Help with making a rotary table more rigid

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RSG
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Re: Help with making a rotary table more rigid

Post by RSG »

So you think this would be rigid enough then Glenn?
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GlennW
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Re: Help with making a rotary table more rigid

Post by GlennW »

I've never used one and it's difficult to tell how rigid it would be from the image, but it wouldn't be as rigid as a RT bolted directly to the table.

If you need the angle feature, it's probably not a bad choice if the chuck with soft jaws id mounted directly to it.
Glenn

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pete
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Re: Help with making a rotary table more rigid

Post by pete »

I'm not sure if this will help or just throw the thread OT. But I've spent more years than I'd care to add up studying everything I could find about machine tool alignment and metrology. Both those are a great deal more complex that it seems, and other than tramming the spindle on a mill most don't seem to understand it's about the last step after many other checks if accuracy is a high priority RSG. Dr.Schlesinger's book Testing Machine Tools is well worth googling and a download. It gives the test procedures and maximum allowable limits for most of the common machine tools. A Youtube poster by the name of At-Man Unlimited Machining posted a series of excellent videos about testing and aligning a decent but used CNC bedmill. Almost all of that could be used on a manual mill. I think we can all agree that compensating for machine tool inaccuracy and cutting tool deflection is a large part of getting good accuracy on the finished part. My logic is if you don't test and align what can be adjusted there's no way to compensate for any unknowns.

How many actualy know on there knee mills how much the knee is off over it's travel in both Z and Y for example? I don't yet know enough about the scraping process to be fully confident of correcting any errors I find. But if I do know how much and where they are there's at least some hope of applying some compensation if the accuracy demands that level of precision.Brand new from the factory my mill was adjusted to within 8-11 thou backlash on both axis's. And about .007" on both from locked to unlocked position on the table locks. Since I had to pull the machine apart to move it then a careful cleaning, removal of the shipping preservitive and then relube with a good way oil, gib and feednut adjustment made sense. The machine was far tighter and much smoother in operation with about .004" in backlash and roughly .003" in table movement between the locked and unlocked positions. I probably could have gone fractionaly tighter I suppose, but not much without running into tight spots I think. The backlash wasn't that important since it's standard to adjust for whatever you have when using the dials, the table movement was important to know. Adding a dro warns you right away the table is moving just from applying the locks. Without one it's easy to miss that unless you do check it with an indicator. Watching the one end of the table drop on the indicator as it goes well past center due to gravity just from the minor working and oil clearances is pretty educational for the reasons working to that half thou is so tough. The usual BP knee mill is to be honest a pretty poor design for fairly high accuracy and especialy rigidity imo. Why there used so much is because there so versitile and not because there the best machine ever designed.

Every rotary table out there no matter how expensive is going to have some inaccuracy besides what the mill already has. Alignment for vertical operation is usually just passing an indicator across the face until it's square to the table travel. How much is it off in the Z axis? Even Moore Tools couldn't make "perfect" feedscrews and nuts using every trick they could think of for there manual jig borers and money was no object. They could measure within millionths what the errors were, but they still couldn't make them error free. The worm and wormwheel on any R/T or D/H are going to have varying amounts of lead and lag errors for sure. I vividly remember John Stevenson posting years ago on the HSM forum that he bought 20 Vertex R/T's for conversion to 4th axis. He stated every "inspection" certificate had the exact same numbers written into each box. Since then I don't blindly trust any inspection certificate. Once I learned enough I did run checks on my 6" Vertex and I never found any measurement that matched my certificate, yet I see lot's of forum posts where people think there a pretty good brand. Unfortunately I haven't yet figured out a quick and easy way to properly check the worms and wheels yet.
RSG
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Re: Help with making a rotary table more rigid

Post by RSG »

Glenn, thanks for the input, I haven't yet made up my mind on the tilt version. I do like the fact it allows me a whole new set of operations I currently can't do but if accuracy suffers too much then it's probably not worth it.

