1. #1
    Xavi's Avatar
    Lives in Portsmouth, United Kingdom. Last Activity: 01-12-2015 Has been a member for 0-1 years. Has a total post count of 2.
    Hi all,

    I am embarking on a CNC project to build a large format CNC Router, mainly for drilling and milling plywood, MDF, and wooden doors. I suspect I would like to mill aluminium in the future too.

    I have a budget of around 2-3k (though preferably closer to 2k) and would like to get as accurate as possible. I was looking at industrial strength models which are accurate to .02. Realise this is unlikely from a DIY job but would be interested to know what can be achieved.

    I have a fairly good idea on the electronics and software factors. I was software engineer and have a fairly good grasp of electronics. The main concern for me at this early stage is the table/frame design. After reading a number of threads on this subject I have decided on steel due to its rigidity and stability and price (whats not to love) and may be bolting it to the floor. Woefully lacking in welding skills though, and have some concerns over the shear amount of drilling of steel required.

    I would like to keep rails beneath the surface of the table for easy access, also flexibility to put a vacuum table on in the future if it works well in the first iteration. If the machine works well I may be using it for short furniture production runs.

    Posted some pictures below. The length of the table is 2500mm. The main structure from 50mm x 100mm 3mm steel tube. Looking at rack and pinion for the X Axis and Lead screw for the Y. No detail on hardware yet just looking at getting the overall form right. In particular how best to attach the X Axis rail and pinion rack to the main structure. Considering bolting the rail and rack onto an angle profile, then bolting onto the sides of the main structure. Comments especially welcome on this. I read on this forum welding the steel surface of the rails is a bad idea due to warping. Attaching on a plate separately would allow for a fair but of shim. Look forward to any comments/concerns/suggestions anyone has. Many thanks in advance.


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  2. #2

    Firstly, welcome!. My apologies for not replying sooner, I did read it yesterday but suspected others would have responded by now.

    Most peoples requirements are generally the same, if you read a few build logs, most people say "I want to cut wood mainly wood" then drop the bombshell "but a bit of aluminmium aswell". After a while as you read through the build logs they begin to realise the machine spec difference required between the two target materials, What Im getting at, is that its hard to have a machine that does both very well.

    I assume, by stating the length of the machine is 2500mm (2.5m) you intend to use sheet stock (ie sheets of mdf or ply)

    I myself, have had two 8x4 machines. One of them was a stock "production" router, running on rack and pinion.

    The other was an upright one I built myself (similar design to what you have proposed)

    Both of them, could potentially "have a go" at milling a bit of alumiumum. However, Not very well.

    I think your design looks about ok, maybe a bit more triangulation of the frame (you'll be suprised how much the thing dances about, and flexes when its moving)
    If your going with an A frame gantry like you have proposed (id space the X bearings further apart. Id maybe even think about making the gantry itself a different shape to promote vertical forces.

    The other thing experience has taught me, is dual steppers for X is a pain in the arse, you really want one motor joined to both sides (with ballscrews, this can be achieved with a big belt) (most recently neil's (njhussey) has adopted this technique) however your saying rack and pinion...
    I intend to build another machine next year, specifically for cutting 8x4 wood and plastic. I intend to use rack and pinion on both sides, -----drive one rotating shaft that runs the width of the gantry, powering pinions at each side of the gantry along racks... I can draw you a picture if necessary

    your budget seems a about right....the yellow one, (used cheap linear rails, and cheap electronics) and cost me 2000 in parts. alot of that was the dual ballscrews and rotating nuts.

    Others will have different opinions... read a few build logs, post us some more pictures of your design...

  3. #3
    Hello and welcome Xavi,

    Ok well straight away can tell you that you need to considerably beef up the gantry and use better design. As you have it now it will be rubbish for anything but plasma machine.
    It's A weak and poor design with not very good balance regards Centre of Gravity (COG) and cutting forces. (Don't under estimatemate the FORCE luke.!!)
    Will need to be much much stronger with more rails and bearings.

    The base frame is resonable but like Matt suggests some more support and triangles will help.
    However the idea of using Angle plate to attach rails is bad idea and will be harder to make work than bolting directly to the steel. The type of linear rail your thinking to use isn't very tolerant of uneven surfaces and angle isn't very flat unless machined flat.
    Also with having the rails on there sides you have the problem of making sure both sides are perfectly parallel and on the same plane in both vertical and horizontal, or if on differant planes both planes are exactly parallel to each other.
    This is why you'll often see rails on flat surfaces which each side can be brought onto the same horizontal plane much easier and setting the rails parallel to each other easier.
    The way you have it Sooooo much harder to do without machining reference edges etc or some way to align etc.

    Don't get too hung up on protecting the rails because they take an awful lot of abuse and crap before they wear out and there are easy ways to help protect if required.
    If you want the rails lower than the surface then simple attach another piece of box section on the frame sides but lower down and sit the rails on those. The Rack can then go on the under side so protected from Crap which is more important than protecting the rails. If unsure what I mean say so and i'll knock a sketch up.
    Doing this will make setting the rails on the same plane etc much easier either via shimming or using something like the Epoxy method.

    I suggest you have a little look around at other builds and take notes on Gantry design and Z axis. Doesn't need to built like tank from armoured steel plate like some you will see but does need to be strong with good design. Esp the Z axis which takes all the cutting forces.

    Hope this helps for now and don't let mine or any other negative comments which will probably come put you off. We are just passing on experience and you will thank us for it if see it thru to end.!!

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