Bridgeport Mill - Gibs

Having just build a new workshop which is nearley completed now .....just waiting for the sparky, I decided to give my Bridgeport Series 1 Varispeed a new x axis power feed, as the old one had finally given up. Anyway, having finally got that installed and working nicely I decided to just check a few measurements/tolerences etc in anticipation of switch on day.

I noticed there was some movement in the table 42 x 9 in the x axis.

I made some adjustments to the Gib, resulting in what we all come to expect nice in the middle, too tight at the ends - slightly worn bed!

So what is acceptable re: movement "slop" in the table ? I currently measure around 10 thou is that tooo much ??

Gary

Ok, So after the great (views) no results and no help or suggestions, thanks everyone....I decided to delve a little further into my measurement technique ....and found my error. I had mounted my indicator in such a way that it was and was in fact measuring "slop, or wobble" whatever we wish to call it from both the table and saddle, y axis?

On re re-valuation. I have mounted my indicator above the y axis slide, adjusted the x axis gib again, and I have reduced my slop to 2.5 thousands better, better better

Further research via u tube found a guy measuring table wobble his so - called machine tolerance was 1.5 thousands on each axis

So I am close, and am happy - thanks for all your suggestions

Gary

I apologise for not giving up my workshop time this evening in order to make an instant reply instead.

I was going to say that the tight at the ends, free in the middle is more likely to be due to a worn leadscrew than worn ways, but clearly that thought is no longer necessary.

Andrew

Hi Andrew, thanks for your post ....sorry I am impatient

But please, could you explain how so your suggestion? I am learning and appreciate your experience

Gary

My BP clone is getting a bit long in the tooth and the table is looser in the middle of travel. The X leadscrew is also worse at the middle. So if I tighten the gib so that the table is nice and tight at the middle, it is pretty stiff at the ends of travel. Similarly, if I adjust the leadscrew nut to eliminate backlash at the middle of travel, it will force the adjustable nut halves apart as the table moves to the far extents.

I have some ballscrews that will pretty much eliminate the backlash issue but I'm pretty clear that I'd need to get the table flats and dovetails reground if I wanted to sort out the gib issue. My workaround is to slightly nip up (and continually readjust) the table locks - but I'm used to doing that anyway so that I can climb mill without fear of the cutter snatching. You soon learn how to do that.

Murray

I used to service bridgports before retiring and when you get slop in the gib strips usually the worst one was the Y axis and it also gives false DRO readings as it crabs if loose. I would always tighten both as much as possible and slacken if it causes problems with a long job and make sure that the lubrication is getting to all parts and slides.

David

David,

Than you for your response. I would agree that the Y axis appears to be the worst offender. I also agree with your recommendations, again, thank you you have quantified my thoughts

Gary

First off, Don't expect a definitive answer or possibly any answer, in just a couple of hours. If people read your post, but don't respond, it's probably because they don't know the answer.

Second, where was the indicator mounted to start with? or have you adjusted the other two gibs to get the best fit everywhere?

1.5 thou of play on each axis may be acceptable for a worn machine, but it means that you'll have to lock the non-moving axes every time you take a cut. The new condition is less than half a thou of movement (typically a couple of tenths) at any point on any axis and no more than three thou of backlash in any feed screws at any point. To achieve this, you'll almost certainly have to learn how to re-scrape a machine.

Basically, adjust the gibs so that movement is a little bit 'firm' at the ends of the travel and live with what you've got. If you're happy with the results you get, that's good. If, with time, you feel you need more precision, then be prepared to learn how to scrape and make precision measurements and plan on taking one to four years on a complete rebuild...

Mea máxima culpa. Some of us are sad enough to actually enjoy rebuilding our machine tools. We probably need pity from those folk that get on and build things with them!

PS:_ I'm a very slow typer

Edited By Mark Rand on 01/11/2017 23:03:56

Mark,

Whilst I appreciate your response, I would like to understand where you get 1.5 Thou of play on each axis as a worn machine. I find it hard to believe that Bridgeport' machines left factories with tighter tolerances than this?

Can you qualify your statement for me please.

Gary

Mark, you are indeed correct. Upon further digging and much googling I found that the spec on a Bridgeport was to be 0.0008 8 tenths of a thou movement between any axis along its entire length when leaving the factory.

Something I just could not contemplate earlier this evening, So yes 1.5 thou is indeed a worn machine!!

SO what I thought was good earlier this evening turns out to be not so good.....

Thats why I am here to learn. But like you pointed out I will adjust the gibs to get the best I can at present and have to live with that, else learn to scrape.

