Tramming range

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

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Mr Ron
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Location: Vancleave, Mississippi

Re: Tramming range

Post by Mr Ron » Thu Apr 23, 2020 9:52 am

I would suggest that the table is higher at the front than in the rear to compensate for the weight of any vise used. A 6" vise can weigh around 70# which is constantly pressing down on the knee. Makes a lot of sense to me. When I'm doing woodworking, I will compensate for cantilevered forces which I estimate knowing it will sag over time. A door on hinges is an example.
Mr.Ron from South Mississippi

Posts: 1923
Joined: Tue Feb 10, 2009 6:04 am

Re: Tramming range

Post by pete » Thu Apr 23, 2020 5:27 pm

I guess it's depends on a persons point of view about how much importance they put on it, but tramming in a mills head and especially so for anything at less than a brand new Bridgeport's quality and price is probably about the last step you'd do once you understand enough to run checks on the whole machines alignment. With any off shore equipment I've learned the hard way to not blindly trust anything at all unless I can personally verify it first. Thankfully those checks only have to be run once and then maybe many years later when testing the possible wear. With a machines alignments verified and any of the inevitable inaccuracy's known, then normal tramming or at least checking it is still true is all that needs doing. And as Russ mentioned it's fairly complex topic to get your mind wrapped around what the books go into a great amount of detail to properly explain. And in my opinion fully understanding the concept that any object can have 6 degrees of freedom isn't really optional either. The Moore Tools book goes into a lot of detail and has about the best explanation about that I've seen so far. But a Google search should do the same for the details. While it's about running a full series of checks on a CNC bed mill, most of the methods shown can also be used to run tests on just about any design of mill so I highly recommend watching this series that was posted on YT a few years ago. And some years ago I think it was Glenn who showed an absolutely brilliant method on this forum of how to test the vertical movement of a mills knee by simply drilling and then boring a true round hole in any piece of plate locked to the table or mill vise and then checking it's absolute X,Y location with a dti swung in the spindle as the knee was moved to various positions up and down the column. To use just the knee ways as an example they can tilt in/out, lean left/right, have twist in them in either or both directions along there length, or even a combination of all of them in respect to the top of the table that you tram the head to.

While I've never operated the same type of mill you have Ron there are a couple of small tricks that help to get the head on a BP type dialed in if yours has the same worm & wheel for at least the tilt (X axis). What I do is get it as close as possible with the head bolts just snugged up, it's the final tightening of those bolts that are what generally makes getting the head really dialed in so tough as they tend to throw the head out of tram as there tightened. So I use a wrench on the worm and lightly tap on it's free end with a small dead blow hammer until the indicator reads zero/zero, add a bit of torque to the head or nod bolts and again tap on the wrench until I get that same zero/zero reading. That gets done for both the X,Y positions. At a certain point the friction on the head or nod mounting surface becomes high enough the head will no longer move as those head / nod bolts get fully tightened. You DO have to use some educated feel and good judgement about just how far and how hard you load that worm and wheel when doing this as you can and will shear teeth off either part. Secondly one of the reasons a head goes out of tram is leaving a pre load on the worm gearing after the head bolts are tightened. Once you know for certain the head is trammed as well as you can get it then or in my case both the tilt and nod worms are backed off so there's no tension on them in either direction. You should be able to spin the worm back and forth with your fingers as the backlash is taken up in the gearing. If those worms are left in the pre loaded condition the normal milling vibrations are what helps move the head out of tram a whole lot easier. That movement isn't very much at the worms, it might only be a 10th or two. But that tiny amount is highly magnified by the time it gets all the way out to the cutting tool. I forget exactly who on the Practical Machinist forms posted that simple but very effective trick or I'd give them credit for it.

It's still a mill and not a jig borer or grinder so there's obviously built in limits in your final part accuracy and what the machine can produce verses the point where the head is trammed in well enough. And since everything is flexible to greater or lesser amounts those tram numbers no matter how good are still only a static condition. Cutting loads alone are going to make alignment changes throughout the whole machine and a lot more than most might think. I'd agree that maybe not everyone wants or needs to run a full set of alignment checks on what ever they have. For some then just tramming the head might be good enough. Myself I want to know WHY something is inaccurate and where that happens. Due to casting movement after the ways were ground and it was already being checked on a very large surface grinder I know my mill table has just about .0015" of twist in them over 32". For vise sized parts that probably has no measurable effect, but I at least know it's there. Of course I only found out about that little detail after the warranty had expired. :-(

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Re: Tramming range

Post by whateg0 » Tue Apr 28, 2020 12:10 pm

Keep in mind too that a thou art 8 or 9 inches is going to be far less on a 3 inch face mill and for a lot of work we do, not even noticable when using end Mills.

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