I just adjusted gibs for the first time. Full disclosure, when you read the full post you’ll say I didn’t need to. True. But by the time I figured that out, it was too late.
OK, so I followed the Tormach knowledge guide to a T. Started with measuring lost motion on the X by moving it out and then back 0.01” at a 5IPM rate. Read .001” difference which is within the spec of <.0013”. Similar for the Y. Moved onto the Z and measured .0025”! Looks like I need to adjust the Z gib. I’m using two techniques now. The lost motion check described above from the knowledge guide as well as checking the number of .0001” movements it takes before the indicator shows actual movement. Working in a direction which minimizes both of these measurements.
I’m honestly not seeing a ton of difference in either direction. And then it hits me like a brick. I don’t have the .0005” indicator in like the knowledge guide said to and I’ve been thinking I had in. I have my .0001” indicator in! Meaning, my X and Y lost motion was only .0002”! My Z, before I started messing with it, was only .0005”! I could have just left it be and been well within the spec of <.0015”.
Long story short, I’m in it now so I might as well find my optimum. Although all my numbers, I now know, are small, they do begin to improve the more I back the gib out / up. I head in this direction until the numbers start getting worse again, and then go back down to where they were the best. They are now at .0004” for lost motion, and it takes .0006” of movement on the controller before you see movement on the dial.
And here’s the weird part. The gib is pulled pretty dang far up / out. This definitely wasn’t where it was before. But this is what the numbers were pointing to. But on the other hand over a range that translates to probably 10-12 full turns of the gib screws the worst I ever saw was .0007” of lost motion and .0008” of movement before it shows on the indicator. This makes me think of a few things
- Did I really, truly find the optimum gib location or are the numbers all so small that the differences don’t really matter and I just happened to find a random location that gave a good set of numbers?
- How in the world did the numbers all stay so small and within spec across such a wide range of gib placement?
Oh. And I ran some tests to try and optimize my feeds & speeds with 3/8” bits after this. Said another way, large diameter bits that my 770M doesn’t usually love working with. And it ran great. No crazy weird or new vibration / issues that could indicate I have a lose Z gib.
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Lost motion is only half of the battle. You have to balance it with axis slop. The looser the gibs, the less lost motion you will have, but the looser the whole axis gets which can cause problems with accuracy/precision of the final part. At a certain point, the gib is loose enough that any lost motion is going to be due to other friction points so continuing to loosen it doesn’t change your lost motion numbers.
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OK, then how do you actually find the optimum point for the gib then if doing it the way the manual states to doesn’t work?
Don’t forget the wiggle at the ends, and tilt if you have a microARc on the same end as the X drive motor. You know when it’s optimum when your parts come out in tolerance.
Put a 0.001” indicator base on the table. Indicator somewhere on the head. With a holder in the spindle, on hand on that, the other at the top, pull and push. What do you get?
I’ll give this a try tonight when I get some time. What sort of numbers should I be expecting? That way, if they’re off, I can go ahead and redo the adjustment while I’m there. Instead of coming back, sharing the number, getting feedback, and then probably waiting for another evening when I have time.
There isn’t a magic number, it’s just a balance. You want axis wiggle as small as possible AND you want lost motion as small as possible. Reducing one will eventually increase the other so the goal is to get both to a place where you’re happy with the repeatability of the machine and the accuracy of the parts.
The book numbers for lost motion are a good place to start though. With yours being so much less than the tolerance, I expect you to have a lot of axis wiggle. I would adjust the gibs closer to the book spec for lost motion, check the wiggle, and go from there. Remember that lost motion can be compensated for to a degree using backlash compensation. A loose axis, on the other hand, can not be programmed out.
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What he said.
These are not linear bearing ways.
Well that was a fun new skill to start picking up. Always new rabbit holes go to down in this hobby.
Where I decided to leave it, I was getting .0005” tilt in the head, lost Z motion of .001”, and backlash of about .0012”-.0013”.
While I did find spots of lower backlash and lost motion values, this was the only one that kept me decently under .001” of tilt while having decent numbers elsewhere.
For anyone coming along later reading this and doing it for the first time. Here’s a couple of observations as a novice.
