PDA

View Full Version : Checking a Bridgeport’s accuracy



Smokedaddy
08-21-2005, 02:43 PM
I tried searching this site and the net but wasn't able to find what I'm looking for. Is there a site available that describes HOW to check a Bridgeport’s Series I mill to see how worn out it is or isn't? If not, I'd appreciate knowing what the procedure would be.

A novice,
-SD:

Norman Atkinson
08-21-2005, 03:05 PM
In a line or two

The Book is called Schlesinger's Limits.

Cheers

Norman

JCHannum
08-21-2005, 06:15 PM
Some very good advice here; http://www.mermac.com/

Also read the information on inspecting lathes and "In Praise of Clunkers". Dave knows what he is talking about.

Forrest Addy
08-21-2005, 06:28 PM
If you referring to a simple test requiring minimum equipment that will give you an diea of a particualr machine's condition I'm not aware of anything on line that really effective.

If you desire a machine survey where the tracking accuracy, ridigity, that is an objectve assessment of a machine tool's condition, you mean a test requiring a reference square, gaging apparatus, a master level and about 4 hours of skilled labor. The result is a document I've always called a "run-off report" that lists the machine's condition and the errors found. There's a good procedure for conducting a machine tool survey outlined in Connelley's Reconditioning Machine Tools".

There are also a number of sofware packages and technical services that can be more or less helpful.

I suppose someone with an air of expertise can spend a minute, peer around the machine, crank the dials, and give you an off hand guess at the machine's condition, servicability, and accuracy. Personally I'd have to see it under power and spend several hours with it with my box of survey equipment before I'd render an opinion.

If afraid I can't give you a short answer to your question. I know of no brief low tech in print or on line that offers a sensible method of machine tool condition assessment. Connely is the best reference I can offer.

Smokedaddy
08-22-2005, 12:20 AM
Okay,

Well I've heard some mention its' a "boat anchor", or "its' a worn out mill", "spindle bearing is worn", "bearing rattle", "ways are worn" and whatever else I can't recall.

Being a novice and the curious type, I just wanted to know what I had and how to check "how good it is in its' present condition" compared to what it once was (whatever that means). Maybe my choice of words in the topic is incorrect.

Thanks,
-SD:

Forrest Addy
08-22-2005, 01:21 AM
No. Smoke there's nothing wrong with your questions. I think you might be looking for too simple an answer. I'm afraid you need to inform yourself on the about machine tools, their uses and their wear modes and learn to treat appropiately the declarations of people who talk through their hats.

The used machine tool market is as much a minefield of hoped for miracles and plain swindling for the neophyte as was once the used horse market a cemntury ago. You might get lucky and buy a pig on a poke and wind up with a pretty good machine for not much money. But then, you're more likely to invest too much in a dud.

The solution for this is study; invest some time in gaining knowledge about machine tools. For that I reccommend you spend a few months - yes months (you won't gain the knowledge you need in a few hours reading during commercials) - surveying local machine tool markets, looking into small local shops, rounding up a few knowledgeable mentors, studying the relevent chapters of teh Connelly book.

There really is no shortcut but the time spent will serve you well over the years as you add to your machine tool inventory and abilities in the machine shop.

Many of us who post here have many year's experience winnowing out suitable machine tools from those the truely may be classed as boat anchors, our experience is hard won by dint of study and hundreds of hours of scrounging.

The contacts you make over your study time are worth maintaining so refresh your acquaintences and exchange favors with them from time to time to nurture your relationship. They will keep your needs in mind and when they stumble across soething you might be interested in they will contact you. It's like having dozens of eyes instead of only yours to see with.

This cashless, back scratching, contact building culture has been around since the days of flint knappers and it's the most productive way of outfitting and keeping a home machine shop running except a bottomless wallet.

[This message has been edited by Forrest Addy (edited 08-22-2005).]

rockrat
08-22-2005, 06:29 AM
<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
Many of us who post here have many year's experience winnowing out suitable machine tools from those the truly may be classed as boat anchors, our experience is hard won but dint of study and hundreds of hours of scrounging.</font>

Translation - We worked on crap machines and found out what we didn't like and thought that we knew what to look for when purchasing a machine tool.

Next, we bought something that we thought was good but didn't pan out because we didn't know that we had to look for this other thing (ie bearings, runout, slop, casting chips in the oil etc).

Then, we hid our previous poor purchase at the end of a rope tied to our watercraft.

Afterward, we went out and spent much time, gas and effort looking for something that would suit us and didn’t find squat. Most likely due to the fact that we wanted perfection in a used tool.

With this knowledge, we found something to "get us by" whilst we were stalking our rare find.

By the time that the one in a million came around, we knew what to look for, bought it, and came here to gloat.

