Bridgeport CNC Conversion – Is It Worth It?

Converting your manual mill (Bridgeport) to CNC is a cheaper way to get into running production runs. While manual mills can still be useful in shops today for certain operations on one-off part, they are virtually obsolete for production runs if you want to make money. However, if you are starting up a small CNC shop of your own or are a hobbyist on a budget, a CNC conversion kit may be your answer. However, lets take a look at what it will take to convert, how much it will cost, and how it compares to a a VMC (vertical milling center).

Conversion?

You can buy a CNC conversion kit and piece it together yourself, or you can buy a mill that has already been converted. Of course buying one that’s all set up and ready to go would be ideal, but you may not have that much cash to spend right away. This is why many machinists end up buying the parts as funds allow.

If you want to convert your Bridgeport (or similar) manual mill to CNC, I suggest doing a full 3-axis conversion. It will be more expensive, but if you are going to do the swap, you might as well go all the way. Being able to program for Z-axis moves in addition to the X and Y-axis will allow for shorter machining times.

Bridgeport CNC Conversion
Bridgeport CNC Conversion

So how much will it cost? A knee-mill (bridgeport) converted to CNC will cost anywhere from 10K-25K. The newer and nicer set-up the more expensive it will be. Shopping around and waiting for deals may help lower that cost, but you should still expect to end up in this margin for a ready-to-go mill.

Buying a VMC

While buying a vertical milling center will be more expensive in most cases, they are much more capable machines. Faster rapid moves, a lot sturdier, more horsepower, coolant, automatic tool changes, and the list can go on. It really depends on how much you want to spend and how big of a machine you want. $20k can get you a used CNC mill, but it will probably be 15+ years old and will need a lot of maintenance sooner rather than later. For another 10-20 grand you can get a newer and nicer machine that will actually last a while depending on how you use it and what kind of deal you get.

What Do I Recommend?

Without a doubt, a VMC is the better choice IF, and that’s a big if, you have enough dough. Of course, many of us that are middle or lower-class citizens cannot just throw $30,000+ at a machine at any given time. This would be a long-term goal, but the capabilities are are vastly greater than a converted knee-mill.

I Should Buy/Build A Converted Manual Mill If I:

  • Am on a budget
  • Have time to convert it as funds/time allow
  • Am just a home hobbyist
  • Want to DIY to save money

I Should Buy A VMC If I:

  • Want a faster, sturdier, more powerful and capable machine
  • Want to make a business out of it
  • Have a bigger budget
  • Have patience to save up for one (If funds don’t currently allow it)

There’s advantages to buying each kind of machine. While I haven’t said which one is better for YOU specifically, I have tried to lay out reasons why you would or wouldn’t want to go a certain route. If you have any questions, feel free to post a comment.

Machinist Toolbox – What To Buy?

Are you new to the Machining industry or considering a career in this field of manufacturing? A toolbox is one of the first things that you should buy. It’s where you will keep most if not all of your tools, personal inspection equipment, and books/manuals such as the Machinery’s handbook.

Kennedy tool boxes are well known in the Machining industry because they are well built and have been around for 100 years now. If you’re looking for a tool box that is made in the U.S.A., a Kennedy is the way to go for a new Machinist, whether it’s a top box, or a bottom as well. These boxes are made specifically for Machinists, so the length, width, and depth of each drawer is designed for specific tools, allowing you to easily organize your set-up; and we all know that time is money when running CNC machines.

Kennedy Top Box
Kennedy Top Box

If you’re on a budget, a standard tool box can work just about as well. It may not be built to hold your machining tools as well, but you can find good deals on sturdy boxes that will save you money for buying even more tools!

There are also some Machinist knock-off tool boxes are on Amazon for a fraction of the cost, such as this 11 Drawer Roller Box. Due to its low price, it won’t have the same quality as say a Kennedy, but some of the same features are there with a similar layout. Like most anything these days, you generally get what you pay for. For something such as a tool box, if you’re a rookie machinist, then a cheap box may work just fine.

What To Look For?

If you are just starting your CNC Machinist classes at the local Tech school and had to buy a tool box without doing any research then it probably won’t make much difference at first. After going to school or working for some time, though you’ll realize what you want in a tool box, and even that may change after being on the job for years.

