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MachinistBlog.com

Plans, projects and how-to's for home machinists

X2 Mini-Mill Mod Reduces Column Flex

Mini-Mill Column Modification

There’s an excellent thread over on the Home Model Engine Machinist forum that describes a modification for reducing the X2 mini-mill’s notorious column flex.  It looks like it would be fairly simple to do.  The hardest part might be cutting a 1-inch thick steel plate to the proper size (or paying to have someone else do it).

You’ll also have to drill and tap holes in the mill, but the author demonstrates how he did it with a hand-held electric drill.

I’m wondering if this mod could significantly improve the mini-mill’s ability to remove metal by reducing chatter so you could make faster or deeper cuts.

Mini-Mill Spindle/Column Alignment

This tutorial was written by Earl Hackett, who graciously gave me permission to republish it here.  Earl found that even though his mini-mill was trammed correctly, different length drill bits were not hitting the same spot on the work piece.  He found that the spindle was slightly tipped and tilted instead of being square with the column.  This article will tell you how to test your mill to see if it has the same problem and how to fix it if it does.

One of Earl’s interests is model railroading and he is modeling the Chesapeake and Ohio Railway as it was in 1952.  He uses his mill to make molds for small parts that he vacuum casts in epoxy.  He often uses very small cutters (ex. .007-inch) so his mill needs to be precisely aligned.

Mr. Hackett can often be found on the C and O Rwy, ChesapeakeOhioRailway, and Mini-Mills Yahoo Discussion Groups.

Photo 2: Dial indicator mounted to align the spindle with the table’s X axis.

Photo 1: Dial indicator mounted to align the spindle with the table’s Y axis.

A nagging problem had been present since I first purchased my mini mill.  After marking a hole with a spotting drill, I would switch to a jobbers length bit to drill it.  When the jobbers length bit made contact with the work piece, it would be noticeably to the left of the original mark.  This could only be explained by the spindle not being aligned with the column.  The spindle casting is mounted on the ways casting with 4 cap screws.  There is no alignment guide so the spindle casting can rotate through a small arc.  It was an error of only 10 or 15 mils so most metals were strong enough to pull a jobbers drill bit into alignment with the pilot or spotting hole.  However, when machining soft materials such as waxes or plastics the jobbers drill would not bend to the correct location and the hole would not be in the desired location.  So while the mill was disassembled for installation of a digital read out, I decided to correct the problem.

The first step was to place a straight shaft (McMaster Carr – ½” ceramic coated aluminum, 15″ or longer (1031K14, $9) in a collet and make sure it is mounted on the axis of the spindle.  A tiny piece of dust on the mating surfaces will knock it out of alignment.  I checked this by measuring the runout as far from the collet as possible.  The best I was able to achieve was ±0.002” about 9” from the collet.

image001

Photo 2: Dial indicator mounted to align the spindle with the table’s X axis.

With the shaft aligned the next alignment was with the table’s Y axis.  The 4 mounting bolts were made snug, but not tight.  A dial indicator was placed on the top of the shaft as shown in Photo 1.

I turned the shaft so the indicated runout was at the mid range and the head was run along the column.  On my mill, the indicator showed a movement of about 0.004” over 8” of travel.  A 0.002” shim at the top of the seam where the two castings join brought the deflection to under 0.001”.

Next the indicator was mounted to measure the error parallel to the table’s X axis as shown in Photo 2.

Again the shaft was rotated to the midpoint of the indicated runout and the head moved along the column.  The indicator showed about 0.016” deflection over 8” or 9” of travel.  I simply twisted the head until the deflection was less than 0.002”.  The head was then run to the top of the column until the two upper bolt heads were exposed.  These were tightened and the alignment was checked again to make sure it didn’t change.  The head was then removed so the two other bolts could be tightened.

Photo 3

Photo 3, the alignment error as shown by the original paint.

The variation in the width of the gap between the original paint and the body in Photo 3 shows the error that was present in the as received condition.


Make or Buy a Power Feed for your Mini-Mill

I took a couple of machine shop classes and got spoiled by the X-axis power feeds on the Bridgeport mills.  Ever since then I have thought about adding one to my Harbor Freight Mini-Mill (Sieg X2).

Beside reducing wear and tear on your arm and the boredom of turning and turning and turning a wheel, a power feed can produce a better finish on your parts because it moves the table at a more uniform speed than you probably can.  It will also feed much slower than your own patience may allow, for those times when a very slow feed rate is called for.

A power feed can also improve your productivity.  Many of them have a rapid feed (traverse) button which can zoom the table back to the starting point much faster than you can by turning a wheel.  It can also give you some extra time to study your plans, make calculations and think about your next operation, although I am not recommending you let one run unattended.

