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The Ultimate Buyer's Guide for Purchasing precision cnc machining part

Author: becky

Jul. 29, 2024

Tags: Mechanical Parts & Fabrication Services

Machining Buyer's Guide

Why do we need to have tolerances on drawings anyway?

In the current age of 3D models, not every engineer has quite got their head round tolerances. In models on a CAD screen, all dimensions are always exact. It is a perfect world where it all fits, everything is makeable, and mistakes are (almost) free to correct.

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You may need a dimension on your part to be 100mm, but what exactly do you mean? There is no such thing as exactly 100.000mm all the time &#; in the real world anyway.

The problem arises because of course nothing is perfect in real life. Everything would be so easy if it were. In real life, temperatures can cause expansion, internal material stresses can relax, cutters can wear and even measurement equipment has a limit to its accuracy.

You have to say how close to 100mm you can accept. The tighter that tolerance, the more cost you are committing to. If you had enough time and a big budget, then you could get close. In most cases though, you do not need it everywhere. To get the most cost-effective part, it&#;s important to agree what is important and what isn&#;t.

How much does a micron cost?

At one end of the spectrum, it can cost a scrapped part or even a warranty claim on an expensive finished product. In the most famous example, a two-micron error on the Hubble telescope mirror resulted in a blurry image and it cost $1.5bn for a Shuttle trip to repair it.

At our more down-to-earth level, microns can still cost big sums. The most likely cause of this is the over-tolerancing of dimensions. There is a whole field of expertise called GD&T (Geometric Dimensioning and Tolerancing). Someone on your team will have expertise on this if it is not you.

There is a range of different manufacturing processes out there, each suited to different tasks. Some are quick and dirty (low process capability) others are more accurate but either slower or more expensive. If there was such a thing as a highly accurate, fast, cheap machine then that would be the only one everyone used.

However, be aware that the tolerances on your drawing tell your supplier which process they need to use to meet your requirements and how they need to measure it. Will a steel rule do the job? Or £1m of CMM (Coordinate Measuring Machine) be required? It also has implications on the temperature control of the machine, the coolant and even the entire internal volume of a huge factory. All of this has a cost.

While this gives some idea of what factors affect the cost, there are further complications. Not all machining facilities have all processes. You might end up in a situation where the suppliers that can meet some requirements and not others, and you end up with subcontract loops. It is not unknown for parts to travel twice round a continent visiting different specialist machinists to get a finished part.

Which process will I need?

An engineering student will be taught which processes can machine to what accuracy, to give some indication of what is possible. It is a gross over-simplification, but it&#;s a start.

Some holes could be milled or drilled, more tightly-toleranced holes will need boring. Going further up the accuracy scale you may need grinding or even honing. The accuracy of all these processes is further affected by:

Temperature
Bespoke tooling
Machine accuracy
Stability of the part
Cleanliness

What can the supplier do to help you up front?

Ideally during the design phase and no later than the beginning of the quotation stage, it is important to talk with experts in you supplier base.. They will usually be quite happy to share experience based on what they have seen before, what works well and where the pitfalls can be.

They will not be able to tell you exactly what level of tolerance your part requires. This is down to you or your designers and will depend on the application, tolerance stack, materials, failure modes and effects and so on. However, they can tell you if it is out of the ordinary &#; in either direction. Building a trusting relationship with experts in your supply chain could potentially save you significant cost and lead time later down the line.

For information on the right questions to ask your supplier, read chapter two below.

How do I know if my part is in tolerance?

It may be written on the inspection report but be cautious and ask yourself these questions:

Have you understood what level of inspection you have asked for?
Does your purchase order ask for 100% inspection?
Of all dimensions?

Bear in mind that a high-performance engine block may have 7,000 dimensions on it.

A Formula 1 customer may ask for a detail report on every one of them, on every part made, using more than one measurement technique and potentially also with CT scans of the internals included.

You may ask for a ISIR (Initial Sample Inspection Report) and then agree a sampling rate for other parts. Every 10th? 100th? Critical dimensions only? What does that mean if a process leads to out of tolerance measurements of parts or dimensions that are not measured?

Also, when is &#;out of spec&#; the same as scrap? You will probably have a concession process to deal with these situations. It has been known however for a customer to refuse to accept a part that is one micron below bottom limit. Ok, if you are machining the mirror for NASA&#;s next telescope then maybe.

You will also encounter very different attitudes in different companies for how they treat out of spec situations. Any professional machinist should be extremely strict. They should be 100% honest and record dimensions consistently according to their process all the time.

It is important your supplier is not only compliant to all standards as well as passing all internal and external audits, but you need to know that you can trust them. Unless you are going to match their measurement capability and measure it all again at Goods In. That is a big cost implication.

What is the difference between machining a casting or machining from solid?

