saddle is moved. There are other sources of backlash so there will be a min value, below which a looser gib will not cause a decrease in the backlash. You might ask why not just loosen the gib up a lot. You can do this and measure the background backlash, but if the gib is left too loose then the x-y axis will be free to wobble and you will not have a right angle of motion between the two axis.

The easiest way to move the gib to a position where the clamping screws are more equally positioned is to take the gib out. Measure its taper from thin end to thick to determine the slope, m, thickness change vs length. Lets assume the thickness difference is 0.1inch per 10 inches of gib length. That means that m=0.1/10=0.010" per inch of length. Also count the threads on the screws so that you know their TPI value. Then you know how far you push the gib in per turn of the screw. Then determine how many threads of the screw you would like to move the position of the screws by. Let's guess it is 12 turns and lets assume that the screw TPI is 24TPI. That means that you want the screw to be moved out by 1/2 an inch or the 12 turns. Since you know the slope of the gib to be m= 0.01"/" via the slope measurement you then know how much thicker the gib needs to be to position it at the mid point. To move the screw setting by the 1/2 " says that you need to add a thickness of 0.005" to the gib.

So having done this you know how much thicker you would like the gib to be to fit at the center position, or other position. Find a shim of the thickness you need and place it on the back side of the gib, that is the surface that is not in contact with the sliding way surface, but is the surface that might have a clamping screw pushing on it, if there is one. After you attach the shim remeasure the thickness of the gib. Any glue that you might use can add thickness too.

I had a gib that was made too small from the factory. I found I needed for the gib to be about 0.007" thicker. I happened to have some Al metal furnace duct tape that was about that thickness. (i.e. Al with sticky tape like glue on one side.) This is not the plastic tape that has an Al coating on it, but is solid Al. (I suppose the plastic would also work as it is essentially in-compressible.) I cut a piece of this just the size of the gib and stuck it to the back side of the gib. Worked well for me. It was suppose to be a temporary fix until I could get a new gib made, but it worked well enough that I have yet to make the gib replacement. You want a shim that does not slide around. So even if you just use a standard shim material, brass, Al, steel, etc you might want to put a very thin layer of glue on it. You can always get it off with a little heat.

If my instructions were not clear enough let me know and I will try again. Maybe a sketch?

If the gib is too thick, you could do the same measurements and calculations to determine how much needs to be removed. These are small numbers and it is pretty easy to simply sand some material off the back side. Just do it uniformly. So measure the thickness of the gib at many locations down the length, very accurately, to determine if the gib is made right to start with. The tapper should be linear! If it is not then you may just need to take off the high points. It is common that the rubbing surface of the gib is scraped. If so you will see a pattern of marks and if there is no wear they will be uniformly distributed. These scrap marks are to help hold the oil and to distribute it during movement. However, if there is wear only at some points, as shown by missing scrap marks, then these are high/thick areas ...or the gib may be thicker at these positions. All of this you can correct if you need to. However, over time the high points will wear down anyway. How long, this depends upon your usaged and the gib material. Some gibs are made from cast iron, while others are made from brass! This largely depends upon the manufacturer. However, even hard cast iron can be sanded down without much trouble. Use fine sand paper (emery cloth), 200-400 or higher, and go slowly. Lots of measurements is good way to judge how fast you are taking material off and if you are taking it off uniformally.

Dave L.

 

[dabear3428]

I will assume that the gib is too thin. That is the thick end of the gib is deep inside the hole. This means that the gib is a bit too thin for the job. If it is the other way around then the gib is a bit too thick. If it is too thick then you may need to make it thinner. If it is too thin then we can add a shim. Let's assume it is too thin.

As far as positioning is concerned, it could be set just right or it could be loose or over tightened. To determine this if you loosen the end that has the screw backed out so you will then be able to determine if you can push the gib in any farther by turning the deep screw head in a bit (clockwise). It maybe already bottomed out? The screw should be firm but easy to turn. Anyway, there are a number of ways people have found to adjust the gib to the best position. I prefer to measure the backlash, then tighten the gib a bit more. If the backlash increases then you may already be too tight. Loosen it and measure again and again until the backlash does not increase any more. Each time, between backlash measurements, you must lock the gib via the screw that is out so far already. Other wise the gib will slide around as the saddle is moved. There are other sources of backlash so there will be a min value, below which a looser gib will not cause a decrease in the backlash. You might ask why not just loosen the gib up a lot. You can do this and measure the background backlash, but if the gib is left too loose then the x-y axis will be free to wobble and you will not have a right angle of motion between the two axis.

