Bridgeport or Knee Mill, VMC, Surface Grinder Owners.

[Richard King 2]

I said I was going to stop, but Charles, the bottom is bent too. The tests I showed above are done on the top and bottom, so when you feed and the bottom of the table is bent concave, the top is convex and as the bottom moves over the top of the saddle it cuts the out side end low. If you use a head gasket it can help the issue.. Or if you never tightened the T-Nuts super tight your table is straight.

 

[Charles scozzari]

I said I was going to stop, but Charles, the bottom is bent too. The tests I showed above are done on the top and bottom, so when you feed and the bottom of the table is bent concave, the top is convex and as the bottom moves over the top of the saddle it cuts the out side end low. If you use a head gasket it can help the issue.. Or if you never tightened the T-Nuts super tight your table is straight.

Richard, thanks for your reply. What you say makes sense. So in the beginning of the conversation It was simply regrind the table flat. There was never a mention of straightening the table prior to resurfacing to insure the table was flat, top and BOTTOM. So what I am taking from all this is the table must be flattened FIRST and then resurface the top. Then why resurface. I really do have to agree with Eddyde's last explanation on how the machines most of the Hobby guys have were shown no mercy when in a shop environment before they became our pride and joy and why I feel there is no problem for the majorty of us Hobby guys, including myself. The pros would not even consider using the mills we own. I certainly would not dismantle my machine to have the table resurfaced ( even if bowed ). I would however replace the feed screws before a resurface. Please help me understand. Thanks, Charlie.

 

[gard]

I think Richard did a better job of explaining than I did. The mill will cut a surface that is parallel to the bottom surface of the table where it travels over the saddle. If the bottom is curved due to the table being bent, you will see this same curve in the top of your newly machined cylinder head. An indicator mounted on the quill will show 0-0 across the head (it is following the cutter). So you will not know that it is not flat until you measure it with a straight edge or surface plate. The main reason in my mind to measure the flatness of the top of the table is it provides some insight as to the likely flatness of the bottom. Grinding only the top of the table will not help and may make things worse in some regards. I suspect most grinding shops would grind all the important surfaces top and bottom. I agree this will not be done by most hobbyist but this is the machine rebuilding sub forum. My mill seems to be in excellent condition except for the bend in the table so this may be the first thing I attempt to fix on it.

I would like to hear more details about the process of flattening a table with a ball pein hammer, what size hammer how hard do you wack it and over how big of an area but that may be best for a new forum topic. Straightening a bent table has got to be way more cost effective than machining all the surfaces. Safe travels Richard.

 

[Charles scozzari]

I think Richard did a better job of explaining than I did. The mill will cut a surface that is parallel to the bottom surface of the table where it travels over the saddle. If the bottom is curved due to the table being bent, you will see this same curve in the top of your newly machined cylinder head. An indicator mounted on the quill will show 0-0 across the head (it is following the cutter). So you will not know that it is not flat until you measure it with a straight edge or surface plate. The main reason in my mind to measure the flatness of the top of the table is it provides some insight as to the likely flatness of the bottom. Grinding only the top of the table will not help and may make things worse in some regards. I suspect most grinding shops would grind all the important surfaces top and bottom. I agree this will not be done by most hobbyist but this is the machine rebuilding sub forum. My mill seems to be in excellent condition except for the bend in the table so this may be the first thing I attempt to fix on it.

I would like to hear more details about the process of flattening a table with a ball pein hammer, what size hammer how hard do you wack it and over how big of an area but that may be best for a new forum topic. Straightening a bent table has got to be way more cost effective than machining all the surfaces. Safe travels Richard.

Cutting the underside of the mill table would mean the dovetails also have to be redone then the saddle comes into play, and while your at it lets resurface the knee. At this point it seems my mill is nothing more than scrap. Before this conversation my mill did what I expected of it and the funny thing is that it will continue to do so for me and many others. I now believe scrapping should only be considered on lathe ways, period. If a used mill is as bad you believe, I would use the cost of resurfacing, scrapping and the incredible amount of labor involved and put it towards another mill. You guys with the bench top mills watch out, there coming for you next.