It is a common mistake to assume all lumps of metal are the same &#; bar, billet, forgings, castings. All of these have different manufacturing processes, and they leave their &#;fingerprint&#;. They all have different internal stresses &#;baked in&#; to the material which you cannot see or measure.

When you machine any piece of material, you are removing something that contributed to its shape &#; not just the stock you have removed but also how that material held the rest of it in position.

Castings are particularly fickle. The way the molten metal solidifies in the mold is part of the pattern makers art. The arrangement of the gating system, risers, chills &#; all have an impact on the final part. A machinist with experience of milling castings will have learned how to predict not only how a part moves when stock is removed, but how to fixture it and minimise distortion.

What will be the cycle time for my part? Speeds and Feeds

How much material is going to be removed in each pass of the cutting edge, and how fast will it happen? Your machinist will be able to show you how these are calculated for different cutters and materials.

However, due to the infinite number of variables that exist, e.g. material, shape, size &#; it is not until the cutting process begins that the optimal conditions can be established. Under proper management from your machinist, a more accurate process can be established by fine-tuning the values based on sight, sound, temperature, and tolerance holding, whilst running the machine. This is a process that can improves with the number of parts machined.

What will be the cycle time for my part? CNC Programming

Potentially a more significant factor in determining the cycle time for machining your part will be the skill of the machinist&#;s engineers. To start with they must select the right machine, part fixturing, machining orientation, machine type, tooling, coolant, and a myriad of other conditions. They will also have to programme the CNC machine.

It is easier and less risky to develop a programme with long tool travel distances (and times), slow approach speeds (where the tool is moving but not cutting) and logical but not optimised tool changes.

What is the real difference between Turning and Milling?

To the casual observer, this is a trivial question. When Turning, the part moves and the tool stays still. When Milling the part stays still and the tool moves.

The real difference though lies in the complexity of the machine and therefore what you can do with it. The machine that does the Turning is a lathe, and this is a reasonably simple machine. It has to have a chuck to hold the workpiece, and be able to spin it on its axis, but apart from that the cutting tool only moves in two axes.

The result is a cheaper machine. Turning also has the advantage that it can use cheaper feed stock &#; bar is easier to make than billet. The shapes you can create though are limited.

Milling on the other hand has up to 5 axes of movement to manage &#; 3 linear ones in the X, Y & Z planes but also two rotational movements. With Milling you can achieve more complex shapes than with Turning.

With more axes you can also drastically cut down on the total cycle time required by eliminating the need to remove and reset the part in a different orientation. For simpler parts or smaller batches though, you can be more effective using a simpler 3 or 4 axis machine.

How many axes do you need?

It is important to understand the differences between 3+2 and true 5-axis machining.

It is often the case that a buyer will approach a supplier thinking they absolutely need 5-axis machining, which can be misleading. The reality is that many parts simply don&#;t require it or are more efficiently machined with 3+2 movement.

This still gives you the benefit of fewer set ups and fixtures due to the flexibility of the machine. Running a simultaneous 5-axis machine can cost twice or three times as much, but for some parts it is the only solution.

True simultaneous 5-axis machining

CNC Buyer's Guide: Which Should You Buy as a Hobbyist?

Comparing CNC machines, wondering which you should buy? In this guide we look at the considerations and options so you make the right choice.

Comparing CNC machines and wondering which you should buy?

Buying a CNC is quite different from buying a 3D printer, or even a laser cutter.

For a start, there is even disagreement on the definitions for what we are talking about, which we will get into in a second.

Secondly, if you have yet to use a CNC, you might not have a clear idea of what you want to do with it.

Third, there are a lot of variables that we need to consider, that might not become clear until having used a machine for real projects, and that are not apparent even after watching and reading reviews.

Keep in mind also, this is not a CNC review, though I will use my own experience of specific machines to inform my thoughts.

Let&#;s get into it.

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What Even is a CNC?

While most people will have an idea in their head when you say CNC, it simply means it is a fabrication machine under computer control.

That probably serves up more confusion than explaination &#; with that definition alone, both a 3D Printer or a Laser Cutter are CNC machines.

When we speak about CNC, however, we are usually talking about a CNC Mill or a CNC Router.

Both interpret computer instructions to progressively carve out material using a spinning endmill to create our end-product, and therefore this style of production is called Subtractive Manufacture, versus additive in the case of 3D printing.

Differences Between a CNC Mill and CNC Routers

So there are two types of machine that people really mean when they say CNC, but how are they different?

CNC Router Vs
CNC Mill

As you can see in these pictures of machines at Protospace, visually they have a lot in common, but the Mill is set up with a high-torque spindle, and more Z (vertical) + Y (horizontal) travel, whereas the Router is on a moving gantery, and is able to move far further in both the X and Y, with some travel in Z but not anywhere near as much.

