Are there adjustments on a Bridgeport I table?

[rszimm]

My head is trammed in to the table. My gibs are adjusted. It seems that there are other things to check and adjust that no one seems to talk about. For example:

1. Is the knee perpendicular to the table top (and thus parallel to the quill). It would be nice to be able to adjust the knee without re-indicating the part. Is there an adjustment here?
2. Is the X axis perfectly square with the Y axis. Is there an adjustment here? Is there an easy way to measure this without indicating on a precision ground square?
3. Does the table sag, dip or tilt as you move it around?

 

[EmGo]

My head is trammed in to the table. My gibs are adjusted. It seems that there are other things to check and adjust that no one seems to talk about.

It's a bridgeport ! if you want to play with yourself, get a moore or a devlieg or a sip ! How about a little realism out there for a change ?

 

[SIP6A]

My head is trammed in to the table. My gibs are adjusted. It seems that there are other things to check and adjust that no one seems to talk about. For example:

1. Is the knee perpendicular to the table top (and thus parallel to the quill). (No) It would be nice to be able to adjust the knee without re-indicating the part. Is there an adjustment here? (No)

1. Is the X axis perfectly square with the Y axis. (No) Is there an adjustment here? (No) Is there an easy way to measure this without indicating on a precision ground square? (No)

1. Does the table sag, dip or tilt as you move it around? (Yes)

 

[eKretz]

All of these questions are why geometry of the machine needs to be good to have everything stay in alignment as the machine is moved through its travels. The machine started with all of this correct. If it is no longer correct, a simple adjustment will not help except at the one place the machine is currently positioned. That's why there are no adjustments for this. As soon as the machine is moved from that position, it will alter the alignment because the way surface is not linear/straight. Geometry of the machine needs to be corrected.

And of course things will shift a bit as the machine is moved around, or as more weight is placed on it. Iron flexes just like every other material. It's just a matter of degree.

 

[memphisjed]

Table sag really doesn’t matter much on Bridgeport. If the quill is near vertical to the knee screw the table will be same height as seen by cutter. Imagine an inch of sag, still the cutter sees one height.
Tilt will make a difference. If the table has near zero tilt front to back then the tramming is parrellel to the knee ways. If parallel to those ways then raising and lowering knee will not move the part x-y.

How square a machine is gets to be relative. If you have a tenths indicator nothing is square over 24 inches- then again flexing a mill a tenth in any axis is not that hard to do with a powerful stare.

 

[rszimm]

I can see sag not being a serious issue.

I don't understand how to true the head to the knee ways. I don't even have a clue how one would go about even measuring that. From your message it seems you're implying that if you tram in the head to the table and both the nod and the tilt read 0 on the label, you just assume that you're parallel with the knee. But it seems possible that the label might not necessarily be true to the knee. Maybe it wasn't applied properly? Maybe the knee ways have worn?

 

[specfab]

OK, so how old is the machine about which you are concerned? What is its history? These factors are large parts of the behavior the machine will now exhibit.

I own (2) Bridgeports, one is a 1958 J-head with 36" table, and the other is a mid-80's 2J with 42" table and chromed ways. I purchased these machines at different times, the 2J machine because I needed a more accurate, LESS WORN machine to do a wider range of work more efficiently, with fewer worries about table slop, wear in the ways, and all the things you mention. The knee ways on a Bridgeport are essentially the foundation on which the overall positioning precision of the machine rests. There is indeed an assumption that this remains fairly unworn, relative to the X and Y axes, and then that one trams the machine head to the table surface for "0" runout, and that's it. If you want to address observed inaccuracies in the slideways, then this becomes a machine teardown exercise and regrinding, or if lucky, only rescraping the way surfaces using all the tools found at the factory. An extensive and labor-intensive project by any measure.

It is unlikely that the knee has been installed incorrectly, but highly likely that it is worn. There is a very noticeable shift in my 1958 machine when I lock the knee, something on the order of .006-.008" of shift in Y-axis of the workpiece relative to spindle. On the newer machine, that shift is more like .0005". Age and history of the machines matter a lot.

