Common Types of CNCs
We decided to go with a moving gantry and stationary base due to the overall size of the machine we wanted to build. The minimum cutting area we hope to achieve is at least a 2ft X 2ft X 4in with overall dimensions of 2.5ft X 2.5ft X 1.5ft. The reasons for a moving gantry style is that it shields the linear components in the base from debris, has a smaller overall size to cutting area ratio (very big factor), and looks cooler (in our opinion)! However this design is trickier to build and tolerances of the parts need to be held tight in order to get a rigid design (how well the machine can hold together when forces are applied on different sides which occur when cutting).
Generally there is a saying throughout the hobby CNC community that whatever you make your CNC out of is the hardest material you will be able to accurately cut. For this reason, we chose aluminium since this will probably be the hardest material we will ever need to machine for home projects or the fourth year design project at the University of Waterloo. To increase the precision of our machine, we plan to use the lathe and vertical mill at the university machine shop in order to maintain tolerances for each part.
Throughout our research, we found that the moving base is more popular among smaller machines since it provides a more rigid setup. The gantry, being stationary, can be build to withstand torsional forces around the z axis if the router is off centre from the y axis and cutting in the x direction. The way to achieve this is to widen the supports for the gantry in the x direction to prevent the gantry overhanging beam from twisting. The biggest downside to this design is that your x axis length will equal your base plate length plus the amount of travel. For example, if you want a 3ft X 3ft baseplate with 3ft of travel, the dimensions of the stationary gantry machine will almost be double that of a moving gantry.
After watching many different diy CNC machines with a moving gantry cut aluminium and even steel , this helped affirm our decision on this style of CNC. At first we were worried about how rigid our machine would be. However, seeing many other peoples success with a moving gantry and milling aluminium with a fair amount of accuracy, we set out to make design plans with this architecture. One reason we are not too worried about the twisting around the Z-axis is because we will not be pushing our machine at ridiculous speeds (1000 inches per minute [IPM]) that will cause large forces opposing the router.
Throughout our research, we found that the moving base is more popular among smaller machines since it provides a more rigid setup. The gantry, being stationary, can be build to withstand torsional forces around the z axis if the router is off centre from the y axis and cutting in the x direction. The way to achieve this is to widen the supports for the gantry in the x direction to prevent the gantry overhanging beam from twisting. The biggest downside to this design is that your x axis length will equal your base plate length plus the amount of travel. For example, if you want a 3ft X 3ft baseplate with 3ft of travel, the dimensions of the stationary gantry machine will almost be double that of a moving gantry.
After watching many different diy CNC machines with a moving gantry cut aluminium and even steel , this helped affirm our decision on this style of CNC. At first we were worried about how rigid our machine would be. However, seeing many other peoples success with a moving gantry and milling aluminium with a fair amount of accuracy, we set out to make design plans with this architecture. One reason we are not too worried about the twisting around the Z-axis is because we will not be pushing our machine at ridiculous speeds (1000 inches per minute [IPM]) that will cause large forces opposing the router.
Here is the website that we used when learning about the two different designs. As you design your own on on autodesk Inventor or some other CAD software, you will be able to see the advantages and disadvantages more clearly.
Frame Material
Now a lot of you are probably wondering how this machine we are making is for a slim wallet if it is built out of aluminium. If you have never made an inquiry about aluminium, a 2ft X 2ft X 1/2in piece of aluminium is at least $120 CAD which will be enough to make the gantry side plates with little to spare for the rest of the machine. Well we got lucky and obtained a lot of aluminium t slot extrusions for free from a company that was clearing out old materials. The aluminium t slot extrusions are most likely made by 80/20 Inc since they have the same dimensions as their "15 Series" extrusions. Also we obtained some aluminium plates (10mm thick) from a friend at the University of Waterloo who occasionally receives donations of used components and materials. All these salvaged pieces of aluminium will allow us to make most of the framework. If you are looking for t slot extrusions, we recommend going onto ebay and searching "aluminium extrusions" and buying lots of up to 10 pieces since it is cheaper than buying from 80/20. You can get all the extrusions you would need for a machine to match ours for around $300 or a little less.
