HB2 – Stepping Up
I am getting closer to a starting point on the HB2 project. I have done one heck of a lot of research. In hobbyist grade machines I have clearly recognized that gantry or table router CNC is NOT the same as machine tool milling CNC. There is crossover to be sure but they are two totally different animals, like apes, chimps and humans.
Total accurate CNC metal machining should be done with a milling machine. Highest accuracy will be obtained with this “old school” design with solid ways, adjustable gibs, zero backlash bearings and drive screws. The limitation is the available space on the table while Z axis headspace is usually outstanding. The little Taig mill I am currently running on CNC is very accurate and repeatable but only in 5.5 x 12 x 6 inches (66 sq/in.)
Large flat table overhead CNC routers offer superior working surface but a sacrifice in absolute accuracy. Coming from the machinist point of view I was at first stunned by the lack of tight tolerances. But then I realized in routing it is of relative importance. Z axis headspace can vary but is generally less, as high gantries have more flex (so are kept short) and most work is on low height flat materials.
High accuracy can be obtained in overhead router CNC design, especially in small sizes or with extremely expensive linear components. I was examining an affordable popular all metal CNC router design with unsupported round steel linear guides only about 2 foot long. The “normal” sag in the middle of the rod was 0.040″ in the Z axis direction! For many uses this might go unnoticed. It would certainly ruin a large lithophane that requires very tight Z direction control.
There is nothing good design and more money can’t fix.
My goal is to be in the slightly over 4 square foot working area class machine which is 24 x24 inches (576 sq/in) or some combination thereof. My hot button right now is a slightly unorthodox design, a little over the smaller limit at 676 square inch working area (10x the Taig.) and should have minimal sag and flex.
My budget for an all metal machine of this size is around $2500. That is motors, screws, all the metal, the linear components, wire. It does not include the controller or power supply, router or the computer. This is for premium linear components such as rails rather than rods.
Less than half this amount could be invested into a wood frame CNC router system using rods and skate bearings. I studied one such popular torque box design. A builder/user of this design posted a note and picture where he literally bolted a chunk of angle iron to each side of the gantry and was amazed at how much more rigid it had become.
There are a lot of options and opinions. I don’t claim what I do and think is any better. If it gives a reader some insight or encouragement, I am glad to share.
I am working on the controller. The stepper motors are ordered (hint: 4). When I find out a little more about the design I am considering, I will order materials and start on a new bench to hold the HB2. Stay tuned.