Ridgidity - 833TV vs. 835 vs. 935

[Pcmaker]

Which machine is more ridgid and can handle larger end mills, can hog largest amounts of steel, can handle hardened steel better?

I've read a few times about the ridgidity issues with J head knee mills when compared to square column bench top mills like the 833.


I've narrowed it down to these 3 mills. I have a PM-25MV mill and I'm looking for a substantial upgrade.

They each have their pros and cons. Travel size isn't that important to me. I've never even come close to being limited to travel on my PM25MV.

 

[kb58]

"In general", go by total weight.

 

[davidpbest]

These lighter-weight knee mills have less rigid Ram/Knuckle setups than larger knee mills. Read the attached document, especially the section on page 5 that discusses my experience with the two types you're asking about. Also watch this video about my PM-935. And I've posted a series of videos illustrating the performance of my PM-935 head which you can view here.

 

[DavidR8]

These lighter-weight knee mills have less rigid Ram/Knuckle setups than larger knee mills. Read the attached document, especially the section on page 5 that discusses my experience with the two types you're asking about. Also watch this video about my PM-935. And I've posted a series of videos illustrating the performance of my PM-935 head which you can view here.

Thanks for the excellent write-up, saved for future reference.


Sent from my iPhone using Tapatalk

 

[AlanB]

The stiffness of a mechanical structure is dependent on many things and weight is only one of them. Comparing the weight of a knee mill to a bench mill isn't a good way to accurately characterize stiffness. Comparing similar designs by weight is fairly valid, but the much longer lever arms of a knee mill require much more material to achieve equal stiffness to a compact bench mill. The bench mill architecture is more efficient of material.

 

[Batmanacw]

I moved to the 835 from a round column mill/drill so it is a massive increase in rigidity. I can't speak to the bench type mills. I do know that in spite of the flaws posted by Mr Best, the mill cuts beautifully. The cross hatching is so fine it is hard to see all the way across because the cut is so good.

I am not entirely sure a couple thousandth of flex is going to hurt that much. I cut working dovetails on a Chinese mill/drill. I think it is more the Indian than the arrow.

 

[Pcmaker]

These lighter-weight knee mills have less rigid Ram/Knuckle setups than larger knee mills. Read the attached document, especially the section on page 5 that discusses my experience with the two types you're asking about. Also watch this video about my PM-935. And I've posted a series of videos illustrating the performance of my PM-935 head which you can view here.

This PDF file is where I got the idea of getting the 833T over the 835 or 935 knee mills. The price of the 833T vs. the 835 is virtually the same.

 

[davidpbest]

The stiffness of a mechanical structure is dependent on many things and weight is only one of them. Comparing the weight of a knee mill to a bench mill isn't a good way to accurately characterize stiffness. Comparing similar designs by weight is fairly valid, but the much longer lever arms of a knee mill require much more material to achieve equal stiffness to a compact bench mill. The bench mill architecture is more efficient of material.

Very well said. I got sucked into the conventional thinking and decided to buy a 1500 pound PM-935 to replace my 600 pound RF-45 in pursuit of increased the rigidity, improving surface finish, and achieving tighter tolerances - specifically in harder materials like 4041 steel and 304 stainless. It was definitely a step backwards In those regards. I have not seen much difference machining aluminum, but I am not pushing material removal rates with aluminum since chip evacuation is the limiting factor with my MQL setup.

 

[kb58]

To get a more balanced view of the 935, please search, as most owners are very happy with them. I am.

 

[davidpbest]

To get a more balanced view of the 935, please search, as most owners are very happy with them. I am.

If you read my document posted above, you will find the following: "The PM-935 is an excellent choice for a baby Bridgeport-style knee mill."

Indeed, I have recommended it to many people depending on their needs. For me, it performs very well within the limitations of it's design. Compared to the full-sized benchtop mills, I like the additional work envelope, having power feed on all three axis, and the quietness of the belt-drive system compared to a full geared head.

But as to the OP's question as to which type of mill is "more ridgid and can handle larger end mills, can hog largest amounts of steel, can handle hardened steel better?", I'll stand by the analysis and direct experience outlined in the posted document.