Struggling to meet max recast

Good day all -

I’m running a Current small hole drill, brass 0.30 mm OD tube, DI water (25 uS) on a cast nickel-based turbine blade. I’m struggling to meet the max recast allowance (0.0015 in) on some fairly deep holes (20x L/D).

I’ve tried different tube IDs to change the flush, but it doesn’t seem to matter (nor does it matter whether the electrode is new or just about used up). I’ve gone up and down an electrode OD. I’ve tried a lot of power settings, dropping down to where the process is only marginally stable.

Most holes easily conform - but a few random holes will show a spot where recast is too high. Location along the hole is also random.

Does anyone have a trick? I’ve got more things to try, but I’m sort of hunting in the dark.

Thanks,
John Z.

If it happens in random areas make sure the burns are very smooth without occational stalling. When it stalls it can create a hot spot.
Some drills actually graph the Z vs time to help spot this.

Hey John
Replace your DI water with Dielectric oil and you should have no white layer.

Thanks … and I had thought of the stalling effect as well … but some holes had stalling and some didn’t, and it didn’t seem to matter with regards to the recast level. However, since I do have the Z position data saved and accessible, I’ll take a harder look at it. Maybe there’s some minimal stalling in all the holes that I hadn’t recognized. If so, the obvious first step is to adjust the process.

As far as going to oil, we run production aerospace parts on this machine of various types. Requalifying everything else in order to accommodate this one part would be a nightmare. And since we run tubes with IDs as small as 0.004 in, I’m not sure there’s an oil out there that would pump the same as water - certainly not with the pumps we currently use. It would be a big productivity hit.

John Z.