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Hirudin
I do not think that type of square will be that good, you really need a granite square, or a ground cylindrical square, to do that kind of testing
Try Traming the Head to the Table first & then do your test
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I think the title says most of my issue, but I'll explain further...
I'm trying to test the squareness of my machine. As of now I have not "messed with" the column; it's exactly as it was when I received it. As you can see in the video I set up a dial test indicator against a square positioned close to the table. I then raised the Z 12". ~3.5" up the square the indicator is reading ~6 thousandths off. To me this seems pretty bad. After that it stays within ~1 thousandth (within a thousandth of the reading at ~3.5" above the table which is between 6 and 7 thousandths of the reading just above the table). I hope this makes sense. If not, I think the video does a pretty good job showing what is happening.
Is 6 thousandths over less than 4" acceptable? Any advice on how to fix this?
I'm assuming it's not the square. It's brand new and is suppose to be accurate to within ~2 ten-thousandths at the top.
I'm a novice so if you see something in the video that I'm doing wrong please tell me.
[nomedia="http://www.youtube.com/watch?v=E3CNdoNdu2k"]YouTube - Novakon NM-200s2 XZ plane squareness test[/nomedia]
Hirudin
I do not think that type of square will be that good, you really need a granite square, or a ground cylindrical square, to do that kind of testing
Try Traming the Head to the Table first & then do your test
The head is trammed in that direction, but as I understand it the tram shouldn't matter for this test.
Anyone know a place to get a good cylindrical square?
Is it possible that what you saw was looseness at the head? maybe combined with lack of lube in one place cause the head to drag on one side. Perhaps once it started moving up it pulled to one side and stayed there. One quick check would be to repeat the test in the opposite direction (downward top to bottom) and see if the error is consistently in the same place. I'd also try twisting the head to check for too much slop in the Z way. If the way is bent it would explain what you are seeing. To check you could tram with Z close to the table then move up and use a spacer say 5" tall to check again. (use the same spacer and move it from side to side.)
Charlie.
Hirudin
That is correct the test you were doing, the head does not need to be Tramed, I thought that machine had linear rails on the Z axes
Mactec54
Charlie
Thanks for the insight! I did actually do the test in both directions and the measurements appear to be the same while moving down as well. I also gave the machine several (too many) pumps on the single-shot oiling system shortly before doing this test I would be surprised if one area is less lubricated than the rest but I'll inspect it as well as I can to make sure.
Thanks for the suggestion about tramming at two different heights. It took me a couple rereads but I now understand what you're saying. Seems like a good idea and I will try it soon. I'm still unclear about what you mean when you say "use the same spacer and move it side to side" though.
I'll see if I can detect any movement in the Z by trying to twist the head.
Mactec54
This machine, the NM-200 series 2, does have linear ways for the Z axis.
About the square. I have thought about it and I can't figure out why this kind of square wouldn't work. It should be accurate to 2-tenths of a thousandth (0.0001 every 6"). As I said, I am a newbie - can you explain why a cylindrical square is preferable for this test?
OK, so I've done this myself and here's what I can say:
1. The type of ways on the Z don't matter when it comes to tramming for this. Tramming is unnecessary because the head can be at 90 it doesn't matter, you're not comparing left side to right side you're comparing travel in Z relative to the XY plane. (I know it seems counter-intuitive, at least it did to me at first. But think about it for a while and it starts to make sense, or your head explodes - either way, problem solved).
2. The type of square you're using is, indeed, not really suitable for this test. Here's a good link to explain the proper way of doing this (note, too, that you need to repeat this test at 90 degrees as well, in other words with the DTI facing forward - more about that below). The reasons why you need either a cylindrical square or (my preferred option) a granite square - note that both are a bit expensive - are probably numerous. These "straight edge" squares are often not truly straight for the whole length of their arm. Instead they bow, typically inwards. They may be spec'd for X variation over Y length, but that can be tip-to-tip square, not along it's length. Plus, of course, many Chinese manufacturers aren't honest about the specs. When I run this against a good granite square I now, after some adjustment, get .0003 one way, .0002 the other over 8" (see here). On my squares like yours I get a "dip" in the centre of several thou.
3. Anything, and I mean anything, under the arm of the square on the table will screw up readings, so be aware of that (though not the dip you're getting)
4. Take readings on different parts of the table as the table is not perfectly flat.
5. I needed in put a shim in the column and carefully move it around a little bit to get everything perfectly (or as near as req'd) square. The 2nd link I mentioned above talks about that. There are other ways of shimming, but this worked for me. One shim, in the right location, corrected both "lean" (left/right) and "nod"(front/back).
