Cutting Slots Oversize or Not?

Still waiting on my order of the Crossfire, but in the meantime I am working on a design for a belt grinder as a kind of test to accuracy, and to learn how to use SheetCam. My question is that this is a weld together design, so I am making slots and tabs to join parts. On CAD, of course, the dimensions are exact, but I’m thinking that I should make the slots a few thou oversized so the tabs fit easily. I really don’t know the accuracy of the plasma cutter. I hear it’s within .005 when dialed in.

Second question is the holes. I can do a pierce or dimple to locate them and then drill them out, which I’ll do on any tapped hole. But I’d rather just cut out any through hole if possible. Bolts are 1/4" and 1/2". 1/2" is probably doable, but will it cut a 1/4" hole in 1/4" plate accurately?

I just got a Hypertherm 45xp. Most of the parts will be 1/4" steel plate, though a few parts will be 1/2". I’m designing on Autodesk Inventor 2012.

Thanks in advance.

It really depends on the material thickness but .02 to .05 added to a slot maybe enough.
Even with the most optimal conditions you’re going to end up with 2%to 4% bevel on your cuts.

Maybe when the stars align and you’re holding your mouth just right. Under normal product production never ever are you going to be in this kind of tolerance zone with plasma. The natural bevel in air cutting plasma alone sends that right out the window.

You could lose this kind of accuracy just in a few minutes of operation as the orifice size increases because of natural wear during cutting

If you’re running quarter inch I’d make those quarter inch holes at least 5/16 and I’d make those half inch holes 9/16 . On half inch I’d make those quarter inch holes 3/8 and I’d make a half inch holes 5/8. That’s probably a good start if you don’t want to have to drill or rework those holes off the table for your through holes.

If you’re cutting holes to tap I would just do a small Pierce and drill out the whole hole so you don’t have to tap through the heat hardened area.

"Cutting Slots Oversize or Not? "
Yes definitely plan to cut your holes and slots wider.


This is always a good example of plasma holes reality: Edge Quality & Minimum hole sizes - #10 by jimcolt

Every cutter will have it’s own level of accuracy or requirement for oversize of bolt holes.


Thanks guys! @TinWhisperer, I’ll up the hole size or pierce and drill. I do have a CNC mill, I may just have to make the pieces with some alignment holes so I can mill the holes quick. This is a learning process to see what I need to do.

On the slots would it work to create a tool profile with smaller kerf size in sheetcam so it cuts the slot oversize? ie, if the actual kerf is .050, I make a tool profile with a .040 kerf and when it cuts the slot will be nominally .020 oversized.

I get the issue with bevels blowing away accuracy. First thing I’ll do is run a bunch of test cards to see what the accuracy is like.

This will be the fun part, seeing how to get the most out of it! Now if the table would just ship…. I got the THC mod the other day, it’s like a tease :roll_eyes:

I would not do it like this.

Have your tool with the proper kerf width and add the allowance to the actual geometry.

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Respectfully, I’m curious why not and what the advantage is?

In my thinking (and I could easily be wrong), In changing the kerf, it allows me to alter the clearance of slots without going through the workflow. For example, if I tried a sample one, and it was a little too loose, I could increase the kerf size on the tool in SheetCam to tighten it up. If I do it with the original model, I would have to go back and redraw the CAD, and then go through the entire CAM process as well.

I guess you could do it that way.

Just as long as those holes happen to be all on the same layer and you only use those tools for the specific holes there are for.

You just need to make sure that nothing else is on that layer or else it will adjust that geometry as well.

It a huge disadvantage in sheetCAM having to backtrack to your design software if you want to make a change to the geometry.

I use fusion 360 so the design workflow in the cam workflow work harmoniously together.

Thanks for the info, it’s truly appreciated!

Yes, they would have to be the same layer. I’ve already learned how to do that. I would make tools labeled with over or undersized kerfs so I wouldn’t confuse them.

So on Fusion… if you have a model on Fusion, and change a dimension, that will automatically transfer to the work you’ve done on the CAM portion? I understand you’ll have to generate new gcode.

I’ve got years on Inventor, so I really like to use that. That said, Fusion is basically same but everything is in different places… which is frustrating :rofl: I haven’t bought SheetCam yet, but do like how it works - very straight forward. I use BobCam for my mill, but no post for that available from Langmuir.

Maybe I’ll just start playing with Fusion. I’m sure it’ll import .ipt files from Inventor - same company - so I could do work on either. There are advantages to that. I was also confused about it’s free status.

free does not mean free…it means limited free use…and can change at any time…


Yes that is exactly like that it’s fully parametric with history.

The free version (hobbyist) license of fusion 360 is is limited to some extent but is completely usable to design a product and produce g code to cut it.

Also there’s probably no other software that is advancing as quickly as fusion 360 is. They’ve just added an amazing plastics 3D interface to fusion and there’s always something new coming and being improved.
When it comes to developing CAD and CAM for product manufacturing nothing else holds a candle even close to fusion 360.

I think they’re only is one or two items that sheetcam has on fusion 360.

One is to center Pierce mark a hole.

And the other is… Maybe some sheetCAM people can shed some light on some of the functions sheetCAM has that fusion 360 does not?

edit ;You can alter the plunge rate that’s another one…

What else…

do you have shares in Fusion…??..ahahahahaha

Thanks! I think 10 active projects in the ‘free’ version? Kind of a pain, but worth a couple hundred a year. Pierce marking is a kind of major one, but can’t you just have it make a .050 hole to pierce a center? :thinking: Plus, I guess there’s no reason why you couldn’t generate a separate Gcode from sheetcam to pierce mark hole centers and run it independently - same 0,0 and don’t move the metal… or just copy and paste it into the Fusion generated GCode.

I sure wish I had shares in Autodesk.

I heard rumors that you had shares in Langmuir on another thread… maybe time to invest in Autodesk as well! :rofl:

You could cut a circle at .0011 or a line .0011 in fusion to have the same effect as a mark. Maybe .00011 is the smallest I forget now?

Fusion 360 like all Autodesk products have an incredible forum site as well.

There’s a shameless plug for the tutorial video series I made (making)

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I’ll check them out, thanks.

@TinWhisperer I may have found a way to cut the slots oversize in Fusion. In the 2D profile in the Passes section, there is a “Stock to Leave” option. It is to leave material that you will later want to clean up on the mill. It says you can put in a negative number to “remove more stock than the model shows”. This should cut oversize instead of leaving extra material to be milled. Should have known. This is a standard feature on most CAM programs.

Good find.

I have never used that option before. I’ll have give it a try and see how it behaves.

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I have to wait to get my table… I’m in the week 5-7 range, so maybe soon :grimacing: