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 336 times

narrow cut.JPG (91.11 KiB) Viewed 336 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.

[4130metalworks]

I did another trial piece today, this had 0.2mm added to the drawing in various ways
Left one was 0.2mm in the middle and straight lines like a triangle
Middle was an arch with 0.2mm larger mid section
Right was straight and had the thc on for all except the 5mm lead into and out of the corners
This was cut the opposite way to previous to check if the movement was a contributing factor.

[2strokeforever]

If you are close to an edge the heat of the cut can cause the steel to expand and warp.
To see this better cut half an inch off the edge of a 3/8 or thicker plate, you will see over an inch of warpage per foot.

I am not saying its your problem, but if you are moving slow this will be an issue, much less of an issue if you are moving 100+ ipm.

let us know your machine, amps and travel speed.

[4130metalworks]

Thats actually a very plausible explanation 2strokeforever, something i never thought of.
I cut 5mm at 1310mm / min at 136volt, those were slowed down 50% all around except for the right one which was full speed
Machine is homemade

[acourtjester]

I find the statement "Machine is Homemade" to be irrelevant. It excepts the theory a homemade table cannot be assembled with or use the same quality materials as a commercial table. I would say there are members here that will dispute that.
Now there is a reason for the error you are seeing, it just has not been found, yet. Many solutions have been found by member of PS, keep plugging.

[cuttinparts]

I still think it has something to do with the speed your torch is travelling at along the various areas of the profile. If you can prevent the torch from slowing down so early and reduce the amount that it slows down I think you might see an improvement. The slow downs are good to smooth out direction changes but when the torch slows the arc removes more material which is why I think this may be a contributor. Many systems allow you to adjust this acceleration and decelaeration, not sure if you have access to that.