Q related to Mach CV settiings

[motoguy]

I'm trying to cut a hole for a fitting. The hole looks like this:

Fitting Hole.JPG

It's a circle, with flat top and bottom. I'll be cutting it out of 16GA steel. When I try to cut the hole with my current settings, Mach turns it into a big skewed oval. Current settings are X,Y,A accel 30, shut off CV > 89, 225IPM feed rate. Lookahead is 200, no other CV boxes checked.

When I slow the feed rate to 100IPM, the issue goes away. However, it's 16GA, so I need to be cutting faster than that. I read in another post that turning up the accel can help with this (CV issues), so I'll play with that, but I don't want to cause damage to the table. It's a 6x12 Bulltear, current (newest) design.

20160328_171602_resized.jpg

If I cut in Exact Stop mode, it looks great (with the ink pen). I'm afraid that extra dwell time will cause divots with the plasma torch.

I'm still extremely green with this...so I thought I'd ask the pros! Tips or tricks on dealing with this?

[acourtjester]

This is not a direct answer to your question, but are you using a lead in and out (turn off THC before end) when cutting this would get rid of the divot.

[Greolt]

As with almost all types of CNC machines, acceleration is king.

If there is room to improve that, then that is the best thing you can do for CV performance.

However being the latest table from a manufacturer, they should have tuning parameters for you to use.

[motoguy]

This is not a direct answer to your question, but are you using a lead in and out (turn off THC before end) when cutting this would get rid of the divot.

I have been using a lead out on everything except circles. Reading some of Jim Colts advise, it sounds like a lead out is not necessarily the best idea on a circle. I have not, however, been turning the height control off before the end of a cut on a circle. That may explain why I'm getting a small divot at the end. Well, that combined with the arc lead in i have been using for circles. Jim's advice states that a perpendicular lead in is the best, if the acceleration rate and speed of the machine can handle it.

[BTA Plasma]

What you don't want to rely on is just acceleration rates alone when your talking about higher speeds. You want to start controlling where the machine makes an exact stop but make sure the machine isn't stopping at every directional change. This is done by using the 'stop CV at angles' in the general config screen in Mach. The reason this works so well is that Anti-dive keeps the torch from dipping. Not all machines will do that and not all machines will cut at book speed. Either not enough power or no anti-dive will limit the higher end federate. Even the bigger Star-Lab machines 6x12-8x24 will indeed cut at book speed but when you get to a certain range you should be using those CV off methods not just exact stop in Mach.

[mechanicalmongoose20]

Not sure on the size, but you could always run DTHC off and slow your feed down to the 100ipm. On smaller shapes and thin materials the acceleration will kill your resolution. Plus it is recommended to run DTHC off on small features to keep the torch from diving, you just may have a touch more dross in that area.

What I have been told is NEVER run in exact stop mode while plasma cutting. The CV off feature will still give you some divots as the machine pauses going around the part.