This invention relates to a device for performing, simultaneously, a combined operation of drilling and reaming.
In other words, this invention performs the function of a drill while achieving the same size tolerance as that achieved by reaming. It can be used for drilling holes of small depth, such as? of 2 mm to 4mm. depth with closer tolerance of 15 to 20 microns.
This invention also reduces the drill drift during machining conditions,
Drilling and reaming are two of the longest known operations in machining. There are various types of drills and reamers, of various shapes and sizes for different materials. Typically reamers cannot generate holes. They only enlarge holes to an extent of 5% of the overall bone size. A drill is known to generate a hole, the size tolerance achieved by drilling ranging from 40 to 100 microns, while that achieved from reaming is within 20 microns. The tolerance range depends on drill size, that is. the drilled hole tolerance is directly proportional to the drill size, material and method of machining conditions.

According to this invention, the device for performing, simultaneously, a combined operation of drilling and reaming of holes, such as, those of 2mm to 4 mm diameter, comprises a drill shank portion and a drill flute portion, the diameter of the shank portion being twice that of the diameter of the flute portion, while the length of the flute portion is less than that of the shank portion by a predetermined ratio; means providing a complete peripheral contact-grip on the device, during operation; the lip angle between the two flanks of the flute portion being 140 degrees.
This invention will now be described in further detail with reference to the accompanying drawings which illustrate by way of example, and not by way of limitation, its embodiments,
Fig.1 illustrating the layout of the drilled hole
Fig.2 illustrating the lip angle, namely, the angle between two flanks of the drill. As shown in Fig.2 the lip angle A is 140 degrees.
The layout of the drilled hole is shown in fig. I. As shown, the hole tolerance required is 15microns. The hole is machined in a CNC Machining Center. To achieve this hole tolerance, the existing process flow is as follows -

Centre Drilling -As the hole is drilled on a curved surface, this operation is done to avoid drill drift and drill breakage

02.5 Pilot Drilling - This hole is made to guide the reamer of 03 as reamer cannot drill the hole

03 Reaming - To achieve final size as shown in fig. I
Limitation of the existing process -
1. Additional cost of reaming process
2. Reamer breakage
3. Capacity constraint This invention:
Following assumptions are made for consistent results from this invention -1. The invention is confined to the particular operation mentioned.
2. The drilled length limits to 3 times diameter (L=3D max)
3. The drill has to be held in Hydro-grip holder as against a regular collet holder. The Hydro-grip is a tool holding device that ensures minimum run out at the tip of the drill.
4. The spindle run-out is not more than 5-7 microns. All standard CNC Machining Centers have spindle run-out less than 4 to 3 microns.
5. Cutting coolant has to be used for consistent drill size over the complete tool life.

The specifications of hole in the present case are -
1. Diameter=3mm
2. Length=4.1 mm
3. Length/Diameter=1.3
Following procedure will be adopted to make drill act as a reamer and achieve size tolerance within 10 to 15 microns
1. Making shank diameter twice that of flute diameter -
This will impart rigidity to the drill, which helps to reduce drill drift while drilling. The drift of the tip of the drill is directly proportional to spindle RPM (revolutions per minute). To overcome this, the drill is held in a Hydro-grip holder instead of a collet holder. Hydro-grip holder has a complete peripheral contact compared to collet holders in which the contact percentage is around 70-80%.
2. Reducing the flute length as compared to shank length -
The flute length will be reduced to reduce the drift at the tip of the drill due to overhang. The ratio of shank length to flute is made between 3 to 4 as shown in figure 2. Preferably, the flute length is < or = 13 mm. This also increases the rigidity of the drill and reduces the effective run-out of the drill at the tip.
3. Changing the Drill tip Geometry -
Lip angle is the angle between two flanks of the drill. As shown in fig. 2 it is 140 degrees. Standard drills have lip angle ranging from 108-118 degrees.

The effect of lip angle is - Lesser the lip angle, lesser the force required to drill. But in this case the drill is susceptible to breakage as the tip becomes sharp and the overall tool life will reduce as a result of this during running conditions.
On the other hand more the lip angle, more the force required to drill. But for this the drill has to be rigid. In this case the drill rigidity has been increased as explained in earlier points. Due to this the tip breakage is reduced and overall tool life will be achieved.
4. Optimizing the process parameters -
The drill is specified to run at process parameters of-Vc (Cutting speed) = 60~75m/min (meter/minute) F (Feed) = 0.08 to 0.1 mm/rev (millimeter/revolution)
The aim is to achieve size consistency over complete tool life. For that the cutting speed Vc will be kept low at 60m/min & feed of 0.085mm/rev. The drill life will be finalized to achieve size consistency.
5. Tool Materials -
For the given material of component, following standard tool materials that can be used for machining are as follows -HSS - High Speed Steel Solid Carbide
Solid Carbide with Titanium Aluminium Nitride (TiAIN) coating We will be using the material Solid Carbide with coating for increased tool life.

The following benefits of this invention are noteworthy:
1. It will eliminate the failure mode of Reamer breakage under following circumstances - Pre drilled hole undersize, Pre hole drill not machined due to drill breakage.
2. Capacity enhancement of the machine due to reduction in Auto cycle time of the process.
3. Tool cost/component reduction as it eliminates scope of pre drill breakage.
It will be appreciated that various other embodiments of this invention are possible without departing from the scope and ambit of this thereof.

We Claim:
1. A device for performing, simultaneously, a combined operation of drilling and reaming of holes, such as, those of 2mm to 4 mm diameter, said device , comprising a drill shank portion and a drill flute portion, the diameter of the shank portion being twice that of the diameter of the flute portion, while the length of the flute portion is less than that of the shank portion by a predetermined ratio; means providing a complete peripheral contact-grip on the device, during operation; the lip angle between the two flanks of the flute portion being 140 degrees.
2. A device as claimed in Claim 1 wherein the said means consist of a hydro-grip holder.
3. A device as claimed in Claim 1 or Claim 2 wherein the length of the flute portion is less than that of the shank portion by the ratio 1:3 to 1:4
4. A device as claimed in any one of the preceding Claims
wherein the depth of the hole to be drilled is not more than three
times the hole diameter.
5.A device as claimed in Claim 3 wherein the flute portion is not more than 13 mm. in length.

6.A device as claimed in any one of the preceding Claims wherein the flute portion is made of solid carbide or solid carbide with coating..
7.A device for performing , simultaneously, a combined operation of drilling and reaming, substantially as herein described with reference to, and as illustrated in, the accompanying drawings.