Reinforce X2 mill

[cncuser1]

This is a summary of my attempt to reinforce the Seig X2.

This mill has a number of well known deficiencies, I tried to address one and this is a summary. Hopefully some lessons can be learned and applied by others.
I wanted to stiffen the mill. I wanted to keep the materials types simple. Space and weight were also a concern.

I measured the deflection of the unmodified mill and compared it to the modified assembly to get an idea how successful I was.

I mounted a dial indicator to the base of the mill and measured the deflection of the head. I used a simple luggage scale to measure the force applied. I applied the force to the spindle. Note I pulled as shown but did not apply a second pushing force. The force that counteracted my pulling went though the floor to the base of the machine.

I have two X2 machines and did the same tests on both. The results were similar but this is my results for machine A.
The stock machine had the following results:
The picture below shows pulling the spindle and the force applied and how the deflection was measured. ( don't pay attention to the readout not matching the results, the picture was a test but looked better)
Pulling in the -Y 56 lbs moved .0104"
Pulling in the -Y 40.6 lbs moved .0070"
Pulling in the -Y; 22.8 lbs moved .0045"




I did the same for the +X direction:
Pulling in the +X 60.4 lbs moved .0047"
Pulling in the +X 41.4 lbs moved .0030"
Pulling in the +X 19.8 lbs moved .0021"

So the stock machine is twice as flexible in the -Y compared to the +X direction.
My goal was to increase the stiffness by a factor of 10, that didn't happen.

I designed a frame to sit under the base and behind the column. It is made from 1"x2" x 3/16" wall steel tube. The plates are 1/4" steel. The column is bolted to the steel plate with qty(10) x 10-24 cap screws as well as the original 1" bolt. I TIG welded the frame together using the X2 mill as a welding jig.




So for the modified machine the results are as follows.
Pulling in the -Y 57.1 lbs moved .0011"
Pulling in the -Y 40.6 lbs moved .0006"
Pulling in the -Y; 20.1 lbs moved .0003"

Pulling in the +X 57.5 lbs moved .0028"
Pulling in the +X 38.8 lbs moved .0019"
Pulling in the +X 20.1 lbs moved .0011"


Comparing before and after stiffness, in the -Y direction the machine is about 10 times stiffer. But in the -X direction it is only about twice as stiff. I did an FEA on the model of the frame and was happy with the results but it was my first time using FEA I and had trouble modeling welds. I think I should have done something on the other side of the tubes ( opposite the 1/4 plates) to take up shear forces in the open areas.

Hopefully this will help other people thinking of doing the same thing. I hope that a discussion will generate improvements, I have a second machine waiting for a stiffening frame.

If anyone is inclined to any calculations of the stiffness of the machine using my measurements that would be cool.
Also if anyone can pinpoint the flaw in my frame that allows for the +X lack of stiffness I would be grateful.

(attached is a model of my frame for others to play with)

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[peteeng]

Hi CNC - For the Y Axis you went in rough terms from 1N/um to 8N/um which is quite good. So you got 8x not 10x.. For the X axis its my guess the new frame is poor in torsion. Its an open section, effectively a C channel which has poor torsional stiffness. Very good bending stiffness in its two principle dirns but torsion no good. Close the back with a sheet like the front and it will improve considerably... Regards Peter

I did the X axis and yes the improvement was small from 2.6N/um to 3.7N/um. (1um = 0.001mm) torsion is tough gig to get right.... interesting your curves droop. They usually strain stiffen. May mean you need a little maintenance somewhere...

[cncuser1]

Pete, do I have to do anything more to tie in the improved upright to the base?

Any guesses how this stiffness compares to proper mill?

[peteeng]

Hi CNC - Lots of small mills like the X2 score1-2 N/um and people do reinforce machines as this is a bit low for some jobs as you have found. 10N/um is a good number. Small commercial VMC's are about 100N/um and solid VMCs are 150N/um. I did read about a bespoke mill for hogging titanium at 650N/um cheers Peter

[tkalxx]

I'm curious how you deal with tramming the head in the y-direction to eliminate nod now that the column is bolted in place? Shim between the head stock and the Z-axis saddle? I don't doubt your welding skills, but I find it highly unlikely that everything was welded at a perfect 90 degree angle.

I made a table from square tubing for my PM25 that is quite large and heavy and I've always wanted to add a big piece of U-channel behind the column to tie it into the table.

Nice write up btw

[cyclestart]

Here's a mod I started on my X2 but never completed. The idea was to have bolts or set screws coming in from the sides.
Obviously I've done no testing.

[cncuser1]

tkalxx, I haven't dealt with that problem. I have about .020" to shim between the column and frame ( at the bottom of the column). I think I can deal with shimming and filling with expoxy in the Y direction without loosing too much stiffness in that direction.

I have a similar amount in X direction. But I don't have an elegant method of tilting the column in the X direction without loosing torsional stiffness. I can embiggen the series of holes in the back plate for the cap screws. But then I would be relying on friction to hold the column to the plate. Tight fit or interference fasteners would be better. My only solutions are, after tramming, either install larger fasteners or interpitch tight fit dowel pins but then I risk making the column back too swiss cheese-like.

The torsional stiffness parameter is a hard to deal with.

[cncuser1]

Cycle: I think I understand what you tried to do. To be honest I am skeptical. I'd say the thick plate on the column would have to be attached ( welded) to something just as strong that is bolted to the entire base. To me you are stiffening the column only, not preventing the weak pivot joint from flexing, that was my goal.

I picked up the luggage scale in the dollar store. An afternoon of before and after testing would tell you a lot.

[cyclestart]

To me you are stiffening the column only, not preventing the weak pivot joint from flexing, that was my goal.

I abandoned the plan for a reason There's the germ of a good idea in there somewhere(?) but I won't try to defend the untested
My mill is in an enclosure so it's difficult to take a picture from the back but it's presently like this first drawing.
The upright part is bar stock that's been added to the rear of the column. The original reason for the bar stock was a need to mount ball screw supports.

The new plan is to buy a longer steel plate and an angle plate like this second drawing.

And that means altering the enclosure. If it wasn't for changing my mind I'd have no mind at all.

[peteeng]

Hi - Using a flat plate is not a good idea. It has to be a SHS or RHS as big and thick as possible. A flat plate is poor in torsion and generally these small mills suffer in the column due to poor torsional stiffness. Peter

[cyclestart]

If 1 inch steel plate isn't up to the job the plate becomes stock for my larger mill, nothing lost.
My guess is something else will start deflecting first and the extra mass may do some good.
One thing for certain given the craptastic nature of this mill is I'm not making matters worse.

[peteeng]

Hi CNC - You can use epoxy putty to set the screws in oversize holes they then become very snug bolts. Wax or oil the bolts before you set the connection. They come out easily if needed. Peter