Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

[ddickey]

@woodchucker No not me. It's a horizontal for sale I was looking at (online). Thought it fit the discussions here.

 

[Richard King 2]

20+ years ago I was demoing the BIAX scraper at the IMTS show in DAPRA's booth and a machine rebuilder from the Chicago area stopped by the booth and he asked me if I had ever peened the bottom of a Bridgeport table to straighten it? I said no and thought the guy was nuts.

When I was a partner in a Used Machinery company 10 or so years ago we bought a Bridgeport and the table was bent. It mic'ed good but was bent about .010. I checked it like the picture I will attach of the Table we have at All World Machinery where I am teaching a class this year. I thought I'd give it a try. I laid the table on the concrete floor and on a 1/2 sheet of plywood. I made a rounded head punch like the end of a ball peen hammer. I started to hammer the punch in the middle of the bottom, not knowing hard to hit it. I must have hammered it about 10 times on the bottom ribs. I stayed away from the dovetails as I figured they might crack.

To my surprise when I set it back on my on the surface plate it had bent .008 and I put it back together. I then power scraped the table top and 1/2 moon flaked it. It looked pretty good and we sold the mill.

I taught a class in Denmark 5 years ago and a student brought a Bridgeport table to the class as a project and it was bent. We laid it on the floor and he peened the bottom and it straightened right it. Since then I have done it a 1/2 dozen times and tomorrow I will do it to the mill table we are going to grind. The one below. I didn't go into taking a ton of photo's, just the way we did it..

 

[Eddyde]

Thanks for that Jwmelvin
I have a Clausing 8520 mill with 24x6 table that uses 3/8 clamping bolts. A straitedge pivots in the center and when one end is lightly clamped to the table a 0.005" shim will fit under the opposite end suggesting a table bow of about 0.0025" high in the center. This is consistent with what I saw attempting to machine straight edges.

View attachment 442259

It seemed possible to me that a high enough T nut clamp force might cause an immediate elastic bending of the table that could be measured. I assume the bending will be limited to small areas under each T nut. A 4 inch kurt vise is clamped near the center of the table. An indicator base is mounted to the table on one side of the vise with a tenths indicator on the opposite side. Here is a conceptual cartoon and photo of the setup. The top sketch is with the vise bolts loose and the bottom sketch is after tightening the bolts and bending the table.

View attachment 442258View attachment 442257

The indicator was recorded as the 2 vise nuts were torqued down. In general I could see the indicator increase as the torque is increased in several steps and return to zero as the bolt is loosened. I repeated the experiment with different T nuts using the same lightly oiled flange nuts and studs. The vise was moved to different locations on the table. For some tests I added thin shims between the vise and table to prevent the vise from straightening the bowed table. I don't have a good explanation of why the results are so variable with deflections ranging between 1 and 4 tenths at around 400 in-lb torque. The less flat T nuts and additional shim seemed to result in more deflection.

View attachment 442260
It is clear a more sensitive indicator is needed to really see what is happening, along with load cell washer under the bolts. Neither of which I currently have access to. I have not attempted any calculation to try to predict this elastic deflection, I may give that some thought.

A rough calculation based on torque and perfect contact suggests about 18,000 psi which should be below the compressive yield stress of most kinds of cast iron. This suggests that if there was perfect contact between the T nut and flange it would be unlikely to cause permanent bending of the table (not impossible).

A couple of questions
Anyone ever try straightening a bent milling machine table for example using a hydraulic press?
Anyone know what kind of cast iron these tables are typically made of? Any estimate of hardness or better yet strength?

Excellent, better setup than I had in my test. I would like to try this approach on my mill, I am curious to see the results. I'm also going to try to get a wider spread from the base to the indicator that should get a higher resolution.

"Anyone know what kind of cast iron these tables are typically made of? Any estimate of hardness or better yet strength"

Better quality mills use Meehanite, a proprietary iron casting and seasoning process that yields a more stable material. Some companies might uses similar processes. My 36 yo mill is Meehanite and the table is still very flat. I know if the iron isn't seasoned properly it can warp from internal stress. I have heard even years later, but I don't know how accurate that is.

 

[jwmelvin]

…you can bend a Bridgeport table in years of tightening a vise if you only center it in the middle of the table.

Moving the vise to other parts of the table, and overtightening it in those other parts, will bend the table in those other parts and make the problem worse.

Using a variety of vise locations will spread out the way wear, but that’s not what we’re talking about here.

 

[Charles scozzari]

Moving the vise to other parts of the table, and overtightening it in those other parts, will bend the table in those other parts and make the problem worse.

