CNC Milling Guide

It's frustrating when dealing with sales representatives who have never touched a CNC mill and often give you a ridiculous quote. To better give an understanding and idea of what it takes I have written this page. 

Here are some basics to try and reduce cost.

Corner Radius - Every machinists nightmare is the corner radius and engineers like to fillet the parts to make them look pretty. The problem is that an inside corner radius requires a tool of that radius, if a tool cannot match then it requires additional tooling with the possibility of more passes, these all add to time and therefore cost. The outside corner radius requires a specially made tool, getting that to match is rather difficult. All of this adds to the possibility of errors and scrap, further increasing cost. Leave inside corners sharp and go with a chamfer on outside corners, this is the cheaper option.

Drilling - A general rule is that drills will go 5X their diameter in depth, so a 0.5 inch drill will go down to 2.5 inches with little issues. The further the depth past this rule requires more expensive tooling. There are many high performance drills that go 10X depth with little issue, however their cost goes up exponentially as opposed to plain HSS drills.

Thin Parts - All forms of milling put some kind of a force on the tool and the part. If the part is not rigid enough then there will be issues such as chatter, tolerances and simply even being able to make the part. There is also a certain amount of clamping force required which if the part is too thin may bend, making tolerances impossible. Sheet is particularly difficult and requires special methods, further increasing cost.

Work Holding - How is the part going to be held to be machined. When designing a part it is best to draw a block as the starting point and then subtract material from it. That gives the user an idea of how it will be machined and each feature is an extra process, and that each facet could be an extra setup. The most common form of work holding is a vise which uses a crushing force, so that has to be considered on thin or hollow parts. Clamping to the machines bed is often the most sturdy but that can hinder the machining path. A 3 jaw chuck is a good choice for round parts and is a very sturdy choice. Some parts require a custom set of vise jaws machining in order to hold it, often to flip the part to machine the other side.

3D Contours - This applies more to mold making as you want a part that flows smoothly and looks pretty. In general we are machining a single plane whether its facing the top of a part of side milling the walls of a part. With that there is a large surface being cut with an appropriately sized milling cutter. When doing a 3D profile you're cutting on a very small contact point of the milling cutter, that means in order to get a smooth part you need to do many more passes. For example if I face the top of a part with a 3 inch cutter I would get a 3 inch wide pass, however if I used a ball end mill I might have to do hundreds of passes to get the same result. So making a part look fancy will greatly hike up machining time.

Tooling - This is a very expensive part of the process and they don't last particularly long. Preferably the largest cutter gives the best rigidity and the fastest cutting times but the cost is very high. A smaller cutter may be cheaper but it has to take many more smaller passes. An example of cost difference is a 3/8" cutter might be $40, a 1/2" around $100 and a 1" around $500. The problem is often that when side milling a part you need the larger cutter in order to get the length. So tall parts with contoured walls start to become expensive past the 1.5inch mark, depending on many other factors as well.

This part was one of the many reasons I chose setup a shop. In my time I did a lot of electronics and had a big interest in motorcycles and cars. I chose to convert my car at the time to turbo and added an extra plate on the intake manifold for an extra row of fuel injectors. 

I spent the timing designing the part, 3D printed a prototype and then wanted to make the real thing, the only problem is that I was not allowed to use the CNC mill and only had access to a manual milling machine.

I reached out to 10 different machining businesses throughout the city, 8 responded and only 3 would give me a quote, that quote being $1000 from each of them. I got a neighbour to waterjet cut the profile for $120 and then used their manual milling to machine face it flat, drill the holes and bore the holes for the injectors. It probably took me 2 hours.

The part for reference is 18 inches wide, 4 inches tall and 1 inch thick. 

If I were to quote this job today assuming I'm given the 3D files it would take 20 minutes to setup and face both sides, 20 minutes to make a jig, 20 minutes to drill the holes, 1 hour to mill the inside and outside profiles, 40 minutes to make a tilted jig and 20 minutes to bore the injector holes. A total time of 3 hours plus $80 of material, so a maximum of $440 for a single part. The accuracy and quality considerably better than the one I made a number of years ago. 

Note that I could have the profile waterjet cut and then run a single pass with the mill to save a bunch of time. I am also basing the timing on a single part. So on a multiple run each additional part may only be $200. When running a single part you're more concerned about getting it right the first time, multiple parts you can focus on speeding up the machining times.

This is a part that is mostly lathed but requires some lobes milled into it. They are anodised afterwards to make them look prretty.

This was a personal project and I required some special tools in a hurry.

I needed to give the ball screw assembly on my wire EDM a service. The pulleys and bearings were held on by special retaining nuts which required special tools. Some people may choose to take these off with a chisel which will get them loose at the expense of damaging them, but the big problem is that they will require torquing correctly on reassembly, ultimately a tool is required.

Both of these tools were made from scrap materials I had laying around and did not require any lathe work. I measured up both of the nuts to create a design with the material I had. In all it took me 1 hour each to design the tool and machine it. The manufacturers tool was triple the price and I would have to wait for delivery from Germany. Using the correct tool meant no hassle and the nuts were torqued to spec.

$120 for each tool not including the cost of materials.