Key Questions to Ask When Ordering Inner Race Milling Machine

If you’re interested in making things (and since you’re reading this, we’re going to assume you are), you’ve almost certainly felt a desire to make metal parts. 3D printers are great, but have a lot of drawbacks: limited material options, lack of precision, and long printing times. If you want metal parts that adhere to even moderately tight tolerances, a milling machine is your only practical option. There is, after all, a very good reason that they’re essential to manufacturing.

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However, it can be difficult to know where to start for the hobbyist who doesn’t have machining experience. What kind of milling machine should you get? Should you buy new or used? What the heck is 3-phase power, and can you get it? These questions, among many others, can be positively overwhelming to the uninitiated. Luckily, we — your friends at Hackaday — are here to help give you some direction. So, if you’re ready to learn, then read on! Already an expert? Leave some tips of your own in the comments!

What Kind of Milling Machine Do You Need?

Before we get into the details of what configuration of milling machine you’ll most likely want to buy, let us first point out that we’re only going to be talking about manual milling machines in this guide. CNC mills are a whole other beast, and they’re going to get a guide all to themselves. Manual and CNC mills share a lot in common (CNC mills are often just converted manual mills), but CNC mills have additional requirements that would over-complicate this article. So, we’re just covering manual machines in this post.

Modern milling machines are divided into two basic types: horizontal and vertical. This determines whether the machine’s spindle axis runs up and down, or side to side. Both types of machine will often have heads, columns, and tables that tilt or swivel, which means both kinds can be used for a lot of the same tasks. However, certain jobs will be easier on one machine than the other.

The difference between the machines, in practice, is more pronounced than just which way they’re oriented. A vertical machine will have the table mounted perpendicular to the spindle’s zero-tilt position, while a horizontal machine will have the spindle mounted parallel to the plane of the table. This introduces a fundamental difference in what kinds of jobs are practical on each type of machine.

A horizontal milling machine’s primary strength is the over arm, which constrains the rotating arbor on two sides. This gives it incredible rigidity, and allows the machinist to take very heavy cuts that would introduce more side load then a vertical machine could handle. The strength is so high that it’s entirely possible (and common) to stack multiple cutters on the arbor in order to cut, for example, a flat table with slots all in a single pass. This makes it well suited to surfacing jobs, cutting grooves and slots, and similar tasks where the part is flat in one axis.

The downside, of course, is that it’s much more cumbersome (and sometimes impossible) to make parts that have cuts in all axes. This is where a vertical milling machine excels: in versatility. You’d be hard pressed to find a job that a vertical mill can’t do — though it’s sometimes a lot more time-consuming than a horizontal mill, depending on the part geometry.

Now that you know the difference, you probably already know which one you want. But, just in case, we’ll say that you almost certainly want a vertical mill. Horizontal mills are great for a small portion of tasks, but those are also tasks that most hobbyists won’t often perform. The versatility of a vertical mill lends itself well to the varied and diverse tasks that hobbyists lean towards, in contrast to the specialty production work horizontal mills are generally used for.

Vertical Mill Features That Matter

Hopefully, you’ve decided that a vertical mill is the best choice for you, otherwise this section isn’t going to be very useful. Assuming you have decided on a vertical mill, you’re probably curious about which features to look for, and are wondering what actually matters. Covering every detail on the subject would take an entire book, but we’re going to go over some of the most important things to consider.

Should you buy brand name?

This is a question that drives a lot of purchasing decisions, and milling machines are no different. So, does it matter? Yes and no. Milling machines have been around for a long time, and there really aren’t any trade secrets when it comes to their construction. It’s well-known what makes a good machine, and what doesn’t. Theoretically then, any manufacturer can follow these design principles and make a high quality machine.

Reality, unfortunately, doesn’t live up to that promise. There are two reasons for this: manufacturing quality and cost. In order to keeps costs down, many manufacturers will cut corners. They might use poor quality materials, under-powered motors, and so on. Even if the manufacturer isn’t purposefully cutting corners, it’s entirely possible that they might just be incapable of high-quality manufacturing. Poorly made lead screws, imprecise machining, and loose tolerances can all leave you with a mill that is frustrating to use and which can’t hold tolerable precision.

