Proper mill cutter and technique

[Susquatch]

Am I correct in assuming that the best ideal placement is to have clamp perpendicular to the T slot with the edge of the work piece right up to the edge of the T slot and the step block of the clamp right up to the other edge of the T slot?

Not really, but that is next best. Best is a full compressed column. A hole in the vise and a flat bottom so that the T-Nut compresses solid metal from top to bottom.

Ideally with the stud sticking up in the canter of the clamp so the force is equal on both sides of the T slot?

Yes, but as stated above, a full column is best. It isn't the equality of both sides. It's the need to avoid tension. Full compression is best.

Are T slots on tables generally considered to be weak or is this mostly a problem only on smaller mills?

Cast iron has poor ductility period. But it is very strong in compression. Think of it a bit like concrete.

I think I'll take the time to make drawings and provide the science behind this issue that others can critique so we can give a consensus of advice. As already noted, there is lots of discussion about it on this forum and elsewhere. Just count yourself lucky to be among those who learned about it from others rather than through experience.

 

[Megar arc 5040dd]

I guess my big question would be if using say the longest strap clamp (please correct me if that is the wrong name) in the set and a work piece and the notched packing block positioned as far apart as possible from the T slot. So basically the worst case set up you can manage with a regular hold down set. How much force would you need to put on the stud pulling up to break the table? I am aware that the nature of threads is that they can apply extreme amounts of force. I am just curious if we are talking like pull to tight on a wrench by accident tight or 2 hands giving it all you got tight? Also how small of a diameter of stud before the stud and or threads are the weak link and not the table?

I am genuinely curious how much of this is in the numbers and how much is speculation based of off what people have seen or heard happen? What I mean is is this a common thing to happen or the results of a 200 pound gorilla trying to get every last degree of turn out of a bolt with a 2 foot snipe? For instance are the forces needed to break the T slots more than the force it would take to mar and ruin the finish on the part being clamped? If it is more then I think most people would stop tightening for fear of damaging the part. However if it is less than it is effeminately something that should always be on the mind when doing setup.

The more I think about this the more fascinated I become. This never would have occurred to me had someone not said something and I am sure it would be the same for most people starting out.

 

[Susquatch]

The more I think about this the more fascinated I become. This never would have occurred to me had someone not said something and I am sure it would be the same for most people starting out.

This is a very healthy mindset.

I did all the math for someone else who asked the same question earlier.

I'll take the time to find it in the process of preparing the thread I proposed earlier. In the meantime, I can at least summarize what I recall.

First of all, it's a lot more common than you might think. It happens all the time. When it does, few are willing to confess.

I'm not a career machinist. I am an accomplished Engineer. Maybe I've seen a hundred mills in my life. Probably a dozen of them had broken T-slots. It's hard to say exactly why - maybe through threaded T-Nuts, maybe poor setup. Maybe gorilla's with tools.

Quite frankly, the math is scary. If you go by the published material properties, it's really easy to pop off the edges of a T-slot. It doesn't take a gorilla to do it. In fact it's hard to believe it doesn't happen more often.

The situation is made worse by the porous nature of cast iron. Casting porosity and casting inclusions are almost a rule. So even very accurate material properties should be considered best case. In the automotive world, cast iron properties are usually derated by a factor of 5 for that very reason. Sometimes even that is simply not enough.

The situation is made even worse by the size of the standard studs used with T-Nuts. For demonstration purposes, I'll just arbitrarily select 1/2" as an average size bolt used on a mill table T-slot. Mine are 5/8. An extremely mild torque of say 10 foot pounds on a dry fastener generates a force of roughly a thousand pounds on the T-slot. This force is not distributed evenly - it gets concentrated on the ears of the slot and further exacerbated by the corner geometry which becomes a stress riser.

It's a whole lot of factors all conspiring to destroy your table. It is simply not a trivial problem that only bites a very select number of testosterone fueled gorilla's. It can and will bite the average machinist if they ignore it.

It's not if it will happen, it's only when. You ignore it at your peril.

And you are absolutely right about the fact that every new machinist is at risk. Nobody would ever think to worry about it on their own. That's exactly why I try to point it out whenever I think someone might not know.

But there is good news. As easy as it is to break a T-slot, it's just as easy to protect them. The easiest way to do that is to always keep your joints in compression. Cast iron is very strong in compression. Avoid tensile forces. @CWret made a very nice T-Nut that solves a lot of problems - it's a beautiful elegant solution. The principle can be easily extended to longer distances.

I don't actually think the worst case is a long step clamp (my name for your strap clamp). It isn't the distance. It's the tensile bending force on the cast iron ear. The bolt and nut are always in line no matter where the ends are. And the ends are almost always applying a downwards compressive force. So it's really only the torque on the bolt that matters and then only for tensile loads not compression loads.

Avoid tensile loads. Simple as that. Study @CWret's vise hold down T-nut - it shows how to span distances effectively. There are many other effective solutions too. Just be careful to avoid the problem of the bolt bottoming out which is just as dangerous.

 

[Megar arc 5040dd]

This is a very healthy mindset.

I did all the math for someone else who asked the same question earlier.

I'll take the time to find it in the process of preparing the thread I proposed earlier. In the meantime, I can at least summarize what I recall.

First of all, it's a lot more common than you might think. It happens all the time. When it does, few are willing to confess.

