Slots smaller in middle

[4130metalworks]

Hi group, I am looking for some help on cutting slots in 5mm mild steel or 3/16

Here is a test cut, pictures are straight from the plasma. After a quick scrape on the table and the measurements from both sides.
My main question is why does my cut always bow in the middle? Also my zero offset straight line is wider at the start than the end, where should my kerf width be taken? Currently it's 1.5mm
All cuts are done with rules through sheetcam and its set for overcutting the sharp corners

[djreiswig]

The bow is probably from the acceleration and deceleration of the gantry.

[4130metalworks]

The bow is probably from the acceleration and deceleration of the gantry.

It could be, I have the acceleration rates turned way down as it was causing unnecessary wear on the table. Might be worth speeding it back up a bit and see if that improves

[acourtjester]

"Also my zero offset straight line is wider at the start than the end"
Are you using a THC on your table, the above could be caused by torch height changing, or table not being level. Meaning the metal surface no being the same distance from the torch tip in different locations.
I don't understand why you have notches in the corners of you cuts. see attached image. There should be only one pierce offset on a cut like those.

narrow cut.JPG (91.11 KiB) Viewed 113 times

narrow cut.JPG (91.11 KiB) Viewed 113 times

[4130metalworks]

The thc was turned off for everything except the outside cut, I usually have less issues on slots with it off, if they were larger then thc would be on. That whole test cut is only 120x120mm

The overcut on the edges is a feature in sheetcam, usually I don’t have it so extreme. This helps with keeping the edges sharp for slots and tabs, most call it dogbone

[adbuch]

"Also my zero offset straight line is wider at the start than the end"
Are you using a THC on your table, the above could be caused by torch height changing, or table not being level. Meaning the metal surface no being the same distance from the torch tip in different locations.
I don't understand why you have notches in the corners of you cuts. see attached image. There should be only one pierce offset on a cut like those.
narrow cut.JPG

Tom - perhaps they are intended as part of the design so that a rectangular piece with square corners will slide into the slot without interference. Just a guess.

David

[4130metalworks]

yes that's correct David, those are just test cuts to set up the actual job. The slots have another part that tabs in and gets welded over afterwards so any overcut on the edges isn't a problem.

[4130metalworks]

also the large slot is meant to be 5x80mm and the smaller one 5x20mm and were done with an inside offset. Could it be the kerf that needs adjusting?

[adbuch]

also the large slot is meant to be 5x80mm and the smaller one 5x20mm and were done with an inside offset. Could it be the kerf that needs adjusting?

If the slot is too narrow, you can either adjust your kerf width setting or adjust the dimension on your drawing to achieve your desired slot width.

Setting the kerf width to a smaller value will effectively widen the slot by the amount you reduce your kerf width setting. So if you reduce it in your software by 0.010", this should result in the slot width increasing by 0.010".

David

[cuttinparts]

I think your slots are thinner in the center because the torch is slowing down near each corner allowing the arc to remove more material than it does at full cut speed along the 2 long straight cuts in the center.