Confusing Results

[garymkrieg]

I created to 1" boxes. Using a .25" end mill, Whitside, I cut an inside profile on one square and an outside profile on the other. I made measurements of the resulting squares in both the X and Y direction. Here are the results:
X direction outside profile the box measured 1.018"
X direction inside profile the box measured .974"

Y direction outside profile the box measured 1.035"
Y direction inside profile the box measured .971"

So clearly my steps per inch needs to be adjusted. The problem is the inside profile would have me adjust in one direction while the outside profile would have me adjusting in the opposite direction. Am I view these results correctly, and if so how do I explain the difference and make the correct correction?

[jimwill2]

You didn't say what the dimensions were supposed to be. Perhaps the bit is not actually .25". Make sure you are cutting outside, inside or on line depending on which you designed it to be. You can get some bit deflection depending on your speed and feed. It could be a hold down issue. Maybe your boxes aren't exactly the dimensions you want. Use the "Set Selected Objects Size" feature to make sure the box dimensions are correct.
I wouldn't adjust your CNC until you are positive the variations aren't cause by something else. Hang in there, CNC work has a heck of a learning curve.

[ger21]

Was one a climb cut, and the other conventional? You'll typically get two different numbers when changing cut direction due to deflection.

[TReischl]

I created to 1" boxes. Using a .25" end mill, Whitside, I cut an inside profile on one square and an outside profile on the other. I made measurements of the resulting squares in both the X and Y direction. Here are the results:
X direction outside profile the box measured 1.018"
X direction inside profile the box measured .974"

Y direction outside profile the box measured 1.035"
Y direction inside profile the box measured .971"

So clearly my steps per inch needs to be adjusted. The problem is the inside profile would have me adjust in one direction while the outside profile would have me adjusting in the opposite direction. Am I view these results correctly, and if so how do I explain the difference and make the correct correction?

I would not be too quick to adjust steps. You have not told us what you are cutting and the feedrate. We do not know how far you have the bit hanging out of the collet.

The way to adjust steps is NOT by taking cuts and measuring. You are introducing all sorts of errors using that method. The first is obvious, the bit size is probably not a perfect .2500 diameter. It may not even be perfectly round for that matter. The second is the collet being used. It may be running out and probably does. Then there is deflection of the spindle and the runout of the spindle itself. As you go farther back along the chain there are tolerances. Nothing moves without some clearance.

The only good way to see what your machine is doing is to use a dial indicator on a stand. They are incredibly cheap these days, you do not need a Starret or Brown & Sharpe. The local HF will have one that will work. You set it up and move the axis watching the indicator. If you command 1.000 of movement and it moves something more or less remember there is a bit of backlash in most motion systems.

Doing it by cutting will result in chasing your tail and eventually lead to mental issues.

A good way to see how large a bit is cutting is to take a scrap board, measure it accurately side to side or end to end. Clamp it down securely and cut it completely in half. Push the two halves together and take a measurement. This will tell you how wide a bit is cutting. Works on a table saw too. Measuring a .25 hole in wood with a dial calipers and declaring it is some dimension is at best hit or miss.

[garymkrieg]

Thanks for all the insight, I will take everyone's advice look at all the possible causes before trying to adjust the steps. Thanks again, I don't need any more mental issues!

[Rcnewcomb]

Thanks for all the insight, I will take everyone's advice look at all the possible causes before trying to adjust the steps. Thanks again, I don't need any more mental issues!

Ted's advice is solid.

Particularly on lighter weight machines there can be a surprising about of flex as speeds and loads increase. You may find that slowing down the feed rate, taking multiple passes, and changing the cut direction may be needed if accuracy is critical. However, if you are making a quick sign for the neighbor's horse stable then accuracy to +/- 0.001" is not needed.