Size is the biggest factor when purchasing a new tool box. Obviously if space is unlimited and you don’t plan on transporting it often or ever, then the bigger the better. If you buy a big box right away then you won’t have to upgrade right after buying a handful of new tools. On the flip side, if you don’t have many tools then you’ll be pushing around a lot of dead weight if you move around the shop on a weekly or daily basis.

If you have a chance to look at and test out tool boxes in person, I would suggest looking at and opening the drawers to see how smooth they are. Ball-bearing drawers slide out nicely compared to the conventional slides. This feature isn’t necessarily a deal breaker, but it’s always nice to have things that perform better.

Castors are the wheels used on tool boxes and are another feature that should be considered when doing research. Putting a lot of heavy tools in your box will put more strain on the castors, so it’s good to have big and sturdy castors.

If you still can’t figure out which one to buy, there’s nothing wrong with having one, two, or three boxes to choose from.

A quick tip on how to prevent tools from getting nicked and scratched up after constant opening and closing of drawers; tape a sheet of felt or lay a thin rubber mat down in each drawer for padding and to help prevent everything from sliding around.

Tool Runout vs. Tool Deflection – What Are The Differences?

There’s a reason tight tolerance parts cost more time. Yes, I use the word “Cost” because time is money, and it can take a considerable more time to set-up and run a part if the tolerances are closed up. Tool run-out and tool deflection can both be an issue when trying to hold a close tolerance. However, they are not the same thing.

Run-out

Tool run-out is how far off of the rotating axis the tool is. While in the machine, you want to check just above the bottom of the tool, as that will give you the most accurate reading and is almost always worse than the top of the tool where it goes in the holder. To check it, put an indicator on a vice and touch the tool off of it. Zero it out and then rotate the tool (usually in a counter-clockwise rotation so it doesn’t catch the cutting edge). If there’s run-out, one side of the tool will give you a different reading on the indicator.

Before we go any further, let me explain what run-out actually does when machining a part with a tool that has it. If an end mill or a drill has excessive run-out, the side (or flute) that is bigger will do more cutting. If you’re milling out a hole with an end mill, it will cause the hole to go over-size if all of your program and offset numbers are right. A drill can also go oversize, as well as drill an oblong hole.

Now lets take a look at what causes a tool to have run-out. A brand new and unused tool can have run-out. Why? Not all tools are made the same, and if you buy cheap tooling, there’s a better chance that it was made with the same precision as a higher quality tool.

Not only can the tool be at fault, but a defective tool holder can cause run-out as well. On the other hand, you may check the tool while in the spindle and see that their is run-out, but certain tools (such as a reamer or drill) will allow you to slightly move it without removing it. This can often get rid of many run-out problems with longer tools.

Tool Deflection

Tool deflection should not be confused with run-out. It is a common term used when side milling with an end mill, and it causes a taper in the part feature that is being milled.

Take this as an example; you’re milling the outside profile of a part (2x2x2.25″) that has blueprint dimensions of 1.950″ wide, long, and 2.000″ tall. Using a 3/4″ end mill with greater than 2 inches of flute length, you mill around the part once. The top of the part is 1.951 all the way around, but the bottom is upwards of 1.954″. This is because the end mill is too long and was ‘deflecting’ because it couldn’t handle the pressure of removing all of that material.

That’s the most simplistic scenario of tool deflection. So, how do I combat this? Great question, and there’s quite a few ways to make sure your part is square, perpendicular, and/or parallel.

First of all, how deep of a cut are you taking? If you’re drilling a 1/4″ hole that’s .375″ thru, you don’t need a jobber drill with 4″ of flute length. When milling, a bigger diameter tool will be stronger and resist deflection better. If you’re milling a feature that is .400″ deep, using an end mill with 1/2″ flute length will achieve the best results. One last thing on tool length is that you should have the tool as short as possible in the holder. Do not clamp on the flutes, but if you choke up on the tool, this will also help prevent deflection.

Feeds and speeds. I’ve said it before, and I’ll continue to say it. Having the right RPM and feedrate for your tools is one of the major keys to success in the Machining industry. Even if the surface foot is close, having a high feedrate will naturally produce more tool pressure and in turn cause deflection.

Slowing down the feedrate can help, but in the end, you may have to take multiple passes to make a feature square, especially if you’re profiling out a part with an end mill.

 

2 vs. 3 Axis Machining – CNC Profiling

Traditional 2-axis milling on a CNC machine is still very common, you adding another axis to the equation greatly expands your possibilities. You can make parts that you might not have been able to before when just using 2-axis programming, and it can possibly reduce cycle times.

