In Search of Lost Zeros

We have more or less completed our Benchmill CNC upgrade project, installing Sieg 2.7 castings in place of the original mill, reinforcing the head, upgrading the power distribution, and installing a 2 HP spindle motor. We have the auto-oiling and pneumatic power drawbar installed, and everything is up and running. As icing on the cake, we are going to take advantage of the Intelitek automatic tool changer functionality built into its CNC software to support a scratch-built sled-style automatic tool changer system.

In designing an ATC system, there are a few options to consider when getting the tool to the spindle nose. The most common approach is to use a rotary carousel where an indexed platter holds the tools. This platter rotates to predefined positions to call up the requested tool, which is then clamped into the spindle using various design-dependent mechanisms. Digging through the config files for the Intelitek software, I saw that it does in fact include settings for a rotary tool changer, but that functionality is associated with a different model (Promill 8000). Rather than run the risk of building something only to find missing wiring connections into the controller that the software assumes are present, I elected to go with a more simple sled-style design.

We at the finish line for our CNC upgrade project (no, really). We have installed the upgraded Sieg 2.7 mill castings in the Benchmill cabinet and have gotten our Benchmill controller talking to the PC running the control software. Now we’ll begin connecting all of the odds and ends to give us a working CNC mill with all the bells and whistles (which, if you’ll recall, was the point of starting with a trainer mill as a base for the project).

There is a lot going on here, none of it particularly challenging but taken together representing a lot work to make the final project polished and usable. Instead of an extended how-to, this post will be more of a tour showing all of the connections as made, since the connections themself are fairly simple.

Having enclosed the upgraded Sieg 2.7 mill using the old Benchmill Enclosure, we’ll now establish power and get the controller and control interface running. We know that everything works independently, so it *should* just be a matter of connecting components.

One initial challenge lies in the fact that we’ve seriously upgraded the stepper motors on all three axes from the little 1.35 Nm motors that came with the original Sieg iXK1 to 4Nm on the Y and Y axes, and 9 on the Z. These draw considerably more power (I have the drivers set to 5-7 amps each). In addition, the new spindle motor draws up to 8 amps at 220v. The original Benchmill transformer tops out (if I recall correctly) at only around 10 amps, so we have some power delivery work to do.

We are nearing the end of our Benchmill 6000 CNC project. The Sieg 2.7 replacement mill has been completed. We’ve reinforced the head, shoehorned in a 2hp servo motor, hand scraped the ways, and installed stepper motors and ball screws on all three axes. Now we’ll begin the process of enclosing the CNC mill using the old Benchmill enclosure.

With the reinforced head, epoxy resin fill, and reinforced column and back plate, I’d estimate we’ve added about 80 pounds to the mill (a good thing). Starting from a 245 pound base weight, that puts us around 325 pounds for the machine alone (excluding the mounting plate in the base and the enclosure). Whereas I was able to manhandle the little iKX1 in the original Benchmill, this took an engine hoist to get it in place.

With the new Sieg 2.7 parts for our Benchmill 6000 CNC upgrade assembled, it is time to build something for the assembly to sit on. We could, well, just put the Benchmill enclosure on a bench, but we’ve increased the weight of the machine to the point that may become problematic. We’ve also increased the Sieg’s power over stock, and it makes sense to match that power with something a bit more rigid than a bench top. Besides, I want something that I can roll around the shop.

That said, I don’t want to spend an arm and a leg on what is essentially a fancy stand. To keep cost down, we’ll use scrap already laying around to build a 2″ square tube box with melamine panels for the walls and cabinet doors. The bottom half of the stand will be a basic cabinet, with one side dedicated to a coolant tank and the other to store a future 4th axis. Up top, we’ll have a slide out tray for the keyboard and two drawers for tooling. The whole thing will sit on adjustable, heavy duty (500lb each) casters.

None of this is particularly rocket science, so feel free to give this a skip if you’ve got cabinet making covered. That said, I have a few of these under my belt and feel that I have the process down to the point that I can construct something relatively sturdy, professional looking, fast, and cheap. If you are looking for design inspiration, read on.

Having reinforced the head and upgraded the spindle motor of the new Sieg 2.7 guts going into our Benchmill 6000 CNC training mill, we’ll now complete the head assembly with an air cylinder-driven power drawbar. This will support the eventual automatic tool changer feature we are planning and, in the meantime, simplify life in the tight Benchmill enclosure.

The power drawbar design we’ll be using sounds complicated but is actually simple in operation: tool changes are made by activating an air cylinder that will press the mill’s drawbar down against a series of Belleville spring washers. When uncompressed, the washers put tension on the drawbar to pull a collet in the spindle nose tight against the inserted tool. When the washers are compressed, tension on the collet is relaxed and the tool can be removed. Here, we’ll be using the Tormach TTS system for the collet and tool holders but the principle would work with any spring-type collet.

After upgrading the spindle and reinforcing the head of the Sieg 2.7 castings we are installing into our Benchmill 6000 CNC project, we can at long last turn our attention to the spindle motor. The original Benchmill sports a 1kW motor, but with servo prices dropping we’ll use this as an opportunity to upgrade our cutting power with a more capable motor to match our upgraded, more rigid platform.

