Bridgeport Constantly Loses Accuracy

bluedog1 · Aug 12, 2018

I have a rebuilt Bridgeport J-head. Every time I tram the head to perfection, the X and Y axis looses it's accuracy pretty quickly. In 6 hours of non use, the X & Y axis lost (slipped).0015". In a weeks time, the x & y values will drop to around .020", respectively. BTW, I tighten the head and ram bolts very tightly but its not enough to hold tolerances. It gets old needing tramming the head often!!

Any ideas?

Thanks!!

thermite · Aug 12, 2018

bluedog1 said:
I have a rebuilt Bridgeport J-head. Every time I tram the head to perfection, the X and Y axis looses it's accuracy pretty quickly. In 6 hours of non use, the X & Y axis lost (slipped).0015". In a weeks time, the x & y values will drop to around .020", respectively. BTW, I tighten the head and ram bolts very tightly but its not enough to hold tolerances. It gets old needing tramming the head often!!

Any ideas?

Thanks!!

How flat, round, wotever and well-fitted are the mating surfaces where it is meant to bear and lock? Are you trying to crush-bind on burr, swarf, or void AIR?

Some of this is "normal". Other parts of it not so much. Might want to grab some high-spot, wrenches, etc. and go looking for poor fitup.

Have to be more realistic as to expectations, too, I'd say? Long, long, LOONG time since I relied on a vertical to earn a crust.. but you at least CHECKED tram on the bitch EVERY SETUP and stood READY to correct it. Long-term stability wasn't taken for granted. If it can be trammed, it must be trammed. It ain't a horizontal.

Might want to look into how consistently you are checking tram as well if it seems to be moving when sitting idle. Also how wide are the temp swings, 24 hour span, all week, all month, your shop?

cyanidekid · Aug 12, 2018

pretty simple really, work out what is moving (I'm not sure from the description), and troubleshoot what holds that piece of iron from moving.

first thing that comes to mind is the rear bolt (of 3) that holds the head in the "nod" "tilt" "pitch" axis. it should bear on a floating plug in the bore around the bolt, because the casting can't flex enough to pinch the trunnion there.

Richard King · Aug 12, 2018

check 3 - 7 page and adjust the gibs. Also be sure to always crank the knee up before locking the knee.

http://www.hardingeus.com/usr/pdf/Knee Mills/FullManual.pdf

also

Checking Gibs For Wear & Needed Adjustments - YouTube

Gib Adjustment on a Bridgeport Milling Machine - YouTube

If the gib screw is adjusted all the way in and it is still loose then you should have the machine rebuilt. Also check the length of the bolt that hold the vise to the table tee nuts and be sure the vise jaws are tight. Your re-zeroing the indicator I hope.

thermite · Aug 12, 2018

Richard King said:
check 3 - 7 page and adjust the gibs. Also be sure to always crank the knee up before locking the knee.

http://www.hardingeus.com/usr/pdf/Knee Mills/FullManual.pdf

also

Checking Gibs For Wear & Needed Adjustments - YouTube

Gib Adjustment on a Bridgeport Milling Machine - YouTube

If the gib screw is adjusted all the way in and it is still loose then you should have the machine rebuilt. Also check the length of the bolt that hold the vise to the table tee nuts and be sure the vise jaws are tight. Your re-zeroing the indicator I hope.

He's had it "rebuilt", Rich.

We hope it was more than a paint-job!

You'd know better than I where they might have not gotten it right, but it sounds like up where the head attaches to all those layers of movable "stuff" a Bridgeport enjoys.

I'd not even have a vise on the table when tramming.

bluedog1 · Aug 13, 2018

Thanks everyone for your ideas! I'm going to follow up with them. Once person asked what am I using as a reference. I have a ProTram from EdgeTechnologies. I tram the BP with it. This morning I trammed it and started taking data points from it and the elapsed time. Tomorrow, I will check to see how much movement occurred over the last 24 hours. I suspect I will be pulling the head off to see what is going on with the connection between the head and the knuckle. My guess is that maybe some contact points were greased when they shouldn't have been.

