Who has had success grinding mill table top???

I got this old bridgeport mill a few weeks ago and I'm thinking about having the table top ground to take out the very many dings,dents, nicks, and drill holes, about .030 should take care of almost all of it. Right now I'm in the middle of cleaning and rebuilding all of the movements X,Y&Z replacing bearings,knee gears, and repainting. For its age 1957, its not in too bad of shape just lots of old grease and grime and the table top is horrendous. once I get it back together I'll put an indicator on it and see how true the table is before I decide to grind or not.

I know conventional wisdom says you have to have all the ways scraped in if you clean up the table top but I'm sure many tables have been reground with out doing all the scraping and suffered no real issues.

Who has had success grinding table top only and not having all the ways scraped?

Alterego said:

I got this old bridgeport mill a few weeks ago and I'm thinking about having the table top ground to take out the very many dings,dents, nicks, and drill holes, about .030 should take care of almost all of it. Right now I'm in the middle of cleaning and rebuilding all of the movements X,Y&Z replacing bearings,knee gears, and repainting. For its age 1957, its not in too bad of shape just lots of old grease and grime and the table top is horrendous. once I get it back together I'll put an indicator on it and see how true the table is before I decide to grind or not.

I know conventional wisdom says you have to have all the ways scraped in if you clean up the table top but I'm sure many tables have been reground with out doing all the scraping and suffered no real issues.

Who has had success grinding table top only and not having all the ways scraped?

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Me, but believe it or not this was the table on a brand new machine I'd just installed.

Later that day the operator came to see me and said " Tyrone, that table on that new machine is tapered ". I said " It can't be, it's brand new ".

He replied " Well I've just miked it on four corners and it has a 0.010" taper from end to end ".

Cut a long story short he was dead right, the table was tapered. That's inspite of the manufacturers inspection test charts all being filled in and signed as correct. If I hadn't have seen it with my own eyes I would have never believed it ! How they managed to make such a balls up I'll never know. I suspect vodka played a big part in it.

Anyway the manufacturers agent was summoned and once he'd seen the problem he agreed to pay for me removing the table and having it ground parallel at a local grinding specialists. I refitted the table later and we never had another problem with it.

Regards Tyrone.

I can guarantee that the table will be hog backed. This is a side effect of years of peening the underside of the T-slots by the T-nuts. Grinding/milling/scraping the top of the table is the first stage. After that the ways will need grinding/milling/scraping to make the mill machine work flat.

I had to scrape .010" off the top of my Beaver mill's 10"x48" table to get it flat. The actual wear on the ways was .003", but they were curved with the additional .010" as well.

Also, don't bother too much about removing the 'war wounds' they are just cosmetic. the flatness is the important part.

1) every day medium size castings like that are milled to .0004" per 40" flatness. usually indicate or probe before removing from table. waviness of <.0003" TIR is commonly expected
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2) normally if you want to machine a part parallel you bolt on pads or blocks to table. pads are machined .001" and indicated to confirm flat to <.0003" before mounting part cause table is never assumed to be flat less than .001" normally. pads or blocks are replaceable
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3) bridgeport table is flimsy. you mount even 100lbs on it overhanging saddle to the side and i would expect it to sag from gravity easily. point being you can put racing car tires on a old low horsepower car that dont make it a race car.

I had a old Wells Index mill table surfaced ground to remove most of the idiot marks from the top. I re-miked the table before and after re-grinding and it measured dead nut for about 75% of the length of the table. For some reason the last foot-foot and a half, it grew in thickness by about .002". This is what I got before grinding the top also. Good enough for the girls I play with. This is the only mill table I've ever had re-ground. I had a No. 3 B & S mill that we had to re-plane the table, both top and bottom because it was that bad. I don't remember the exact measurements, some where around .045-.060" we had to take off to get it straight again. Oh, we used to have a old Gorton mill that we bought that the table top had been re-ground on, and was dead flat from the best we could check it back then. I remember a No. 5 K & T mill at a place I worked at that you could eye-ball the eight foot table and see the droop from end to end.

I would say it's a crap shoot on every mill out there if the table will stay straight, bend from grinding, OR straight on top and curved on bottom.

Ken

You'll want more than just the table top flat.

Bridgeport tables are light enough that they are an easy job to rescrape the top and ways. Have done several tables. Time taken depends on how drooped and how much wear in the ways. Wouldn't make sense to me if more that 7 or 8 thou of table top droop or more than 4 thou wear in the ways. Doubt it would profitable for a pro rebuilder to do, but for a guy with a surface plate and some spare time I'd say do it yourself. No risk of grinder heat warpage and save your money for other needs.

L7

I Redone my enco mill I bought new in 93. oilers never did work good , that’s what I initially pulled it apart for. Found out nothing was straight on it (big surprise) I surfaced it on a big Rottler block machining center with a cbn cutter basically a 16 inch fly cutter. Creates very minimal heat so keeps the warp from that down. Leaves a finish that rivals grinding. Scraped in the underside ways. Man what a difference just doing that. After finished the table would indicate 4 corners at .0005 to .001 with no weight on it. Now if the rest was as nice


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Thanks for all the replies, I think I will grind the top and hope for the best.

It has to be flat to the ways not the bottom, send it out to be ground to the bottom and you are going for a crap shoot.
Roll the dice as you want.
When doing such I spec to the grinder how I want the tavel shimmed but that is a complicated process of measuring the machine and table run.

CarbideBob said:

It has to be flat to the ways not the bottom, send it out to be ground to the bottom and you are going for a crap shoot.
Roll the dice as you want.
When doing such I spec to the grinder how I want the tavel shimmed but that is a complicated process of measuring the machine and table run.

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When I did mine I should have elaborated more that the whole table was in a banana shape. I blocked the ways on the bottom, took a minimal cut to take twist and some of the warp out.
Set it upside down on my surface plate and check for any rock and measured the way error. Then machined the ways flat, flipped and machined, top flat to the way surface. Probably not the right way but worked for me. I figured flipping and moveing the operation around helped the bound up stresses in the table. Sounds good on paper. I did match scrap the bottom of the table to the carriage as the ends of the x Axis ways were worn.


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Generally, you want to machine the top first, in order to relieve peening stress. I don't remember where, possibly in the Connelly book, there was a recommendation for the initial cut to be deep enough to cut only 50-60% of the top surface, since the rest is likely to spring back closer to flatness.

Paolo