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JohnGi

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Posts posted by JohnGi

    Cheap CNC Milling Machine - It's Arrived!

    in 3D Printing, Laser Cutting, CAD & CNC

    Posted

    Hi,

     

    I have a second hand Roland MDX-15. So far I have only used it for cutting/engraving plastics and foam for

    buildings. I intend using it for rolling stock in the future. It can (apparently) cut aluminium, brass and NS

    if you go very slowly, but I notice that Roland don't advertise their more recent machines as having that

    capability- perhaps they realised they were on the limits of what was practical with the machine.

     

    I have experimented with ABS, polystyrene sheet, acrylic sheet, foamex/palight, foam board (expanded

    polystyrene  core, and Kapa brand foam board (PU core).

     

    With cutting plastics the main problem is heat. The chips heat up and then reattach themselves leaving

    you with a nasty burr and also a buildup of plastic around the cutter. If you are engraving or milling a 

    pocket the melting/remelting can also leave an unsatisfactory finish at the bottom.

     

    The cheap Chinese carbide cutters with 1/8 inch shanks are really intended for milling circuit boards. I

    feel they're not all that sharp and they create a lot of heat.

     

    I get my best results by using Clarke FC3 throwaway cutters (about £5 each, and nice and sharp) to remove

    the majority of the material. These give a nice finish at the bottom of the pocket. Then use a small FC3 to 

    remove the last 0.1 mm from the edge, and finally a Chinese 0.5 mm carbide to go into the corners. I have

    a non Chinese 0.5 mm cutter (much more expensive, but hopefully sharper) but so far i haven't needed to risk it-

    its really easy to break the small cutters, even on plastic.

     

    The FC3 cutters have 6mm shanks and fit the machine perfectly. The machine came to me with a shop made 

    adaptor for 1/8 inch shank tools, but I found this had excessive run out, and tended to overcut into the corners 

    of pockets (when using the tool path I just described). I was able to make a replacement for this. I also found that

    the machine had excessive backlash on the x-axis which could sometimes be compensated for in the tool path

    but not always. I was able to remove most of this backlash once I got hold of a service manual for the machine.

     

    I use Fusion 360 software for the CAM. It allows a lot of control over the tool paths and is free for hobbyists.

    Unfortunately they expect backlash compensation to be done in the machine firmware which is not the case

    for my machine. 

     

    Be aware that the stepper motors only give open loop control of position. With a laser cutter this is not a big

    problem because the mechanical forces are unaffected by the material being cut, but with an engraver/mill

    the forces depend on the material and depth of cut. More expensive machines control position with servos and

    encoders and can compensate for this, with steppers you will have to be more conservative with

    cutting speeds and depths.

     

    I hope you find this relevant. I will take some photos later today and upload them.

     

    John

    Re-motoring Unimat SL

    in Kitbuilding & Scratchbuilding

    Posted

    Hi, 

     

    I fitted my unimat 3 with a 24V 150W scooter motor a couple of months ago. I use a 24V 200W supply.

    Using the orange or green polyurethane belts I find that the belts slip before the motor 

    stalls.

     

    As mentioned by others there is a lot of extra information available through the yahoo unimat sl and unimat 3 groups.

     

    John 

    Foamboard vs PVC foamboard

    in Scenery, Structures & Transport

    Posted · Edited by JohnGi

    When I looked into this myself there seemed to be three types of product being used by modellers.

     

    1) Closed cell PVC sheet. Brand names Forex, Palight and others. There is a thread on this

     

    http://www.rmweb.co.uk/community/index.php?/topic/70240-modelling-in-foamex/

     

    [the OP of that thread  referred to it as foamex, although the trade name was actually Forex, which might

    cause some confusion]

     

    2) Card and foam sandwich with expanded polystyrene core. Common name foamboard.

    Widely available in craft shops etc, various trade names.

     

    3)Card and foam sandwich with polyurethane core, Brand names Kapa and Foam-X 

     

    As Arthur said the polyurethane core has advantages compared to the more common 

    expanded polystyrene. Some of the work done by David Neat (linked by earlier poster)

    used the Kapa Line product. I have some of this myself and its definitely got a denser core

    than ordinary foam board. But not as dense as the closed cell PVC board.

     

    John

    EBay madness

    in Ebay Topics

    Posted

     

    The one that was sold before is a different (though similar) engine. This one actually has photos, and can be seen posed

    on what seems to be proper broad gauge track. Photos are not brilliant, but it looks like a nice model.

     

    The two engines don't seem to have been that successful in reality. I suspect the models also would

    not have great traction due to lack of weight over the driving wheels, but that's just how the

    prototypes were designed.

     

     

     

    Thunderer was the first of a pair of locomotives (the other being Hurricane) built for the Great Western Railway (GWR), England, by R. & W. Hawthorn & Co. whose design was very different from other steam locomotives. In order to meet Isambard Kingdom Brunel's strict specifications, an 0-4-0 frame carried the 'engine', while the boiler was on a separate six-wheeled frame. The driving wheels were geared 10:27 in order to reduce the cylinder stroke speed while allowing high track speed, in line with the specifications.

    The locomotive was delivered to the GWR on 6 March 1838 and ceased work in December 1839[1] after running only 9,882 miles, but its boiler section was kept as a stationary boiler

     

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