Why We Chose This Configuration

Here is a summary of why we chose this machine type and these main components. The calculations behind the numbers are in Component Calculations and Structural Analysis Calculations.

Machine Type: Gantry with an Open Bottom

• We want to drill holes in large welded parts (up to about 750×715×330). Instead of moving the workpiece to the machine, placing the machine onto the workpiece is faster.
• With a gantry open at the bottom, we can mount it directly onto the product using a jig. Small parts are machined on top of a stand.
• The outer dimensions are 1000×800 for storage convenience. With a spindle reach of 656×456, a 640×450 hole pattern can be reached in a single setup.
• We keep the machine weight down to around 200 kg. It is hoisted with a 250 kg hoist.
• Reaction forces loop back through the machine, jig, and product in that order, so we place the jig clamps close to the holes. To keep the frame from distorting, we adjust the supports with shims.

Frame: Built by Bending and Assembling Plates

• Steel welding is our day job, so we build it ourselves. Welded structures always distort, so we build in this sequence: welding, stress relief, outsourcing the machining of mounting surfaces, and assembling the guides.
• Compared to square tubing with the same stiffness, assembling bent plates saves 4 kg. It also lets us choose the plate thickness for each location. Channels would be heavier for the same stiffness, and an open beam cannot be used.
• For mounting surfaces, instead of machining thin steel tubing, we weld on flat bars and machine those.
• We also make the base from steel. We do not machine aluminum because its chips would mix with steel. Analysis confirmed there are no vibration issues (the lowest natural frequency is 70 Hz with t12 steel, and 72 Hz with t25 aluminum).

Guides: Roller Guides We Already Have on Hand

• We use IKO LRXDG30 on hand (box label: LRXDG 30 C4 R4892 SL S1 E585). We have four sets of 4892 mm rails with 4 blocks each, which is more than the required amount (4.29 m of rails, 12 blocks).
• Reading the part number: G means long, SL means stainless steel, and S1 means interchangeable specification. There is no preload code, so the preload is "zero or slight." E585 is not in the catalog tables, so we will check with the manufacturer.
• Catalog values: H 45, W 60, L 134, mass 1.18 kg. Rail width 28, height 28, 5.01 kg/m, pitch 80. C 53,200 N, C0 96,700 N.
• Since rigidity values are not listed in the catalog, we assumed 200 N/µm. We will measure and verify it along with play.

Spindle: Belt-Driven BT30 Head

• Instead of a high-speed cartridge type, we drive an unpowered BT30 head with a servo. This is well-suited for drilling and tapping that require low-speed torque.
• We chose one with angular contact ball bearings. Heads with deep groove ball bearings have low axial rigidity, and their runout specs are also larger.
• The candidate is an HLTNC ATC BT30 (outer diameter φ90, flange φ126, pull force 280 kg, 5M-50T). We will measure runout after receiving it. The cylinder is not included, so we will prepare it separately.
• The servo is an A6-RS1000 (1 kW, 220 V). Geared 2:1, it delivers a rated 6.36 N·m and a maximum spindle speed of 3000 rpm. A φ12 drill (2.4 N·m) and an M10 tap (5.3 N·m) fall within the continuous rating.
• Drilling thrust pushes the tool holder into the taper, so it does not need to be compared with the pull force. What matters is the axial capacity of the bearings (unverified).

Feed: Ball Screws and Closed-Loop Steppers

• Rapid feed of 3 m/min is plenty, so we do not need expensive servos.
• The X-axis is driven by two screws, one on each side. This keeps the gantry from racking. The block spacing is only 200 mm, which is short relative to the 800 mm beam.
• All ball screws are preloaded with double nuts. This prevents climb milling with an end mill from pulling the tool into the cut.
• The motors are folded back with a 2:1 belt reduction to fit within 1000×800.

Tool Changing

• We are designing it from the start with BT30 automatic tool changing in mind. Until the rack is finished, manual changing is fine.
• Spindle orientation is done via servo position commands. With a 2:1 ratio, the spindle can stop at two orientations 180° apart, which works if the drive keys are symmetrical. Ratios like 3:1 would require a sensor on the spindle side.

Threading

Vibration Notes

• Natural frequencies start around 70 Hz, so avoid around 2100 rpm with a 2-flute end mill. This is well away from 1250 rpm (42 Hz) for φ12.
• At low feed rates (87.5 mm/min for φ12), watch out for chatter caused by stepper motor ripple and roller friction.
• Milling is limited to light cuts. Test sequence: shallow slots in SS400, thread milling, face milling.