# Ridgidity - 833TV vs. 835 vs. 935



## Pcmaker (Nov 14, 2020)

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.


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## kb58 (Nov 14, 2020)

"In general", go by total weight.


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## davidpbest (Nov 14, 2020)

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*.


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## DavidR8 (Nov 14, 2020)

davidpbest said:


> 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


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## AlanB (Nov 14, 2020)

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.


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## Batmanacw (Nov 14, 2020)

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.


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## Pcmaker (Nov 14, 2020)

davidpbest said:


> 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.


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## davidpbest (Nov 14, 2020)

AlanB said:


> 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.


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## kb58 (Nov 14, 2020)

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


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## davidpbest (Nov 14, 2020)

kb58 said:


> 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.


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## wrmiller (Nov 14, 2020)

If your highest priorities are: "can handle larger end mills, can hog largest amounts of steel, can handle hardened steel better", may I suggest you buy a small bed mill and be done with it. Something in the 4000-5000 lb range might work for you.


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## Batmanacw (Nov 14, 2020)

Pcmaker said:


> 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.
> 
> ...



You are going to think I'm full of crap but your current mill is far more rigid than my old round column mill drill and I have modified lathe insert holder and cut lots of stainless on it without issues.

You seem quite new to machining and I'm guessing you've read a lot of bad advice on here and elsewhere. 

The machine you have now will cut stainless like butter. You just need to avoid rubbing and keep taking a chip. You can buy a very rigid mill and not have any better results. 

I humbly suggest you learn to cut stainless on your current mill before you spend huge money and run into the same issues on a $5000 machine you have now. Cutting stainless is easy. So is hardened steel with the right cutter, feeds, and speeds. Your current mill might just need snugged up a bit.

I am not being critical. Just trying to help.


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## Pcmaker (Nov 15, 2020)

Batmanacw said:


> You are going to think I'm full of crap but your current mill is far more rigid than my old round column mill drill and I have modified lathe insert holder and cut lots of stainless on it without issues.
> 
> You seem quite new to machining and I'm guessing you've read a lot of bad advice on here and elsewhere.
> 
> ...



I need to work on my speeds and feeds. I still have no idea how to hand feed at a certain rate. I've just been feeding slow


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## Batmanacw (Nov 15, 2020)

Pcmaker said:


> I need to work on my speeds and feeds. I still have no idea how to hand feed at a certain rate. I've just been feeding slow



At 0.1 per revolution your table moves 6" a minute at 1 revolution per second.


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## 7milesup (Nov 15, 2020)

I like what Dave Best and Batmanacw had to say.  I think they summed it up nicely.   I have the 833T and other than the gear train noise, I really like it.


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## Pcmaker (Nov 15, 2020)

Does a heavier table help when machining? You can't lock the axis you're using and would it help when climb milling?

The 835 looks like it has a lot more mass than thr 833


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## davidpbest (Nov 15, 2020)

In my experience, table mass is much less important than adjusting the split-nuts to remove backlash and getting the gibs adjusted properly so the table is restrained from lateral movements during climb-cutting.   I routinely climb-cut stainless on my RF-45, and when machining stainless, the key is to keep the cutter chewing - go too slow and the material will work harden quickly and burn up your end mills.  The best way to control feed rates is to add a power feeder.   Having a DRO that displays the feed rate is also very helpful.  I'm not sure I would be very successful machining stainless without a power feeder - consistent feed rates are important with that material.   Same goes for drilling stainless - keep plunging, and here too, power downfeed on the quill is very useful for stainless drilling.


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## Batmanacw (Nov 15, 2020)

davidpbest said:


> In my experience, table mass is much less important than adjusting the split-nuts to remove backlash and getting the gibs adjusted properly so the table is restrained from lateral movements during climb-cutting.   I routinely climb-cut stainless on my RF-45, and when machining stainless, the key is to keep the cutter chewing - go too slow and the material will work harden quickly and burn up your end mills.  The best way to control feed rates is to add a power feeder.   Having a DRO that displays the feed rate is also very helpful.  I'm not sure I would be very successful machining stainless without a power feeder - consistent feed rates are important with that material.   Same goes for drilling stainless - keep plunging, and here too, power downfeed on the quill is very useful for stainless drilling.



I agree.


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## Pcmaker (Nov 15, 2020)

Thanks. 

I'm leaning towards the 835 now because of the price and the ease of tramming the tilt and nod of the head.


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## Batmanacw (Nov 15, 2020)

Pcmaker said:


> Thanks.
> 
> I'm leaning towards the 835 now because of the price and the ease of tramming the tilt and nod of the head.



The Align power feed is sweet! You might want to have them install it. The shims have to be just right. I love my 835 so far. 

I noticed tramming is easier when you get it right and leave no pressure on the worms before you snug it down tight.


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## Pcmaker (Nov 15, 2020)

On my cart, I have the X and Z axis power feeds, and the power draw bar


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## Pcmaker (Nov 15, 2020)

Batmanacw said:


> The Align power feed is sweet! You might want to have them install it. The shims have to be just right. I love my 835 so far.
> 
> I noticed tramming is easier when you get it right and leave no pressure on the worms before you snug it down tight.



Did you put a VFD on your 835? I don't see how anyone can put up with the belt changes to change speeds.


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## Batmanacw (Nov 15, 2020)

Pcmaker said:


> Did you put a VFD on your 835? I don't see how anyone can put up with the belt changes to change speeds.



Not yet. I'm 6'2". It's not hard at all. I will someday.


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## davidpbest (Nov 15, 2020)

Pcmaker said:


> Did you put a VFD on your 835? I don't see how anyone can put up with the belt changes to change speeds.


You’re going to just love the back gear.  Lol. 
Batmanacw is right, the Align feeders are indeed tricky to setup. Details *here* if you decide to DIY*. *


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## Pcmaker (Nov 15, 2020)

davidpbest said:


> You’re going to just love the back gear.  Lol.
> Batmanacw is right, the Align feeders are indeed tricky to setup. Details *here* if you decide to DIY*. *



I'm paying extra for them to install the feeders, I learned that the hard way when I installed my DRO on my PM25MV. 

I paid them to install the DRO on my PM1127


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