Mini Mill Overview

There’s an old saying that free advice is seldom worth it. While I am not a milling machine expert by any means, this article is based on my research on mini mills and my experience with the one I bought. I am not endorsing any brand of mill and am not being compensated in any way. The Harbor Freight mill that I bought was not my first choice. So why did I buy it? The one I wanted was back ordered for months. I wanted to get started on my project and I got tired of waiting.

Naturally, a few weeks after I received my mini mill and had started my project, my first choice was available, but only for a week or so and then it was sold out again. It’s obviously a popular model and I know why. When I purchase tools, firearms, or whatever, I look for the best value, that is, the most bang for the buck. In my opinion, this model was the best value based on price, features, and quality for the type of work that I had planned.

The mills that I discuss in this article are referred to as bench top mills, mini bench mills, or just mini mills. These models typically weigh between 100 and 130 pounds and can be a suitable alternative to a full-size mill while being smaller and less expensive. Two things to consider when buying any power tool are how will it be used and how much will it be used. If you spend a lot of money on a machine and don’t use most its capability or it spends 99% of its life sitting idle, it’s a waste of money.

A smaller machine may be able do the same work but just take a little longer. On the other hand, if you have a busy hobby or business that requires parts on a regular basis, then you can justify a larger mill. The main factors in selecting a mill are table size, axis travel, and cutter capacity (power). If you primarily intend to make small parts a few inches in size, table length and width and axis travel aren’t as critical.

Having extra capacity comes in handy and can make setting up a job easier but it comes with a higher price. When buying a mill, you will also need to purchase several hundreds of dollars of tooling. This includes end mills, collets, T-slot clamping sets, machining vise, center drills, 1-2-3 blocks, cutting fluid, etc. I learned that all mini mills are made in China in the same factories. They are sold under many brand names: Grizzly, Little Machine Shop (LMS), Micro-Mark, Harbor Freight, SIEG, Eastwood, and others.

If you want a domestically sourced mill, start with Bridgeport. While the mini mills come from the same factories and look similar externally, internal components like bearings, lead screws, etc., can vary in quality and affect performance and reliability. The Harbor Freight 2 Speed Benchtop Mill/Drill Machine (model 44991) was the mini mill I purchased. It has worked fine and I was able to complete my project without any issues.

I mainly work with aluminum up to oneinch thick and my mill was able to do what I needed. It cost $800 which is at the lower end of the price range. Shipping was $90 and it arrived only five days after being ordered. It weighs 110 pounds and after uncrating it, I was able to pick it up and put it on my bench.

Speed control is on the left and the Zaxis fine control knob on the right. I set the base on a pair of 2x4s to get more clearance for the Xand Yaxis handwheels and bolted it to the bench. It has the tilting column, PWM motor, and two speed gearbox with plastic gears. I’ll describe the pros and cons of these as compared to other designs. Design Basics Most mini mills have a tilting column, with the column attached to the base with a big bolt and nut.

Loosening the nut allows the column to be tilted left or right, up to 45 degrees. This is a drawback in my opinion as it decreases the structural stability of the column. If you tilt the column to do angled milling, you must tram it again when returning to the vertical position (zero degree tilt). It’s better to use an angled fixture to hold the work piece.

There are a few versions with a solid column, such as the Hi Torque Mini Mill models from Little Machine Shop (LittleMachineShop.com, and SIEG in China (SIEGind. com). Tramming a mill means ensuring that the vertical spindle axis is exactly perpendicular to the worktable, particularly along the Xaxis (left-right table movement) and is critical for precise machining. A dial indicator is attached to the spindle with the appropriate fixture and the indicator is swung from one end of the table to the other.

The column is adjusted until there is very little difference in the readings, typically less than 0.002". On a tilting column machine this means loosening the big nut and tapping the column left or right until the column is aligned. Of course, tightening the nut throws off the alignment and you get to do it over and over until it’s good. A solid column is trammed by shimming up one side or the other at the base of column.

Since the Yaxis travel is about half of the Xaxis, tramming the front to back tilt (referred to as nod) is less critical but can be done on the solid column machines. There are two types of DC motors used in mini mills: conventional rotor motors with PWM (pulse width modulation) and brushless motors. I’m not an electrical engineer so I won’t try to explain the difference.