Finally, we pulled up the watercraft-mooring device from the start of our adventure and gave it to Hoffman for restoration. http://bbs.homeshopmachinist.net//biggrin.gif

And the rest of what Forrest said is solid too....

rock-

andy_b
08-22-2005, 11:45 AM
i'm far from being an expert on anything, let alone machine tools. i have found though that you can compensate for most deficiencies by getting to know the machine. the one exception would be for the spindle bearings in a mill or lathe. no matter how much fancy maneuvering you do, you will never outwit a bad spindle bearing. if your B-port truly has bad spindle bearings, i'd look at replacing them and then learn to use the machine from there.

i recall several past threads dealing with how to check spindle bearings on various tools. a search should find them.

andy b.

spope14
08-22-2005, 01:43 PM
XYZ are fairly easy to check out with a 1 inch travel indicator on a magnetic vise, and a four inch gauge block. Find a reference place on the table, and put th indicator on it. best to use a parallel or something flat to start this off so you are not measuring a potential ding or scrape mark. set the back lash into the ref surface. Using Z as an example, move the table down 4 inches by the dial, re-setting the backlash at the 4 inch mark to the same direction, then put in the gauge block. The difference is your accuracy (multiply by three for per foot error, of divide by 4 for your per inch accuracy).

I just looked at a mill this morning, a very old Bridgeport / Brown and sharp machine hybrid. Three turns of backlash in the Y axis, did a few things with the handles and thrust bearing type adjustments (the idea of such), and got it to about .050 backlash. The same was true with X and Z. The spindle showed minimal wear. Oiled it up a bit, and found the accuracy to be off by .002 in the Z, .005 in the Y, and right on (less than .001) in the X over four inches. The original asking price was about $700.00, had the guy down to $500.00, but with my checking, the guy all the sudden hemmed and hawed, and now he has decided to keep the machine, which is all well and good, for I don't have room for it yet.

Guess I did this more fo the hobby than my own benefit.

Smokedaddy
08-22-2005, 05:17 PM
spope14,

Sorry for being such as dumb @ss but I'm not sure I understand. For starters, I didn't know they made a 4" gage block. &lt;duh&gt; I saw one on MSC by SPI, Grade 2, material is Steel, + .000024/- .000012 for $54.81. So I assume that's what you're talking about?

BTW, Thanks for the help,
-SD:

wierdscience
08-22-2005, 06:58 PM
A few simple checks of the table will tell you a lot and you don't need anything fancy,just your hands.

Run the table to the center of the saddle and notice how much force you need on the crank,then run it to each extreme end of the travel.If it starts out easy in the middle and gets progressivly tighter towards each end,barring the presence of rust or dry ways the ways are hour glassed and will need scaping at a minimum.

Twist the spindle by hand back and forth and side to side,if it's loose or has lots of slack then it may need bearings or have wear in the quill splines.

Check and shift through all speeds gears etc.Engage the power feed even if the machine isn't powered up the hand feed will work on a B-port,if the hand feed works 90%of the time it will also work under power.

[This message has been edited by wierdscience (edited 08-22-2005).]

Davis In SC
08-23-2005, 10:39 PM
<font face="Verdana, Arial" size="2">Originally posted by wierdscience:
A few simple checks of the table will tell you a lot and you don't need anything fancy,just your hands.

Run the table to the center of the saddle and notice how much force you need on the crank,then run it to each extreme end of the travel.If it starts out easy in the middle and gets progressivly tighter towards each end,barring the presence of rust or dry ways the ways are hour glassed and will need scaping at a minimum.

Twist the spindle by hand back and forth and side to side,if it's loose or has lots of slack then it may need bearings or have wear in the quill splines.

Check and shift through all speeds gears etc.Engage the power feed even if the machine isn't powered up the hand feed will work on a B-port,if the hand feed works 90%of the time it will also work under power.

[This message has been edited by wierdscience (edited 08-22-2005).]</font>
Another thing that will make cranking tighter at each end... Lead screw is worn in the center area, & nut has been "Snugged up" to reduce backlash.... This is a sign of a well-used mill...

knutz
08-23-2005, 11:00 PM
<font face="Verdana, Arial" size="2">Originally posted by Davis In SC:

Another thing that will make cranking tighter at each end... Lead screw is worn in the center area, & nut has been "Snugged up" to reduce backlash.... This is a sign of a well-used mill...[/B]</font>

If the mill is just used for general machining this can easily be fixed for little to nothing if you have a good lathe.

Pull out the nut and replace with a new one or split the nut completely in half and reinstall. By splitting the nut completely you get more adjustment.

Then take the lead screw and set it up in the lathe. Most screws are 5 tpi but measure to be sure. Or better yet check the dial. thenb grind a tool that will fit down in the thread and only cut the sides. Keep your compound at 90 degrees to the chuck.