Some of the descriptions you hear are applicable to each machine, but if the workpiece moves in all but the up and down (z) direction with the spindle static, it is likely to be a mill.

Mills are more robust, therefore are often used for metal work, or very hard plastics, creating highly precise and repeatable machine parts, for example.

On the other hand, while a router could well cut metal, it would be commonly seen cutting acrylic signs, or in a cabinet maker workshop, cutting and carving MDF drawers and pine doors.

While neither of the above machines would be seen in a home workshop, when we talk about hobbyists, we are almost always talking about a CNC router.

What Do You Need a CNC For?

What can you do with a CNC?

As we have already seen in the use-cases for a CNC mill versus router, different machines are suited to different tasks.

In determining the specific machine you go for, and the budget you ought to expect, you need to know at the very least:

What you intend to make.
The kinds of materials you wish to work with.
The size of the materials you wish to work with.
How quickly you need to finish jobs.
How precise you need your work.

What You Can do With a CNC that 3D Printing and Lasers Can Not

Many people consider CNC routers as being obsolete now that lasers and 3D printing are gaining capabilities, but that is not true yet, and might never be entirely true.

That said, there are many times I consider using a CNC and decide instead to go with 3D printing or laser, just out of comfort and convenience.

Where CNC shines is when you consider materials and scale.

I have a laser that is powerful enough to engrave metal, and can possibly even cut very thin metal, but on my Nomad 3 I can cut and engrave aluminium well, and make pretty decent circuit boards, including cutting them out.

Even with multiple passes I could not cut anywhere close to the thickness of materials that I can cut on my CNC, and there are materials that would be toxic even if I could.

While CNC is subtractive, just as lasers are, it can do 3D using STL files like 3D printers can. This can be useful for simple things like bevels and chamfers, all the way up to intricate 3D carves.

3D test carve of an STL file from Fusion360

3D prints are more capable when it comes to shapes, some fine details, and geometry, but CNC is more capable when it comes to size and materials.

CNC Negatives

As mentioned, there are times I turn to my other &#;digital fabrication&#; options such as 3D printing and lasers over CNC, and here are some reasons:

CNC produces dirt and dust. Even with a dust shoe and my enclosed Nomad 3, it is still a consideration.
They are noisy. I have seen reviews for some more recent machines that are less so, but remember you are using essentially a high powered motor with a drill bit, and using motors to move everything around, and on top of that you have dust collection.
It&#;s not just fire it up and go, because you need to hold your work down, you need to change or select the correct bit, you might need to program in new feeds and speeds, and any mistakes means often spoiled material and perhaps a broken bit, or worse.
They are expensive so you had better be sure your existing machines can not accomplish your objective by thinking in another way. For example, using a 2D laser you can make an assembly or create layers that fit together to make a larger 3D object.
CNC is a whole new learning curve. Yes, there is a lot of overlap with other types of making, for example you can still use Adobe Illustrator or Fusion360, but that is only a small amount of what you need to know to be productive with CNC.

Considerations for Buying a CNC Machine

Ultimately, the decision between a budget CNC and a fifteen thousand dollar machine depends on your specific needs and goals:

Determine your intended use for the CNC machine.
Consider your budget and whether you plan to start as a side hustle or expand into a larger operation.
Evaluate the level of precision and production rates required for your projects.

Keep in mind that there really is an element of you get what you pay for, but you might be paying for more performance than you need:

The number one factor in a machine&#;s performance is rigidity.
A fifteen thousand dollar machine is built with more substantial materials and components compared to a three thousand dollar machine. Pretty much everything else is upgradable.
Make sure you don&#;t over-estimate how large your projects will be. I loved my confusingly-named Longmill (it is actually a router), but I didn&#;t have space for it so I sold it.

GRBL vs Mach 3 and CNC Software

All of my home CNC machines run on the open source GRBL software. You can be almost certain yours will too, but Mach 3 and other software is out there and highly popular, especially at makerspaces and manufacturing, so we should touch on this.

GRBL runs on Arduino-based CNC controllers and reads GCcode, much like Marlin in the 3D printer world. This means both the firmware and the hardware can be open and free.

Because it is hugely popular, it offers a wide-variety of software options, including UGS, Chili Pepper, Open Build Control, Carbide Motion, Easel, Cool Term, CNCjs, and more.

Mach3 CNC Software Comparison with GRBL

Mach 3 is proprietary, and $175 at the time of writing, but does have a limited demo license. It uses your desktop computer and communicates via a parallel port. Yes, it only works on a desktop PC with a parallel port, not a laptop.

While not as beginner-friendly, and of course more expensive, Mach 3 does have advantages in terms of capabilities, for example it can control a 6-axis CNC machine. It also supports macros via scripting, which for a regular hobbyist is not a big deal, but for small-batch manufacturing starts to become interesting.

