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Earl Sigurd
12-20-2013, 01:08 PM
I have been putting a mill back together after dismantling for transport and cleaning. I ran a fine stone over the table to remove the lumps surrounding the dents put on it over the last 40 years or so. (1969 machine)

I put a level on the table, centre and ends and was surprised to see that both ends bend down at about 0.35mm/m or 0.004"/ft.

The table length is 48" and the saddle is 22" wide.

Is this a normal event over time?

http://i1297.photobucket.com/albums/ag34/sigurd3/20Dec2013014_zpsb220e437.jpg (http://s1297.photobucket.com/user/sigurd3/media/20Dec2013014_zpsb220e437.jpg.html)http://i1297.photobucket.com/albums/ag34/sigurd3/20Dec2013015_zpsd78183a3.jpg (http://s1297.photobucket.com/user/sigurd3/media/20Dec2013015_zpsd78183a3.jpg.html)http://i1297.photobucket.com/albums/ag34/sigurd3/20Dec2013018_zps101bbd3e.jpg (http://s1297.photobucket.com/user/sigurd3/media/20Dec2013018_zps101bbd3e.jpg.html)

flylo
12-20-2013, 01:32 PM
I talked to a guy that rebuilt or scrapped mill, he said he never saw one you could not turn top down & spin it like a top.

willmac
12-20-2013, 01:36 PM
Earl -

With that type of mill (Bridgeport?) it is quite common to see that type of pattern. It is probably not actual 'bending' of the table but a combination of rocking and wear in the ways that support the table. You should be able to diagnose the actual cause with careful use of a DTI. Your table surface looks to be quite good, with no obvious divots and cutter marks. Badly abused tables can get un-flat, but I doubt that yours is like that. You can do a quick check with a good straight edge and some feeler gauges.

The original Bridgeports had a smaller table of about 32 inches length. The support for this smaller sized table is not too bad, but it gets a bit narrow when a full 48 inch table is used. This tends to promote wear which looks like this.


That is a nice level - British Aerospace originally?

Doozer
12-20-2013, 02:18 PM
OK, it is not gravety. Iron will not cold flow like that.
It is the peening in the t-slots from the t-nuts being tightened
year after year. It would be noce if the saddle was longer,
(like a jig bore) but I don't think that would eliminate the issue.


-Doozer

Earl Sigurd
12-20-2013, 02:36 PM
Bill,

The measurements were taken with the table centralised and stationary. I also repeated the test with a second sensor in the centre and placing the other at either end. The table is not moving so it looks like the top is bent.

It's a Beaver mill incidently. The level was off ebay, 20 notes. Ex MOD by the tags. Fluid filled vials as per normal level but with electrodes read by an ac bridge I presume.

Doozer,

I can see the peening at the bottom of the slots, there is a ridge of metal forming a burr along the edge where the T nuts clamp. I will have to have another look and make some more measurements.

j king
12-20-2013, 03:27 PM
My mill was bowed. Whether it was from gravity or t-nuts it doesn't matter.I put a straight edge across the table and it had .008 bow.
I recut the top and bottom and it is flat now.When I leave it sitting for extended periods I crank it off center several inches and then lower table to sit on a prop rod at the longer end.I do the the same on my Cincinnati mill too.LOL. Can't hurt me figures. ; )

jlevie
12-20-2013, 04:18 PM
Bowing from stretching by tee nuts sounds plausible, but cast iron is hardly ductile. However, it isn't unusual to have a piece of cast iron warp well after it was made. The good news there is that it probably will never move again. If there is no twist in the table it would not be hard to compensate for the bow when milling long objects clamped to the table with shims. The bow probably won't matter when using a vise in the center of the table.

boslab
12-20-2013, 05:48 PM
M
Bowing from stretching by tee nuts sounds plausible, but cast iron is hardly ductile. However, it isn't unusual to have a piece of cast iron warp well after it was made. The good news there is that it probably will never move again. If there is no twist in the table it would not be hard to compensate for the bow when milling long objects clamped to the table with shims. The bow probably won't matter when using a vise in the center of the table.
I dont know but was under the impression that the tables were made of ductile iron?, i have a cincinatti and i must admit there is a bit of bow there, not enough to worry me really, and i cant quantify it without a 5 foot straightedge really.
I suppose in the end it would be nice to get it reground, but itll do for now.
Mark

goodscrap
12-20-2013, 06:08 PM
i had my bridgy re-ground (by slideway services in nuneaton, they do lot of them, for one of the main re-conditioners;)), anyway i was informed that when you grind the top of the table it will stress releave it, so you have to grind the top, then the ways, then re-do the top again.

Brian

Richard King
12-20-2013, 09:37 PM
In my 40+ years of rebuilding I can say all of the Bridgeport tables I have seen were bent. I had never heard the idea of the T-Slot peening or stretching until meeting Professor Archie Cheda who is retired and just moved from out east that to CA. He explained the metal stretches and makes the table bend and has nothing to do with it bending from lack of support. I have seen how iron bends and twists when your leveling a machine so I believe it's a combination of both bending and peening plus wear. In a recent scraping class we tested a 42" BP table and it was bent .019" high in the middle. We flipped it over and peened the bottom ribbing using a rounded 3/4 wide chisel and BFH our host Jamie used when he straightened crank-shafts. He beat on the bottom and we measured it again and it was down to .009", almost 1/2 of what we got the first test. He did it again and got another .002" but it did not move again.