First, we must establish the difference between two and three axis machining. Most CNC mills these days should be able to accept and perform programs with 3-axis machining. Two and three axis machines both have an X, Y, and Z axis, but using that third axis for milling profiles can allow you to profile the surface of a part.

In 2-axis milling, you can move in the X and Y-axis at the same time if you’re milling the outside or inside profile of a part. If you’re using the third axis, you can make X and Z-axis moves while milling a profile, such as a waving contour. You can also move in the Y and Z-axis if you simply change the plane that you program in.2 vs. 3 Axis Machining - CNC Profiling

If you’re hand-writing the program, G17, G18, and G19 are the CNC commands for selecting which plane you want to machine on.

G17 is the XY plane.

G18 is the XZ plane.

G19 is the YZ plane.

Other than that, programming is virtually the same as any other G-code program. If you want to make a positive Z and negative Y move, an example would be:

G90 G20 G19 (To set the YZ plane and absolute)

 

G1 Y-.5 Z2.23

If you want to go back to the traditional XY plane, a line with G17 will be needed.

If you have rendering software or a program that simulates your program, I strongly recommend using it on a new program, especially if this full 3-axis machining is new to you. Good luck, and go experiment! The best is experience is with machine time and trying new things.

What Is Lean Manufacturing?

Any company in the machining industry has to incorporate Lean Manufacturing in their business and process plan to survive these days. To put it in simple terms, lean manufacturing is the production practice of being efficient by eliminating any ‘waste’ in the process plan. Though they may not call it this, all companies strive to be lean because it makes their customers happy, and ultimately, more money.

Although there was seven original “Wastes” involved in lean manufacturing, we’ll look at eight of the most common ones in a machine shop. Most of them are simple, but it can take a lot of work and orchestrating to implement them all. There is no perfect company that has it all figured out. There is always room for improvement, which is why employers seek new ways to lean out their manufacturing process. The acronym for this practice is “DOWNTIME”. Now you could just look this term up on wikipedia, but it won’t give you a real perspective or example of what it means. Those are just general illustrations in the manufacturing industry, but working in a machine shop may produce different scenarios.

Defective Production:

If bad parts are made, it takes more time to either re-work it or make another one. More material, more machine time, more tooling wear, which can add up to almost double the cost of a part. The machine shop pays for this and not the customer, and that is why it is on the list of big “wastes” that companies try to eliminate. We are only human, so mistakes do happen occasionally, but the goal is to decrease the percent of defective parts.

Overproduction:

I believe that this one can go either way, but if space is expensive, then overproduction is definitely considered a waste. If you make extra parts for a customer, it costs more time to make them, and the excess supply of parts costs more to store because it takes up more space in between machining and shipping, plus the time it will take for the customer to order more. If something happens to the customer and they discontinued that part or went to a different vendor, your extra inventory has now turned into a complete waste of money.

Waiting:

There’s two ways you can look at this; the parts that are waiting, or the machinist that is waiting. This happens whenever you have stock waiting to be cut or for operations on a machine. There is usually a waiting time in between finishing the parts and shipping. This takes up valuable time, as well as space. Although it may not seem like a big difference if parts are waiting an extra day or two to be worked on or shipped, the quicker you get the parts out your door, the quicker you can move on to another part/order.

Non-used Employee Talent:What Is Lean Manufacturing

This should be an easy, but too many employers miss it. If you have an employee that is skilled multi-axis machining and/or programming, putting them on a grinder or running simple mill parts is a waste of talent. Even if they need a little more training, it’s much more efficient to move that employee to the more complicated work instead of hiring another person, which you may have to train-in anyway.

Transportation:

Transportation is all of the unneeded movements of parts and materials. The shortest route from point A to point B is a straight line, anything else is wasting time. While it’s not always possible to do this in a machine shop, the shorter the distance parts and material have to travel the better.

Inventory:

This is similar to overproduction because having too big of an inventory takes up space and takes more machine time to run. If you’re making more parts than the order requires, it is considered wasteful inventory.

Motion:

Much like wasteful transportation of parts, a machinist should reduce wasteful motion as much as possible to be efficient. If you’re setting up a job, all the tools should be set-up and ready to go or on the workbench next to the machine. This can be done during cycle time of the previous job to save time. When loading and unloading parts in the machine during production, as well as part deburring, having everything close by or within reaching distance will reduce motion and save time in the long run.

