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Thread: Tricks for adjusting gibs?
05-03-2008, 02:04 PM #1
Tricks for adjusting gibs?
I am in the process of cleaning up a Tree Journeyman 330 CNC knee mill. It doesn't appear to have had a very hard life. I have just been cleaning it all up to make it nice and pretty before I start running it. I removed the gibs in order to clean them and check for wear/damage/chips.
Now that I have them back in place, I am wondering how to properly adjust them. Is it something that is done by feel? Indicator? I am still fairly new to machine work, so I have never really had the need to dial in a machine. This mills has a box way for the Y-axis and a dovetail for the X. With two gibs on the Y, do they need to be adjusted in order? I'm guessing that the gib on the side is the more crucial of the two.
Anyway, if anyone has some pointers that they can give me, please let me know. The machine is coming together nicely, and I want to ensure that I don't create any inaccuracies through my incompetence. I also considered replacing the balls in the screws, but that will be a project for another time. Thank you for any help!
05-03-2008, 03:43 PM #2
This is a question I was going to ask for a Tree 325. The manual punks out with 'gibs are pre-set at the factory...have a service technician make adjustments'. Can't imagine why this is not within the capacity of a machine shop to adjust their machine gibs.
If someone knows...can help out two people....
05-03-2008, 04:29 PM #3
someone correct me if I'm wrong, but I usually tighten them up until I can feel them just start to drag a little when I turn the hand crank.
05-03-2008, 05:59 PM #4
Loggerhogger's suggestion matches the technique that I use on Bridgeports.
The comment about two gibs on the Y axis leads me to believe that one sets the squareness of the table and the other sets the clearance, i.e. it fills the empty space. If this is true, then the adjustment sequence could be critical, and the two gibs may not be identical.
05-03-2008, 06:18 PM #5
Since its a Tree I assume you mean tapered gibs.
While it may be tempting to run the gibs up snug to definite drag, remember that there is practically zero clearance when a dovetail exhibits detectable drag; a pile of warm chips on the table can expand it enough to increase the drag - possibly to the point of galling. A small working clearance is as important in a way bearing as it is in any other kind of plain bearing. If you need a little drag snug the clamp slightly.
There's a number of approaches for adjusting gibs and which you use depends on your mode of operation and the condition of your machine. You want the minimum drift possible if axis linearity is to be preserved; OTH you want to preserve the machine's longevity. One works against the other; the world is not perfect.
New machine tools have zero wear so gib adjustment is a snap, run the gib in until you meet resistance then back it off a bit just until the drag disappears. Usually this means a bit less or a bit more than a thousandth depending on how well the gib is fitted. Gibs usually have a 1/4 per foot taper (metric machine 1/50). As an indicator, withdrawal of the gib a bit less than 1/16" results in about 0.001" increased clearance.
Before you start, make sure the ways, the gib seats, and the gibs are clean. Even a thin film of varnish or paint residue or a thread escaped from a rag affects how a gib fits.
Over time, ways wear hour glassed, that is, they wear more in the center of the travel than the ends. The more use the machine sees the more pronounced the hourglassed condition. If the gibs are adjusted for best running in the center they tend to bind at the ends. If you need the full range of travel you'll have to adjust the gibs for best fit at the max travel and live with the excessive clearance in the middle. Practiced operators of worn machine tools quite often keep the tools to adjust the gibs handy so they can be tweaked on the fly so to speak. The operator has to be vigilant. A machine slide feeding under power past center soon meets increasing drag and the operator much be quick to slack back the adjustment. In a worst case the slide may jam or the gib adjustment broken.
Setscrew adjustable gibs are trickier. Here you not only have to treat a setscrew as an ungraduated micrometer you have to estimate the "pull back" when snugging the lock nut. Starting with the setscrews backed off a part turn so the slide is loose. Run the end setscrews up snug together for the moment. This holds the slide against the opposite way so the center setscrews can be adjusted.
