Wednesday, January 23, 2008

Getting started

On Monday I arrived in Medellin ready to get this project rolling. The first thing I did was work on recruiting more resources. I visited the space at the university and it is great. I will post some picture tomorrow. I also narrowed down my list of materials to use on the basic frame. I settled on 135x135x5mm tubing. 165x60x3.4mm tubing and 300x100x7mm tubing I purchased 4,4,1 6 feet length of each. The total was around 1550$, it was easy to find since this typical structural tubing for construction. Again the idea is to use easily available materials. If anybody is curious the company name is Ferrasa. I also when around some other place looking higher quality steel for some ideas I have in mind but had limited luck. It seams McMasterr Carr is cheap compare to the prices here for specially steel.
I ended up Monday night creating a project plan, it looked tight and the first item on the list was to get the base design and constriction underway by feb1. The there is the axis of motion and last the spindle around the 2 week of feb.

Tuesday was the day I when tot he university, looked at alterative shop space with a company call Hybritech. A good friend from Boston is starting his own company and gladly offered warehouse space. The university although it has some limitations on schedule is much better equipped, not fully equipped so we will have to work some creative magic. I also spend a good part of the afternoon visiting the steel suppliers. I found out for example that no one takes my AMEX card so I will also have to get creative in that area. Alter in the night I mold over my design options. I finally got something simple and that will do the job.

Wednesday morning I was a little stressed. I had requested the full approval of the budget so that I didn’t have to authorize each individual purchase. So everybody was on hold and waiting. Finally before noon I got the go ahead. I was off with a different credit card to make the first purchases. Initially my card won’t go through, I tried calling the credit card company and after 10 failed attempts finally got thought. I know I am off to rough start but slowly the momentum is going.

Wednesday afternoon I was sailing smoothly detailing the base frame. SolidWork 2008 acted up a few time but after dinner I hasn’t given me any trouble. I also spend some time worried about simple decision like choosing which screws to standardizes. I chose m4, m6 and m8. So I am preordering this as well as taps, clearance and tap drill, and not to forget the washers. One I say the base semi complete I started looking for a t-Slot table. I am primarily worried about the height and the lead time. Initial web surfing gave me some hit but nothing like what I need. It occurred to me that with thick enough material I could simple machine the table flat with the machine it self. I plan to put killer bearing on this things so it should be a breeze. With the proper backing one can make a very nice table this way, I am even thinking of grooving it for drainage. The table base will probably be made from the 300x100mm tube with a threaded hole pattern for attaching vices and work pieces.

During the morning as I was discussing the budget with my boss I also made an important decision. Instead of relaying on find a spindle I was going to make it. Yes I am crazy. Not really the problem is the cost and the lead time. The cheapest I have found it 5350 plus shipping, import taxes, and other extras like a pull up bar and cylinder and an oil cooler and not to mention that I won’t get it on time (5 week lead time) have driven me way. I will keep looking in case anybody knows someone but I will make my own. I was thinking about it being low rev and making it with regular bearings or build it with hybrid ceramic bearings but I will first try a hydrodynamic spindle I even came up with two ways of make in the two basic designs with tools at hand.

As it turns out spindles and ball screws are two things people here have been trying to make with out any luck. I got a solution for the ballscrew but right now its not worth the time and effort. I am getting the call screws from THK. I know THK is expensive so if anybody has any ideas that have short lead times let me know.

I also fond some nice glass slides to make this machine very accurate, it will be tricky to make them work with other encoders but will try. Now I have work on matching the motors/controller to take the linear feedback. I found then in Travers. By the way I hope to have the time to set up a site wit the BOM for the machine so that all information is in one place.

Sunday, January 20, 2008

Availible resources

One of the first things I did as soon I decided we could do this in 6 weeks was call up the crew in Medellin. And I asked to call or go to local steel suppliers and asked them what they had at hand that we may use in the project. The guys came back with this list

I then turn the list into to this diagrams, which when printed on a regular printer was to scale. This is a great way to get a feeling for how big things are, will they hold a thread or welds and most importantly what will fit into what. Remember the viscous shear effect, it relays on concentric structural members with narrow gaps in between.

Next step will be to turn this into a cad library, this way we can easy insert tube members into the design. Juan is working on doing just that over the weekend.

This is a nice segway into talking about design, which I have been thinking and sketching a lot in the last few days. This is the part with lot of drawings so it should be fun. I usually start with pen and paper, before going to cad I like to have mental image o what I want. As the mental image gets more complicated I recourse to pen and paper as a quick way to keep track of all the detail. Plus if you have a hard time drawing something then their may be physical constraints you may not be awarded. The paper or note book drawing are also very important to conveying ideas to the team in Medellin. As I draw out a concept I simply take a picture with cell phone and email it right ways. This way next time we talk they know what is in my head.