Excessive Processing:

Like mentioned before, time is the biggest money breaker or maker, and if you’re spending too much time trying to perfect parts or orders when it is not needed, then you’re wasting time. If you have wide open tolerances on some or most of the part features, spending extra set-up and/or cycle time to try and get it right at the nominal number is waste. As long as all of the parts are within tolerance of the blueprint, they’re good. If the part doesn’t go together during assembly or function properly, it’s that customer’s job to fix the print, not the machine shop’s job.

Now, how can YOU as a Machinist benefit from all of this? This can help boost your reputation at your current job, as well as your resume. The harder you work at being more efficient, the more your boss/foreman will notice. This may result in better raises, a promotion, or benefits in various ways.

Although not all of these factors directly relate to you, suggesting them to higher authority may give you better recognition in the long run.

How To Get Fired/Hired On Your First Week As A Machinist

Starting or transitioning to a new job can be difficult. New people, different machines, new shop, and a different way of doing things (to an extent). If you’re already hired and can actually do what you put on your resume, it’s not very likely that you’ll be fired in the near future. That is, unless, you have the same routine at every job. Allow me to explain…

It is easier to train one guy to work like the other 20 machinists, than to train the 20 to work like one machinist. If you already have a way of doing things as a machinist, be prepared to change it up a little bit. Just because you learned how to do something at your previous job or jobs, if your new company does it a different way, YOU are the one that has to change.

A company is not going to change how they do things just because that is how you learned it. One of the quickest ways to get fired is by disagreeing or trying to force your own techniques onto the new shop. Unless they are very open to new ideas and strategies on how to manufacture their parts, they will only keep you so long.SCCCNC1.JPG

So, how do I keep my job after the first week or even month? By working with them and doing things how they want done. Now this doesn’t mean that every technique has to be the exact same, but the process and finished part should be identical. If you think they are doing something wrong or less than efficient, politely suggest to them how they could improve, and ultimately, save money. If that doesn’t catch their attention, then so be it; either deal with it or move on.

If they have a improper machining practices and aren’t willing to accept any better methods, then maybe it’s not so bad that you find another company to work for. It’s often hard to find out how a company operates during one short interview. You don’t really know how they do things until you start working with them.

CNC Programmer Salary – How Much Am I Worth?

Are you good at programming on CAM/CAD software? Programmers are in high demand in most areas around the United States. The salary, or wage, for CNC programmers depends on a couple things. There’s a wide spectrum of programmers because not everyone has the same experience. However, just because you have 10 years of programming ‘experience’ doesn’t mean you’re worth more than the other guy that gas only 3 years. More on this later…

Master The Systems

While a programmer with less years on the job can get paid more, the more you know the better. Having 8 years of experience on Mastercam is great, but if you know how to program on Gibbs, Surfcam, as well as multi-axis on CAM systems, you’re worth more to certain companies.

Knowing how to run multiple CAM or CAD systems gives you an edge over the competition if you’re looking for a job because you’ll fit the “requirements” for a lot more companies that are looking for a programmer.

CNC Programmer Salary - How Much Am I Worth
Programming on Mastercam

Even if you haven’t worked with a certain software that a company you want to work for uses, having experience with multiple other systems makes you more versatile, which will making it much easier for them to train you in on their programming software. Every CAM/CAD system is different, but you’re essentially doing the same thing with all of them; programming. If you’re used to being acclimated to a new software, learning how to use one more shouldn’t be an issue.

What’s the Demand?

If there’s no demand in your area for a programmer like you, don’t expect to get a high paying job offer in the near future. Big cities with Machining, such as Houston, Minneapolis, and Ohio have a high demand for any and all kinds of Machinists or Programmers. Finding a job in these areas isn’t too difficult today because the manufacturing demand is high.

If you want to get started as a programmer, you can find easier jobs at shops that will start you at the bottom and work your way up if you only have school experience. It’s not going to be a wealthy salary, but it’s better paying than most operators.

Let’s See Some Numbers

Okay, this is probably the real reason why you’re here… Programmers are usually paid more than machine operators, which is a big reason why so many machinists go to school for programming. Before I throw any digits out there, you should know that these are not set in stone. Every market and area of the country/continent is a little bit different. These numbers are a general starting point to show you what a CNC programmer can make for a living.

