Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

[Shotgun]

Would a better solution be to relieve the center of vices so that the bolt down points don't get "pulled down"?

 

[jwmelvin]

Would a better solution be to relieve the center of vices so that the bolt down points don't get "pulled down"?

The effect Richard points out is high clamping pressure causing table deformation. That’s certainly possible on either side of the interface. But like anything, it will depend on the details.

Imagine a large relief on the vise bottom. Now clamping it down would induce bending stress in both the table and the vise. That’s certainly undesirable. I’d think you want the contact surfaces to match each other.

 

[Charles scozzari]

Can cone pressure be canceled or equalized by the use of a stud and and flanged nut along with a large, thick washer when securing your vise.?

 

[jwmelvin]

The cone is a model for stresses inside a material and how the pressure after some thickness tends to disperse. So a flanged washer is good, by starting with a larger area (less pressure for a given force). But a smaller-diameter spacer under it reduces the starting point of the pressure cone through the vise body.

The T-nut is a little different from a classic cone, because the applied stress is concentrated at the top 1-1.5 diameters of thread length (so pulling upward on the top 0.5-0.75” of thread in a 1/2”-13 threaded nut). But the pressure applied to the table is through the nut flanges, so you’d have to consider them as bending beams, with the bolt force applied in their center along the beam width (not too far from the thread. The opposing force comes from the T-slot edge pushing down near the beam root. It seems pretty complex really, but I guess some modeling would show the trends.

 

[Charles scozzari]

So what I got from my uneducated view is if most, if not all mill tables are not flat. Then how can you tram the head if the surface is not true. If you rely on your vise being true and the table is not flat either is the vise. If this is a major factor even a $150.000. cnc milling center is subject to this problem, How do they deal with it ?. My feeling now is I didn't know of it before and for what I do its not a problem. I want my work as best as I can achieve and I am satisfied with what I make on my mill and I didn't even know if it's not flat, and am not going to check it. Moving the vise from side to side along the table, to me just may make the table worse. I am not by any means a machinist, I just have a mill to help me solve problems and increase my ability to make nicer things. In short, that bump in my well used Bridgeport mill is not a problem. True we all want to produce well made/looking work and have been doing it on our not so perfect mills. It did make for interesting conversation and I did learn something from it. At best for me and I would suspect quite a few others with lumpy mill tables full of holes, it is not a problem. Life go's on, work with what ya got. This is just my opinion and you know what is said of opinions. Thanks, Charlie.

 

[Richard King 2]

In a few weeks I am going to teach a class In Rosco IL at All World Machinery and one of the students is bringing a Bridgeport table they are going to grind for him. I will bring a 3' camel back straight edge along and I will document this to the folks who doubt what I am saying. I will set the Bridgeport table on a granite surface plate and put the 4 corners on 4 - 1 2 3 blocks and show you how the center is convex.

Also I will set a height gage on the granite plate and indicate the table all around. I have been seeing errors like this for 50+ years.

 

[Charles scozzari]

In a few weeks I am going to teach a class In Rosco IL at All World Machinery and one of the students is bringing a Bridgeport table they are going to grind for him. I will bring a 3' camel back straight edge along and I will document this to the folks who doubt what I am saying. I will set the Bridgeport table on a granite surface plate and put the 4 corners on 4 - 1 2 3 blocks and show you how the center is convex.

Also I will set a height gage on the granite plate and indicate the table all around. I have been seeing errors like this for 50+ years.

Hi, I have no doubt at ALL it happens, but as for myself and maybe others who're not aware of the problem, it's not a problem. I am not going to have my table resurfaced, take it home put my vise on it only to bend it again. If the table only reacts to tightening once and will not react to each successive tightening by increasing the bend more and more than grinding is not a solution. Again, I'm not saying it's not happening, I don't see a solution in resurfacing. Maybe by running your vise at 90 degrees on the table will help. But I for one would not want my vise with that set-up. Thanks again, Charlie.

