Powermax 85 settings for c220 commercial bronze

[vchild]

Hi all. I'm new to the forum. I was wondering if there is anyone out there cutting bronze on a CNC plasma table. I'm trying to cut 1/4" c220 commercial bronze plate into small curved shapes approximately 2"X3". Unfortunately I can't find any consistent recommendations on the settings for 1/4" bronze. My equipment includes:

Arclight Dynamics 5'X5' CNC plasma table with water table
Hypertherm Powermax 85
Command CNC controller/software with digital torch height control
Sheetcam software

The system is new and I am new to plasma cutting (have owned it and been playing with it for several months). So far I've had great success cutting mild steel, stainless steel and aluminum but I have recently been requested to cut 1/4" bronze. I'm aware that bronze sucks up heat pretty quickly due to the high copper content. One recommendation was to use the settings for 1/4" aluminum but slowing the feed rate. Another suggestion was to use the settings for mild steel, again slowing the feed rate. I did some 6" straight line tests cuts with new consumables each time and neither one worked very well. The results varied from cuts that were sloppy and irregular on the backside with lots of bevel and heavy dross, to cuts with very narrow kerf width that looked okay for the first 4" but then failed to achieve full penetration on the last 2" (more like a gouge on the last 2"). In both situations the shield became caked with thick dross, although the nozzle was spotless afterwards. I was using a 45 amp nozzle and settings.
Any advice would be greatly appreciated including whether or not to leave torch height control on or off (very small pieces cut out of very flat plate).
Also should I be cutting with the water table fairly full as usual to stabilize the temperature of the metal, or would that draw away too much heat?
Thanks

[jimcolt]

There are a lot of different bronze alloys, and you are right it is tough to cut due to the heat transfer, much like aluminum is. On Bronze it is always best to start with the aluminum specs. It is important to maintain the physical cut height of .06", so every time you change cut speed you will need to adjust arc voltage so the cut is achieved at .06". You can try 45 amp shielded consumables, but may need to go to 65 amps for best results. I would set the torch at .06" with a thickness gauge, start on an edge (so you don't have to pierce at .06" which will damage the nozzle), and cut a series of 4" long straight cuts at different speeds....eventually you will see improvement.....once you like what you see, observe the digital voltage readout during the cut at the ideal speed and fixed height.....and there you will have the voltage and speed setting that works best for your alloy. Jim Colt Hypertherm

[vchild]

Thank you for the advice Jim. I'll give that a try.
Tom

[vchild]

Update on the test cuts:
I did a series of edge start 4" test cuts using the 65amp nozzle and modified settings for 1/4" aluminum at 65amps. I first confirmed 0.06 inch torch height by measuring and turned off the torch height control. I incrementally decreased the feed rate by fixed percentages from the original setting of 105 ipm. I recorded video of the screen during each cut to capture the average torch volts on each cut. The first three cuts were at 90%, 80% and 70%. None of the first three cuts even penetrated the 1/4" bronze plate. They just gouged the surface for 4" although the 70% feed rate did discolor the bottom of the plate a little under the gouge. I then tried larger decreases in the feed rate. The next three cuts were at 40%, 30% and 20% of the aluminum feed rate. The 40% rate barely penetrated the plate but did so for the entire 4". There was significant hard dross on the top and bottom. Also, the bottom fourth of the cut surface was extremely rough which indicated to me that the torch barely made it through the plate.
The test cut at 30% (31 ipm) was much better. On the top of the plate there was no dross on the right side of the cut (the part side) and moderate dross on the left side (the scrap side). The cut surface was consistent from top to bottom and significantly smoother than the previous cut. Not smooth like mild steel but no rougher than aluminum. There was a minimal amount of bevel.
The last test cut at 20% feed rate had significant dross on top and especially on bottom. The cut line wandered a little and the cut surface was slightly irregular. There was also noticeably more bevel.
So far the winner is 30% of aluminum feed rate. I guess I could continue fine tuning with additional test cuts but the price of 1/4" bronze plate is astronomical and I have just enough left over right now for my part to be cut twice (in case I botch the first cut). I'll go for the 30% on my first attempt.
Thanks again.