Bridgeport broken T slots

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

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spro
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Re: Bridgeport broken T slots

Post by spro » Wed Nov 08, 2017 12:03 am

That is so true. Another point Steve M made; It is a clamping surface, not a puller surface. In any procedure where longer bolts hold a tall item, there should be enough thread above the table, for another nut and plate to Clamp the bolts to the slot.

earlgo
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Re: Bridgeport broken T slots

Post by earlgo » Wed Nov 08, 2017 12:14 pm

spro wrote: In any procedure where longer bolts hold a tall item, there should be enough thread above the table, for another nut and plate to Clamp the bolts to the slot.
I am a little confused about the plate part. Where does that go? It would seem that no matter what is put on the top of the table the tensile load on any clamp bolt will be transferred to the t-nut(s) and therefore to the bottom side of the t-slot. What am I missing here?
--earlgo
Deja Poo - The feeling you have seen all this crap before.

John Hasler
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Re: Bridgeport broken T slots

Post by John Hasler » Wed Nov 08, 2017 6:22 pm

The plate needs to be somewhat wider than the bottom of the T-slot. That makes sure that the overhang of the T-slot is loaded in pure shear rather than as a cantilever.

Harold_V
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Re: Bridgeport broken T slots

Post by Harold_V » Thu Nov 09, 2017 2:34 am

That's still not a good repair, as the bulk of strength would be derived by the method of fastening the plate. In my mind, it would be by steel fasteners, where you're at the mercy of the strength of the thread. It might look better, but it would lack strength.

One thing is sure---I would avoid any welding, regardless of procedure. Welding on a precision surface in the hopes that it won't change is pretty much futile. The intense localized heating would not only introduce stresses, but would also harden the HAZ due to rapid cooling. All in all, welding on a table is not a good idea.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Winn SpeedLock
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Re: Bridgeport broken T slots

Post by Winn SpeedLock » Thu Nov 09, 2017 8:00 am

I agree with John Evans the stud went through the t-nut and bottomed out in the t-slot. To repair it, I think the best way would be to mill out the damage and make a piece to fit into it. If you use socket head cap screws to bolt the repair in, go a little deep with the counter bore and plug it. If you stone it afterwards, you will be surprised the way it will blend in. Within a couple of months of use, it will be hard to notice. You may want to make some longer t-nuts to disburse the load for those areas.

earlgo
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Re: Bridgeport broken T slots

Post by earlgo » Thu Nov 09, 2017 9:28 am

Mr. Hasler: I understand what you are saying about shear vs bending stresses, and I failed to consider bending stresses at the first thought. If the inside under corner of the table slot is sharp then there is a considerable stress concentration factor to consider in de-rating the allowable stress of the table material. It might be interesting to make a rudimentary calculation of the relative difference between the pure shear and the bending stresses.
--earlgo
Deja Poo - The feeling you have seen all this crap before.

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Richard_W
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Re: Bridgeport broken T slots

Post by Richard_W » Sat Nov 11, 2017 11:37 am

SteveHGraham wrote:
John Evans wrote:Often caused by using T nuts with threads clear through instead of last thread incomplete . Specially when using bolts rather than studs.
Now there is a hazard I didn't know about.
I found the other thread where this is mentioned.

http://www.chaski.org/homemachinist/vie ... ot#p170961

little giant
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Re: Bridgeport broken T slots

Post by little giant » Mon Dec 25, 2017 1:28 pm

You could make a sub-plate that goes on top of the table with tapped holes for the size of the studs and you don't need the t nuts.

pete
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Re: Bridgeport broken T slots

Post by pete » Sat Dec 30, 2017 8:12 pm

Uh it's a Bridgeport, sort of the small block chevy of the machine tool world and the older ones are being scrapped every day. Kansas is a lot closer to the rust belt than I am and I'd still look for a table in semi decent condition before I'd try fixing what will never be as good as a replacement table would be. $200- tops $400 for almost new shape plus shipping would be my guess if you can find a real good one. Call it half the price of a decent milling vise and people seem to find the money for those often enough. A mill eats tooling money and you'll forget the cost long before you quit using it. I'd email a few machine tool rebuilders and I bet they have some just sitting there from scrapped machines.

shorttrackhack
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Re: Bridgeport broken T slots

Post by shorttrackhack » Fri Jan 05, 2018 2:30 pm

Brand new tables are still available from H&W for about $1500.00.

pete
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Re: Bridgeport broken T slots

Post by pete » Fri Jan 05, 2018 6:50 pm

Hmmm, about the price of a top end Orange vise for new then. A lot of money, but still a bit less than I thought new might be. I doubt any table would be a true drop in fit with used parts on a different used machine. But if the search turned up one in very good condition you could delete years of wear as well. Doing a proper survey of what the OP has and what he's willing to live with would be my first step I think. Pulling the table isn't tough at all, and the gibs current adjusted position would tell a lot about how much wear his table has. I'm sure any reputable machine tool rebuilders would know and be more than willing to say just what condition any used tables they have are in. Ebay, Craigslist might be a shot in the dark about what your getting. My estimate of used prices may or may not have been a bit low, but I've never priced out a used table yet.

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