Unisig Gundrilling machine

Unisig Gundrilling machine

The component is positioned on the table using packing and slips, utilizing the table's tenons as datums. The gundrill spindle has its own slide-way and feeds the gundrill into the component.
This type of machine can also be fitted with fixtures for holding components, a counter-rotating head fitted with a chuck or collet, a tail-stock, and a steady for drilling long bars between cones or centers.

The component is held in a chuck or collet, sometimes mounted on a counter-rotating head. This gives better concentricity. The gundrill spindle has its own slide-way and feeds the gundrill into the component, or in some designs, the table carrying the chuck and bush plate moves forward into the rotating gundrill. Some machines have a tail-stock to enable long shafts to be drilled.

Usually designed for a unique component. The component, typically a shaft, is manually, hydraulically, or pneumatically clamped between cones, and the gundrill is fed through the start-bush and into the component, utilizing its own slide-way. The cones and component are often counter-rotated to improve hole concentricity.

The component is placed on a table with X and Z axis movements, and sometimes also a Y axis movement. This enables the start-bush to be butted up to the component or moved away to allow the component to be moved to the next drilling position. The gundrill spindle has its own slide-way and feeds the gundrill into the component.

The component is placed on a table with X-axis movement and sometimes a Y-axis movement. The gundrilling spindle is carried on an arm mounted on the column. The arm moves up and down the column, providing Z-axis movement. The spindle head has its own slide-way to allow the drill to be fed into the component. Column machines are used for very large components. The component is placed on a stationary table, and all the indexing movements are carried out by the machine’s spindle head.

The basic drilling principle is the same on all machines. Filtered oil is pressurized by a high-pressure pump, piped to the drill holder, or if the drill is rotating, to a high-pressure rotary union, which feeds the spindle head with high-pressure oil. The spindle head is mounted on a slide-way, which feeds the gundrill through whip guides, if necessary, into the chip-box. Oil and chips are separated in the chip-box and then passed into the start-bush, which is mounted in the bush plate.
The start-bush should butt up to a machined face on the component being drilled to achieve a watertight seal. The start-bush should be machined to very fine limits, with the ID being +0 to +1 micron on the gundrill diameter, and the OD of the start-bush should be a light push fit in the bush plate. The concentricity should be within a few microns.

Most modern standard machines have a bush plate that is hydraulically or pneumatically moved a small amount forward to seal the bush against the component. It then retracts to allow the component to be moved to the next drilling position. Older machines rely on the operator to ensure the bush is sealed against the component.

For best results, the coolant should be neat gundrilling oil. The coolant pressure requirement is dependent on the diameter of the drill; smaller diameters require higher pressures. Pressures above 100 bar (10 MPa) are not uncommon for small-diameter drills. Refer to the graph in our catalog for minimum recommended pressures for each diameter of gundrill. Sufficient coolant pressure is of the utmost importance. Lack of sufficient pressure is often the cause of premature failure of the gundrill due to chip compaction, pickup on the drill, or heavy wear on the cutting edges.

Note that coolant filtration is very important and has a direct effect on the surface finish of the hole.
As drilling is taking place, ensure there is a constant flow of chips. If the flow of chips stops for more than a few seconds, the feed should be stopped, and the drill should be withdrawn until the blockage is cleared. Since the only part of the drilling process that can be visually observed is the oil and chips ejecting from the chip-box, much information can be gathered by listening to the resonance of the cutting action. A singing noise is not unusual, but tone changes can indicate trouble. If a load monitor is fitted, try adjusting the speeds and feeds to give a stable reading; this will result in a better surface finish.

The chips should be examined on a regular basis to ensure the drill is cutting correctly. If the chips suddenly change shape or color, the drill should be inspected to confirm it is not chipped or worn. The ideal chip is a small needle-like or conical flake. However, when drilling stainless or alloy steels, the chip tends to form a series of conical flakes joined together, exiting the hole in lengths of 20 to 80 mm. As long as the chips continue to exit the hole and do not compact, all will be well.

To break up the chip, the normal procedure is to increase the feed in small increments until the optimum chip is achieved. The speeds and feeds advised in our charts are conservative starting positions. However, we advise caution if increasing the feed to more than twice that stated in our tables.

After drilling the first hole, inspect the carbide tip for wear on the nose grind and for pickup on the circumference. The nose grind initially shows signs of wear, but this wear will stabilize after a few holes. Heavy wear on the cutting edge indicates that the speed is set too high for the hardness of the material. Pickup or material welding on the circumference of the drill indicates insufficient lubrication. The drill should be Ø1, and the component should be Ø1 enough to touch. For stainless steel series and high-alloy type steels, we expect approximately 3 meters of hole per re-grind.

If it is a through hole, drill through only enough to complete the hole.
For CNC-controlled machines, if it is a blind hole, back off the drill from the cutting face by 1/2 mm.