6. Make sure you're using a good DTI, make sure it's secured well in the spindle, and if possible lock the spindle somehow. I rigged up a clamp system at the drawbar with my trusty visegrips which had the advantage of being able to turn it in 90 degree increments.
7. Think about what's happening during travel to locate the cause of the error & correct it. If you think about it a bit your intuition will probably lead you in the right direction. Remember to measure lean & nod each time before a correction. Otherwise you could fix one and exacerbate the other.
8. Don't expect miracles. Set a goal of what you realistically need and try to hit that. I fluked upon the right setting on mine to get the accuracy that high. I probably would have been happy with .001 over 8"
Hope that helps!
Last edited by JoeBean; 02-07-2011 at 09:00 AM.
Thanks for the reply as well JoeBean!
The square I'm using is a Starrett 50136 (American made). I bought it from Amazon so I highly doubt it's counterfeit or something like that. Also since it's brand new and was shipped in literally five layers of cardboard (including the Amazon box) I figure the chance that it was damaged in shipping is slim to none. If I can't trust that it's square I wonder if I can trust anything. The DTI is a Mitutoyo 513-504T (also new from Amazon) and is graduated in tenths-of-thousandths - an entire rotation of the dial is 0.010. I think I can trust the square and DTI to be accurate to a much greater degree than I can expect to achieve.
Reading the internet some it looks like people don't think Mitutoyo DTIs are very good. Maybe I'll throw my Shars DTI on there and see what it says.
I could be convinced that the shape of the square could be a slight concern, but at the moment I really can't see how. I cleaned off the table and the square (not that either was dirty in the first place) before doing the test. I'll test again (recording the readings every 0.5") and then turn the square around and test on the left side of the spindle. If I get different readings I'll take a hard look at the square and indicator. I'll also try different parts of the table as you suggested. Once I start shimming I'll surely alternate between the XZ plane and the YZ plane.
I've read that page on CNC Cookbook you linked to. Thanks for it anyway though; I'll read through it again. I can't stress enough that I am a beginner so maybe now that I have a tiny bit of experience the importance of one of the tips on that page will become more apparent. Although (and maybe because) I'm a beginner I still have to know *why* some things are the way they are. If a cylindrical square really is the only and best tool to use I'm perfectly willing to accept that fact, but I would really like to know why they're better suited for this job.
In case this might matter: the table is also pretty much perfectly level, or it's as close as I can get it. When I put my electronic level on the table it says it's less than 0.03º off.
At the moment the spindle isn't secure. I will take your advice and clamp it down somehow. I don't think it's moving, but I guess it certainly could be. Though I only recorded one, I moved the head up and down several times and got what appeared to be the same readings every time.
If I could get under a thousandth that would be great! I'll settle for anything less than 2 thousandths without complaining too much.
My suggestion to use the same spacer was to eliminate any chance there was a height difference between to different spacers. You don't need anything super precision just easy to position when you tram from one side to the other. You could also mount your indicator to an extension to bring it closer to the table when checking the tram a few inches up.
Charlie.
Originally Posted by Hirudin
Oh, I see now. Thanks again.
Starrett's quality has been going downhill for some time (not just them, my beloved B&S has been heading the same way). Not that that proves there's a problem with this square, but just search for Starrett over on PM and see what problems have been encountered. As well, a lot of their product is being made in China, but still stamped with their US location (ie. not "Made in America"). Not that that proves it's off either - from some comments I've read over the last 2 years some Chinese made stuff from them is more accurate than the American made stuff.
So again, I'm not saying that yours is a POS, I'm just saying don't trust it automatically. If you have a good known surface plate and the equipment needed to check it that can give you some more confidence in the tool.
Mitutoyo are trouble to rebuild (apparently - I have never taken one apart, just going from what I've read) but they're accurate and stay so until they're used up. That doesn't mean they're not accurate. FWIW check out Long Island Indicator for info on brands and models when shopping - they rebuild instruments (more than just indicators), sell parts, and have a HUGE wealth of info on the differences between brands and models. I use them all the time when looking to buy a new instrument and they've never let me down. And if you buy from them their prices are quite reasonable and their service fast.The DTI is a Mitutoyo 513-504T (also new from Amazon) and is graduated in tenths-of-thousandths - an entire rotation of the dial is 0.010. I think I can trust the square and DTI to be accurate to a much greater degree than I can expect to achieve.