The picture bellow are pretty self explanatory

Thinking about the Machine's Design

Machine Specifications

One of the first things that was clear about this machine is that it would be nice for it to hold a CAT40 tool holder, if only for popularity reasons. With 12,000 rpm max, maybe 15,000 rpm should be enough for most jobs. By deciding on a CAT40, the max rpm is set and other characteristics of the machine are also set. This machine will not move very fast (800in/min but rather more like 300in/min) when cutting. Given the rpms, it will be a strong machine in the sense that to make it an efficient machining center it will need to take larger bite - hence requiring more torque on the spindle and thrusting force while cutting. This is very different from the other end of the spectrum – high-speed routers that are kind of flimsy. But given their high cutting speed, they can experience reduced cutting forces to maintain accuracy. The CAT40 will allow for large diameter tools which are ideal for some jobs. Plus in the future we should be able to have a different tool holder.

Now what about work volume? Most entrée machine start at 20x20x16in and this sounds like a good place to start.

What about machine configuration? Should we go for a traditional C shape (Bridgeport style) of gantry style? After a lot of thinking and reading about this I really like the gantry style. You don’t see it on entrée level CNC machining centers but given a new choice of material (which I will talk about later), and the fact that I want to design a style of machine that could scale to larger size, I think it is a perfect fit. Gantry also is perfect for limiting thermal distortion caused by hot chips and reducing the need for way covers if all the moving components are kept high above the work piece. I also like the fact that there is less variation of translating mass when you move the work head around instead of the work piece.

Construction Choice

Lets talk a little bit about the vision. One of the goals is for this machine to be accessible to many. And by this I mean almost anybody. So I am also focused on the fact that I want to make a design that can be built in any moderately equipped shop. Also the cost of material is low ($10k), so some traditional technologies will have to be abandoned.

First, iron castings: Although at productions scales they can be cheap, this is not a technology that scales down nicely - so castings are out. Maybe components like T slotted tables (which you can get as a standard component) may fit the bill but a machine base will not. So what will I use instead? This takes me back to some of the work done by one of my favorite college professors and precision machine design guru Alex Slocum. He came up with base designs with 4x improvement in vibration absorption performance over concrete filled in steel structures. Here is link to the article it basically consists of using thin layers of high viscous shear damping material in-between hollow sections. Sounds fancy, but it is not. These materials are everywhere from honey to tar. And if used in the appropriate place in a structure they can significantly dissipate energy. In the machine world this translates to reduce transmission of cutting vibration force and results in smother finished surfaces. Square and rectangular steel tubing will be ideal for this application given the low cost, availability and easy of manufacturability.

Second, nice quality linear slides and precision spindles: These are expensive. I want this machine to be as good as it can be for $10k and to keep the cost down. To do this, I will have to resort to two strategies. I can look for these components in China - hopefully their price has come down or I can down grade them accordingly, but that will be my last resort. There is an alternative, which I plan to develop in parallel: Low viscosity hydrodynamic bearing design, again by Slocum. Hydrodynamic bearings have been around for ages, even before ball bearings. The concept is to slide on a thin layer of fluid. In the past, the fluid of choice has been oil, but oil had its disadvantages like high resistance at high speeds and specialized skill involved in the manufacturing of race ways. Although hydrostatic oil bearings are still used in the high-end precision machines, ball bearings with their high efficiency, relative low cost and cleanliness have taken over the market. They are not as precise but, although cheaper, they present a challenge when trying to keep the $10k budget. The alternative I am proposing is pressure assisted low viscosity hydrodynamic bearings. These are bearings that use water or a fluid like water instead of oil - they don’t require the manufacturing tolerances of oil bearings and they can move with little resistance, given the fluid’s low viscosity. If low cost spindles and linear ways are not available, I am positive this is the way to go. The plan is then to design a hydro-block that anybody with a Bridgeport can make, and to reuse them all around the machine. These bearings also have very high load capacity - if done right, they will never wear out and have much better vibration damping characteristics over ball bearings.

Thursday, January 17, 2008

The Challenge is on

I am starting this blog to keep track a personal challenge. I am setting out to put a project that I led off track back on track, its a personal CNC machine for ~10k USD and I got 6 weeks to accomplish the task.

Here we go........


This project started about 2 and half months ago. Since then I've been doing tons of research, hiring contractors to help with the projects and setting up a relationship with an university in Medellin Colombia to help with the project. The university name is EAFIT and I am contracting with a local Design firm call "De Lapice a Cohete" which translates to From Pencil to Rockets. They have three member, Pedro, Juan and Esteban.

Following I will be blogging about the design process, decision, discussion and general communication between the group and mi self. By the way am based in Cambridge MA but will be spending some time in Medellin to work more efficiently.

Here are some helpful links to start of with

Components resources

My favorite


I want to be like this guys