  • You can buy power feeds specifically designed for the Sieg X2 and X3 mills.  Littlemachineshop.com sells one for the X2 in a number of different configurations for about $180 and Micro-Mark sells it without a feedscrew for $196.  I have heard from multiple sources that the X2 feed works, but it could use a more powerful motor.  It also does not have a rapid feed capability.
  • LMS and Grizzly sell the same power feed for X3 and Super X3 mills for $230 and $320 respectively.   It does has a rapid feed button and you can learn more about it by downloading the owner’s manual on Grizzly’s site.  I have not heard anything about how well they work.  If you have one, please leave a comment and let us know.
  • Of course, people with machine tools like to build things and some have made their own power feeds.  I am very impressed with the one The drill’s clutch is used to prevent damage if the table is run to the end of its travel and his design includes a quick release that disconnects the power feed if he wants to use the manual feed wheel.  The connector he machined to attach the motor to the feed screw looks easy to make, although if your mill has an older “short” feed screw, you may have to buy a replacement from Littlemachineshop.com for about $21.  Power is supplied by a 12-volt power supply for a portable electric cooler.  The choice of power supply for any kind of homemade feed may require some careful consideration, because almost any kind of suitable motor is probably going to require more voltage and current than the typical easy-to-find “wall wart” can supply.  You also have to consider electrical safety and use proper sized fuses and wiring to avoid a fire caused by overheating.

I have an extra cordless drill and I’m really tempted to see if I can build a power feed as good as Keith’s, but I have decided not to.  I bought my mill because it came with a CNC conversion kit that the previous owner never used.  I held off installing it for a couple of years because I wanted to learn how to use the mill manually first, but I have just started the conversion.

Addendum (2/7/2017) – Fixed broken link, updated prices & added a related link
Addendum (11/17/2010) – I removed some information about Harbor Freight’s power feeds.  They no longer seem to sell them.

Related Links

An Easy Way to Reduce the Mini-Mill’s Z-Axis Backlash

aluminum_tape_20090312

The typical Sieg X2 mini-mill [HF 44991, Grizzly G8689, Micro-Mark & others] has a lot of backlash in the Z-axis. In my case it was .053-inches, which was almost a full turn of the fine-adjust knob.

I heard that putting a shim behind the rack on the column would help by causing it to mesh tighter with the pinion gear in the head. So I decided to give it a try. For shim-material I used adhesive backed aluminum tape that was .0035-inches thick. The tape was easy to apply once the rack was removed. I just stuck a piece on the back of the rack (after cleaning it with brake cleaner to remove any oil), then I burnished it with a piece of metal to make certain there were no wrinkles and trimmed the tape to size and around the screw holes with a utility knife.

I applied only one layer at a time and then reassembled the rack and measured the backlash. After the forth layer of tape the backlash was down to .035 inches, a 34% improvement. But the backlash didn’t improve any more after applying the fifth layer. Plus it became difficult to move the head and very difficult to engage and disengage the quick-adjust wheel. So I peeled that layer back off.

It was probably not necessary, but I applied a final layer of thin clear packing tape over the aluminum since it is generally not a good idea to have dissimilar metals like steel and aluminum in direct contact.

Removing the rack was not hard. I have a short rack held on by just two metric flat head screws, although I have heard that some machines have longer racks with more screws. I recommend that you take care not to lose or damage your screws as you may have trouble finding replacements locally. I managed to drop one of mine in the large hole in the top of the head, just in front of the rack, and it took me a while to fish it out. So you might want to plug that hole with a rag or something. By the way, you can get replacement screws, or almost any other part for your mini-mill, from Littlemachineshop.com.

The screws and rack are easy to remove once the head is dropped down as far as it will go. But to get it low enough you have to disconnect the spring torsion-arm supporting the head. It is not hard, as the spring is not that strong, but there is definitely a danger you could pinch your hand or the bolt could go flying into your face, so take precautions.

I want to make it clear that you have to disconnect the torsion arm from the head, and not the spring itself, which is mounted on the column. I found it easiest to remove the single attachment bolt when the head was positioned so the torsion arm was horizontal. I also put a pile of rags on top of my vise to help protect it and the spindle if the head dropped on it after the spring was removed.

Once the head is down all the way it is simple matter to remove the screws and slide out the rack. When you’re done, just reassemble everything and then lift up the head and turn the quick-adjust wheel until the gears mesh again. Then reconnect the torsion arm.

I don’t know how hard it is to find aluminum tape like I used. I think I bought mine at an auto parts store a long time ago. I know you can buy it online in the US here. There are plenty of other materials you could also use as a shim. Please leave a comment if you try this mod and let us know what you used and what your results were.

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