- Changes weren’t perfectly predictable. In fact, sometimes readings would go slightly up when I thought they’d go down after a gib change. And then if I did another change in that direction they’d head back in the expected direction again.
- Going back and forth between tightening and loosening to find a spot I liked I also noticed that if you, say tightening the gib by one turn, measured, and then loosened it back 1 turn and measured. You wouldn’t necessarily be back at your original measurement. Said another way, it’s a slightly imperfect art.
- When measuring lost motion it greatly matters which direction you were headed in before performing the act. Let’s say you’re going to zero out your Z axis, then run G1Z.01F5 and G1Z0F5. That’s a positive Z movement. If when zeroing out your indicator you came up to the zero point, you’ll get much less lost motion than if you came down onto the zero point. The difference in these two measurements is generally slightly less than whatever you measure for backlash. For example, where I left my machine, I got .001” of lost motion if I came down to my zero point. I got .0001” if I went up to my zero point before running the tests. Since my test traveled up to Z.01!
Either way. I guess I’ll run some test cuts with it soon too and see what I get. But a 1/2 thou of head tilt, under spec on lost motion are both great I think, based on my other readings at other gib locations. Not in love with .0012” of backlash. But we’ll see if it makes a ton of difference in cuts.
My experience is that more precision in measurement yields more demand for precision in manufacturing. Most of what I make can easily handle ±0.005” if not more from part to part.
My machine readily performs at ±0.003 without any real effort. So based on that, my parts are effectively perfect without even trying. Yet, when I measure them and I see something is off by a thou, or 15 tenths, I have a moment of annoyance that it’s under or oversized, not because it’s a bad part, but because I’m able to see the error.
If I ignore the fancy measuring devices and simply assemble the parts, they fit and function and look good so at that level of precision, I am thrilled with the results. It’s only after looking closer that I find “problems”
Point is, unless you have a real need for sub 0.005” precision, your machine is going to make good parts that you will be happy with, so while I understand the desire for perfection, it’s not a worthwhile endeavor in this case.
FWIW, sub 0.001” precision is possible on these machine when necessary, it just takes a willingness to walk in those critical dimensions each time. I often find that multiple spring passes, while horrible for tool life, yields amazingly accurate final dimensions. Of course, that’s when I break out the .000005” digital mic and get mad all over again because it’s still not perfect.
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I worked in aerospace composites for 8.5 yrs. Engineering would come out with tolerances like 0.0005" for a composite part. Machine shop manager would ask them what the current machine specs were. Read used VF8, minimal maintenance, with carbon fiber, fiberglass dust throughout. Stuff penetrates everything. True position? Drill under, measure with laser tracker, move hole, rinse and repeat. Great fun.
“My experience is that more precision in measurement yields more demand for precision in manufacturing. Most of what I make can easily handle ±0.005” if not more from part to part.”
Running a PCNC 1100 for making prototype molds. Always running 0.001” and under, and always tinkering with the Z and X axis to keep it that way. For some reason the Y axis is rock solid. Next machine will have linear rails.
I think some of y’all are misunderstanding my intent. I misread my dial. Modified the gib when I probably should have left it alone. And this was merely an attempt to get it back to an acceptable setting that works well. My goal isn’t ridiculous accuracy improvements. I’m happy just getting it back to the level it was before I messed with it and shouldn’t have. Ha!
But since the process that was recommended in the knowledge docs and here involves taking measurements and finding a happy medium between them, subject to subjectiveness, then that’s what I’ll do. Or said another way, the reason I stopped where I did was because that seemed like the best tradeoff between the 3 ways of measuring that I came upon as I was adjusting. Simple as that.
Intent understood. My post was that if you want too keep high accuracy you have to keep on top of the adjustments. The machine accuracy seems to drift with amount of work done.
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This is expected really. Any machine that uses dovetail ways and gibs is going to experience wear over time. That’s why it’s a good idea to move setups around the table rather than defaulting to the middle all the time. This helps to keep the wear even across the whole range of motion.
@Nota_Sharps No misunderstanding of intent on my end, my comments were just sharing my experience with chasing tenths for no reason other than because I could measure them. I have every expectation that the numbers you’re getting now will give you very good results on your finished parts.