The motor we’ll be using is a Hangzhou Bergerda Nema 42 model (number 110F-0625TDL). It is a 1.5kW AC servo with a rated speed of 2,500 rpm and should be capable of delivering constant torque across that speed range. The Benchmill software is set up for a 5,000 max speed, so we’ll gear the motor at a 2:1 ratio using 40 and 20 tooth HTD 8M timing pulleys that take a beefy 25mm belt.

Having successfully installed the CNC stepper motor mounts and ball screws for the Z-axis and Y-axis, it is time to finish with the X-axis. We’ll follow a similar design to the one we chose for the Y-axis, with an angular contact bearing held in a mounting plate attached to the left end of the mill table and a bracket tying the ball nut to the saddle.

The stepper motor will be mounted in reverse configuration using a steel mounting plate, tucked under the front of the table, and will turn the ball screw using a belt to achieve the 4:5 gearing ratio we need to use the Benchmill software with the new 1605 ball screws (the original machine uses 1204 screws). Unlike the Y-axis, where we used a bronze bushing to hold the floating end, we’ll use a regular sealed ball bearing held in a mounting plate on the right side of the table.

The main (left side) mounting plate will be milled from aluminum, the motor mount and the ball screw mount will be milled from 3/8″ mild steel, and the floating end (right) bearing mount will be 3D printed using carbon fiber filament, which will save us time and will be plenty strong for the floating end of the ball screw.

Having reinforced the Sieg 2.7 bench top mill column, reinforced the head, and dry fitted the Z-axis and Y-axis CNC components, it is time to finally pull base, column, and head together. Before doing this, it is time for a final modification to further improve rigidity and cut down on vibration.

By moving the Z-axis ball screw to the outside of the column, we’ve made it possible to fill the entire column with epoxy granite to add weight and stability. Recall, we inserted a piece of 1/2″ threaded rod where the Z-axis lead screw used to go, and secured it so it will extend into the base to provide an additional point to secure the column to the base. We’ve also used extra long bolts when fixing components to the column, to provide more real estate for the epoxy to grip.

Previously in our Benchmill CNC upgrade project we reinforced the Sieg 2.7 head casting and hand scraped the base. Now we’ll take our first steps toward adding CNC capability to the mill, starting with the Y-axis.

For ball screws we’ll be using a 360mm “C5” 1605 double nut model. I put the C5 accuracy designation in scare quotes because the ball screw I ordered was rolled, not ground, and most real C5 accuracy screws seem to be ground. Color me a bit skeptical. Still, the screw and nut that arrived were clearly of higher quality than the $30 Ebay specials one tends to see, and the nut/screw combination was significantly cheaper than a true ground C5 screw (at about $120).

In the last installment of our Sieg Chinese mini mill CNC conversion we installed linear rails and constructed a sliding saddle mechanism tied to the head assembly to increase rigidity. Now we will complete the Z-axis assembly by installing a heavier duty 2005 ball screw and Nema 34 Stepper Motor.

I intentionally decided to mount the ball screw nut on the top side of the linear rail saddle to keep the attachment point as close to the head as possible. To do this, we need to raise the bearing housings through use of two riser blocks.

I designed these in Fusion 360 so that the stock bearing blocks would bolt directly to the risers. This was made more complicated than it needed to be by the spacing of the bolts holding the back plate to the column and by the bolts mounting the rails to the plate (dictated by the wide spots in the castings and pre-drilled rail mounting holes). While designing the risers, I added through holes so that the blocks could be tied together with a threaded rod and spacer assembly.

We’ve identified a lack of rigidity in the head and column of our import Sieg mill castings as the primary areas of concern for our CNC mill project. In a previous post we added a half-inch plate along the back of the column and made sure it was flat and parallel to the front ways. Now we’ll return and add a mechanism to stiffen the head along the column and increase overall rigidity.

The design is simple: we’ll install a pair of linear rails on the new half-inch steel column back plate. To that we’ll attach a mounting plate that rides on the rail trucks and to that, in turn, we’ll mount the ball nut holder for the eventual rear ball screw. To the sides of the mounting plate, we’ll attach two thinner (3/8″) plates that pass along the sides of the column and tie the assembly to the head casting. This will allow the ball screw to elevate the head, while adding to the overall rigidity of the setup.

Having initially scraped the saddle flat, the next step in converting our Sieg 2.7 mill castings into something actually accurate for the Benchmill 6000 CNC upgrade is to get the base casting flat and parallel to the bottom, and to make sure the base dovetail ways are flat and parallel to each other.

Throwing the casting on a surface plate showed that one corner was 7 thou (!) higher than the others. This is too much to attack comfortably using a hand scraper so I started by gently flattening the bottom of the casting with a die grinder turned at a 15 degree or so angle. Between rounds, I inked the bottom using Prussian Blue, just as one does with hand scraping, and checked on the surface plate between grinding rounds to make sure I headed in the right direction. When it was within 2 thou or so of being parallel to the top, I finished with a hand scraper.

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