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MachinistBlog.com

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

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

Clamping Thin Work on the Mill – Two Methods

eccentric_clamps_being_used

I recently needed to mill down the thickness of  a 7-inch long piece of steel from .25-inches to .20-inches.  As the metal became thinner I started getting chatter and a bad finish on the ends because they were not being supported enough by my 3.5-inch wide precision screwless vise and 6-inch parallels.

The obvious solution was to clamp the metal directly onto the milling table so its entire length would be fully supported.  But that also presented a problem, because even though I had some extra length to work with, it was not enough to keep my clamps out of the way of my fly-cutter, which I wanted to use to get a nice final finish.

So, I sought advice from the Mini-Mills Yahoo Group about how to clamp a thin piece of metal to the table without any clamps protruding above it.  Many suggested I attach my workpiece to a larger wood or metal base plate using double-sided tape, glue or solder, and then clamp the base plate to the table.

That’s what I did, using double-sided carpet tape to attach my work to a nice piece of flat dense particle board.  Then, just to be safe, I also used a hot-glue gun to apply a fillet of glue all the way around it.

It worked great, until it was time to get the work piece off the board.  It was really hard to pry it off the tape. I ended up getting cut and I am lucky I did not bend the metal in the process.  I also had to scrub the tape residue off with acetone.  I am not giving up on using tape again in the future, but now I am shopping for some that does not hold quite so well.

I only used the hot glue gun when I did the flip side and it worked fine.  The glue was also hard to remove and the next time I may try softening it a little with a heat gun.

Someone also suggested using eccentric clamps to hold the work piece.  Unfortunately, I could not find out much about these clamps until I accidentally came across this article (PDF) by Rick Sparber that tells how you can make a set easily and inexpensively.

Rick shows a simple way to turn down button-headed hex screws on a lathe so the heads are offset from the threads and will operate like a cam when they are tightened.  He then tells you how make some cup washers out of a drill rod with a sharp ridge around the outside.

To use them, you take a couple of steel plates that have holes threaded for the eccentric screws and clamp the plates on your milling table so your work piece is held between them.  You then tighten the eccentric screws, which forces the cup washers against your work, causing the ridges on the cup washers to dig in and hold your work even tighter.

The Mini-Mill Head Drop Problem

Everything was fine until the head dropped down and dug in

A problem with the Sieg X2 mini-mill [HF 44991, Grizzly G8689, Micro-Mark & others] is that the head can suddenly drop down while you’re milling.  Everything will seem fine until the head suddenly drops a bit, which causes the end mill to try to cut too much metal at once, causing the table to start shaking as much as the play in the gibs allows.  When you finally hit the power switch and raise up the head you find that there is a big gouge in your workpiece.

This article discusses why it happens and how to prevent it.  Continue reading The Mini-Mill Head Drop Problem