The Mini Mill platform
The Mini Mill is a great platform for home shop, hobby and light machining. With it’s affordable price, small size and large community support, it has become an extremely popular machine. The community of machinists using the Mini Mill platform has grown to include retired machinists, novices, hobbyists, gunsmiths and light manufacturers. The future of decentralized machining and manufacturing is bright due to the availability of small machines. Many Mini Mills have converted to support CNC automation for rapid prototyping and light manufacturing.
The key to the success of the Mini Mill however, lies in understanding it’s uses, and it’s limitations. In this article, we will address two of the most common issues faced by users of the Mini Mill. Mass and Rigidity.
Mass, Rigidity and understanding the limitations of the Mini Mill
The mass of a milling machine can most easily be thought of as the weight. Some call it the “Beef” factor of the machine. As those of with a Mini Mill already know, it does not have a lot of weight. This is both good and bad. Good in that it is easy to move, cheap to ship and can be mounted on a suitable workbench. Weighing in at approximately 180 lbs, the Mini Mill is quite light. In contrast, larger, industrial sized mills often weigh in around 1500 to 2500 lbs. The mass of a machine affects the the rigidity. Without lots of material in the mills base, column, table, head, etc, it is difficult to have a very rigid machine.
Rigidity is one of the most important drivers of accurate, repeatable and high quality machining. Rigidity is called by some “Stiffness”. In short, this means that the machining platform must have enough mass to resist vibrations and movement caused by the machining process. For example, when a tool bit, such as an end-mill, is cutting into a work piece, there will be vibrations and resistance to the cutting. This resistance and vibration must be dissipated somewhere. We want the vibrations to be minimized and the resistance to our tool bit to be taken on by the work-piece, not the milling platform. In other words, we want the mill to be completely stable, and not flex, or move or vibrate while cutting a work-piece.
Since the Mini Mill is small and lightweight, it naturally does not have much mass. The lack of mass results in less than desirable rigidity. This limits the accuracy and repeatability of the machining that can be done on the Mini Mill. It also limits the speed with which work can be done. Lastly, the lack of mass and rigidity results in reduced quality finishes on parts.
Increasing the Mass and Rigidity of the Mini Mill
Over the years, owners of the Mini Mill have come up with numerous methods of increasing the mass of the machine, reducing vibrations and increasing the rigidity. Most of these modifications revolve around the column of the mill, which, is the primary weak point in the mills design. For those folks with a tilting mill column, the rigidity issue is even more pronounced than those machines with a solid column.
There have been many methods used to increase the rigidity of the Mini Mill. Two of the most common approaches are:
- Column Stiffening: One of the more extreme and irreversible approaches fills the column hollow column with an epoxy/granite mix. Some folks have mixed metal cutoffs, springs, bolts and gravel into the epoxy mix. The purpose is to stiffen the column so that it does not flex along it’s length.
- Column tilt elimination: Some folks have bolted a steel plate to the mill base and the column so that the column will no longer tilt and will have a more solid connection to the base.
These methods both provide a measure of stiffening and additional mass. Folks who have implemented these modifications report significant improvements in the quality of their machined parts.
While it’s difficult to increase the mass of the individual manufactured parts of the mill, such as the cast iron base, table, or column, there are some approaches that allow us to increase the mass of the machine as a whole. When considering modifications to the machine, it is helpful to consider the entire machine as a single unit. Ask yourself how a change to one portion of the machine will affect the operation of the entire platform. You probably noticed, I am referring to the milling machine as a platform. That is intentional. The entire machine must be taken into account in order to maximize it’s utility.
Mini Mill Platform Mass increase and Stiffening
While both approaches above are good, we will be approaching this problem from a slightly more systemic, and platform based view. Our approach takes into account the mill base, column tilt and column flex issues by making use of concepts from both of the common solutions.
The Platform Base
We will be adding mass to the platform base by placing a steel plate under the base and bolting the base to the plate. The plate should be larger than the mill base. The plate should be securely bolted to the bench on which the mill sits. Use as thick of a steel plate as you can afford. This will provide a sturdy platform on which to mount your mill. Ensure that the base plate extends behind the mill column by at least 3″ as we will be using that space to mount a column support. Since the mill base is securely bolted to the new base plate, the mass of the milling platform has been effectively increased to include the mass of the base plate. I used 2 steel plates, each 6″ wide by 16″ long, to create a base that is 12″ wide by 16″ deep. The plate was 1/4″ thick, since that is all I had available.
Now that the mill base has been bolted to the base plate and the base plate bolted to the bench, we begin move vertically up the mill to the column itself. We will be bolting a steel plate to the back of the column. This will require drilling and tapping holes in the back of the column.
I used a 5″ wide by 14″ long steel plate for the column attachment. The plate is 1/2″ thick. Using a Silver and Demming drill bit, I made a 15/16″ hole for the column pivot bolt to extend through. With the 1/2″ thick stiffening plate, there is still enough thread on the column bolt to securely attach and tighten the “Big Nut”.
In the picture the column stiffening plate is mounted to the column. There are 6 holes for mounting the plate to the column, in addition to the Big Nut. Only the top 2 mounting holes are currently shown attached in this picture. As time allows, I will drill, tap and bolt the other 4 attachment points.
There is a cutout for the mill head weight-bearing spring. If your mill has the air spring kit, then you will not need this cut out.
Note the flat washer on the Big Nut. I found that this was necessary in order for the Big Nut to provide more pressure against the column, much like the original Bellevue washer had done.
The small wires attached to the column plate are for the Shars digital DROs. The flexible mounting of the wires to the plate is made by using the EPDM rubber strip and a magnet on each side. This has worked well since it does not bind the wires and allows them to flex naturally.
The column stiffening plate can now be attached to the platform base using a thick piece of 3″ by 3″ angle. This angle is 1/4″ thick and was all I had on hand.
This will securely mount the column plate to the base plate. Effectively creating a single unit out of the mill base and column.
In the picture, the angle has been bolted tot he base plate using 4 bolts. The 4 base plate bolts extend through the bench.
The angle is attached to the column plate by 2 bolts.
Finally, there is 1 center bolt with a jam nut to adjust the Y-Axis tram. The angle is tapped for the tramming bolt. Adjusting the tramming bolt places more or less pressure against the bottom of the column pate which pivots the plate against the Big Nuts column bolt. This pivoting action results in the mill head leaning further forward or backward as required to tram the mill. To adjust the head forward, loosen the tramming bolt. To adjust the head backward simply tighten the tramming bolt.
When adjusting the tramming bolt, the 4 base bolts must be fully tight, while the 2 column bolts may be less tight. As the tramming bolt is adjusted use a Dial Indicator or Dial Test Indicator on the mill spindle and table to determine the alignment of the mill head to the table. Once the mill head is aligned, tighten the 2 column bolt and the tramming bolt jam nut. Place a drop of Locktight red on the bolt exterior (but not on the thread). Always use lock washers on the nut side of the bolts to reduce the chance of loosening over time.