Pete, thanks for the comments - You do have a vast knowledge of tools and tool makers....I try to test all my equipment to the best of my ability but there are some tests as you mention that I'm guilty of not testing for such as the knee and I would imagine it's not that difficult to test either. I will research Dr.Schlesinger's book and I am familiar with At-Mans videos.
Vision is not seeing things as they are, but as they will be.
pete
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Re: Help with making a rotary table more rigid

Post by pete »

Nope I don't think it's vast at all, I've barely learned enough to have some idea about just how much I don't know. :-(

The best addition to my test equipment after a 10ths indicator that I found was a decent cylindrical square. It's a basic reference surface and if your lathe is in half decent condition and tweaked to turn as parallel as possible then making one more than accurate enough to test a mills spindle stroke and the knee movement isn't all that tough. There also fairly easy to check for absolute squareness if you've got even a small surface plate. Don at Suburban Tools did a great video showing even a worn inaccurate cylindrical square is still perfectly square at one position. If that's known and marked it can be used at that position. B & S purposly made there 558 cylindrical square with one end ground at a slight angle, there's dots running up and down the square that allow accurate measurements that show how far off something is from being square and there's one point that is marked as being square. Making one by turning between centers a cylindrical square is then a great tool for running a quick check and realignment if you ever off set the lathes tailstock.

If I was making one? I now think I'd make two. At least a 12" and one 6"-8" long. With the two you can align the long one with the X axis, butt the other against the first at 90 degrees then test the Y axis and how square it's movement is to the X. Years ago I think it was Glenn who showed an extremely clever way to test a mills knee travel without a square in one of his posts here. Just drill then single point bore any 1"-2" diameter hole in any piece of plate held in the mill vise. It's then perfectly aligned with the spindle C/L and as round as the spindle bearings allow. Lower the knee to what ever position you want to test at and sweep the hole with a dti by turning it while held in the spindle. That show's any inaccuracy in X or Y right away. Obviously the spindle stroke can be checked as well. The Moore Tools book Foundations of Mechanical Accuracy has the best detailed description I've seen so far about the complex and possible misalignments machine tools can have. Expensive to buy but for anyone with an interest your local library should be able to get it through there lending program with other librarys.

The worst part about doing detailed checks like this is the possible severe depression when finding something you wish you hadn't :-)

Like Glenn I've not used one of those tilting R/T's either. That one in your link shows decent sized trunnions so it should be fairly rigid. There might be at least some truth to there guaranteed test numbers since that's roughly about what my Vertex table shows. They are playing on peoples perceptions of accuracy tho. They don't say it's repeatable accuracy is 10 seconds, they just say it has a 10 second vernier. In other words that's only it's resolution not repeatable accuracy. One thing that's not been mentioned yet is looking for a decent condition but used optical table. From my reading they were mostly used on high accuracy jig borers and today usualy sell for very little for what they are. Roughly the price of a new off shore R/T or sometimes a bit less. I'd want to buy one that shows decent pictures of the table and mounting faces and I wouldn't buy anything that showed any milling marks or that it wasn't looked after. Any rust at all showing on the ground and lapped surfaces would be a bad sign imo. Optical tables like that and because of what they were used for were made to very high accuracy tollerances and many didn't get used all that much. All the one's I know about used a small light bulb inside to light up the scale and it can now be tough to find what were in a lot of them special proprietary bulbs. But it's not tough to replace the socket and add something else that will work just fine.
RSG
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Location: Ontario, Canada

Re: Help with making a rotary table more rigid

Post by RSG »

Good info and nice read Pete, You do like to write in detail.
Vision is not seeing things as they are, but as they will be.
RSG
Posts: 1541
Joined: Fri Feb 04, 2011 9:59 am
Location: Ontario, Canada

Re: Help with making a rotary table more rigid

Post by RSG »

I've been researching tilting rotary tables and found one from Shars Tool that appears to be good (or at least as good as Chinese can be). I've seen the same rotab from a few different suppliers too and they are all about $1000.00 US then I stumbled upon this site - http://www.utoole.com/8-Precision-Tilti ... _p_53.html and found the price was less than half! So obviously I'm leery, but after talking to the guy on the phone he sent me a link to the manufacturer in China showing all the specifications for the rotab and then learning after it's the same one Shars sells (after they told me so) I have to wonder why it's so cheap! The guy from Shars thought it might be that Utoole buys more at a time filling a shipping container as opposed to their purchase a handful each year, making it cheaper.

My question is has anyone every bought anything from Utoole online? If so what was your experience.

Thanks in advance.
Vision is not seeing things as they are, but as they will be.
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