If you’re looking for a first just as a programmer, even if you’ve been a machinist for a few years, you’ll probably start towards the bottom of the pay scale. Right now, most full-time programmers start out around the 18-20 dollar an hour mark. It won’t make you rich, but if you’re looking for a good starting job, there’s usually a good amount of room for improvement.

If you look in the classifieds of Job listings, you’ll often see Programmer wanted ads with 3-5 years of experience. With more experience, comes more money (for the most part). The compensation is usually in the low-mid 20s for wages, and if you’re working overtime like a lot of shops do these days, that could be a pretty decent paycheck. However, like mentioned before, not only do you have to have the years of experience qualification, but also the right kind of experience as far as programming on CAM/CAD systems.

For you programming masters out there, high 20s and into the $30+/Hr mark is not uncommon. Some machine shops just need a lead programmer that can do it all, and to some companies, that person may be worth 35 bucks an hour.

Lathe and basic mill programmers are usually at the bottom of spectrum because they require the least amount of skill and training. The high-end jobs are usually multi-axis or special milling programmers that require a lot more training.

Once again, these salaries aren’t going to make you a millionaire. But that’s not why you chose CNC Manufacturing as a career, otherwise you would have gone a different and probably less interesting route.

If you have any questions, feel free to post a comment…

CNC Machinist Training Near Houston, Texas

CNC Machinist Training Near Houston, TexasFor you to get a job as a CNC Machinist, you will need to have some training and/or experience in the industry. While there are no specific requirements that each state has for Machinists, certain cities have a much higher demand for them, and Houston is one of, if not the, biggest one.

For the most part, you must:

  • Be at least 16 years of age
  • Be able to read and understand English
  • Have basic machining skills and knowledge
  • Be able to pass a background and/or drug test
  • Have your own tools (varies per company)

In order to meet these requirements you have to get some experience or training, and the easiest way to do that is by going to school for CNC Machining. Depending on the school and what degree you want to graduate with, it can take two semesters, nine months, or 2+ years for the highest degree.

Below are colleges/tech schools that have a CNC classes and/or a Machining program:

UTA – University Texas Arlington

While the U of Arlington doesn’t have a full-on machining course, they offer multiple programming, blueprint, and shop math classes. Unfortunately, there are no machines available to practice on. This would be a good place to go if you already have basic machining experience and want to learn CNC programming.

Fort Worth ISD Adult Education Center

The Adult EDU center in Fort Worth has a small machine shop with manual machines, along with basic machining classes to go with. However, the classes are geared towards people that have at least a few months of experience/training in machining already. Fort Worth has CAD/CAM, CNC programming, and 3D programming classes, as well as Blueprint reading and shop math. The equipment isn’t as high tech as a full on Machining course at a Tech school, but it may be worth it to take some classes here to get some knowledge on these.

MT Training Center

MT Training center is a vocational school that has a complete CNC Machining program for anyone that has graduated high school. It covers almost everything from the basics to CNC programming and machine set-up.

Texas State University

TSU doesn’t have a specific CNC Machinist program, but it does have a Manufacturing Engineer degree with CNC classes. The school has a couple of CNC machines that students can practice on after learning how to use CAD/CAM software, as well as manual programming. It may not be a full-on machining course like a tech school, but it’s a good place to go for programming and machine set-up since they have quality equipment/software.

Tarleton State University

Much like Texas State U, Tarleton State offers various classes for CNC machining and programming. There’s several mills and lathes to practice set-ups and run parts on after learning how to program. There’s some machining 101 classes here, but you may want to have a little experience before committing to this school.

Texas State Technical College

Texas State Tech has a few different courses for Manufacturing. There’s a couple 18 month-long associate degree programs for manufacturing, and a CNC-based program is 12 months. They have all of the necessary classes, starting with basic machining, blueprint reading, shop map, as well as CNC programming, and CAD/CAM classes. There’s a CNC shop with computers and machines to learn on. If you’re looking to go to school for machining, this is a good place to get your career started.

Going to school is one of the easiest ways to get started in a new career, especially for Machinists since you can be done in two years or less. Plus the fact that most programs cost much less than going to a regular four-year school. It’s never too late to get started, whether you’re 25 or 45. You can complete an entire program from start to finish, or just take some night classes to boost your knowledge and move up in your workplace.