What it means to the user is the PWM motor has less torque versus speed capability and uses a two-speed gear box to get the full range of speed (0-2500 rpm) with adequate

The collet and drill chuck are retained in the spindle by the draw bar. torque. The gearbox has plastic gears and is known for breaking the teeth when making heavy feeds, particularly in steel. The gears are noisy and I wear hearing protection due to the high pitched whine. Again, most mini mills use the PWM motors and gearbox. The brushless motor has enough speed and torque to use a belt drive system which is more durable and much quieter. A brushless motor increases the cost but has definite benefits.

Models from the Little Machine Shop offer a belt drive conversion kit with stepped pulleys to get the two ratios otherwise provided by a two-speed gear box to replace the plastic gears in the PWM motor mills. The larger containers leak at the cap so I use the smaller bottles. I mentioned table size and axis travel earlier. A larger table and more travel allow you to mill larger work pieces. A machining vise is very handy to use and can make setting up the work piece simple and easy.

The vise must be trammed when it’s installed and aligned with the Yaxis. If the vise is removed to make room to clamp apart to the table, it must be retrammed when reinstalled. This can be a pain. Planning the sequence in howa part is milled can reduce this pain. The advantage of a larger mill table is the vise can be installed on end of the table, leaving room to clamp apart on the other end. This maximizes the mill flexibility. Spindle height is another potential limitation.

It’s very frustrating when you can’t use the correct size drill bit or cutter because the work piece is too tall and there’s not enough spindle height. Using a vise raises the part and reduces the available spindle travel as the drill chuck eats up about 3" of spindle height. Most mini mills have R8 taper spindles which is an industry standard, so tooling availability is not a problem. Digital Readouts (DROs) The Xand Yaxis handwheels have dials graduated in thousandths of an inch to measure the table travel.

Unfortunately, the lead screws have backlash. When you turn the handwheel in one direction, you can track how far it moves by using the dial. No problem until you reverse the direction; the handwheel turns but it does not move the table for several thousandths due to the backlash of the leadscrew.

Normally the vise would not be on the table, I did not want to remove it for the photo as I would have to retram it. If you want to reverse the travel to the original dimension, you must keep track of the amount of "free play" it has and add it to the desired travel. The spindle travel is controlled with the drill press type handles for large amounts of travel or with the fine control knob on the front of the mill that has a graduated dial. The fine control has backlash as well.

The backlash makes preci- A low cost version of an Xaxis power feed. sion work more difficult and time consuming and that’s where DROs make life a lot easier and more precise. The DRO on each axis measures the actual travel so the backlash problem is eliminated. If you do a lot of milling, particularly precision work, DROs are worth the investment. They can be added to the mill at any time so the cost can be pushed out to a more convenient date.

Videos/Tutorials There are a lot of mini mill videos on You Tube, some are very good, many are not. The best ones I’ve seen are from Frank Hoose and Quinn Dunki at Blondihacks.com. They both have many excellent videos that cover a lot of details and are very educational. They cover the basics on mini lathes as well as mini mills and show several projects start to finish. Quinn is an excellent teacher, has a great sense of humor, and isn’t afraid to show her mistakes in the videos and how she addresses them.

There are vids on how to install DROs as well. So, what was my first choice? For a reasonably priced and capable mini mill for the type of work I planned to do, my first choice was

the LMS Hi Torque 3990. It has the solid column, brushless motor, and the larger table and axis travel. In my opinion, that is the most bang for the buck. The Micro-Mark 84630 has a brushless motor and tilting column, however, according to the specs on the website, it had less travel than the Harbor Freight mill but was the same price as the Hi Torque mill. So, I decided to save some money with the Harbor Freight mill and use the left over cash for the tooling.

The specs for the three mills are shown in the comparison table. The costs are what were listed at the time when I bought my mill. Obviously, prices and availability are subject to change. As I mentioned earlier, my mill works fine, I was able to complete the intended project with no problems. I took my time in setting it up and learning how to use it, taking light cuts until I felt like I knew what I was doing. A machinist handbook was very helpful.

It explained the difference between "up milling" where the work is fed into the cutter opposite to the cutter’s direction of rotation and "down milling" where the work is fed into the cutter in the same direction as the cutter rotation, and why it’s important. It covered the best ways to clamp parts to the table, when to use two and four flute end mills, etc. Mini mills are very capable and versatile tools and can make all kinds of cool stuff. If anyone learns something from this article, it has served its purpose.

AG