Spruewell · Aug 13, 2018

After you tram the head to the table and tighten down the locking bolts, do you back off the pressure on the worm screw? If so, then that may be allowing the head to sag. You may also want to check that all the bolts used to lock the head in place are the correct length to allow them to tighten down before bottoming out and not so short they are pulling the threads out of your machine.

thermite · Aug 13, 2018

bluedog1 said:
Thanks everyone for your ideas! I'm going to follow up with them. Once person asked what am I using as a reference. I have a ProTram from EdgeTechnologies. I tram the BP with it. This morning I trammed it and started taking data points from it and the elapsed time. Tomorrow, I will check to see how much movement occurred over the last 24 hours. I suspect I will be pulling the head off to see what is going on with the connection between the head and the knuckle. My guess is that maybe some contact points were greased when they shouldn't have been.

Not "just" greased or oiled.

If the surfaces look great, individually, but are not a mating fit, it is damned hard for any amount of bolt-tightening to hold. There are no gear-notches, detents, nor "star washers" in there.

It's all down to plain-old ignorant FRICTION writ tightly. On a Bridgeport or most clones, anyway. They are made to be moveable, and this is part of the price to be paid for the extra flexibility in-use.

My one (Quartet) has no nodding knuckle, just rotation. Even that has a great, fat, tapered locking pin, so "tram" can be automagic. "Nod" needed to mill an angle? That wants a tilted fixture on the table. My problem. Head ain't moving, doesn't give a damn. I'm good with that. Dasn't move in the cut, either.

One checks tram anyway. S**t wears.

CarbideBob · Aug 13, 2018

"I have a ProTram from EdgeTechnologies. I tram the BP with it."
I will not touch the many reasons this type system does not and never will work or repeat.
It all looks good on paper but ..... I live in the real world.
Your head may in fact be moving, I don't know but this is a terrible method to check it.
Bob

dlane · Aug 13, 2018

I had one of those Edge Pro Tram gizmos I sent it back and used a real DI to tram and haven’t had a problem since. The edge pro tram uses the cheapest Chinese DI I’ve ever seen mine weren’t repeatable. I have since made my own protram using decent indicators. Not sure that’s your problem , just sayin.

Edit :Bob’s a quicker typer than I

thermite · Aug 13, 2018

"indicators?"

Elitist bastards.

Rich f**kers probably even own SHOES and flush-toilets!!

Nobody never taught yah how to use a damned PLUMB BOB?

World's chock full of used, but still right decent Tesa, B&S, Mahr, Mitutoyo, Compac, Interrapid, Starret, and my choice of Hamilton, Dorsey, and either one for P&W-branding.

Also proper co-axials, spindle mounts... and a lot more.

Tramming is not to be trusted to the shop-floor, sacrifice to the coolant, crush, impact, and gravity Gods disposable Chinese Kleenex DI's.

Which I have for just that reason. Disposables. No tears. Just go and get another one out of the drawer. A decent drill costs more. And is worth it.

Spruewell · Aug 13, 2018

I have a nicely ground flat plate I put on the table, then use a co-ax indicator to tram the head on my BP style mills. It allows me to dial in both x and y at the same time. Then you can use tenths indicator if you need it super right on.

thermite · Aug 13, 2018

Spruewell said:
I have a nicely ground flat plate I put on the table, then use a co-ax indicator to tram the head on my BP style mills. It allows me to dial in both x and y at the same time. Then you can use tenths indicator if you need it super right on.

There's fotos, "right here on PM" of the older skewl use of the outer race salvaged from a large Timken bearing. Really common approach.

A flat plate and a mill's table need a formal introduction. That can get tedious, done carefully when mill is getting much actual use.

A ring cuts down on how much surface area has to be worried over, makes any trash or raised burrs MUCH easier to detect and clear-off or stone.

