Reducing Mini-Mill Column Flex (and Column Y-Axis Alignment)

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Nov 26, 2008
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After I finished the spindle-column alignment on my Harbor Freight mini-mill (, I still didn't get a good tram in the Y-axis. It measured about .004 (all my measurements are in inches) out of alignment, as if the column was tilted to the front just a tad. I also had a significant flex in the column. With a firm push or pull on the motor or top of the column in the Y-direction, I could get almost +/- .010 (2.5 tads) of up and down movement of the spindle relative to the table. I wanted to fix, or at least reduce, the flex, and planned on correcting the column-table alignment while I was at it.

From what I had read about the column flex, the consensus was that the major cause was that big Belleville washer under the big nut at the back of the column (with maybe some compression of the column wall under the washer thrown in), and by replacing the washer with a flat plate you could reduce the flex. Some folks added angle brackets and braces to the plate to make the column even more rigid. I decided on a simple flat steel plate bolted to the back of the column to replace the washer and act as a reinforcement up the column axis.


I made a column reinforcing plate (hereafter called a column plate) out of 3/4" thick, 4" wide steel, and drilled holes for some 3/8" bolts plus one 15/16" hole for the big pivot shaft. I used the center of the plate as a reference to locate the bolt holes. Most of you probably know I shouldn't have. The pivot shaft is not on the centerline of the column. It is centered on the ways, but the ways are shifted over about 1/4" on the gib side of the column. I never noticed it before. Must be a lesson in there somewhere. Anyway, my hole locations were far enough in from the sides of the plate to still work on the column, it's just a little off-center.


I match-drilled holes for the bolts into the back of the column, tapped them for 3/8" threads, and bolted the plate to the column. The flex was reduced to less than half of what it was, now about +/- .004, but still more than I wanted, and I hadn't fixed the Y-axis tram yet either.


I drilled four holes in a piece of 1/2" thick, 1" wide steel, one at each end for 3/8" bolts, and two near the center for 7/16" bolts. The end holes were far enough apart to clear the edges of the column plate. This would be a pressure plate to push on the bottom of the column plate.


I drilled and tapped holes into the base of the mill itself to match the 3/8" outside holes in the pressure plate, and drilled and tapped through the column plate only for the 7/16" holes, not into be base.


I bolted the pressure plate to the base of the mill with the outside bolts and tightened them until the column was pulled in enough to tilt the column back and eliminate the .004 Y-axis tram error. I then inserted 7/16" bolts through the center holes and screwed them through the column plate until they were snug against the mill base. With the end bolts pulling on the column, and the middle bolts pushing on the column, the bottom of the column was locked in place and the flex was reduced to about +/- .0015.



Some final notes:

With the bolts in place, I can no longer rotate the column side-to-side to mill at an angle. I have never used that feature anyway, so it was not a concern to me. Also, my mill was trammed in the X-axis to my satisfaction before I drilled the holes at the bottom of the column plate, but I left plenty of clearance around the 7/16" bolts in the pressure plate to allow some slight angle adjustment for tramming, if necessary.

My mini-mill Y-axis alignment required the bottom of the column to be pulled in toward the base, so the 3/8" outside bolts were tightened first, as evenly as possible to avoid any twist. Then the inside 7/16" bolts were screwed in just tight enough to eliminate any flex. If the required correction was the other way, the center bolts would need to be screwed in first, pushing the bottom of the column away from the base, and then the outside bolts tightened to eliminate any flex.

If I were to do this mod over, I would use a thinner column plate, say 5/8" or 1/2" so the big nut would be sure to grab all of the pivot shaft threads. The 3/4" plate I used just barely allows that. Also, my plate was 12" long. If I were planning to fit my mill with an air spring kit like the one sold by Little Machine Shop, I would shorten to column plate to maybe 10" long so it wouldn't be in the way of the hole in the column required by the kit.

The remaining minor flex in the column assembly is probably due to bending of the pivot shaft and/or the pivot plate. I couldn't think of any practical way to eliminate it entirely.

Drilling into your mill might void any existing warranties. I never had an extended warranty on my mill, so it wasn't a concern.

I think that's about it, thanks for reading.

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'With the bolts in place, I can no longer rotate the column side-to-side to mill at an angle. '

Does anyone use this feature?
Personally I would prefer it wasn't there as it just allows the mill to slip out of adjustment easier.

I always like reading peoples mods as I'm very much a follower rather than a leader in machine mods.
interesting mod
Tmuir .

I have actually used the angle tilting function IIRC had to put holes in some 6 foot long pieces at a 37 degree angle . but for model work it seldom if ever gets used. IMHO it is usually easier to tilt the work piece than the mill head. although tilting the head to the correct angle then tramming it back to square was a required exercise in trade school.
IMHO tilting the mill head rather than the column would be more useful.
Great thread Rudy. Really good.

While I suspect (hope) that someday I can move up to a better lathe, I think my mill will be around much longer. I have the exact same mini-mill so your mods have been very interesting too.

How did you drill/tap into the base of the mill? Drill press? Take the table off?

zeeprogrammer said:
... How did you drill/tap into the base of the mill? Drill press? Take the table off? ...

zee, actually I left everything in place and used my trusty old electric drill. I used a scrap piece of steel that I milled down so it would fit between the plate and the base (from an earlier abandoned attempt to put a wedge between the plate and the base; it's a long story...) for a drill and tap guide. Two sizes of guide holes, first were sized to fit the tap drill (size "Q" as I recall) and I drilled the holes in the base. (Slow going in the cast iron, but not too bad.) Then I removed the scrap, enlarged the guide holes for a close fit on the 3/8" tap, replaced the scrap, and tapped the holes in the base. I clamped the scrap in place so it wouldn't slip. Used the same method to do the holes in the plate.