Therefore, it’s a good idea to buy a proven machine. Usually, that means going with a respected brand. But, some less expensive brands still produce quality machines (often clones of more expensive models). They may have fewer features, or less robust motors, but could be enough for your needs. Just be sure to read some reviews from people doing real work with them.

Size Matters

Small desktop milling machines can be tempting, but it’s best to avoid them if you’re planning to mill metal. Think about the last time you had to drill a hole in steel, or had to cut off a piece with a hacksaw. It’s difficult work, and takes a lot of force. Your milling machine needs to be able to apply that kind of force without flexing at all — even a little bit of flex with ruin any chances of milling a part with respectable tolerances.

For that reason, the frame of the mill needs to be as heavy and rigid as possible. A small desktop machine will almost certainly be unable to mill anything harder than aluminum, and even then it’ll be imprecise. The wisest choice, if you want even moderate precision, is to buy a mill that’s as large and heavy as you have space for.

DRO and Power Feed

A DRO (digital readout) is a module that can be added to each axis of a milling machine. Some machines come with them, others have them available as upgrade packages. Kits are often available to retrofit mills that didn’t originally have them as an option as well. A DRO gives you a display that tells you how far you’ve moved the table (or quill), which makes pretty much every operation much easier.

Using a DRO isn’t strictly necessary, as all mills have dials for measuring movement. However, reading them can be cumbersome and time-consuming. This is especially true when you consider backlash (slop in the screws), which is easy to compensate for when you have a DRO, since it only tells you how much the table has actually moved, as opposed to how much the handle has moved.

Like a DRO, power feed is something that can be added to each axis, and which many mills come with from the factory. It allows you to toggle a small motor which moves the table for you, so that you don’t have to crank the handle yourself. This can dramatically lower fatigue, but can also give you a better surface finish on your cut as the speed stays consistent throughout the cut.

Spindle Motor Horsepower

Like car enthusiasts, machinists make a big deal about horsepower. And, this isn’t completely unwarranted — the last thing you want is the motor stalling in the middle of a cut. That said, virtually all mills will have some way to gear down the motor to gain torque at the cost of speed. Milling steel requires high torque and low-speed, while aluminum needs the opposite.

So, you can certainly compensate for a motor without a ton of power. That may be a good idea, as motor horsepower makes a huge difference when it comes to cost. That said, you should probably avoid a mill that has any less than 1HP. It’s also difficult to find high horsepower electric motors that aren’t 3-phase.

3-Phase? Huh?

We don’t have the room to get into how multi-phase power works, and what it’s advantages and disadvantages are. But, suffice it to say that you almost certainly don’t have 3-phase power at home. If you’ve got an industrial space, you may have 3-phase power available, but even then you may not. The point is, many industrial-grade tools have 3-phase motors, which cannot be run on standard household single-phase power on their own.

That means that most of you will be limited to mills with single-phase motors. However, that often makes it possible to find 3-phase machinery for significantly cheaper than single-phase machinery. If you find such a machine that strikes your fancy, it is possible to replace the spindle motor with a single-phase unit, or to buy or build a phase converter.

What about a mill/drill machine?

A mill/drill machine is basically a drill press that has had a 2-axis table strapped onto it. They’re significantly cheaper than true milling machines, but that’s for a reason. They really don’t have the rigidity necessary for real milling, and are really only good for precise hole drilling and very light milling. Other than saving a little cash, there is no reason to buy one, as an actual milling machine can certainly drill too.

Buying Your Mill

By this point, you should have a pretty good understanding of what matters in a milling machine. Now it’s time to dive into how you should go about buying one.

New or Used?

You should seriously consider buying used. Quality mills are machines that are designed to stand up to serious abuse for decades, and you can save a lot by buying used. Local industrial auctions and Craigslist are good places to look. Inspecting used machinery is kind of like inspecting a used car: make sure everything is working, that there isn’t excessive wear, and that you can see it’s been taken care of.

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Dirt and grime are okay, that can be cleaned, as long as the important bits are clean, lubricated, and not too worn. Make sure the spindle spins smoothly, has no play (measure run out if possible), and sounds good. Grab the table and jiggle it as hard as you can, and make sure you can’t feel any play. Take a look at the screws to see if they’re clean and unmarred. Make sure the ways (the smooth metal that the table slides on) are clean, lubricated, and don’t have gouges. If all of those things are good, and you don’t notice any other red flags like cracked castings, the machine is probably solid and entirely usable. Age isn’t generally considered a problem as long as it’s been maintained and serviced.