 

[vtcnc]

I am unlocking this thread. It has been viewed over 2k times and has been stickied in the Noteworthy threads section. Like many of you, I find it to be interesting and informative. Unfortunately, there were some problems with the content and we had to lock the thread temporarily to clean up some of the issues.

Please see the following link regarding the OP of this thread: Notice regarding a Sponsor Member. Members should understand that while the sticky thread remains, Richard cannot reply to your posts, so please do not tag him expecting a reply moving forward.

 

[Flynth]

Good this thread is open

I've recently removed a table from my horizontal knee mill (a well used German made heckert fw250) as I wanted to pull the leadscrew nut and I was very surprised to find original scrape marks visible on the central part of the table ways. They are worn off on both sides so I certainly didn't expect them there especially that the leadscrew in mostly worn in the middle.

This thread provided some food for thought. However my mill lived all it's life in a local rubber hose factory and there were no milling vices in sight in their workshop(just drilling vices and general vices). But they made lots if their own work holding, t-nuts, long t-nuts, long bars with elongated holes in the middle etc. When I was buying the machine I searched through that workshop as I was looking (and found) a vertical milling head for the mill.

So I wonder, has anyone else seen a wear pattern like this on a machine where vices were not used with?

It is indeed very interesting. It is the only place on this machine I have found any scrape marks on the ways.

Perhaps the table has been bowed from the factory?

 

[Charles scozzari]

Good this thread is open

I've recently removed a table from my horizontal knee mill (a well used German made heckert fw250) as I wanted to pull the leadscrew nut and I was very surprised to find original scrape marks visible on the central part of the table ways. They are worn off on both sides so I certainly didn't expect them there especially that the leadscrew in mostly worn in the middle.

This thread provided some food for thought. However my mill lived all it's life in a local rubber hose factory and there were no milling vices in sight in their workshop(just drilling vices and general vices). But they made lots if their own work holding, t-nuts, long t-nuts, long bars with elongated holes in the middle etc. When I was buying the machine I searched through that workshop as I was looking (and found) a vertical milling head for the mill.

So I wonder, has anyone else seen a wear pattern like this on a machine where vices were not used with?

It is indeed very interesting. It is the only place on this machine I have found any scrape marks on the ways.

Perhaps the table has been bowed from the factory?

Hi, how wide is the table and how wide is the knee?. Thanks

 

[Flynth]

Hi, how wide is the table and how wide is the knee?. Thanks

The table is 1m long (40in), 25cm wide(10in). The knee top is almost double the table width. Slightly under 50cm (20 in). The knee top is almost square and it is about 40cm tall (16in).

The machine is very similar to this (it is its bigger brother, my machine is 1000x250, this one is 1600x400 the configuration is almost the same.

 

[Dabbler]

So I wonder, has anyone else seen a wear pattern like this on a machine where vices were not used with?

I have seen this wear on almost every well used BP style milling machine I've ever seen.

It comes from the table overhangs and running the lead screw to each extreme, putting more pressure on the outside ends of the saddle dovetails. This produces a crowning of the saddle dovetails, and then the cast iron table 'relaxes' due to gravity to conform to the crowned saddle.

In all my years machining I have never seen any BP style mill suffer from over-tightening vise studs in a way to bow the table, It would have to deform the underside of the T slots enough to move from elastic deformation to plastic deformation - at about 60,000 psi Since the majority of the force is in a conical strain zone, that means putting around 40,000 lbs or more of tension on the studs. I'm guessing over 300 ft-lbs of torque on the nuts, or thereabouts. The maximum recommended torque on a 1/2-13 is somewhere around 85 foot lbs.

 

[Charles scozzari]

Hi, thanks for your reply. That is a beautiful machine. With the table centered, and just a 10" overhang ( roughly ) on either side, I wonder if it's a victim of its own table weight. Thanks.