learned a lesson about RF30 style mill

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I've heard about the problem of holding zero on a round column mill when raising or lowering the head. I know about it and have been able to work around it so far with no problems. This weekend while trying to mill out pocket in a piece of aluminum, I noticed my slot was moving. First cut was ok, second cut was offset about 30 thou. moving back to my starting point, my DRO's were off by over .250"

Long story short, the head was not tight enough and was turning as the table moved.

Round columns have pitfalls, and I found one I hadn't heard about before. The fix was to properly tighten the head and take lighter cuts. To be honest, I was taking about a .100" deep cut in 7075 with a 3/8 end mill. I was just hogging it out, but it was a bit ambitious.

Tighten the column and take realistic depth of cuts.
 
This fixed it for me. Original column bolt pictured above, cut copy paste with the beef bolts below. No more slips.
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I have heard of using fine thread bolts as replacements, these look like the same threads. What makes these work better?
 
I guess it's hard to see in the photo, but the new studs are 5/8-11 up from 1/2" stock. The new studs have a much larger clamping area, and the cup washer tolerates misalignment from the Taiwanese casting. Plus, it's still a direct fit for the nut recesses in the left side of the column clamp. It's about $25 worth of hardware and it fixed my issues. I can use a normal amount of torque, and now my mill head stays put under load.
 
I think some head to column fits are tighter IMG_20190624_121320879.jpgthan others. I just hand tighten mine with a lever I made - no issues.
 
Another thing to be aware of is that when you tighten the two bolts the head moves by a few thou. Mine moves 0.0025" as the bolts are tightened and the head straightens up on the round column, and it doesn't matter if you tighten the upper or lower bolt first. I tighten my bolts to 35 lb/ft of torque to keep things solid and consistent. I have not had the head move even once so I guess it works.
 
Mikey I was hoping you jump in. I tighten mine "as tight as I can" with a 3/8 ratchet, no idea what foot pounds.

I never thought about it moving while you tighten it, it never came up. I always tighten and then re-indicate as needed.

Pontiac, I am pretty sure mine are already 5/8-11. Somewhere I seem to remember reading that it was easier to tighten up, as in would tighten up a bit easier with fine thread bolts. That's why I say I have 5/8 now, I went looking for fine threaded bolts that size. Somehow I made a mistake and the bolts I bought were about an inch short and didn't extend past the other side, I couldn't get a nut on them. I never revisited that.
 
I may have misspoke when I said the stock bolts were 1/2", they may have been 9/16".

My only argument about going fine thread is that these are pinch bolts that are routinely handled by the operator, similar to a Kipp lever. If the threads are coarse, the lever throw is short, and if the threads are fine the throw becomes longer. With improved contact under the bolt heads, even with the coarse thread I get good holding power under modest torque (a humane pull on a 10" breaker bar does it). All I can really say about it at the end of the day is I had problems with the head slipping about the column, and now I don't, and I'm not doing anything else differently.
 
We're better off with coarse threads; they handle heavy loads better than fine threads.

Scott, I wouldn't have thought to check head movement when tightening the head bolts either but I'm in the process of evaluating the True Line 8 and this is one of the things I had to contend with. More on this eventually but the torque on the head bolts does make a difference in how accurately the head aligns, or realigns. I think if you torque the bolts consistently and with adequate torque then you should be good. The question is what is adequate. I found that 35# is adequate; below that and the readings get more inconsistent. Just a FYI.
 
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