The easiest way to move the gib to a position where the clamping screws are more equally positioned is to take the gib out. Measure its taper from thin end to thick to determine the slope, m, thickness change vs length. Lets assume the thickness difference is 0.1inch per 10 inches of gib length. That means that m=0.1/10=0.010" per inch of length. Also count the threads on the screws so that you know their TPI value. Then you know how far you push the gib in per turn of the screw. Then determine how many threads of the screw you would like to move the position of the screws by. Let's guess it is 12 turns and lets assume that the screw TPI is 24TPI. That means that you want the screw to be moved out by 1/2 an inch or the 12 turns. Since you know the slope of the gib to be m= 0.01"/" via the slope measurement you then know how much thicker the gib needs to be to position it at the mid point. To move the screw setting by the 1/2 " says that you need to add a thickness of 0.005" to the gib.

So having done this you know how much thicker you would like the gib to be to fit at the center position, or other position. Find a shim of the thickness you need and place it on the back side of the gib, that is the surface that is not in contact with the sliding way surface, but is the surface that might have a clamping screw pushing on it, if there is one. After you attach the shim remeasure the thickness of the gib. Any glue that you might use can add thickness too.

I had a gib that was made too small from the factory. I found I needed for the gib to be about 0.007" thicker. I happened to have some Al metal furnace duct tape that was about that thickness. (i.e. Al with sticky tape like glue on one side.) This is not the plastic tape that has an Al coating on it, but is solid Al. (I suppose the plastic would also work as it is essentially in-compressible.) I cut a piece of this just the size of the gib and stuck it to the back side of the gib. Worked well for me. It was suppose to be a temporary fix until I could get a new gib made, but it worked well enough that I have yet to make the gib replacement. You want a shim that does not slide around. So even if you just use a standard shim material, brass, Al, steel, etc you might want to put a very thin layer of glue on it. You can always get it off with a little heat.

If my instructions were not clear enough let me know and I will try again. Maybe a sketch?

If the gib is too thick, you could do the same measurements and calculations to determine how much needs to be removed. These are small numbers and it is pretty easy to simply sand some material off the back side. Just do it uniformly. So measure the thickness of the gib at many locations down the length, very accurately, to determine if the gib is made right to start with. The tapper should be linear! If it is not then you may just need to take off the high points. It is common that the rubbing surface of the gib is scraped. If so you will see a pattern of marks and if there is no wear they will be uniformly distributed. These scrap marks are to help hold the oil and to distribute it during movement. However, if there is wear only at some points, as shown by missing scrap marks, then these are high/thick areas ...or the gib may be thicker at these positions. All of this you can correct if you need to. However, over time the high points will wear down anyway. How long, this depends upon your usaged and the gib material. Some gibs are made from cast iron, while others are made from brass! This largely depends upon the manufacturer. However, even hard cast iron can be sanded down without much trouble. Use fine sand paper (emery cloth), 200-400 or higher, and go slowly. Lots of measurements is good way to judge how fast you are taking material off and if you are taking it off uniformally.

Dave L.

nice write up but did you forget the other possibility?
ie it is almost new and the gib is just right and at the beginning of its travel, that is how mr grizzly g0758 is the gibs are at one end and it is almost new as well.

 

[durableoreo]

I'd guess that it's new. Unless there's some problem with the table of the mill, I'd recommend forgetting about it and making a bunch of parts instead. In the words of Mr. Crispin, don't take you lathe/mill to bits. Once you take it apart, putting it back together might become a full-time hobby. If you got the mill to make parts, make parts. If you want to take a stab at machine building and reconditioning, there is no end of improvements you can make to a small machine tool. And from the car enthusiasts, I've learned that you need a daily driver first, then a project. Taking your mill apart will results in needing a mill to fix the parts you took off. Same for lathes. You can see how this could get out of hand Congrats on the new mill.

 

[B2]

that is how mr grizzly g0758 is the gibs are at one end and it is almost new as well.

Of course, they maybe ok from the start. On the other hand the gibs, especially the z-axis gib, on my PM940M-VS-CNC, was totally messed up when I got them new. The z-axis gib was too thin and it was cut off too short. Who ever installed these did not know what they were doing or just did not care! As @durableoreo says make some parts, but I would add keep a close eye on things. The improper gibs and the installation caused my CNC to move different amounts in the two directions, up vs. down. My first project was to drill a series of holes. Each one was starting out at a lower point and getting deeper. Only because I had programed a large clearance height between moves did the moves not break off the bits and ruin the part. Observing this as it was happening was required to know to intervene in the program! By the way, the other axis also problems which I am still dealing with.