Run a center setscrew up to the gib and feel for snug. Note the position of the driving slot. Lightly snug the setscrew, release the end setscrews, and try the feel of the lead screw. If the table moves freely, hold back the locknut with a wrench and make slight setscrew adjustments until you feel drag. Then tighten the locknut slightly and feel etc, working setscrew against locknut until the adjustment is snug and the final adjustment of the locknut removes the drag. The locknut tends to loosen the setscrew adjustment provided the setscrew is immobilized when the nut is adjusted.
Repeat for the remaining screws. Sounds like a PITA but you soon get the feel for the task. The same cautions for hourglassed dovetails apply for setscrew gibs too.
05-03-2008, 06:48 PM #6
Thanks Mr Addy- you've a rare gift for clear explanation.
On a different aspect of the question, and in the more general case:
On smaller machines where the size and weight permit, my feeling is that it's much easier to achieve a nice result along the entire travel distance if the feed screw is removed, allowing the table to be slid directly by hand.
I probably wouldn't bother doing this on a big table if it had ballscrews, but in the case where the screws are plain it could still be worthwhile dropping off the screw and rigging a temporary lever to move the table, so you feel only the frictional forces of the gibs.
In the case of setscrew gibs, sometimes these are retained longitudinally only by shallow dimples engaging the end of the setscrews. These tend to be sticky when you initiate a movement due to the camming action, and this will get worse with the accumulation of time since last strip and clean.
This effect can be eliminated by fitting a short peg to prevent endwise movement of each gib relative to the moving element. (table, compound slide or whatever)
05-03-2008, 07:21 PM #7
Cincinnati Milacron explains in their manuals how to adjust gibs on a CNC machine by saying that you should tighten the gib until it increases observed backlash, then back off until the increased backlash goes away. I'm experimenting with that right now for the first time on a machine with box ways. I put 2 .0001" dial indicators on the ways, one bearing on each side of the carriage, and adjusted the gibs until backlash was even on both sides and so that loosening the gibs did not decrease backlash. When I tighten them from this point, I can see stick/slip happening, so I think this is the right point to set them. I also measured the torque needed to turn the ballscrew. With the gibs backed off, it took 4 ft/lbs to turn, now it takes 5 ft/lbs. This is on a lathe where the carriage is way too heavy to move by hand - it takes a loong bar and blocks of wood to budge it with the ballscrew disconnected.
I'd appreciate comments on this from those more experienced than I.
05-04-2008, 01:07 AM #8
On a machine like that I would stick to the manufacturer's recommendation, Mud. When I set the gibs on our HBM's, I usually use a big prybar and dual dial indicators and check the gibs by basically checking "endplay" from the ways to the machine table. There'd be no way in hell I could feel the change in drag on a 120" long by 48" wide cast iron table that weighs 5000 pounds or more.
05-04-2008, 07:24 AM #9
Clean surfaces with thinner AND benzine. Check that the gib is dead stright.
Oil with some very light mechanism oil. "push" the gib in hard with one screw. Lock hard with the other one. Back of the first one until slightly loose and re-tighten it just a bit over snug. Check for reasonable drag.It is very important that the gibs are straight and most of the time they are not. There may be small areas where the gib binds onto. Scrape ( the gib) them away. Do not try achieve the ultimate fit - it is not necesary. Unless exagerated, slight play in the gibs is not the ultimate factor in machine tool accuracy by a long shot. The difference between the gib being just right and being a bit too tight is HUGE as drag is concerned. In other words you can't go wrong.
05-04-2008, 12:19 PM #10
Is there anyone that knows the actual Tree service process for adjusting gibs? These are fairly heavy CNC machines. Testing for drag doesn't seem to apply since the axis contained with ballscrews. There must be some sort of proceedure.
05-04-2008, 01:09 PM #11
I've got 8 or so SERVICE manuals for european machines. They're not even remotely similar.
I'd be very curious what the Tree service manual says. The manuals for 2 german and one swiss machine are clear in that gibs must be tightened until play is eliminated. They say the boxways should never be adjusted.
As to drag : I've got a very fancy german shaper made in 1989 - a steal from german eBay.
If I adjust the ways with thew proper clearance for the ram I can not move it by hand. If I double the clearance to some 0.01mm feels as light as a feather.