 

[Charles scozzari]

The only solution I see is a much wider mill knee or mounting the vise in the center of a 9"x 40"x I" piece of CR and then clamping the plate (with the vise on it) at the end of the first and last "t" slots only which will render the rest of the table useless but just might control the warp. Take your choice. (there's really none). Charlie.

 

[Provincial]

The first thing I would like to say is "Thank you, Richard, for bringing up this topic. This is an important issue, and one that should be widely discussed."

One thing that I get from this discussion is that in my observations, most machinists over-tighten the fasteners holding work to the table. This is especially true with vises. The prevailing practice is to tighten the nut by pulling on the wrench (the longer the handle on the wrench, the better) and then yanking on the wrench by throwing full body weight against it. If tight is good, tighter is better.

The idea is that the fastener must be as tight as possible in order to keep the item (vise, workpiece, fixture, etc.) from shifting when making a cut. This is how I was trained back in the Dark Ages, when I first went to work in a Job Shop.

The problem with this practice is that there is a specific amount of clamping force necessary to hold the workpiece firmly against the cutting forces, and exceeding this only abuses the affected parts. Unfortunately, there are no guidelines widely available for determining the optimum clamping force.

The amount of clamping force necessary is determined by many variables. The number of fasteners, the contact area of the fasteners, the materials from which the fasteners, workpiece (including vise), and table are made, the cross section of the table (including not only the T-slot area, but the rest of the table), and the amount of force applied to the workpiece by the cutter. I'm sure that there are even more variables. With this many variables, the default is to overtighten the fasteners.

When taking heavy cuts on a mill, it is often possible to see the table bounce as each tooth enters the cut. There is considerable force necessary to cause the feed screw to visibly stretch. Is it reasonable to expect two T-slot fasteners to hold against that amount of force? In most cases, no, it is not.

There are ways to spread out this force. More fasteners, toe clamps, blocking to buttress the work (or vise) against the force applied, and other solutions exist. The point being, don't just accept that the two points built into the vise design are the only way to secure it against cutting forces.

I would like to see a study of the design of T-nuts and T-studs. My gut feeling is that the surface area transferring the clamping pressure may be inadequate for the forces applied in the real world. This would lead to over-stressing the table material. This is very similar to warping an engine block by applying excessive, uneven torque on the cylinder head fasteners.

Richard's point of moving a vise around on the table is one way of distributing the overstress, but I am thinking through how to keep from overstressing the table in the first place. Now that I only do "non-paying" work, and have no pressure to get the job done in the minimum time, I tend to use less aggressive feeds. I should be able to reduce the clamping force, since there is less force applied to the work. The question is, how much clamping force is necessary, and how do I determine it?

A practical way to determine this would be somewhat complex, but actually quite doable. It would require two separate tests. The first would be to put a strain gauge on the fixed end of a feed screw, then record the amount of force applied to the screw when machining a variety of materials using different feeds and cutter designs. By cutter designs, I mean tooth number, type of cutting edge material, rake, chipbreaker, etc.. For example, hogging with a large, negative-rake carbide shell mill will apply more cutting pressure than a smaller, 4-flute HSS end mill.

Once cutting pressure is determined, a test rig could be set up to apply a measured force to see how much clamping force is required to resist movement. For instance, set up a vise on the table and see how much torque is needed on the two nuts to prevent movement . If it only takes 50 ft./lbs. of torque on a 1/2" T-nut to hold the vise in place, then anything in excess of that would be excessive, and abusive.

It makes sense that a recommended fastener torque should be determined. The question is, who is going to take on the task, and how do we spread the word. Richard has taken the lead in pointing out the problem. Now we need to follow up and determine how to prevent warping our tables.

By the way, I am in the habit of moving my vise around on the table. I often put it on one end, because it allows me to hold work directly on the table without removing the vise. I also use a combination wrench to tighten the T-slot nuts, instead of a longer wrench, like a Double Box End, or socket with a long handled drive. It would give me some piece of mind if I had a recommended torque to apply.