References:

CNC Concepts

Manual CNC Programming – Pros & Cons

Manual programming for a CNC mill or lathe has been the most common way to program over the years that CNC machines have been used. Manual programming is done either by hand and transferred to a computer file, or punched in by hand on the machine controller.

Pros

Lets look at the good stuff first… Manual programming is limitless when it comes to developing a part program. You can set and adjust every fine detail to your exact touch, whether it’s tweaking feeds and speeds, changing cycle parameters, or using different cycles.

Another big advantage is that manual programming teaches strong discipline. The machinist writing and/or editing a program must grasp the skill and techniques of CNC programming to be proficient. Every number, every letter, every decimal, must all be to perfection for the program to run smoothly. In addition, being able to do all of your programming gives you an edge over the competition when it comes to looking for a job.

A bonus if you’re going be programming on a CAM/CAD system, is that you will be able to understand what exactly is going on and why. Programming software can get pretty complex these days, and it’s your job to know every little detail on how the part is being programmed.

Cons

Manual programming definitely has its disadvantages. The time it takes to make a complete program is probably the biggest downfall. Compared to making a program/toolpaths on a CAM system, hand writing can take considerably longer, depending on the machinist. Not to mention the time it takes to run the program out and make sure everything works.Manual CNC Programming - Pros & Cons

Another important disadvantage would be the number of errors in the program. When you’re writing a program by hand or punching it in the controller, it’s very easy to make a mistake. All it takes is on wrong number, letter, is a misplaced decimal.

Other areas that make manual programming inferior include: not being able to see your toolpaths, as well as the complication of needing to edit a program and more.

While there are some huge downfalls to manually writing a CNC program, having the ability to manual edit a program is still extremely important in this industry. Just like giving a kid a blow torch; not knowing the basics before using high-tech software is a bad idea.

Mini Desktop CNC Mill – Pros & Cons

CNC mills have progressed so far these days that you can have your own milling machine that can sit on desk or benchtop; hence the name ‘desktop cnc mill’. Depending on how much you want to spend, you could have a set-up for the price of a new computer.

If you are willing to dedicate a good amount of time to learn how to program and use software, having a desktop cnc machine can be a great asset, whether you’re looking to make your own parts/tools as a hobby, or actually turn it into a small business.

So, lets look at the what makes a benchtop CNC Mill an advantage over a full size machine, as well as how they are at a disadvantage…

Pros

Cost

Compared to the price of a new 100,000 dollar milling machine, you can get a mini CNC mill for a small fraction of that. How much do they actually cost? I can’t really give you a good estimate because there’s so many options available. You could buy a basic kit that’s complete for less than a thousand bucks, or you could be a bigger and better machine that’s fully enclosed with more bells and whistles for a few grand.

Fully Enclosed Desktop CNC Mill
Fully Enclosed Desktop CNC Mill

Technology

For the price of a desktop mill, you get a lot of technology, especially if the package comes with a good CAM/CAD software. In the past, you’d have to buy a big manual mill or lathe and convert to a 240 set-up for electricity in the garage/shop. They are not enclosed, so after machining a part you’ll have a big mess to clean up, whereas a desktop CNC can be enclosed so you have little to no cleanup afterwards.

Size

A big advantage of a desktop mill is how small they are, hence the name “desktop” or “benchtop CNC”. Instead of requiring a large garage or pole building to set up a milling center, you can put one of these in your house!

Cons

A desktop CNC mill may be able to machine parts just like a full-size milling center, but there are some pretty big disadvantages. This may or may not affect you, though, depending how what kind of machining work you plan on doing.

Mini Desktop CNC Mill
Mini Desktop CNC Mill

Size

The size of these machines can be a good thing, but it can also be a downside depending on how large the parts are that you are going to make. If you want to make motorcycle rims, a desktop machine just isn’t going to cut it. Not only is the machine itself too small, but you may run into rigidity/machine capabilities if you are removing large quantities of material, which brings us to its next downfall…

Capability Woes

Unfortunately you won’t be able to hog out stainless or hard still parts on most of these mini mills due to the fact that they are not nearly as rigid as a full size machining center. A lot of them are only built to machine plastics, wood, and soft metals such as aluminum. If the metal is too hard, it will cause a lot of spindle/tool vibration, causing poor finishes. It may also end up breaking your tools because the machine cannot handle the extra tool pressure.