05-04-2008, 04:30 PM #12
the machine tech
the guy that works on our fadal told me that you tighten the gibb until it wont move. use mag base and indicator. then loosen until you get .0005 playl. use a pry bar in a safe area and push against the play. then adjust all the gibbs the same.
hope it helps.
05-04-2008, 04:46 PM #13
First I would make sure the ways are clean of any debris and also way oil. Way oil can give you a false reading. Tighten gib until you feel it is a little tight. Move axis from one extreme to another. Either by feel ( manual machine) or amperage draw ( CNC machine) find out the area that gives you the most drag or tension. While axis in this area make gib snug as possible so that the machine can still function properly and travel from one extreme to another without jamming or servo motors getting hot to touch. That is the best you can do without scraping or grinding the high spot.
05-04-2008, 05:00 PM #14
Wells Index suggested: Tighten the gibs until a .001" table deflection is noticed under pressure and the table goes back to it's original position when the pressure is removed. You make the measurement between the table and saddle, and saddle and knee.
I thought that was a pretty good idea, because that's where you want the table under working conditions.
Henry Ford had it right when he had all his manufacturing machines re-made to close tollerances WITHOUT gibs. He did this so his people would quit messing with them.
I thought that was funny. But how true.
05-04-2008, 06:31 PM #15
Problems with "feel" method
my experience says that a gib after its been in use is no longer flat anyway, they tend to bow one way or the other. (not by wear but the metal tends to take a set) Generally, snug it up till movement is impaired with hand weel and you get what you get till your willing to do the scraping thing and put everything back true. (ways and gibs) usually on cnc machines a DC amp meter used instead of feel and that takes some charting from the previous readings that should have been done over the years. each axis has it's normal load.
05-05-2008, 04:34 AM #16
A remark on tappered gibs
The shorter the distance between fully blocked and smooth running the better is the fit
05-05-2008, 07:05 AM #17
I've always just adjusted them for a tiny ammount of drag.
On the machines I have, I've always just adjusted the gibs to where they just barely give you any drag you can feel. Move it from one extreme to the other, making sure there's no binding. If I've taken the mechanism apart, I clean everything SPOTLESS with brake clean, and apply a very light coating of way oil (the way oil will be there when the machine is in use anyways). The lathes both have set screw gibs, and they are tricky. The best way I've found to do those is to torque the middle one first (they have 3) until snug, then do the edges. The gib will be too tight like this, then I use the lock nuts to back them off to where they run smooth. Just barely a noticable bit of drag, if I crank on the top slide, I can get about .0003 deflection with my hand. Seems to work the best for me, seems gibs are a matter of preference. I prefer them to be tight enough to keep things accurate, and from chattering. A tiny bit of wear over time is acceptable. Some people like to keep the gibs with a little more play in them for less wear. I suppose if a machine was used for production, where tolerances weren't an issue, I suppose you'd adjust the gibs just tight enough to keep everything from chattering, and avoid wear.
Anyways, with the tapered gibs on the mill drill, there's an adjustment screw at one end, and that's it. I just adjusted it until it was snug, then backed the screw off a tiny bit. Then I move the table the opposite way the gib tightens, and it slides out against the screw. Then I see if the table moves with a bit of drag. If it's still to tight, I back it off a bit, and move the table so the gib comes out, and repeat until it's set. What's weird is I'd think the gib would tighten itself with nothing to lock it in place, but it doesn't. The table moves about .0005 at the very end in relation to the y axis when I push it hard with my hand, with the gib set the way I like it. The Y axis moves .0003, seems to work well that way. My B&S mill hasn't been adjusted since I've had it, the guy that owned the shop I bought it from adjusted well enough, I haven't needed to touch it.
An idea for adjusting CNC machines, how about measuring the current the servo motors draw?? You can adjust the gibs until the servo motors start to draw a bit more current. Let them detect the drag for you. Put a meter on the input side of the servo controllers. You should be able to see the current increase with load, even a small increase in load. It's either that, or use the indicator method.