Using a variety of vise locations will spread out the way wear, but that’s not what we’re talking about here.

I got it, I'm making a 10" fly cutter and fixing my own table. I was gunna use my belt sander but the fly cutter won out.

 

[Eddyde]

Again, the vast majority of home machinists had no idea this is a proposed problem for them. I will NOT even check my table because to makes absolutely NO difference to me and I would think quite a few others either. If its that important we better let the aero space people know as soon as possible. Until I'm asked to make gyroscopes for the next space shuttle I won't lose any sleep over my very well used mill table is out a few thou. Richard, in my opinion maybe this isn't the right forum for your cause............ Charlie.

I totally agree, the "Problem" is way overblown for most hobby and small machine shops. If one has a worn, warped mill table and hasn't noticed it affect their work, it probably won't ever be an issue. If it is a problem, one has many workarounds available, shimming the vise or work, etc. The last resort would be regrinding and scraping in. Most new machine owners are not apt to abuse their investment, and with moderate use, the issue would probably never manifest itself. Most of those crapped-out mill tables are probably form huge manual production shops & factories of a bygone era. New CNC machines are not likely to be subjected to those abusive conditions.

FTR, My interest in this issue is merely to gain a better understanding of materials, machines and physical dynamics that affect them.

 

[Charles scozzari]

I totally agree, the "Problem" is way overblown for most hobby and small machine shops. If one has a worn, warped mill table and hasn't noticed it affect their work, it probably won't ever be an issue. If it is a problem, one has many workarounds available, shimming the vise or work, etc. The last resort would be regrinding and scraping in. Most new machine owners are not apt to abuse their investment, and with moderate use, the issue would probably never manifest itself. Most of those crapped-out mill tables are probably form huge manual production shops & factories of a bygone era. New CNC machines are not likely to be subjected to those abusive conditions.

FTR, My interest in this issue is merely to gain a better understanding of materials, machines and physical dynamics that affect them.

Your reply makes so much sense. You summed it up beautifully.

 

[gard]

I don't think it is way overblown. If your mill is not making flat surfaces this would be a good thing to check. If the table is warped and you are trying to machine a large flat surface on a rigid part, it will end up with about the same curvature as the table. I do not see how this can be fixed with shims or vises. I want to thank Richard for bringing this up and also suggesting a way to fix it, by bending the table flat.
I have used my mill to machine surface in preparation for scraping flat. I think I now know why the parts did not come out of the mill very flat. So I have learned something. If I ever buy another mill I will bring a straitedge with me when I go to look at it. A dial indicator on the spindle will not show it. I am not as sure I understand exactly how my mill table got warped over the last 60 years or so. Is it just over tightening or could T nut design and interface flatness be a big contributor? I agree it is an interesting subject.

 

[Eddyde]

Yes, I stand corrected. If one has to do a long very flat or straight cut that must be done in relation to the Z axis and the table is warped beyond the desired tolerance, it could be a problem that only straightening the table will fix. However, I maintain this is not going to be a very common situation. Anyone doing exacting work, probably won't buy a worn-out mill in the first place.

 

[Charles scozzari]

I don't think it is way overblown. If your mill is not making flat surfaces this would be a good thing to check. If the table is warped and you are trying to machine a large flat surface on a rigid part, it will end up with about the same curvature as the table. I do not see how this can be fixed with shims or vises. I want to thank Richard for bringing this up and also suggesting a way to fix it, by bending the table flat.
I have used my mill to machine surface in preparation for scraping flat. I think I now know why the parts did not come out of the mill very flat. So I have learned something. If I ever buy another mill I will bring a straitedge with me when I go to look at it. A dial indicator on the spindle will not show it. I am not as sure I understand exactly how my mill table got warped over the last 60 years or so. Is it just over tightening or could T nut design and interface flatness be a big contributor? I agree it is an interesting subject.

So, you are going to resurface a cylinder head on your Bridgeport mill, you secure it to the table at both ends using the tapped accessory holes at each end of the head. Naturally the head is now suspended above the table to achieve a level flat position for the cut to be taken. The plates at either end holding the head in suspension are appropriately 22" apart, what's the problem?, there is none, you are spanning any bump on the table. I use a cylinder head as an example, people do clean up heads on their mills. As for what you experienced prepping for scrapping I would think your work may have been suspended and if not would you not have done the final scrapping with the part in position on the intended machine. I have never seen a table ground but I would think you would have to grind the underside also. Thats got to be some trick. Just saying. Thanks, Charlie.