[Jay2Jay]

Need help/comment from expert out there...

My company is developing gun drilling on a CNC Swiss type with high pressure coolant pump 120 bar (max). We are using gun drill from TIEFBOHRTECHNIK GmbH. The Gun drill size is Ø2.48mm with kidney bean-shaped coolant hole, using vegetable based coolant vascomill10, material is SUS 17-4 (Ø1/2 inch) and the hole depth is 100mm.

Pilot Hole - Ø2.485mm with 4mm depth.
Gun drill - Feed into pilot hole with turn the spindle speed on 600 RPM, when achieve depth 3.5mm, turn on the high pressure coolant and increase the spindle RPM step by step till 6500 RPM.
Drilling Method - Pecking 5mm each 25mm depth.

Hole Requirement.
- Concentricity max 0.10mm

Defects.
- Concentricity more than 0.10mm (max 0.18) on and off. Best achievable 0.020mm
- 50% reject rate.

I feel that the coolant pressure is too low.


Any help/comments are appreciated.
Thanks!

[hoecken]

Why are you peck drilling? Gun drilling you do with a constant feed.

[G59]

Drilling Method - Pecking 5mm each 25mm depth.

As previously mentioned, No pecking when gun drilling.

What is your feed rate?

What pressure are you running the coolant through the tool?

SUS 17-4 equivalent AISI Stainless 630

Machinability of AISI 630 Stainless is, Long, gummy chips which characterizes this alloys machinability.

[underthetire]

Believe it or not, the shop I used to help out with high pressure systems found sometimes too much pressure caused this, but there systems would get up over 200 bar. We put inverters on the pumps to be able to control pressure without bypassing oil, since that creates a lot of heat.

[G59]

Yes, and the whole point of pressure is to flush the chips out quickly so they don't interfere with the cutting edge or build up around the shank causing plugging and deviation or over sized holes. Gun drilling requires good lubricating coolant so the skates(guides) of the drill don't end up rubbing on the bore of the newly drilled hole.

[Jay2Jay]

Why are you peck drilling? Gun drilling you do with a constant feed.

At the first beginning, we drilled with constant feed but the concentricity is out of specification.
Then adjusted to pecking method to flush the chips out.
The feed rate is F0.0025