Getting it Home

Milling machines are heavy; they can be anywhere from several hundred pounds on the light side, to several thousand pounds on the hefty side. Getting one back to your home or shop isn’t a trivial task. If you’re transporting it yourself, make sure your truck or trailer can handle the load, and that it’s securely strapped down. You’ll also need a forklift of some kind of both sides of the delivery (to load and unload the machine).

If, like most hobbyists, you don’t have a heavy flatbed truck and your own forklift, you can hire riggers to move the machine for you. Expect to pay at least a few hundred dollars (and sometimes a lot more) to have it moved, even over a short distance. If you know anyone who has this kind of equipment, it’s definitely worth calling in some favors — you’ll need that money for tooling.

Accessories and Tooling to Purchase

Once you’ve got your milling machine home, there are a number of purchases still to make. First and foremost, you’re going to need a decent machinist’s vise. You don’t necessarily need to spend hundreds of dollars on a Kurt vise, but you’ll want something that is well machined and which isn’t going to loosen. A swivel mount is nice to have, but isn’t a necessity.

Next, you’ll need a way to hold your end mills. For this, you’ll need collets or end mill holders that match the arbor on your mill (this information should be readily available in the specs). You’ll probably want to start with two or three of them in various sizes, so that you can use end mills with different shank sizes.

Speaking of end mills, you’ll want to order a bunch. You can buy them in bulk cheaply via eBay, which is a good idea when you first get started. Buy carbide, and a handful in a few different sizes. Make sure the shank sizes match the collets/end mill holders you bought. Generally, 2 flutes are recommended for aluminum and 4 flutes are recommended for steel. If you’re not sure what you’ll be cutting, you can get some of each, or get some 3 flute end mills as a compromise. Expect to pay $10 and up per end mill (yes, tooling gets expensive).

Finally, you may want to consider a coolant system. These come in a few different variations, but flood coolant is usually the easiest to get setup. Despite the name, coolant does a lot more than just cool; it also lubricates and flushes away chips. Coolant systems can fairly easily be added to any milling machine, and many come with them from the factory.

Start Milling!

Now the fun finally starts! You’re going to want to do a lot of experimenting in the beginning. Do research on feeds and speeds (and get a decent calculator for them), what different end mills are good for, and how to perform specific kinds of operations. Don’t get discouraged if you break an end mill, that’s why you bought extras. Practice, practice, practice, and soon you’ll be coming up with excuses to make precision metal parts for all of your projects.

A considerable amount of time and investment goes into finding the right CNC solution to efficiently and effectively machine composites. The last thing manufacturers want is to invest in the wrong machine and be paying for that mistake for years to come. With little room for error, it is important to understand how different materials, machinery characteristics, and machinery options affect machining speed, precision, service, and overall production.

1.  Spindle HP & RPM - Consider high RPM, low torque

Material density plays a key role in determining the best spindle RPM, HP, and torque for an application. To aggressively machine hard materials, such as steel and Inconel® , some traditional machining centers typically are equipped with high torque, low RPM spindles that operate at a maximum of 12,000 RPM. When it comes to composites (such as foam, tooling board, or carbon fiber), a high torque, low RPM spindle is too slow to reach optimal chip load and thus results in inefficient (slower) composite machining. Instead, high RPM (18,000 - 24,000), low torque spindles are more efficient in reducing cycle time, lengthening tool life, and improving overall spindle reliability. For the best of both worlds (composite and nonferrous metal machining), consider a spindle capable of running at 20,000 - 24,000 RPM and higher feed rates for lighter-duty materials but also can operate in the 10,000 - 12,000 range for harder materials that need more torque. 

2.  Dust Containment / Collection - Improve employee safety and machine longevity

Machining composites often creates a large amount of dust and debris that can cause health problems to workers and damage to the machine. Some composite dusts can lead to lung damage if inhaled and some are electrically conductive, so they can damage machine circuits and cause spindle or machine wear at an increased rate. Thus, choosing a CNC solution with sufficient dust collection and properly sealed and covered components is imperative when working with composites. Some additional considerations for handling abrasive material machining include air knife systems, downdraft tables, and/or full enclosures.