 

[Charles scozzari]

The first thing I would like to say is "Thank you, Richard, for bringing up this topic. This is an important issue, and one that should be widely discussed."

One thing that I get from this discussion is that in my observations, most machinists over-tighten the fasteners holding work to the table. This is especially true with vises. The prevailing practice is to tighten the nut by pulling on the wrench (the longer the handle on the wrench, the better) and then yanking on the wrench by throwing full body weight against it. If tight is good, tighter is better.

The idea is that the fastener must be as tight as possible in order to keep the item (vise, workpiece, fixture, etc.) from shifting when making a cut. This is how I was trained back in the Dark Ages, when I first went to work in a Job Shop.

The problem with this practice is that there is a specific amount of clamping force necessary to hold the workpiece firmly against the cutting forces, and exceeding this only abuses the affected parts. Unfortunately, there are no guidelines widely available for determining the optimum clamping force.

The amount of clamping force necessary is determined by many variables. The number of fasteners, the contact area of the fasteners, the materials from which the fasteners, workpiece (including vise), and table are made, the cross section of the table (including not only the T-slot area, but the rest of the table), and the amount of force applied to the workpiece by the cutter. I'm sure that there are even more variables. With this many variables, the default is to overtighten the fasteners.

When taking heavy cuts on a mill, it is often possible to see the table bounce as each tooth enters the cut. There is considerable force necessary to cause the feed screw to visibly stretch. Is it reasonable to expect two T-slot fasteners to hold against that amount of force? In most cases, no, it is not.

There are ways to spread out this force. More fasteners, toe clamps, blocking to buttress the work (or vise) against the force applied, and other solutions exist. The point being, don't just accept that the two points built into the vise design are the only way to secure it against cutting forces.

I would like to see a study of the design of T-nuts and T-studs. My gut feeling is that the surface area transferring the clamping pressure may be inadequate for the forces applied in the real world. This would lead to over-stressing the table material. This is very similar to warping an engine block by applying excessive, uneven torque on the cylinder head fasteners.

Richard's point of moving a vise around on the table is one way of distributing the overstress, but I am thinking through how to keep from overstressing the table in the first place. Now that I only do "non-paying" work, and have no pressure to get the job done in the minimum time, I tend to use less aggressive feeds. I should be able to reduce the clamping force, since there is less force applied to the work. The question is, how much clamping force is necessary, and how do I determine it?

A practical way to determine this would be somewhat complex, but actually quite doable. It would require two separate tests. The first would be to put a strain gauge on the fixed end of a feed screw, then record the amount of force applied to the screw when machining a variety of materials using different feeds and cutter designs. By cutter designs, I mean tooth number, type of cutting edge material, rake, chipbreaker, etc.. For example, hogging with a large, negative-rake carbide shell mill will apply more cutting pressure than a smaller, 4-flute HSS end mill.

Once cutting pressure is determined, a test rig could be set up to apply a measured force to see how much clamping force is required to resist movement. For instance, set up a vise on the table and see how much torque is needed on the two nuts to prevent movement . If it only takes 50 ft./lbs. of torque on a 1/2" T-nut to hold the vise in place, then anything in excess of that would be excessive, and abusive.

It makes sense that a recommended fastener torque should be determined. The question is, who is going to take on the task, and how do we spread the word. Richard has taken the lead in pointing out the problem. Now we need to follow up and determine how to prevent warping our tables.

By the way, I am in the habit of moving my vise around on the table. I often put it on one end, because it allows me to hold work directly on the table without removing the vise. I also use a combination wrench to tighten the T-slot nuts, instead of a longer wrench, like a Double Box End, or socket with a long handled drive. It would give me some piece of mind if I had a recommended torque to apply.

Hi, is your table warped, and have you checked it ?. And yes, I have seen guys tighten clamps like they were gonna hold the crack of day light. This I would think is a thing related to Bridgeports and I bet nobody knew or cared what effect it had on the work in a production shop. I would love to know the condition of the table of a bench top mill. Charlie.