Tweakie.CNC - Making stuff - Routing and Profile cutting

Some notes on making a welded steel frame.

Welding steel box section is a quick and low cost solution to producing the basic framework necessary for any CNC machine. With the right equipment and some practice it is not difficult to master the techniques and achieve very acceptable results. (A Google search for 'welding techniques' should bring in sufficient information to get anyone started). There is however one major drawback to welding which, due to heat and metal expansion with it's subsequent contraction upon cooling, introduces distortions into the finished work and If these distortions are considered to be inevitable then some way of correcting for any errors must be incorporated into the design.

My framework was constructed as accurately as I could, by carefully cutting each part squarely to the correct length and using jigs and clamps to support the work during the welding process. The completed frame was as square and true as I could measure with basic tools but the important rails to which the linear slides were to be attached were not flat to within +/- 0.25mm. To re-gain the required flatness I chose to use alloy shims on each of the four ways. These shims were bonded to the steel and then lapped flat, square and true. Lapping was achieved using a retired (300mm x 600mm) iron surface plate to which I fixed, with double sided sticky tape, emery paper and then turning the plate upside down it was slid back and forth along the ways, checking for square, flat and true at regular intervals. This process was extremely slow, however, the end results were well worth the effort.

Although it may well have been easier and simpler to have made a frame from proprietary alloy extrusions, bolted together and thereby avoiding any induced distortions I am certain that it would have taken around the same amount of time yet cost much more. If and when I do build my next CNC machine I will still opt for a welded steel construction for the framework but I think, that by careful design, I can confine the distortions to less critical areas.

Fig.1

Fig.1 illustrates a typical type of distortion (somewhat exaggerated) which is induced by welding two pieces which were initially touching each other. If the base plate was strong enough not to have deformed then tremendous stress would have been induced in the weld which at some time in the future would almost certainly be doomed to failure.

Rather than getting bogged down with stress relief techniques, I think that time spent looking at alternative approaches is the way to go.

The example of Fig.1 would have turned out differently had a small gap been left between the two parts prior to welding. I have observed professionals using the stub of a welding rod to produce the gap for the first tack, then removing the stub and leaning the upright to increase the gap slightly more, apply the second tack (on the opposite side to the first). Cooling of the second tack draws the upright back to the true position. A similar process is repeated for more tacks then the weld proper is completed. This technique allows the upright to be truly upright and minimises distortion to the base.

(1) Welded steel frame made from 25mm square box section just sitting there waiting for the paint to dry.

(2) Alloy strips 20mm x 2mm(shims), bonded to the frame where the linear slides are to be fitted, these are lapped flat and true to correct for the distortions introduced in the frame by the welding process (each pair of linear slides need to be fitted perfectly parallel and true to each other).

(3) To avoid the need for tight tolerance working, oversize clearance holes were drilled in the frame and tapped 'nut plates', made from steel strip 14mm x 5mm, slid inside the box section and used to secure the linear slides by way of M5 cap head screws. This method of construction allows for precise, parallel and true alignment of the slides during the final assembly.

(4) Nut plates for the X axis slides. The cut-away in the frame is for the X axis stepper motor. The original concept here was to use a belt drive between the steppers and lead screws, however this idea was dropped in favour of direct drive thus necessitating the cut-away to make space for the motor. It is not always possible to get everything right first time.

(5) Ball screw for the Y axis. Again oversize clearance holes in the frame to allow for precise alignment in the horizontal direction during final assembly. Both the X and Y axis ballscrews are of the pre-loaded type with twin circuits of ball bearings - the advantage here is that they truly have zero backlash.

(6) Flexible plate used to connect the nut of the ball screw to the table. The plate is wide enough to resist torsion but flexible enough to compensate for any misalignment between the ball screw and the slides in the vertical direction. It is a lot easier to build in some form of flexibility than it is to use shims.