5-Axis Programmable Dust Hood. Watch a demo.

Fully Enclosed CNC Router. Reduce harmful debris and airborne particles. Watch a demo.

Air Knife. Remove Dust & Debris with CNC-Controlled Airblasts. Watch up close.

3.  3-Axis vs. 5-Axis - Depends on the part’s geometry

When machining complex 3D composite components, a 5-axis machine is more efficient than a 3-axis machine and provides a greater return on investment over time. With a 5-axis machine, multiple sides of a part can be machined without having to manually reposition material or swap out tooling for angled heads. A 3-axis machine can perform multi-face machining as well but at a much slower rate as it requires an operator to stop the machine and reposition the part after each operation. To achieve 5-sided milling, drilling, tapping, and/or sawing operations without a 5-axis machine or without having to manipulate parts, consider adding a 4th axis and utilize angled heads, known as aggregates.

4.  Rigidity -  For speed, tool life, and machine longevity

Due to the abrasive and unique structure of composites, it is worth investing in a CNC machine with fully reinforced structural integrity and rigidity. A sturdy design reduces vibrations and tool deflection to provide top acceleration/deceleration speeds, long tool life, and low maintenance and repair costs over the lifetime of the machine compared to less rigid, light-duty machinery. From an upfront cost perspective, a machine can be made cheaper by reducing the structure and quality of its components; however, it will fail to have the longevity of a better built machine and ultimately will result in a higher total cost of ownership.

5.  Work Envelope - Make sure it fits, and consider the benefits of larger tables

In addition to the type of composite material being machined, factor in its size, and select a work envelope accordingly. At a minimum, the work envelope needs to be larger than the largest part being machined. However, depending on production goals, buying a machine with twice the work envelope that supports pendulum processing, where materials are safely loaded/unloaded while the machine is still performing cutting operations on another part, will significantly speed up overall production time. Other advantages of large tables include batch processing which allows you to fixture multiple parts at one time and machine continuously without having to unload and reload.

Note - When searching for 5-axis machining solutions, be sure to consider the work envelope while the machine’s spindle is at 90 degrees.

6.  A Second Spindle (option) - Doubles the throughput

A second spindle is valuable when machining small or long composite parts at high volume. Adding a second spindle doubles the machine’s throughput without increasing its footprint. Depending on the machine manufacturer, take it a step further by adding up to 8+ spindles for maximum part production.

7.  Two Tables (option) - Continuous operation

Similar to a pallet changer for a traditional CNC machine center, a second table allows an operator to load composite material onto, or unload finished parts from, one of the tables while still machining on the other table, so production never has to stop. Additionally, on some machines, two independent tables can be electronically “locked” together to process extra-large parts.

Dual Process -  Combine a multi-spindle option with a multi-table option to unlock the ability to perform what is known as Dual Process machining. With this technology, one spindle performs an operation on one of the tables while the second spindle performs a completely separate operation on the other table, essentially turning one machine into two in the footprint of one machine.

8.  Return on Investment - Planning for the long haul

The cost of the CNC machine matters, but more importantly is the return on investment. A cheaper or lesser machine that fails to meet the unique challenges of machining composites and that has to be replaced after only a few years can cost a company more in the long run, not just in direct costs but in lack of part quality, production downtime, program delays, and added frustration. As John Ruskin says, “There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man’s lawful prey.” It is worth the time to work with experts in the field to ensure the machine being purchased is the very best for the specific composites manufacturing application.

9.  Support and Service - Questions to ask before buying

Asking the right questions provides an understanding of the service and support offered by an original equipment manufacturer (OEM) or dealer. Before making a purchase, some key questions to ask include:

• How many dedicated CNC technicians are there (not for other machinery)?
• Is there 24/7 support for emergencies?
• Is there remote login support?
• What is the lead time on replacement parts?
• What is the lead time on replacement spindles specifically (in the event of a crash)?
• At what point do machine parts become obsolete and are no longer made or carried?
• Where is the service department located?
• Where are spare machine parts manufactured and/or stored?

10.  American Made vs. Imports - Some food for thought

Buying an American-made CNC machine means:

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