I am no scientist or metallugist, but it got better. We did not grind the table, but am sure if we had it would have straightened out. Another rebuilder Axel Fors out in Utah tells me he runs a T-slot cutter up the T-slot and the table bends back to with-in .005 before grinding. Give it a try and let us know what happens. Rich

Earl Sigurd
12-21-2013, 02:52 AM
Thank you for all the replies. It would seem that this is a not uncommon, perhaps even a universal fault.

If the top of the table shows a bend then the bottom slides will almost certainly show the same curve plus wear in the centre. It would seem likely that the saddle slideways have worn into a curve to match the table. I can measure the slope on the exposed lower slide surfaces without too much difficulty. The wear in the middle of the slide surface of the table is minimal as the flaking/scraping marks are only slightly less visible than the ends.

It would be interesting to measure the elastic sag of the table when supported in the way it is when installed.

Richard- I may have a T-slot cutter that would fit. I would have to do this in sections with the turret rotated left and right.

Mike Nash
12-21-2013, 08:13 AM
Umpteen years ago I bought a Penn State scroll saw mail order. It is one that has the table slotted to the front so you can slide the blade out or in with the workpiece. It came with a considerable warp in the cast iron table where one side was seriously higher than the other at that slot. After much internal debate, I applied a prying instrument (large screwdriver) and 'warped' that sucker back in line ('lign). I was very surprised that it worked being cast iron, but it did and I still use it. We were both "relieved." :)

Richard King
12-21-2013, 10:00 AM
Thank you for all the replies. It would seem that this is a not uncommon, perhaps even a universal fault.

If the top of the table shows a bend then the bottom slides will almost certainly show the same curve plus wear in the centre. It would seem likely that the saddle slideways have worn into a curve to match the table. I can measure the slope on the exposed lower slide surfaces without too much difficulty. The wear in the middle of the slide surface of the table is minimal as the flaking/scraping marks are only slightly less visible than the ends.

It would be interesting to measure the elastic sag of the table when supported in the way it is when installed.

Richard- I may have a T-slot cutter that would fit. I would have to do this in sections with the turret rotated left and right.


I would really like to hear about your results of cutting the T-Slots. A bit of a hassle to machine it yourself on the machine, but do-able. Yes the ends of the saddle top wear hard from the table being curved. It helps to have a granite table to indicate the table and saddle on. If the table gib is kept adjusted right, I could see the sage would be less as it's not pulling the table down. A lot to think about and to be a detective machinist :-) Merry Christmas. Rich

Mark Rand
12-21-2013, 03:39 PM
I've spent a fair amount of spare time this year scraping my 48" Beaver milling machine's table. It had 10 thou of hog in the middle but a total of only 3 thou of wear (based on changes in thickness). It's had a bit more than a pound of metal scraped off it. Luckily, the column, that I've just started on, has only half a thou of wear on each face.


PS, I'm jealous of the Electro-Level kit:p

partsproduction
12-22-2013, 02:12 AM
I didn't read all the posts, but expansion of the top because of workpiece and T nut peening is the culprit aided by gravity of course.

The fix of milling the top may not be the best answer because the dovetails are bowed as well. Finding an old table to experiment on would be nice to try peening the bottom to bring it back to flatness. I guess if it worked (And it should) I'd go past flat a thousandth of an inch.

Milling the underside of the T slots to remove the expanded surface where the T nuts press against might work too, but peening the bottom between the dovetails seems like a better fix, one worthy of experimentation anyway, what's to lose?
I've owned and run my Supermax long table mill for 20 years or so. A few weeks ago I milled a shaper dovetail prior to scraping it, the surface plate really shows how flat a mill cuts. It was very evenly blued when I checked it, I was pleasantly surprised.
Then again though some people over tighten hold down nuts, 8 out of 10 people in my experience, and those guys might make a table bow much more than other people over the same number of years.

Richard King
12-22-2013, 11:14 AM
I wrote Professor (Archie) Cheda and asked him to comment on here yesterday, I see he hasn't. in so many words he did tell me he said to be careful not to mill the slots to thin and he has gone to buying longer T-nuts to spread the pressure out. I have seen T-nuts that are up to 6" long. I also talked to a rebuilder friend of mine and he said he would grind the table top and sides for $300.00 and do the complete table including the doves for $500.00. He's A&D Machine Reuilding (Rick) in Roberts WI. I would think most rebuilders would charge about the same. Merry Christmas. Rich

garyhlucas
12-23-2013, 06:59 PM
I don't know if the top of the table is flat on my bridgeport, but the table ways are worn like a banana. Make 12" long cut and your part will vary 0.009" from center to end!