There is no practical method of checking the spindle travel or head tram to the knee ways directly, at least that I'm aware of, after the machine is completely assembled. The table surface becomes the proxy for aligning the head.

 

[stephen thomas]

There is no practical method of checking the spindle travel or head tram to the knee ways directly,

There is, but trying to accomodate any found discrepancy compromises other factors that might be more important.

Put a dowel or test bar in a collet in the spindle.
"mess around" with it until you can get it to tram parallel with the quill going up and down In practical terms, make it concentric at both ends of travel.

Next lock the quill and all the travels & square the head perpendicular to the table as well as you can get it.
Perhaps with an indicator attached to the stout test bar you already made concentric.
You were going to do the above anyway.

Now put the indicator on the table, lock everything except X. Run the table back and forth with indicator tip on front side of test bar to find center. Lock X

loosen knee as little as possible (You did already adjust gibs before all the prep for tramming anyway?)

Run knee up and down with indicator point on test bar in spindle.

If there is a steady divergence in 4" or so, it will inform you how much the knee displaces in that 4" section of travel on the column. IOW how far in Y a hole centered at the top of the travel will be moved at the bottom of that same travel.

You can relocate the indicator to the side, find the center with Y, lock it, and do the knee thing again to discover any displacement in X with different knee positions.

But now you have a problem:
Will you change the head tram to mirror knee motion & drill holes and make features so they are parallel with the knee motion so they are not displaced when the knee is moved? How often do you drill with the knee? Is the angular difference enough to bind the drill when you do?
Or do you keep the head square with the table so those parts are square with the bottom (reference surface) of the parts as is generally much more important and practical? Split the difference?

If you bore (as opposed to drill) with the knee, holes will be ever so faintly oval, but there is no way you can measure it reliably relating to a BP.

So yeah, i have sometimes trammed a knee to see where it is. It is useful info
But as others said, there is nothing possible to do directly to change it, except rescrape.

smt

 

[specfab]

Good point and good explanation of process for actually checking relationship of knee travel to head. A good illustration of how important it is to think through the possibilities before writing about them (palm=>forehead).

 

[rszimm]

ahh, so you're only indicating over 4" or so of knee travel. That makes more sense. I was having trouble imagining a bar mounted in the spindle that was long enough that you could indicate over the whole length of knee travel, but that somehow wasn't in the way of the table when you brought the knee up.

My unit was built in the late 80s. It has an Anilam 2 axis CNC retrofit that likely was put on when the mill was new. My guess is that without the motor on the Z axis, the knee didn't get used much. It had an HP asset tag on it, which is odd because I don't know of an HP facility in Phoenix (maybe there was one 30 years ago?). The guy I got it from said they bought it from HP about 7-8 years ago.

I picked the machine up about a month ago and have been just going through everything and was surprised that "setting the machine up" seemed to stop at gibs and tramming the head to the table. There's just so many more things that seemingly could come out of alignment.

I'm not surprised that the knee doesn't have any adjustment, but I've got to admit I'm really surprised that there's no way to adjust the X perpendicular to the Y. That seems like something that could come out of alignment, and is super critical to any machining operation I can think of. I remember being a young engineer in an automation shop 25 years ago and the other engineers would all bring their carpenter squares in from home and get the machinists to "make them square" on their Bridgeports. Makes me wonder now if their squares left the shop less square than when they came in.

 

[neanderthal mach]

Properly understanding the concepts shown in the Moore Tools book Foundations of Mechanical Accuracy that clearly demonstrates and explains how any machine tool can have 6 possible degrees of inaccuracy or variable combinations of any or all of them is imo important. Secondly the best reference I know of would be this PDF, https://pearl-hifi.com/06_Lit_Archi...s/Schlesinger_Georg/Testing_Machine_Tools.pdf since he literally wrote the book about the subject. And in one way or another, I'd say the machine tool manufacturers still base there test methods and machine certifications on it.

