Opinion about the structure of the router ...
This is a discussion on Opinion about the structure of the router ... within the Gantry/Router Type Machine Building forums.
- 30-04-2012 #11
I made a few changes in my router project. after taking account of the costs, I came across the need to make some adjustments. My budject is smalest. But what I ask is that comment on the change in the yy axis (the botom linear guide).
My YY axis is all in alumineo, 20mm thickness, is better than steel tube? probably more expensive... but less heavy.
I also change the ball-screw diameter and pitch instead 2005 I use 1610. I could control the resolution with pulleys.
Sorry my English ...
- 01-05-2012 #12
part ejection pins that are cylinder ground with the mould parts hardened then fitted after surface grinding to within 0.005mm tolerances way beyond gantry or any other mill capability. The tolerances needed for mould making far exceed that of most other precision engineering needs
It is possible to make a simple mould with a gantry mill providing the moulding material is quite thick. Clay moulds are an obvious option, however if you need to build the two sliding parts (Back and Front) of a mould accurately, you will have to apply all your engineering skills in precision part manufacturing for the mould part construction to achieve a working result from a gantry mill (Router).
I too have a Gantry Mill and have increased its accuracy simply because it reduces the hand work needed to make the parts fit properly. Just finished my Kitchen where machining the new gas hob and new sink aperture in a worktop was a doddle on the CNC machine. If you want to increase the accuracy of a gantry mill you need to apply the following:
- Use high precision linier bearings on each axis
- Reduce the spindle head moment distance as much as possible
- Keep the work piece inside the bearing boundary. (I noticed that your design allows the Z axis to drop below the X axis which will work ok but will increase the moment and reduce the machining accuracy.)
- The spindle head needs to be high power variable speed AC motor using a VFD frequency controller and at least 3kw able to draw 8-10 amps under load. High load side pressure bearings with a collet run off less than 0.005mm
- When machining more accurately “slow down” the faster your machine runs the more overrun is used.
- Become familiar with tool speeds and feeds for different material machining.
This gantry type design is easy to construct but has an inherent problem as each axis compounds the axis tolerance accuracy error, so the closer the spindle is to the Y axis the better. When the tool head (6) is under Z+ pressure any head movement depends on the Z tolerance which depends on the Y tolerance which depends on the X tolerance so the head movement is compounded by the bearing tolerances’ and overall design. Spindle wobble can be as much as 0.2mm, I use the type of bearing in the photo above which has much more strength and accuracy than the round type. If each axis baring has a tolerance of 0.02 movement this compounded by the distance from each baring to the next X axis has 0.02 by the time the moment reaches the top of the Y axis (4) the moment is increased by the width and height of the side, 40% is estimated this is then increased again by the Y axis to Z (5) where another 20% is applied and finally this is increased by the Z to the tool bit (6) there again half the distance from the side giving another 20% not forgetting that these are compound slackness errors. Starting at the X axis with 0.02mm movement at the top of the Y site this will be 0.03mm movement, then the Y moment to Z 0.02 added to this is 0.05 and then back down the moment for the Z axis to the tool bit could end up with an X axis movement of 0.1mm (0.005”) and that is without any pressure on the tool (6). This is the sort of tolerance expected from a router for machining wood, which is of course what they are designed for. Another area of concern with this type of design is the accuracy in terms of X and Y axis squareness, this design lends itself to almost no adjustment and of course this is the most important aspect of any CNC machine. If you make a part that is not precisely square its fitting ability is seriously affected. It is important to use the right tolerances and material hardness, in my experience building press tool and injection moulds even the standard milling machines used in a tool room like a Bridgeport mill, even though design to overcome temperature changes and are very rigid, are still not accurate enough for mould making as they can not machine hardened surfaces or produce a sliding surface for a mould part to work without gaps and problems. I can see in your design, the Z axis is inside the Y bearings between (1) and (2). However, the design will still compound axis bearing tolerance but at least when under pressure the load will be between the X axis bearings (1 and 2)
Other things to consider are:
- The affect temperature has on the frame design; the coefficient of linier expansion will increase and decrease the frame members potentially putting your job position in a different place on a hot day or if the machine increases in temperature from its use.
- The distance between bearing 1 and 2 in the diagram make a big difference on the machine accuracy, however the longer this is the less relative X axis movement you have.
- Fit linier scales and a closed loop system. I have fitted a DRO and linier scales and am working on a MACH3 positioning monitor to dynamically adjust to the scales position.
- Square X and Y; the only way to check this is to machine along the Y axis and back down the X axis after setting the test piece parallel to the X axis with a DTI and flip it over and check the run out with a DTI along the Y axis. The reading difference will be double the machines error.
I have managed to reduce the “moment” on my router to half of it original design and I now get an accuracy on +- 0.0254mm (0.001”) with much effort put into adjusting the X and Y squareness I can now produce a successful circuit board and flip the board over only compounding a squareness error of 0.04mm run out over 500mm. The machine is now also rigid enough to machine aluminium, so I have changed the spindle to reduce the RPM needed for light engineering work, now making new parts for my CNC machine.
Hope this all helps and is beneficial.
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