2CW

rons · Aug 13, 2018

bluedog1 said:
I have a rebuilt Bridgeport J-head. Every time I tram the head to perfection, the X and Y axis looses it's accuracy pretty quickly. In 6 hours of non use, the X & Y axis lost (slipped).0015". In a weeks time, the x & y values will drop to around .020", respectively. BTW, I tighten the head and ram bolts very tightly but its not enough to hold tolerances. It gets old needing tramming the head often!!

Assuming that you know what you are doing with the tram operation. Tram the head. Now move the table down and up a few times. But don't crank the table to change the X-Y position. Recheck the head orientation to the table. Your tram points should be exactly the same. If something changes then your table/knee/column has issues.

Red James · Aug 13, 2018

Nothing wrong with your tramming device . . . except it has two different indicators. Remove one indicator and rotate the spindle so the remaining indicator checks to right, then to left of table. Repeat in the Y direction.

To prevent the indicator bouncing over tee slots, put a clean, ground block on your table and slide it under your indicator point at each x,-x y,-y position. On a Bridgeport it is important to tram Y axis first, then X. Repeat.

JRIowa · Aug 13, 2018

First of all, there is a Bridgeport section here. Probably 100s of threads about tramming a BP.
Second, throw away that damn thing that's supposed to help you tram the head. They don't work.
Thirdly, read this thread: https://www.practicalmachinist.com/...nge-mills-and-lathes/tramming-bp-head-117024/

If you're still having issues, then, post some pics of how you're trying to tram the head (in the BP section) and we'll help you. This ain't rocket science! But there are tons of newbies and hobiests that really struggle with it.
JR

thermite · Aug 13, 2018

JRIowa said:
First of all, there is a Bridgeport section here. Probably 100s of threads about tramming a BP.
Second, throw away that damn thing that's supposed to help you tram the head. They don't work.
Thirdly, read this thread: https://www.practicalmachinist.com/...nge-mills-and-lathes/tramming-bp-head-117024/

If you're still having issues, then, post some pics of how you're trying to tram the head (in the BP section) and we'll help you. This ain't rocket science! But there are tons of newbies and hobiests that really struggle with it.
JR

I kinda wonder if we don't need a section on exorcism of Demonic Possession?

I mean.. he's got a mill that moves around when he isn't using it or watching it?

Maybe it used to be a Plymouth Fury named "Christine"?

Christine (1983) - IMDb

JRIowa · Aug 13, 2018

thermite said:
I kinda wonder if we don't need a section on exorcism of Demonic Possession?

Either that or he's on more drugs that I am. That's bad Bill, all of your posts are starting to make sense to me!

JR

thermite · Aug 13, 2018

JRIowa said:
Either that or he's on more drugs that I am. That's bad Bill, all of your posts are starting to make sense to me!
JR

Damn. That's worrisome, Jim.

I keep a loaded .380 Auto in the bedside stand just in case they ever start to make sense to ME!

You know the drill:

An ethnic catches his wife shagging another man. Pulls out a pistol, puts it to his temple.

She still loves him, or doesn't want the carpet soiled. Same thing, some wimmin.

She says: "No, No, don't do it!"

He says: "Shut up BITCH! You're NEXT!"

Newman109 · Aug 13, 2018

CarbideBob said:
"I have a ProTram from EdgeTechnologies. I tram the BP with it."
I will not touch the many reasons this type system does not and never will work or repeat.
It all looks good on paper but ..... I live in the real world.
Your head may in fact be moving, I don't know but this is a terrible method to check it.
Bob

You said the magic word "repeat". A very important factor in any measuring method, device or system is the ability to repeat or its "repeatability". I wondered about accuracy and repeatability when I first saw the ads for the ProTram unit. With two dial indicators, I just don't see how it could ever repeat when checked over and over again.

I have a big Webb mill and regardless of whatever anyone might say or think about Taiwanese machines, I can guarantee that it doesn't move when I'm not using it.

Bridgeport Constantly Loses Accuracy

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