While there's certainly other methods that can be used, I bought a pretty decent 12" long cylindrical square that can help with a lot of the 3 dimensional test checks on any mill. With the head trammed square and as close as possible to the table and the same for the fixed jaw on a good mill vise, plus a 10ths indicator, checking X,Y,Z squareness is fairly easy. At the end of the day, it's still a flexible and in reality rather poorly designed for high repeatability and locational accuracy tool. If you want / need better, then any BP and especially something used that could have variable amounts of wear sure isn't the best choice. But I still think running those test checks and at least knowing where any mills inaccuracy's are and by how much is worthwhile since you can't hope to even try any compensation if you don't know how much might be required. If I'm being honest though, that really doesn't tell the whole story, since any of these test checks are still static, the actual machine and component dynamic loads during machining operations add another level of uncertainty.

 

[EmGo]

At the end of the day, it's still a flexible and in reality rather poorly designed for high repeatability and locational accuracy tool.

Bottom line : it is what it is, and fussing over that isn't going to make any difference. If you want something better, go buy it.

 

[stephen thomas]

ahh, so you're only indicating over 4" or so of knee travel. That makes more sense. I was having trouble imagining a bar mounted in the spindle that was long enough that you could indicate over the whole length of knee travel, but that somehow wasn't in the way of the table when you brought the knee up.

".....only indicating over ( z") of knee travel" *at a time*

You can map the entire knee vertical travel so long as the test bar is accurate & stiff enough in whatever 4" or 6" stick-out within your ability to make it parallel/ concentric with the quill/spindle.

Drop the knee some more, set the indicator on box parallel or some other not too heavy item on the table, and tram again.
Drop it again, put the indicator at the end of a longer arm from the base, etc. You should find center of the bar each time indicator is relocated but that is trivial.

There's also the question of ram deflection and whether it is different elsewhere than you checked the quill-to-table squareness originally.
Or how square the top of the column is to the knee, so when you rotate the ram housing it does not lean one way of the other in relation. I bet BP's are quite good. Clones might be as good, better, or worse when it gets down to cost.

Moore jig borers are fairly cheap these days though they have now also gotten scarce.
I was even offered a SIP double column a few years ago after someone bought it just to strip off the DRO's, but could not work out housing for it.
DeVlieg seems to maintain value. All the above also have funky tooling (tapers) that can add cost. I gather most machining centers are accurate & stiff. It depends what you want to make.

smt

PS: second the motion to get yourself Moore's book FoMA..
Also want to appreciate that Schlesinger is the best book (PM'r) Frank Dorion ever gave me.

 

[???]

I never lock any of the axis when I am doing accurate work as it pushes the table around. If you want to check this . Indicate a round object on the table using an indicator in the quill, lock the X and Y and observe the movement.

Below is the easiest way that I have found to check how square the knee is.

• Tram head by swinging indicator.
• Use bearing roller or parallel under indicator at 0, 90, 180 and 270 degree position when checking.
• Fasten round pin to table and indicate to zero, I use a V-Block to hold the pin.
• Lower table and re-check position of pin, this can be checked at a number of different heights.
• I always wind the table up slightly after dropping it when checking.

Table sag can be a major issue and cause huge problems. I drilled, bored and reamed a punch holder plate on a Bridgeport. I proceeded to use every single Fibro punch that the supplier had in stock due to broken punch heads. Took the punch plate off and checked for vertical on the punches, the further from the center line the more they were kicked over. Punch heads were flexing under load and eventually fatiguing and snapping off.

Bridgeport's are an extremely versatile machine, however they have never been as rigid as a Deckel etc. If you want micron accuracy toss the Bridgeport and buy something else.

If you prefer Bridgeport type machines I personally would purchase a Kondia, Lagun or even a lot of the better Taiwanese machines as they are more rigid.

 

[memphisjed]

Any decent machine is rigid within a range. The knee mill split the atom and sailed rockets. They are versatility and ‘good enough’ rigidity and accuracy are signs of its ingenious design.
It is +- 2000 pounds with 7 axis- I would hope a column machine with 10 x the weight and 4 less axis is better at whatever it does.

 

[Cole2534]

Good point and good explanation of process for actually checking relationship of knee travel to head. A good illustration of how important it is to think through the possibilities before writing about them (palm=>forehead).

The same process, loosely, in the horizontal plane is how you parallel the turret/ram to the Y axis so the head will tilt/nod correctly in its respective planes.