Drive System

The three main drive systems on CNC machines are ball screws, rack and pinion, and belt drives.

It is not true any longer that hobbyist machines will always use belts, but it did used to be the case because belts and stepper motors are cheaper, and are easier to manufacture well.

That said, a premium will be paid for more powerful motors and drivers compared to a less expensive machine.

Belts vs ball screws

My X-Carve 500 has belts, but the belts are sufficient for the work it is expected to do. My Nomad 3 is built more like a mill, it has a smaller, enclosed work area, but uses ball screws.

Ball screws are preferable because belts are stretchy and can cause imperfections due to backlash.

One of the things I liked about my Longmill was even though some of it was not super rigid (eg. 3D printed parts), it used threaded rods instead of belts (but not ballscrews unfortunately).

Ben helping me assemble the Longmill

Spindle

Almost every entry-level router will use an off the shelf handheld router as its spindle, or a powerful 24v DC motor. In the former case, the speed will be set manually using the dial on the casing, and will be turned on and off manually too.

A premium built CNC will have a specifically designed spindle that features more power and has programmable speed control.

My CNC Journey

I came to CNC via wanting to prototype circuit boards, because I am scared of nasty chemicals. It was only after owning a machine that I branched out into other things, and eventually took the certification at my makerspace so I could use (and later teach) the big scary Mach3 machines.

This means my machines have almost always been on the smaller size, and my one larger machine as I mentioned earlier, I found I used but didn&#;t really have space for.

My first machine was pretty bare-bones and has many names, but I knew it as &#;T8 CNC&#;. Really, it is just for PCBs, but didn&#;t do a very good job of that because the accuracy and rigidity isn&#;t there. What it did do, at a low price, was take away my fear of the machines.

Next was the Sienci Mill One. Sadly, it is no longer produced, but it was a great machine for me. Again, only small, but much more robust and capable. For a teaching situation it especially proved itself, it was bashed around and abused at the other maker space for a long time and survived.

I tested all kinds of materials on that little guy, and eventually outgrew it. First I upgraded to a slightly larger 500mm x 500mm X-Carve, which I still have and enjoy.

Longmill and X-Carve

Eventually I upgraded in size again to the Longmill. At the time I got it there was a deal where we got an early version at a discount, so I paid under $2,000 CAD for a 30&#;x30&#; machine.

Would it have been nice to just lay down a full sheet of plywood or MDF and get cutting? Sure, but even this machine in our two-car garage had to live hung on a wall.

Many of my friends thought I was losing it when I finally (for now) bought a Nomad 3. This machine has a small work area, and can not even accept larger materials because it is all enclosed.

The thing is, it suits 80% of what I use a CNC for, while keeping all the dirt (MDF and carbon fiber especially) and dangerous flying objects mostly inside. When half an endmill goes ping at high speed, that piece of plastic is an eye-saver.

While we had a workshop I was tempted to get a larger machine again, but now we are space-limited I am glad I didn&#;t take the plunge. I just wish there was a makerspace nearby as well-equipped as the ones I left behind in Calgary!

Bigger, Better and Newer CNC Options

If you are looking to make stuff for yourself, maybe to sell at craft fairs or Etsy, you do not need to be spending $15,000 on a machine like the Avid etc.

While I am a big fan of open source and community, and especially saving money, if this is your first CNC then I would guide you towards one of the well-known brands with a tried-and-tested new customer onboarding:

X-Carve: The stripped-down machine costs $1,395, including Makita router. The 4×4 Pro is $5,995 including VFD.
Shapeoko: The basic Shapeoko 4 starts at $1,800 with a router, and they go up in size and capability from there. The 5 Pro is $3,750 and you can select the basic trim router at +$80 or $750 for the VFD.

Both come with free software and work with the popular third party applications, such as Aspire/Vcarve.

For ease of use, I especially like Inventables Easel, which is browser-based, compatible with pretty much any GRBL-based CNC and is free for the basic license. Optionally there is a pro version.

Another option for CNC brands is Onefinity. I have heard good things about these, and they seem comparitively priced, but I have no personal experience of them.

If those are too rich for your budget, then cutting your budget right down to the level where you won&#;t be too upset if it doesn&#;t work out makes sense.

A budget option I have heard good things about is the Two Trees TTC-450, which at the time of writing can be got for under $650 including spindle, but again I haven&#;t got any direct experience of this machine.

For me, today if I had the budget and the space, I would likely choose the Shapeoko 5 Pro, because even with VFD, and even though shipping is more than X-Carve, it is still less expensive than the X-Carve Pro.

I don&#;t think you can go far wrong with either the X-Carve or the Shapeoko, though! Would be sure interesting to see them go head-to-head.

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