Just carefully skim the side of the thread untill it is taking a little bit off all the way down. Adjust your cut by moving the compound. take as little as possible. Then once you have cut the front side switch or grind a new tool and do the backside of the thread going backwards.

What you are doing is basically turning the whole thread down to the same size as the most worn part...and by splitting the nut completely you can then take out all the slack with the addied adjustment. Yes it will wear out quicker than a new nut and screw combo but it does work.

I've done this a couple of times and it works great. And the machines were able to repeat and hold fairly decent tolerances. Course if I was trying to hold true position of .001 I wouldn't be using a bridgeport anyways.

No it's not as good as installing a new thread and nut and probably not as accurate.(although I've not had any complaints) But for bringing an older mill back into shape it works wonders.

I hope that made sense.

jeff

Norman Atkinson
08-24-2005, 06:17 AM
Maybe I am finally losing the rest of my marbles but I thought that I had corrected the name of the book.
Correctly, it is :-

The Testing of Machine Tools by Georg Schlesinger.
It was published in 1961 by the Machinery Publishing Co.Ltd

The copy on my desk came from the local Public Library so there may be a more up to date edition.

I note that the agents in the US were
The Industrial Press in Worth St, New York 13

I keep wondering about whether I dare attempt to copy it.

Comments?

Norman

J Tiers
08-24-2005, 04:13 PM
Trouble is, usually you don't have a lot of time to make a decision.... deals are there, and then they are not. Maybe no time to run under power several hours, maybe no power there.

I like a combo of Weird's and the Mermac advice.

Usually if it looks beat, it is beat, and if it looks well cared for but not freshly painted and shined, it is probably ok if you don't see / feel anything major externally.

The best looking and feeling used machine is gonna have SOMETHING wrong with it...

I am sure there are many exceptions, but that has worked for me. I have been burnt ONCE... and that was on something I knew about but thought I could fix or deal with. Wrong on both counts, but it wasn't a surprise, rather a calculated risk.

aboard_epsilon
08-24-2005, 04:33 PM
if the machine is yours already and you own it.
I would just go ahead and use it .......
it's then that you will find its failings over what maybe a number of months.
I think this is the best course of action.
Many things can be just adjusted out as you go along.
all the best..mark

madman
12-26-2005, 10:24 PM
Im surprised that people would wish to machine to 50 millionths on a crappy old bridgeport. If you have a old machine install a DRO on it you are one lucky happy home shop guy. The rest is up to you for the accuracy to do a precise jobé Measure twice cut once.

Mcgyver
12-27-2005, 12:38 AM
Norman, the choice to break the law, which most people regularly do, depends upon your judgment on 'in doing so would i be committing an act that is against my moral or ethical make up and/or 'can I live with sanctions if i get caught'. Clearly the biggies are off the radar screen by both measures for most of us, but for the little infringements, this is why we're able to drive 10k over the limit on the highway and still look at ourselves in the mirror.

If the book is long out of print and the author dead, I'd have no moral crisis, regardless of copyright rules, and would hold little fear if sanctions. on the other hand making the book again accessible would help a lot people and machines in a sector that the changing world has left otherwise left behind , in other words a reprint or competing title is unlikely. Ask yourself why wouldn’t you copy it?

The law is not sacrosanct, its just a bunch of made up rules, and only you can decide if copying it materially harms people and/or if you can ‘do the time’ if you ‘do the crime’ lol

cmiller231
12-27-2005, 10:05 PM
If I were to attempt to judge a b-port in a short period of time ,i would snug the table locks till a very slight drag is felt cranking the table. The table is centrally positioned over the knee .If it locks up before it gets to the end of it's travel ,the ways are worn or as mentioned some body may have tried to remove end play from nut. try again with the locks loose.Repeat for the y direction.Another check to see the condition of the ways is to crank table all the way one way and measure over the dovetails with a pair of suitable size dowels .5/8" I think . Then measure at the end of table and compare measurments. I would probably pass on mach more than .003. Now i know that a lot more wear can be tolerated, The previous owner of my boat anchor did.It currently resides in pieces in one corner of my garage .It was being used with .018 wear measuring over the dovetail ways on top of knee, metal banding added for gibs
If i could not hear the spindle run i would discount the value of the machine .
It's easy to get caught up in the excitement of buying a new mach.But be honest with your self ,it is what it is . There is a lot of junk out there.
The above is only a rough check if time is limited.
I also have the book "Machine Tool Reconditing" a great asset.
Chris Ypsilanti Mich

Millman
12-27-2005, 10:14 PM
Madman, you got it! Even a worn machine will make great parts with time and experience. Personally, I would at least want to hear it run, just listening will tell you a lot.Old machines, measure 3 times; wouldn't want one for production though!

------------------
Dave da Slave