Today I have been testing with 1/2" for the first time as I'm using that for a fabrication table top. I'm doing 1/2" holes on 4" grid. Anyway, I've got the hole quality pretty damn good other than a flat spot at the beginning/end of the cut.
I'm at 60% speed and using early torch off on a .15" overcut. Perpendicular lead in with no lead out. I'm cutting inside offset on .525" holes. All other settings per Hypertherm book. Any suggestions or is this as good I can expect?
The beginning/end of the actual hole is the right side (3 o'clock) in this pic. (Bottom side of hole. Top is pretty much round.). I can fit a .5" bolt through with no cleanup.
You have my vote for best thread title on this site.
Have you tried arc lead in/lead out? I've read Jim Colt's suggestions of perpendicular lead in, small overt, and no lead put. However, I seem to end up with flat spots like you have.
I do an arc lead in (equal to material thickness if I'm piercing), small overcut (. 1" to . 2" depending on material), and a slight arc leadout (1.5x kerf width or so).
I've been happy with that on 1/2" plate, but I wasn't doing 1:1 (hole diameter to plate thickness) like you are doing.
Very interesting. I just tested your settings and it's by far the roundest hole, but leaves a divot on the bottom edge of hole. With your settings, but adding in "early torch off", I get a very slight flat spot and no divot.
I was going off of Jim's suggestions so far. I'm going to run a few more test holes as I've lost myself on which are which. Then it's time to hopefully not ruin a $300 plate.
Hole #1 - .5" Arc lead in, no lead out, .2 overcut, early torch off
Hole #2 - .5" Arc lead in, no lead out, .2 overcut
Hole #3 - .25" perp lead in, no lead out, .15 overcut, early torch off
Holes #1 & 2 are almost identical. No flat spot, but there is a slight "crowding" (for lack of better term) on the bottom of the hole around the 7:30-10:30 o'clock position. I attribute this to the dross on the top side being closer to the actual circle from the arc lead in. I believe this is why Jim Colt pushes the perp lead in.
Hole #3 is the same as before, flat spot at the 6 o'clock position. The rest of the hole is nice and round on the bottom since it's not running into lead in dross on top.
Comparing holes using a bolt, hole #3 is superior in that when the bolt is inserted from the bottom, it is more centered in the top hole. This is due to the "crowding" on holes #1 & 2. The crowding causes the bolt to pushed towards the 3 o'clock position.
When the bolt is inserted from the top, Hole #2 wins for bolt fit. It has slightly less crowding than #1 and is the tightest fit of the three.
At this point on a 1:1 hole, I think it's a half a dozen/six. I'm way overthinking this as usual and for it's intended purposes I think any of the three would suffice. I'm going to use the settings for hole #3 simply for the hole being more centered top to bottom. I am going to keep your settings in mind for larger diameter holes where the arc lead in dross isn't such an issue.
Overall I'm actually impressed with the quality achieved. They are no drilled holes, but they will be more than sufficient for what it is.
