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Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

Richard King

Diamond
Joined
Jul 12, 2005
Location
Cottage Grove, MN 55016
Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

Here is a tip for you. In all my 50+ years of Rebuilding Machines 99% of used Knee mill machines the table is bowed up in the middle. Or the Mill tables is bent convex averaged .006" If you had a straight-edge and set it on a stoned table it will pivot in the middle. I used to think that was because the table is longer then the saddle and over time it bends.

I was partially right, but I met a Engineering Professor named Archie Cheba (spelling may be wrong) who was a member here told me the main reason is when the operator tightened the vise on the table over time the T-slot iron gets stretched or peened. So this tip is to buy or make longer 3" T-Nuts then the average T-Nut that is about 1" long. Also move the vise from side to side and not always in the middle.

You can also do this on machining centers and grinders, don't always set small parts in the middle of your table. It wears more evenly.
 
Joined
Apr 19, 2006
Location
Manchester, England
Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

Here is a tip for you. In all my 50+ years of Rebuilding Machines 99% of used Knee mill machines the table is bowed up in the middle. Or the Mill tables is bent convex averaged .006" If you had a straight-edge and set it on a stoned table it will pivot in the middle. I used to think that was because the table is longer then the saddle and over time it bends.

I was partially right, but I met a Engineering Professor named Archie Cheba (spelling may be wrong) who was a member here told me the main reason is when the operator tightened the vise on the table over time the T-slot iron gets stretched or peened. So this tip is to buy or make longer 3" T-Nuts then the average T-Nut that is about 1" long. Also move the vise from side to side and not always in the middle.

You can also do this on machining centers and grinders, don't always set small parts in the middle of your table. It wears more evenly.
Good advice. On another point if you are working at one end of the table and somebody comes up with a “ rush job “ you can set that up at the other end without disturbing the set up of your original job.

Regards Tyrone.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Another tip along those lines: clean your t-slots! Chips getting in between the t-nuts and the underside of the t-slot can also push in and displace iron. And be certain that your t-nuts are making good contact on the flat surfaces in contact under pressure. I can't count the number of times I've seen machines with t-slots that are knife edged down inside. That is often caused by t-nuts that are only making contact in the radius of where they were machined to make them into the 't' shape. Cheap or shop-made t-nuts may not have a sharp (enough) inside corner there and that can cause trouble.
 

jbacc

Hot Rolled
Joined
May 5, 2009
Location
New Jersey
All good advice, but my Bridgeport only has a 32" table so there isn't much overhang and little extra real estate to work on one side or the other without removing the vise. I do have a K&T Horizontal Milling Machine that is substantially longer/larger and the suggestions made here are quite useful...Thanks.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
Similarly, I have found a number of surface grinders that the long direction of the table is bent up at the ends from way too much tightening of the chuck hold-down t nuts..you can see it in the wear pattern of scraped oil ways. it is not very much and possibly it may spring back with releasing the overtightening.
Some new machines are delivered with too much tightening.
 
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Richard King

Diamond
Joined
Jul 12, 2005
Location
Cottage Grove, MN 55016
I just tested a Bridgeport table last week to see if it was bent. First we stoned the table top, then set it on 3 points on a granite surface plate T-slots down, then measured all 4 corners and moved the single point end in and out to get all 4 corners to read .0005". Then I measured the top and bottom with 2 height gages. The T-slot side was bowed down in the middle (on the plate) .006" .High on the 4 corners The top flat ways tested .008". Now if the table was on the saddle, then the would mean the table was bent convex or high in the middle. Our old teacher (member) here who was a professor of Engineering Archie Cheba taught me this is caused by tightening the T-nuts to tight and the T-lots got peened and that stretched the cast iron on top and the table bent convex. The bottom stayed the same and the top stretching made the table bottom go concave. The flat ways were .002" more then the table top meaning it was worn .002". I will add a picture later as it says the file is to large now.
 

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AdamC

Cast Iron
Joined
Jun 16, 2011
Location
SE PA
I have 2 questions:
1) does this matter? How much does this matter? If you are machining a long part, I guess it would be not flat? I‘m guessing you won’t see this with a test indicator in the spindle. Can you (gentlemen) talk more about what this means to the machinist. Guessing this is not a new problem.

2) can it be remediated by peening/planishing? Richard, what are your thoughts on peening BP tables.

mine is off my mill. It’s got a high spot in the center back top due to some gorilla banging the back of the table with a hammer because it was “stuck” (y axis nut came loose in the housing. Like to maybe try peening it back?
 
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eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
I have 2 questions:
1) does this matter? How much does this matter? If you are machining a long part, I guess it would be not flat? I‘m guessing you won’t see this with a test indicator in the spindle. Can you (gentlemen) talk more about what this means to the machinist. Guessing this is not a new problem.

2) can it be remediated by peening/planishing? Richard, what are your thoughts on peening BP tables.

mine is off my mill. It’s got a high spot in the center back top due to some gorilla banging the back of the table with a hammer because it was “stuck” (y axis nut came loose in the housing. Like to maybe try peening it back?

Yes it makes a difference. The parts you make on any given machine will basically mirror whatever shape the ways are in. The smaller the part the less it matters because the deviation from flatness will be small over a short length. No a dial indicator mounted in the spindle will not really show this problem with table travel. A straight edge or surface plate check of your finished parts or the table itself will give you some idea. As Rich's check showed, the table top will usually mirror the ways pretty closely.

Peening can get you closer to straight but scraping is needed to do a really good job of restoring the geometry. Grinding can get pretty close if the grinder operator is good, but scraping and flaking shouldn't be skipped.
 

Richard King

Diamond
Joined
Jul 12, 2005
Location
Cottage Grove, MN 55016
Many make the mistake and indicate from the spindle as eK says. It will read zero and not look bad as the indicator is measuring the same place. Many can't figure it out and want to argue. They call me (or did when I was rebuilding for a living) and say the straight part was crooked after they bolted to the table. Many assumed the spindle was moving because the indicated in the part and it was straight. They didn't understand the table was bent and when he tightened down a straight part on the bent table it bent, then after he machined it it bent back straight and t was screwed up.
As far as peening table straight. For years I never peened them and thought it was sacrilegious doing it, but as time has gone by I do it. In the old days I would mill or grind the table straight. Only when I was in my 20's working for my Dad, He had me scrape the tables. As more shops in the area had surface grinders we got the ground. A company named Checker machine they had a big Elb grinder and they could grind the whole table even the dovetails as they would set the table on the sides and dress a wheel with a 50 deg. angle. Do one side and then flip it over and grind the other side..

My friend Rick at A&D machine Rebuilding in Roberts, WI grinds the top and sides and then Planer mills the bottom and dovetails and then scrapes off the machine marks. Bridgeport had way grinders that they only ground the tables and never scraped them.

On his photo gallery, the first picture is a set of my King-Way Straight-edges.
 








 
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