CNC Mill – First Cuts

In January 2018 my Openbuilds C-Beam mill became operational. It’s now early February and I thought I’d post on what I’ve learned in the last four weeks.

This is what the finished product ended up looking like:

IMG_0310

Hardware

The mill itself is based on the Openbuilds C-Beam plate maker design. I ordered the parts from Maker Store in Victoria. The extrusions I ordered in 1000mm and 1500mm lengths and cut to size myself. I did make one modification to the standard design, the original plate machine was 250mm high. I made mine 300mm, in the hope that I would be able to get better clearance. This is probably a mistake and will require future correction.

The standard spindle bracket has a 72mm hole. In the USA this fits the Bosch Colt router. In Australia the equivalent is the Bosch GMR 1 Professional. I considered using either the Makita or DeWalt but both have 65mm barrels and require some sort of adapter make them fit the bracket. This may be something I want to do in future, especially now that I have the ability to machine one out of acrylic.

The Bosch router had some issues with the collet. It was supposed to be 6mm but would not tighten up sufficiently to hold any 6mm bits. I tried 1/4″ but these wouldn’t fit, too tight. Bosch Australia were unhelpful. My patience wore thin and I eventually bought after market collets from Elaire Corp. Am very happy with the Elaire products.

Stepper motor control is handled by a Syntheos TinyG v8. The mill uses NEMA 23 steppers, some of which are at the upper end of the TinyG’s power capability. I have experienced no issues due to thermal overload. The mill has run comfortably in my shed on hot summer days where the temperature was around the 40C mark. I run the board without any cooling:

IMG_0313

The other board in the photo is a Beaglebone Green (bottom left). Right now it is not being used due to RFI problems with the WiFi. I suspect the spindle motor is blowing it off the air.

I am using my laptop to control the machine using the USB cable visible on the left.

Software

The TinyG works well with Chilipeppr, an open source control system that runs in a browser. Chilipeppr is feature rich and offers several ‘niche’ control specialities, PCB milling for example.

I tried using LibreCAD for drawing. It worked well for the drawing part and I really liked the way you could enter shape details via keyboard. Other CAD packages rely heavily on the mouse, something I find awkward.

A Ben Heck video I saw on YouTube made the case for a toolset based on Inkscape (SVG) passing on to JsCut, a browser based GCode/toolpath editor. Tried using Inkscape on Mac and found 1) it’s extremely powerful (big learning curve) and 2) not Mac native (uses XWindows). JsCut is very particular about the SVG structure; requiring SVG paths instead of objects. When combined I found this toolchain to be ‘clunky’. Inkscape is a graphic designers tool, not really a CAD package. My application right now is machining small components out of MDF/acrylic and aluminium.

I finally settled on CamBam. Admittedly it is not free software but it appears to get the right balance between CAD/CAM input and usability. Right now I am part way through their trial period. You get to launch it 40 times for free, after that you have to pay.

With CamBam you edit two separate models, the drawing and the operations. You start out by drawing a plan view of the thing you want to make (i.e. CAD). Once you have drawn the shape outlines you apply machining operations like drill, pocket and profile to generate the tool paths and GCode (CAM). I tried using LibreCAD for the CAD part but found making changes to the drawing and then re-importing it was creating a whole lot of rework on the CAM side. I’ll keep using CamBam for now.

First Cuts

My first cut was to the worktable itself. I wrote a small Java program to generate GCode for a 10mm grid and used an engraving tool to cut it into the surface. This worked well but I found the cuts were slightly deeper in parts of the table. That is, the table wasn’t completely in plane with the cutter path.

I wrote a second program to drive an 8mm end mill in a square spiral, thus truing up the table surface. The third program plunged a 3mm end mill into the table surface to create a bunch of pilot holes for material hold downs (a future project).

IMG_0311

Ignore the large hole in the centre, that was the result of a machine crash that I have yet to explain.

Lessons Learned

Openbuilds use a machined plastic (Delrin?) component as an anti-backlash nut for the Acme lead screws. They come with an adjustment grub screw and a locknut. The grub screw seems to be 1-2mm short and the locknut didn’t fully engage with some of the screws. The lock nut was quite loose on the grub screw as was the fit of the grub screw into the anti-backlash nut itself. After about 30 minutes of operation the X axis lock nut and grub screw shook loose.

A second problem became apparent after 4 hours of machine time. The Y axis (worktable) worked itself loose and could be pushed 2-3mm backwards and forwards with little force. This turned out to be due to the anti-backlash nut mounting bolts somehow working loose. Not sure how this happened, the nut is secured using nyloc nuts and these seemed to be quite tight on the tread. Maybe the plastic nut ‘settled’ a little onto the aluminium spacers.

In hindsight I should have applied Loctite to the anti-backlash nut mounting bolts and adjustment grub screws as part of the regular mill assembly. This would have been much easier than tearing the machine apart after 5 hours of operation.

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