Another option for aluminium extrusions is ordering from Misumi. They have fairly cheap extrusions ($13.60 for a 610mm length slot with a profile dimension of 30mm X 60mm) but shipping could run your costs significantly higher. Our guess for shipping cost would be about $10 USD at least per extrusion. You can check the approximate cost of shipping by entering your orders info into the UPS shipping calculator with the weight Misumi gives you for your order when you request a quote and approximate dimensions.
Aluminium is quite expensive and if we hadn't gotten materials for free, we would have settled with a machine made of MDF. A 2ft X 2ft X 3/4 inch piece at Home Depot sells for around $22 CAD which is doable. Extend one of the 2ft dimensions to 4ft (2ft X 4ft X 3/4) and its about $35. Buying two pieces of 2ft X 4ft X 3/4in should be enough to complete most of the frame of a machine. Another type of aluminium framing that can be considered is square tubes, a 2in X 2in square (1/8in thickness) and 6ft length can be bought for around $55 on McMaster-Carr. We have yet to look up the costs on Misumi but they might have it slightly cheaper. Currently they have a promotional $150 off your FIRST order. This ends March 31, 2014.
Another option for aluminium extrusions is ordering from Misumi. They have fairly cheap extrusions ($13.60 for a 610mm length slot with a profile dimension of 30mm X 60mm) but shipping could run your costs significantly higher. Our guess for shipping cost would be about $10 USD at least per extrusion. You can check the approximate cost of shipping by entering your orders info into the UPS shipping calculator with the weight Misumi gives you for your order when you request a quote and approximate dimensions.
Aluminium is quite expensive and if we hadn't gotten materials for free, we would have settled with a machine made of MDF. A 2ft X 2ft X 3/4 inch piece at Home Depot sells for around $22 CAD which is doable. Extend one of the 2ft dimensions to 4ft (2ft X 4ft X 3/4) and its about $35. Buying two pieces of 2ft X 4ft X 3/4in should be enough to complete most of the frame of a machine. Another type of aluminium framing that can be considered is square tubes, a 2in X 2in square (1/8in thickness) and 6ft length can be bought for around $55 on McMaster-Carr. We have yet to look up the costs on Misumi but they might have it slightly cheaper. Currently they have a promotional $150 off your FIRST order. This ends March 31, 2014.
If you are not in a hurry to get your materials, look around and try to go to some local scrap yards or search on sites like kijiji for people selling metal or giving away free wood. Emailing manufacturing companies near by could help out a bit on the budget. One recycling company we stumbled upon is foxy recycle located in Ottawa but they do shipping as well. These recycling businesses might have some aluminium plates on hand that have a few holes but still reusable for making some parts. Also if you go to home depot, they sometimes have some wood with defects like a piece of MDF with a crack down the center. They sell these pieces at a steep discounted price.
If you plan to make your machine without access to a machine shop and need a way to make straight cuts, we have CADed a jig in this video (click here). The materials you will need are some MDF, linear rails (which you will need for your CNC machine anyways), a hand drill (for making holes unless you want to use your router), and hex sockets/screws or whatever fasteners you can find.
Estimates of Different Frame material costs:
Aluminium - be prepared to spend at least $400 for an entire 2.5ft X 2.5ft X 1.5ft frame if you order plates that are 1/2 inch thick
MDF - A 2.5ft X 2.5ft X 1.5ft frame will cost ~$80 if you purchase the 2ft X 4ft X 3/4in boards and some other small blocks
Summary of my basic design:
Material: Aluminium
Style: Moving gantry, stationary base
Cutting area: x-axis: 2ft (approx 609mm)
y-axis: 2ft (approx 609mm)
z-axis: 4in (approx 102mm)
Budget: $1000
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