As for trusting it, nothing's a sure bet.
See above.Reading the internet some it looks like people don't think Mitutoyo DTIs are very good. Maybe I'll throw my Shars DTI on there and see what it says.
[/QUOTE]I could be convinced that the shape of the square could be a slight concern, but at the moment I really can't see how. I cleaned off the table and the square (not that either was dirty in the first place) before doing the test. I'll test again (recording the readings every 0.5") and then turn the square around and test on the left side of the spindle. If I get different readings I'll take a hard look at the square and indicator. I'll also try different parts of the table as you suggested. Once I start shimming I'll surely alternate between the XZ plane and the YZ plane.[/QUOTE]
There's an easier way to verify if it's the measuring tool's fault: As I mentioned in the earlier post, do one to test to check lean (as you're doing) then another to check nod. If they both show the same dip the problem is with your measuring equipment. If it only dips on the lean then it's your machine.
As for shimming, you should only need one appropriately sized shim. Loosen the 6 bolts on the sides of the column riser & remove the 2 bolts inside the column facing up (accessible by removing the bottom rear cover plate). Use some means to tilt the head slightly (this is why the bolts on the sides are only loosened slightly, a few turns at best: It keeps alignment and prevents you from pulling the whole head over). Push the head to tip it and insert the shim - either on the left or right, depending on lean and either forwards or rearwards depending on nod. Of course in the center would give you no adjustment of the nod or lean. You should need a very small shim - I used a .0005 feeler gauge. Once you've got it in place lightly tighten the 6 bolts and run your tests again. Once they're good tighten the 6 bolts all the way and retest, and again retest after installing the 2 lower bolts.
Cylindrical squares are not the only tool for the job. As I mentioned above granite squares work great as well.I've read that page on CNC Cookbook you linked to. Thanks for it anyway though; I'll read through it again. I can't stress enough that I am a beginner so maybe now that I have a tiny bit of experience the importance of one of the tips on that page will become more apparent. Although (and maybe because) I'm a beginner I still have to know *why* some things are the way they are. If a cylindrical square really is the only and best tool to use I'm perfectly willing to accept that fact, but I would really like to know why they're better suited for this job.
The thing that both of them have going for themselves is that they're very stable. The likelihood of them going out of tolerance over time, barring any damage being inflicted upon them, is very low. On the other hand, with standard squares you have the joint that can be out (though that wouldn't account for your numbers,as I mentioned earlier), you have a thin metal arm that can "settle" over time after having been ground, and you have an item that most companies seem to be manufacturing sloppily. Yes, a proven good square would work, but they're not easy to come by that way to begin with and often require reworking to get perfect. Cylinder squares and granite squares are less likely to be off - not saying they can't be though! And, of course, their shear weight makes it easier to test - no clamping required, their weight will keep them from moving.
But you're correct, assuming the square is perfect in all dimensions there's no reason it can't be used for the test and give accurate results.
This is mostly a convenience to prevent it from turning and popping off the edge. In your case, one benefit of using a thin edge is you'll know right away if that happens. On a cylindrical square it can be more subtle.At the moment the spindle isn't secure. I will take your advice and clamp it down somehow. I don't think it's moving, but I guess it certainly could be. Though I only recorded one, I moved the head up and down several times and got what appeared to be the same readings every time.
Try doing as I mentioned above - run the test again 90 degrees out, ie in the YZ plane (you already know what it will say in the XZ). If it gives the same, or comparable, dip then you know there's something wrong with your equipment, likely your square. If it gives no (or little) error you know there's something wrong with the mill. BLAMMO! Answer found!If I could get under a thousandth that would be great! I'll settle for anything less than 2 thousandths without complaining too much.
On a related note, if you're not already familiar with the process, if you have a good surface plate & a surface gage you can check the square for arm straightness & squareness.
Also, how far down on the Z rails are you when you're starting this travel? I'm wondering if it's happening only at the very bottom of Z's travel, which would obviously be less serious.
Your actual error, btw, looks to be about 5 thou/6thou when compensating from cosine error (at least from the best measurement of the angle that I could make).
Finally, 2 other thoughts come to mind: What happens if you turn the square around and run indicator up the other side of the square (obviously you'd be starting with the indicator slightly higher, but you have the room with your setup)? And secondly, if you have decent 123 blocks that are spec'd for squareness have you tried running up one of their long sides to see if that also gives you the error? It should give you nearly the same over the 3".
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