Operational Steps for Gundrilling Process:

Stop the spindle rotation (rev)

Cease the rotation of the spindle to stop the gundrilling process.

Withdraw the gundrill from the hole

Gently withdraw the gundrill from the hole to prevent damage to both the drill and the component.

Turn off the coolant

Shut off the coolant supply after the drilling process is completed.

Material Considerations and Drill Wear

Speeds and feeds are highly dependent on the material type and hardness.

The maximum hardness for efficient gundrilling is typically 50 Rc (Rockwell C).

Hard materials, above 40 Rc, require the gundrills to be frequently re-ground due to faster wear.

Hole Quality

Burnishing Effect: The circumference of the gundrill slightly burnishes the hole, providing a smooth finish. A well-produced hole also acts as an ongoing guide for subsequent parts of the hole.

Hole Concentricity: Gundrills should produce holes that are true within 1:1000. For example, a hole 500mm deep should not have more than 0.5mm run-out at the far end.

Gundrill Specifications

Manufacturing Range: Gundrills are custom-made, with diameters ranging from 3.0 mm to 42.0 mm (in 1mm increments) and lengths up to 3 meters.

Surface Finish Requirements: For very high-quality surface finishes, it is recommended to PVD coat the carbide tip, such as using TiN gold or TiAlN black. The coating helps prevent pick-up and assists the burnishing process, although it’s not necessary to re-coat after re-grinding the cutting facets.

Information Required When Ordering Gundrills

To help us manufacture gundrills to the best specifications and advise you on the correct speeds and feeds, please provide the following details when ordering:

Diameter of the gundrill or the diameter and tolerance of the hole.

Overall length of the gundrill, including the driver (see drawing on page 4).

Driver code or the driver diameter and length, and dimensions of any flats.

Type of material being drilled and the specification (if possible).

Hardness of the material being drilled.

Type of coolant used.

Coolant pressure.

Regrinding Gundrills

Gundrills should be re-ground before they become badly worn. The typical stock removal on the nose grind should be approximately 1mm. Ensure that all wear and chips are removed.

Consult the catalogue for drawings on the recommended regrinding procedure.

Important Advice During Operation

Tip Positioning:
Make sure the gundrill tip is inside the bush when the spindle head is withdrawn. If the spindle head is not withdrawn far enough, the next component will chip the gundrill tip. If it’s withdrawn too far, the tip will not be supported when the spindle is turned back on, potentially resulting in a broken tip.

Whip Guide Calculation:
Take note of the calculation and graph in the catalog showing the maximum length of unsupported gundrill before needing a whip guide.

Replacing a Gundrill in the Spindle:

If using poly whip guides, fit them over the tip and onto the tube, ensuring bearings are fitted before assembly into the machine.

Move the spindle head to the rear and manually position the gundrill through the rear chip-box bush and into the chip-box.

Fit and position the splash seal.

Manually, gently push the gundrill through the start-bush for approximately 150mm.

Position and fit the whip guides into their holders.

Jog the spindle head forward to the point where the gundrill driver is.

Manually position the driver (part held in the spindle) and lock up the driver retaining screw.

Jog the spindle head back until the gundrill nose is approximately 1 mm inside the bush. Set the return stop at this position.

Drilling Through Holes:

Clamp a cup over the end of the component to retain the coolant during breakthrough.

Alternatively, if using a tailstock, use a good-fitting cap with a center hole.

Whip Guide Fit:

Poly whip guides should fit well on the gundrill tube. For a 10 mm gundrill, about 1 kg of force should be needed to move it up or down the tube. For smaller diameters, less force is needed; for larger diameters, more force may be required.

Start Bush Replacement:

Worn start bushes should be replaced when they are approximately + over-size or if the hole starts to wander or become bow-mouthed.

Ensure the start bush has a sharp corner with no cone or chamfer in the bore where it butts up against the component.

Emulsion Oil Use:

If using emulsion oil, it should be oil-based with high-pressure additives, not synthetic. The concentration should be at least 10%. You may need to reduce the recommended speed by 20% when using emulsion oil.

Component Tendencies:
Remember that gundrills will always tend to move toward the thinner section of a component. This tendency can be reduced by keeping the component cool.

Material Hardness & Properties

Below are Brinell Hardness (HB) numbers and their corresponding Rockwell C (HRC) and Vickers Hardness (HV) values for various materials:

The figures in parentheses require a modified Brinell test using a tungsten carbide ball when the Brinell value exceeds 450 HB.

Speeds and Feeds for Most Common Materials

Speeds and feeds will vary depending on the material being drilled. Below is a general guideline for gundrilling:

Drill Diameter: Ranges from 2-38 mm and is dependent on material hardness.

Feed Rates (mm per revolution): Different feed rates are provided based on the material’s Brinell hardness.

The materials being drilled will determine the speeds and feeds used for optimal performance.