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
Posts: 2126
Joined: Tue Dec 22, 2009 12:36 pm
Location: Vancleave, Mississippi

Re: Tramming range

Post by Mr Ron »

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
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Tramming range

Post by pete »

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. https://www.youtube.com/watch?v=4HWuhbhySpw 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. :-(
whateg0
Posts: 1114
Joined: Sat Mar 28, 2009 3:54 pm
Location: Wichita, KS

Re: Tramming range

Post by whateg0 »

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.
LouStule
Posts: 86
Joined: Fri Sep 11, 2020 7:20 pm
Location: Washington State

Re: Tramming range

Post by LouStule »

Harold_V wrote: Tue Mar 31, 2020 3:39 am The overall accuracy of a mill begins with the column, knee, and way surfaces. Here's what I recommend.
Begin by checking the gib on the knee. It should be snug, but not overly tight. The knee should be able to just settle by its weight, not driven downward by the screw.
When the gib is adjusted (a snug gib ensures the knee is not leaning downward at the front), you should then dress the table of your mill. Do that by using a single cut or smooth file, one that has seen some use, so the original sharpness is gone. Note that a new file offers the opportunity for metal to be removed where it shouldn't be.

The file is placed at an angle on the table surface, no handle, and drawn by your palms parallel to the T slots, covering the entire table surface, side to side. By doing this, any slight projections (usually caused by clamping on a chip, or from dropping small objects on the mill table) will be removed, leaving the mill table surface untouched otherwise. I do not recommend using a stone for this purpose, as it isn't selective and will remove, albeit very little, metal where it shouldn't. Once you have deburred the table, it should feel silky smooth when stroked by hand. If it isn't, repeat the file operation.

With the table centered on the saddle, side to side, use an indicator mounted in the spindle to sweep a circle. I make sure my indicator crosses the front and rear T slots, so I make the circle as large as I can. I don't like using anything between the table and indicator, as that offers a slight chance of introducing error, especially if one skips the deburring of the table. When adjusting the head, the restraining bolts should be snug, but still allow the head to rotate. If they are not, when you tighten the bolts you most likely will see a change in your setting.

When you place the indicator, it is best applied such that the contact addresses the table such that you can rotate the indicator with the tip trailing. I often don't do that, but to ensure that the crossing of the T slots doesn't alter the indicator setting, I make contact by only a thou or two, so the contact moves very little. I also use the largest diameter tip for the indicator. This allows the indicator tip to address the T slots from the side without losing registration.

The larger the circle you sweep, the more precise will be your setting. New mills are made with the front edge of the table ever so slightly higher than the back, to address settling as the machine wears. If your mill is not new, ensure that you don't leave the table high at the front. I prefer to make it even, front to back, just as I do side to side.

There's no reason why you can't get a head within a half thou sweeping a large circle (about 8"). That degree of precision will yield no issues when moving the knee, keeping in mind we're talking about a milling machine, not a jig borer.

Dialing the top of a vise isn't recommended. If, by chance, the vise top is not dead parallel with the table surface, you will defeat the purpose of dialing in the head. Considering the fact that the large surface is also the moveable surface on a mill vise, it most likely won't be dead parallel.

H
Great info. Thanks!
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