I'm not too good at drilling straight holes or tapping freehand, so I use guide holes drilled in the drill press whenever I can. Also, the holes in the base were located so they were just inside the side walls of the base. I didn't want to have to tap into blind holes.

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Thanks Rudy. And while you didn't know it at the gave me a clue as to what to expect when I work on my Spindex. Drilling cast iron will take some time.

You are quite welcome zee. I should have said "with MY drill bit it was slow going." YMMV. Always glad when a post is helpful to someone, and always hope it's not as a bad example. ;D

Very nice work! and great photos. I have one question though, how do you keep your shop sooo clean? Seriously, my shop is a mess and I clean it regularly.

Thank you Bill. The shop isn't normally that neat, just happened to be between projects and cleaned up that area to work on the mill and lathe. Usually tools and paper towels and chips and scrap and other assorted odds and ends scattered about.

Model Engineers' Workshop (monthly British publication) had articles in issues 150, 151 & 152 (May, June & July 2009 respectively) about modifications to various X1 mills.

Relating to rigidity - or at least, vibration damping - the author, David White, filled the base & column with "granite epoxy." It was obtained as a 2-part liquid epoxy/hardener & he added the granite chips. Near as I can determine this material would be similar to the material that is used for kitchen countertops; however, it is in the raw state and not obtained by breaking up old countertops.

I have not tried this myself. Merely passing the info along fwiw. Above author got some pointers from

Disclaimer: I am not affiliated with any sites/commercial entities mentioned in any references in this post. Merely passing along the info.
Well, I am new owner of a Sieg X2.
I found this wonderful thread (thank you rudydubya) and the other one about spindle column alignment and I want to implement the rudydubya's ideas.

I want to show you the results of my implementation.

Sorry for the quality of pics (and the resizing didn't improve them) and for the quality of my language but english is not my mothertongue. Again, I apologize but I will use metric system. I will try to convert to imperial but I am not always sure about conversions.

Let's start.

In this pic you can see the standard setup of X2. Belleville washer. Just base and column, no milling head.


Upon the column I put a clamp and on that clamp I put a weight (a simple toolbox, weighing about 3 kg - about 7.5 pounds) to create a torque against the big bolt of the minimill.

I did put a DTI on the cross table and zeroed it against the column.


As I add the toolbox to the clamp, the DTI indicates 5/100 mm (2 thou).




Then, let's proceed with the experiments. Instead of the Belleville washer, I put a stainless steel plate. Dimensions are 200x100x10 mm, i.e. about 8''x 4'' x 25/64.
(I used stainless beacause I had no choice...usually I HATE stainless and drilling the big hole was a kind of nightmare. Luckily I found a workshop who did it).

No pressure plate at this stage, so the stainless plate just distributes the load of the big nut.


I check the zero on DTI, put the load on the clamp...and the DTI indicates again 5/100 mm. (2 thou). I was quite confused.
What is happening? The plate has no effect at all.

Well, then I proceeded by adding pressure plate (stainless steel too).



Zeroed the DTI, added the weight...again DTI indicates 5/100 mm (2 thou) flex. Well, now I am scratching my head.


The plate seems have no effect. No improvement on the column flex.

I kindly ask your opinion about this. Where is/are my mistake(s)?

Congratulations on the mill Emanuele. I have the same type.

I can't answer your question but I'm sure you'll get some people chiming in.

2 thou with a weight that far from the column doesn't seem bad to me.
It wasn't clear to me from your post...when you removed the weight...did the DI return to 0?
There's still quite a bit of column left above the plate...could the flex be in that?
zeeprogrammer said:
2 thou with a weight that far from the column doesn't seem bad to me.

Of course, I would like to compare my results with observations from other guys owning that kind of mill.
Maybe 2 thou is just great, so I can be happy :)

zeeprogrammer said:
It wasn't clear to me from your post...when you removed the weight...did the DI return to 0?
There's still quite a bit of column left above the plate...could the flex be in that?

Yes, the DI returns to 0.

About the flex in the column: I read the document from John Pitkin (Column Flexing in the Minimill - revision 1.pdf, I can post it if there are no problem in posting PDFs); his tests say that the column doesn't bend. And that the flexing is in the base to column junction.

I trusted his tests but I didn't verify his results.

This is to say that the column shoud not bend. But of course I am not sure about that.

You did a much more extensive test than I did. I approached it a little different. In looking at the design of the mill the weak link looked like the way the column was mounted. When you get down to it the entire column is supported on about a 2" base bolted down which is just not wide enough to do the job properly. My opinion is that most of the flex occurs here and not in the vertical column. Along that same thought; the mount has its bolts to the front of the machine. I would expect more flex of the unbolted back side, so putting weight to the front of the column is measuring the weak side of an already weak mount. Since drilling and milling puts a force to the rear of the machine and not the front I think I would hang the tool box off the rear and see what you get. Of course that is just my non-engineer, non-professional opinion which comes without a warranty or certainty of correctness. ;D
I've read somewhere that some folks have filled the centre of the column with an epoxy-granite mix. This apparently helps to dampen vibration at the same time stiffening the column.

This is a mod I plan to make on my X2 in the coming months along with the same replacing-the-big-washer that you have already done.

I've also seen that others have attached a "I" beam to the back of the column, but that is a bit overkill in my view.