Homemade Carbide Shank Boring Bar

    This project originated from a scrap find. In our carbide scrap at work, someone had blown up a long reach carbide end mill. Only the flutes were damaged and a considerable length of untouched carbide was left behind. Being a pack rat who found a large piece of his favorite material, the carbide shaft went into my tool box for a future project. At the time I didn't know what I was going to do with it.

Finished Boring Bar

   That was well over a year ago and not much happened with it. I had decided early on I wanted to make it into a carbide shank boring bar. The boring bar decision came out of a frustrating boring job on the lathe where I didn't have a good boring bar in the right size. Boring is one of my favorite operations but it can be finicky without a good bar. Work doesn't have and good boring bars. Carbide shank boring bars are the bees knees but are well outside my price range. Looking around online, many bars use odd or proprietary insert designs, or don't have a good selection of insert grades and geometries available. I do high mix work so the ability for one tool to preform well in a range of materials in critical. 

    With my first bar design ideas, I chose CCGT21.5 inserts because they are inexpensive and meet my generalist criteria. With some quick 3D models and a plastic prototype, I saw that it wasn't going to work. The clearance between bar and wall was too tight and insert pocket shape was less than ideal. I didn't save any of the models, prototypes or programs from that first design so I can't show it here.

    Shortly after this prototype, life got busy and I put away the project for a long time. Only recently, my schedule opened up and some A2 landed in my lap so I decided to give it a second shot. This time around I chose to use DCMT21.5, which are also available in a wide range of geometries, coatings and grades, and come with the added benefit that my workplace stocks them. Starting with the insert pocket and working out, I iterated through two versions to have a final design. I scrapped my first design after my modeling program broke down trying to handle blending all the curved surfaces I generated.

Model of boring bar head

    I did all design and programming in Fusion, mostly due to it being my CAM platform and an integrated CAE package is hard to pass up. Unlike my first attempt, the insert pocket walls on the final design have a 7 degree taper to match the insert walls. I had to make a decision on the joint between the steel head I was making and the carbide shank; notched or tapered. Some feature had to be made to increase the surface area for the joint. The notched joint has the benefit of simplicity, but leaves a translational axis of movement that needs to be monitored during assembly. The tapered joint restricts all axis of motion and is easier to form on the shank, but greatly complicates the machining of the head. 

    The biggest influence of the solution came from the bond type. Most bars use a brazed joint, with is preferred. I lack the experience and proper materials to effectively and cleanly braze carbide to steel and I wanted to finish this project. I decided to try something new and glue the head to the shank. Loctite 380 is well known for its high strength and impact resistance so I thought I would try it. The notched joint ends up having slightly more surface area than the tapered option I modeled. The notched also has the added benefit of a large part of the joint being subjected to high shear loading, which glue is strongest in. Some quick math suggested with my chosen geometry, the joint can support ~100kg of down force on the tip of the cutting tool. I don't expect to ever exceed. My only long term concerns with the glue joint is heat resistance. In a heavy cut with no coolant there is a chance the joint could become hot enough to fail.

Checking insert fit before cutting head off from stock

    If this happens I'll just glue it back together and keep using it. This whole bar was supposed to be a fun project so why not try something totally new. All the machining was programmed in Fusion and run on a Trak 2Op. The 2Op is an interesting machine. Its sold as a VMC that can be moved on a pallet jack and it has a great, compact work envelope. While intended for secondary and finishing operations, we use it at work for primary work because its rare we do more than 6 of the same part at a time. 1000mm is great in X if you are doing mold and die work, but when 95% of your parts fit in a 300mm cube, you don't need a huge machine. I love its compact size and the fact it has a tool changer for such a small machine. Despite this, the machine steel feels a bit 'hobby-ish'. Something more professional than a Tormach, but greatly lacking in some obvious areas. A 15k spindle, 25000mm/min axes, and a more powerful control and coolant pump and I would buy one. 

Operation 1 after roughing

    Tooling selection was pretty standard; an assortment of small, carbide endmills. My design did require a 7 degree/side tapered endmill. This wasn't explicitly necessary - I probably could have gotten good insert registration on square walls - but if its worth doing its worth overdoing. I ground a single lip tapered cutter on the surface grinder. The HSS blank used was an old tap from a job several years ago that I have been carrying around with my tools ever since; good to finally put it to use. A custom tool presented a problem for CAM programming; tool tip diameter. I took the tool on the optical compactor and measured both the angle of the cutting edge, and the diameter reduction from the shank. This allowed me to model the tool in Fusion and program the contours. I started out leaving material on the walls, and then slowly cut the walls back checking the insert fit relative to the locking screw hole until there was a perfect fit.

Custom ground cutter, HSS has the advantage that I can easily sharpen it

    After machining, it piece was shined up a bit with a sanding sponge and files to give a pretty snazzy looking part. The harder part was chopping the head off of the parent stock and adding the notch. Cutting off, easy on a band saw. To cut the notch I decided to set up on the surface grinder and plunge grind it. There is barely enough flat land next to the insert pocket and before the angled wall to clamp on, but I managed it. The ground notch was just centered by eye and checked against the carbide shank.  

    The vice in vice method of compound angles is by far my favorite. Its clean and easy. To set my 45 degree angle, I clamped this little 90-45-45 square I made to the jaw. For the grinding a dressed the side of the wheel square with the face of the wheel by clamping a diamond to the side of a block and traversing across the wheel.

Grinding set up

Homemade 45 degree square setting the angle

    This wrapped up all the machining operations and it was off to hardening. I brand all tools and projects I build for myself and this was no different. I hand engraved a small version of my logo above the insert pocket. I hardened the head with a MAPP gas torch. For such small parts, naturally aspirated torches have enough Watts to heat the part evenly. Naturally aspirated torches also have the advantage of producing a neutral flame. Oxy-acetylene torches - having oxygen forced into the flame - have the possibility to be oxidizing flames, that is you can supply too much oxygen for the stoichiometry of burning acetylene. This extra oxygen reacts readily with the hot metal and causes pitting and damage to the material surface. This doesn't happen so readily with naturally aspirated torches. 

Back side of the finished bar

    Oxides still formed on the surface of the head but they were very even and didn't damage the surface. I really quite like this matte grey finish the heat treat left, and it stayed through tempering. A quick clean up on the joint surface and some glue and the project was done. Eager to know if it worked, I bolted it up in a tool post and gave it a test cut. It preformed as best as I could have expected. 1 mm radial cut in aluminium at 7 x D overhand and not chatter. Light finishing passes caused chatter but I honestly attributed that more to a old, sloppy lathe and poor tool holding.

    I don't regret working on this. It was a lot of fun and I have a great tool at the end of it. It also gave me reason to exercise some of my little used tool grinding skills. Hopefully this tool will last for years to come.