I'm not a career machinist. I am an accomplished Engineer. Maybe I've seen a hundred mills in my life. Probably a dozen of them had broken T-slots. It's hard to say exactly why - maybe through threaded T-Nuts, maybe poor setup. Maybe gorilla's with tools.

Quite frankly, the math is scary. If you go by the published material properties, it's really easy to pop off the edges of a T-slot. It doesn't take a gorilla to do it. In fact it's hard to believe it doesn't happen more often.

The situation is made worse by the porous nature of cast iron. Casting porosity and casting inclusions are almost a rule. So even very accurate material properties should be considered best case. In the automotive world, cast iron properties are usually derated by a factor of 5 for that very reason. Sometimes even that is simply not enough.

The situation is made even worse by the size of the standard studs used with T-Nuts. For demonstration purposes, I'll just arbitrarily select 1/2" as an average size bolt used on a mill table T-slot. Mine are 5/8. An extremely mild torque of say 10 foot pounds on a dry fastener generates a force of roughly a thousand pounds on the T-slot. This force is not distributed evenly - it gets concentrated on the ears of the slot and further exacerbated by the corner geometry which becomes a stress riser.

It's a whole lot of factors all conspiring to destroy your table. It is simply not a trivial problem that only bites a very select number of testosterone fueled gorilla's. It can and will bite the average machinist if they ignore it.

It's not if it will happen, it's only when. You ignore it at your peril.

And you are absolutely right about the fact that every new machinist is at risk. Nobody would ever think to worry about it on their own. That's exactly why I try to point it out whenever I think someone might not know.

But there is good news. As easy as it is to break a T-slot, it's just as easy to protect them. The easiest way to do that is to always keep your joints in compression. Cast iron is very strong in compression. Avoid tensile forces. @CWret made a very nice T-Nut that solves a lot of problems - it's a beautiful elegant solution. The principle can be easily extended to longer distances.

I don't actually think the worst case is a long step clamp (my name for your strap clamp). It isn't the distance. It's the tensile bending force on the cast iron ear. The bolt and nut are always in line no matter where the ends are. And the ends are almost always applying a downwards compressive force. So it's really only the torque on the bolt that matters and then only for tensile loads not compression loads.

Avoid tensile loads. Simple as that. Study @CWret's vise hold down T-nut - it shows how to span distances effectively. There are many other effective solutions too. Just be careful to avoid the problem of the bolt bottoming out which is just as dangerous.

View attachment 58491View attachment 58492View attachment 58493

Thank you I appreciate you taking the time to help me better understand this. This is a little bit of a tangent that isn't really relevant but I feel you may know the answer. Would cast steel have this same problem and if so would it be any stronger or would it be weaker? I ask out of curiosity. My background is in welding and though it has been a long time since I have done repairs on either I do remember them being quite different materials at least from a repair perspective.

My second question is about the clamps pictured. I can tell because of the angle is there a step on the left side of the bottom piece as well? I am assuming that is what the red circle indicates? As I mentioned in an earlier post I need to make some dedicated hardware to hold the vise down and this looks like it would be pretty easy and quick solution. Just want to make sure I understand it first. Would having a T section down the center still work in order to have more threads engaged? Or would that ad unwanted rigidity to the nut?

Sorry to keep hammering you with questions. I really wish I had access to an old mill table that was worn out and scrap. I would love to test different configurations and sizes of studs and see what happens.

 

[Susquatch]

Thank you I appreciate you taking the time to help me better understand this. This is a little bit of a tangent that isn't really relevant but I feel you may know the answer. Would cast steel have this same problem and if so would it be any stronger or would it be weaker? I ask out of curiosity. My background is in welding and though it has been a long time since I have done repairs on either I do remember them being quite different materials at least from a repair perspective.

Cast Steel has much better tensile strength than cast iron and isn't nearly as easily broken. However, it's yield strength would allow the bed to deform which isn't a good property for a mill table.

My second question is about the clamps pictured. I can tell because of the angle is there a step on the left side of the bottom piece as well? I am assuming that is what the red circle indicates?

Yes. The photos are @CWret's. The red circles were mine. Both ends of his T-Nut are raised.

As I mentioned in an earlier post I need to make some dedicated hardware to hold the vise down and this looks like it would be pretty easy and quick solution. Just want to make sure I understand it first.

@CWret's method is perfect for a vise without holes for tie-down bolts.

Would having a T section down the center still work in order to have more threads engaged? Or would that ad unwanted rigidity to the nut?

I'm not sure what you mean?

Sorry to keep hammering you with questions. I really wish I had access to an old mill table that was worn out and scrap. I would love to test different configurations and sizes of studs and see what happens.

That's funny cuz I was thinking of doing exactly the same thing for the thread I proposed. It would be subject to significant variation caused by the non-uniform material properties of cast iron. But still worth doing.

 

[Megar arc 5040dd]

The best way I can try to describe what I mean is if you were to take a regular T-nut shape and relieve the middle of both sides where it contacts the underside of the T slots. So just like the piece in your picture it would only contact the underside at the four corners of the nut. It would just have the section still that goes up in between so the bolts would have more than just a few threads engaged. This may be completely unnecessary it just looks like the pieces are quite thin and would flex when tightening. I could also be wrong I have issues telling material thickness and sizes from pictures.