THANKYOU for making me question you, and look further

Gary

Don't get hung up on comparing your machine with what it might have beenbrand-new out of the factory. The test is whether any wear that you might have is more than you can live with when making parts. 1.5 thou on any axis is bloody hard to notice on most finished parts, and if you lock the un-moving axes when making a cut, then there's a very limited opportunity for clearance to cause any problems. If, after using the mill for some time, you decide that you really can't live with or work around any issues, then is the time to look into rebuilding.

I was blessed/cursed with getting a Beaver Mk1 VBRP mill that was, to put it kindly, clapped out. 10 thou horizontal and vertical wear on the X ways, 20 thou horizontal and vertical on the Y ways and 10 thou on the knee to column.

After four years effort it's down to less than 2 tenths total deviation from square and 1 tenth play in all axes, but I didn't make anything with it in that time. It was unusable when I started, but I'd have probably been better off to buy a machine that wasn't a basket case to start with.

Basically, use the Bridgeport to make stuff and work out whether you need any better after a year or so, rather than worrying too much to start with.

Have fun, it's what machine tools are really for.

Hi Mark,

Thank you for your advise. Yes we all want our toys to be perfect. As I said in my first post I have just about finished my new machine shop, and have been fiddling about with the mill prior to powering up...thats where it all started!

For what I want it will probably be fine, and looking at some posts people have certainly had more of a problem than me.

I now need to actually start making something worth posting about rather than fiddling with the machine all the time.

Again, I appreciate your time and posts

Gary

If I had a machine tool with 1.5 thou slop in any given direction I'd be pretty durn happy. I would not call that badly worn, more like average working condition. It should do anything you are likely to want it to, provided it is used correctly. Always allow for backlash by moving the job in the same direction. If you go too far, don't just wind it back to the mark on the handle (or the DRO), take it back well past that point and then approach the setting from the same direction as before. And keep all axes locked at all times except those in use for this particular cut. And avoid climb milling, especially into corners where the cutter will grab the job and put a nice divot in it. Enjoy your almost as-new mill with only 1.5 thou movement, a whole 0.7 of a thou more than it left the factory with.

As Mark and others have said, adjust up as best you can and live with it. In my working life I have used plenty of worn machines and produced some good work. When I was managing a tool room I bought around 14 or 15 new Bridgeport mills, average life for us was 5 years. Towards the end we had some really shocking new machines assembled at the Bridlington works two of which where replaced. Bridgeport started suffering from the far east competition and instead of manufacturing all in house subbed out stuff and quality suffered. We started to have our earlier machines rebuilt with some good results.

Certainly you should get on and start making chips to see if it's good enough for your purposes before you fiddle with it. This glib talk of scraping the ways is all very well but when you lay the table out on the bench (not forgetting saddle will also need attention), you will see the magnitude of the work that would be involved. Hence my suggestion of grinding. As I said, you can make up for it by keeping the slide locks nipped up - or locked when not required to move.

I regularly climb mill on my worn out machine. Unless you only wish to cut only in one direction and then waste ages returning the table between cuts, it's a perfectly viable method. You soon get the feel of loosening the lock to the point where the table almost self-feeds, beyond which you mustn't go! Nip it up a little from that point and you are set. Climb milling gives a better finish and a better tool life and actually takes very little skill or risk.

Murray

May I be permitted to quote the Bridgeport Operation and Maintenance Manual?

Table Screw Assembly Backlash adjustment... tighten until 0.004"-0.005" is obtained!!

In reality it is nearer 0.0015". Both X & Y. Do not overtight the anti backlash nut on the leadscrews, they only wear faster. Look at the leadscrew end bearings as well./

Table Gibs, "adjust table gib screw until slight drag is felt.

Of course the Y axis displays more slop than the X axis because the gib is that much shorter.

Not much he can do about the backlash on the leadscrew. Most of us would settle for 4-5"'! The issue with "slop"(?) is that if the gibs are tight at the ends, they will be loose at the middle. Either you live with it our you adjust it according to the position you are operating at. There's really not much choice surely.

It's possibly a bit late to be worrying about wear from dragging gibs, especially given how little use our machines actually get. Much of the usage seems to happen in the armchair workshop.

Murray

Andrew Johnston,

You are a bad,bad boy so there !!

Of course you should have replied at once !

Dont you know that time is of the essence?

I dont know what the world is coming to,I really dont !

PS On that note,do you think it,s about time I started to do something at 11.15 am?

As they say,"What a difference a day makes", or does it?

Larry, its nearly beer o'clock - thats when its time to do something

Gary

Yeah, yeah, I know. But then again I've never claimed to be a model anything, let alone model engineer. Far too many vices for that.

Andrew