A Thought On Round Column Mills

[Maddogmech1]

So I came up with an idea (not sure if it's original or not) about my round column RF40 mill. I'm having the age old problem of losing center when I lift/lower the head, and was searching for a simple solution. So my thought is, what if I make to clamp plates, say that clamp to the column, one on top, one on bottom, that extend out to the rear, with a 1" rod between them, then make a slide block or linear bearing that attaches to the back of the head where it clamps onto the column that slides on that bar? In theory it would keep center as the head moves up and down, as long as everything was precise and tight.
I rarely need to move the head side to side, so I doubt I'd ever need to take the thing off, but it wouldn't be that hard to if needed. Thoughts?


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[TomS]

Get a copy of the current Home Shop Machinist magazine. It has an article on how to do exactly what you are describing.

Tom S

 

[RJSakowski]

So I came up with an idea (not sure if it's original or not) about my round column RF40 mill. I'm having the age old problem of losing center when I lift/lower the head, and was searching for a simple solution. So my thought is, what if I make to clamp plates, say that clamp to the column, one on top, one on bottom, that extend out to the rear, with a 1" rod between them, then make a slide block or linear bearing that attaches to the back of the head where it clamps onto the column that slides on that bar? In theory it would keep center as the head moves up and down, as long as everything was precise and tight.
I rarely need to move the head side to side, so I doubt I'd ever need to take the thing off, but it wouldn't be that hard to if needed. Thoughts?


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Sounds like a good idea. There is no practical reason for rotating the head. Doing so just reduces the inward Y axis traverse. One concern would be flexing of the bar. One inch sounds substantial but there will be movement which will be magnified unless the the guide bar is positioned a distance equal or greater than the spindle distance from the column.

Another hitch comes about because of the tightening mechanism for the head. As it is being tightened, friction prevents it from settling in to a true position.

For the former a double set of bushings/bearings separated by as large a distance as practical will make your bar more rigid. For the latter, I haven't got a good solution. I my radial arm uses a similar mechanism with a pin riding in a Vee groove to maintain alignment. When tightening, I gently rock the head from side to side to get it properly seated. It sometimes works and sometimes not. But I am not positioning to thousandths either.

I had the same problem as you with my old mill/drill. It is very frustrating to go through a bunch of operations, only your last tool is too short or too long to complete the project.

I have put a piece with a reference hole on the table and located it relative to the spindle with a dowel pin. After the table was adjusted for height, I tightened the head. It usually gets you within a few thousandths. Carried further, make the reference hole 1/2 or 3/4" in diameter and use a collet mounted pin. It should be better able to resist the rotation which occurs when tightening the head.

Anyway, I will be interested in seeing a report on your success when you have completed the mod. In recommending a mill/drill at work, I specified a square column to to avert this problem. Your solution would be a boon to many fellow machinists.

 

[Maddogmech1]

So after reading these comments, I'm thinking it will be a good idea to up the size to maybe 1-1/2" bar, or using dual 1" bars. I've noticed on my mill (and probably all column machines) that after moving the head, and retightening, it will still be out on the X. I assume this is from not torquing the bolts the same every time, or maybe the top one first one time, the bottom one first the next. At any rate, this upgrade would hopefully cut down on that as well by holding the head straighter to the column. Now the only question is: clamp plates aluminum or should I do the extra work of making them in steel? Also would linear bearings be better than a slip bushing setup?


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[RJSakowski]

The y axis changes very little if you a close to perpendicular to the x axis. The x axis is the on you need to worry about. On the heads that I have seen,the collar is split and the clamping lever draws them together to grip the column. Depending upon the localized friction at any position, the near side may move further than the far side or vice versa. The outrigger bar will help because now the bending moment placed on the bar will provide a reactive force to the unbalanced frictional force. One thought would be to lubricate your column. Hopefully, that would reduce the a sliding friction to the point where your outrigger dominates. I don't know how that would affect the clamp force for the head though.
Regarding the slide bearings. There is a necessary clearance for bushings which will translate into play of the head position. A good set of linear bearings will reduce that play. One way to reduce this play would to put two bars instead of one. Configure them about six or eight inches apart and their center to center plane parallel to the x axis. Now instead of working with an angular displacement over the diameter of the bar it will be over the separation distance. Basically, the design strategy of a dual beam height gage. I would mount all the bearings/bushings on a single plate.
I would try to use steel if possible. If you are using aluminum, beef them up well. One advantage of steel , aside from its superior mechanical properties, is that you can weld up more complex structures instead of carving them out of massive blocks of aluminum.

 

[kwoodhands]

So I came up with an idea (not sure if it's original or not) about my round column RF40 mill. I'm having the age old problem of losing center when I lift/lower the head, and was searching for a simple solution. So my thought is, what if I make to clamp plates, say that clamp to the column, one on top, one on bottom, that extend out to the rear, with a 1" rod between them, then make a slide block or linear bearing that attaches to the back of the head where it clamps onto the column that slides on that bar? In theory it would keep center as the head moves up and down, as long as everything was precise and tight.
I rarely need to move the head side to side, so I doubt I'd ever need to take the thing off, but it wouldn't be that hard to if needed. Thoughts?


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I don't really have a problem with the work off center when the head height is adjusted. I just move the X & Y axis back to center with a wiggler. Rarely takes more than a minute to re-align the work.
mike

 

[Dave Paine]

A YouTube channel "Cuppa Joe" has a number of videos modifying his RF30 mill. One was installing a vertical guide to keep the head aligned when moved vertically.

The first video, you can get to the second part from this link.