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Session 8: Toolpaths, Offsets, Step-Over & Step-Down

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Session 8

Toolpaths, Offsets, Step-Over & Step-Down

Your Box Can't Cut Anything Yet

Session 5's box is a model. A router can't read a model. It moves a spinning bit along a path, one line of instructions at a time.

Turning a shape into that path is called CAM: computer-aided manufacturing. In this course it happens in OnShape too, in a tab called CAM Studio.

CAM Machines a Saved Version

CAM Studio never cuts your live design. It cuts a version, a saved snapshot of it.

Change a dimension afterward and the CAM tab keeps cutting the old shape until you save a new version and update it. That is how a team machines last week's design without noticing.

Three Toolpath Types

Diagram showing three toolpath types side by side: Profile (cuts along an outline), Pocket (clears an interior area with a raster pattern), and Drill (plunges straight down at a point)

CAM Studio Has Its Own Names

Those are the names every machinist uses. CAM Studio's menus use different ones, so learn both: you'll hear the left column and click the right one.

What it's calledWhat CAM Studio calls it
ProfileType Edges, Pattern 2-Axis profile
PocketType Edges, Pattern 2-Axis rough
DrillType Holes, Pattern Drilling

Drill Has a Catch

A Drill toolpath plunges the bit straight down. So the hole it makes is exactly one bit wide, no more and no less.

Drill Has a Catch

Your arcade cabinet's buttons want holes of roughly 28–30 mm. The Genmitsu's bit is 3.175 mm and the ShopBot's is 6.35 mm.

So no button hole is a Drill toolpath. It's a Pocket or an inside Profile instead, and knowing that now saves you a failed job later.

The Bit Has a Diameter

It's not a mathematical line. It's a spinning cylinder with real width.

Run its center exactly along your drawn line, and the bit removes material on both sides of it.

Profile Offset

Diagram showing three profile offset options — Outside, On-Path, and Inside — with a dashed line showing the drawn boundary and a teal line showing where the bit's center actually travels for each option

Profile Offset

Cutting a part out: Outside. Everything the bit removes is waste, so the part comes out full size.

Cutting a hole: Inside. The bit stays inside the line, so the hole comes out the size you drew.

On-Path ignores the bit's width, so it's rarely right: a part comes out small and a hole comes out big.

In CAM Studio, an Arrow Makes the Choice

There's no Outside or Inside menu. When you pick an outline, CAM Studio draws a small red arrow across it, pointing to the side the bit will travel on.

The arrow points where the bit goes. Pointing away from the part is Outside. A Reverse button flips it to Inside.

In CAM Studio, an Arrow Makes the Choice

On-Path has its own setting. Under Offset, change Compensation type from Computer to None, and the bit's center runs right on the line.

Leave it on Computer. That's the setting where the arrow means anything.

Predict the Damage

The template's sample plate is drawn 150 × 100 mm, cut with a 3.175 mm bit. Somebody flips the arrow to Inside by mistake. How big does the plate come out?

Predict the Damage

Inside keeps the bit's whole width inside the line, so every edge of the plate loses a full 3.175 mm. Left and right both lose it, and so do front and back.

150 − 2 × 3.175 and 100 − 2 × 3.175: 143.65 × 93.65 mm.

Same idea as Unit 1 Session 9: a spec that's right on a screen doesn't guarantee a correct real part.

A Pocket Can't Be One Pass

A profile follows a line. A pocket has to clear a whole area, so the bit sweeps it in passes, each one overlapping the last.

Diagram comparing an even step-over, where overlapping passes clear the entire pocket area, against a step-over set too wide, which leaves ridges of uncut material between passes

A Pocket Can't Be One Pass

Step-over is how far the bit moves sideways between passes. Set it too wide and thin ridges stand between the passes, even though the toolpath "covered" the whole area.

Depth Comes in Layers Too

Cross-section diagram of material being cut in three progressively deeper passes before reaching final depth, with an arrow labeling the step-down distance between passes

Step-down is how deep each layer goes. A small bit cut too deep in one pass snaps.

Depth Comes in Layers Too

CAM Studio's default step-down for a pocket is 10 mm. The Genmitsu's bit is 3.175 mm across.

That default would bury the bit three diameters deep on its first pass. It snaps. The class template is set to 1.5 mm, and that's why the template exists: nobody gets a default by accident.

Something Has to Hold the Part

When the outline cut finishes, your part is free. With nothing holding it, it can shift into the spinning bit or be thrown off the bed.

Many CAM programs leave tabs, small uncut bridges. CAM Studio doesn't have tabs. It does this instead:

Something Has to Hold the Part

Cross-section of a board with the bit down in its cut. The cut stops 0.5 mm above the bottom, so a thin skin of wood still joins the part to the waste

Onion skin: the outline's last pass stops 0.5 mm above the bottom. The part stays held until the job is done, and you snap or sand the skin off after.

Two Things to Remember

Ask "am I keeping this or removing it?" before you set the arrow.

Keeping it means the arrow points away. Removing it means the arrow points in.

Two Things to Remember

Step-over is side to side. Step-down is top to bottom.

Ridges in a pocket floor mean step-over. A snapped bit means step-down. Don't fix the wrong one.

Today

  1. Copy the class CAM template into your own OnShape
  2. Break its offset on purpose and watch where the bit goes
  3. Find the step-over and step-down, and see what the 10 mm default does
  4. Find the onion skin

Next Time

Session 9: Apply CAM to a Practice Piece

Your own part in the template, toolpaths set up from scratch, and simulated before anyone trusts it.

'F' → Fullscreen

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Build

Step 1: Copy the CAM Template

  1. Open CAM Template - Genmitsu from the link your instructor shares. You can view it but not change it.
  2. Make your own copy: ≡ menu → Copy workspace… Name it Session 8 - your name.
  3. In your copy, open the CAM Studio tab. Under My part → Setup you'll find the bit and two toolpaths. Write down which toolpath type each one is.

Step 2: Find the Arrow

  1. Double-click Two axis profile to open it.
  2. Scroll to Drive edges and open it. The plate's bottom outline lights up orange, with a red arrow across it.
  3. Open Offset and write down the Compensation type.
CAM Studio's Two axis profile panel with Drive edges open: the plate's bottom outline is highlighted orange, and a small red arrow crosses the front edge pointing away from the plate

Which way does the arrow point: away from the plate or into it? Using the arrow rule, is this Outside or Inside?

Step 3: Break the Offset on Purpose

Before each change, write your prediction on your sheet: will the cut land outside the plate's edge, on it, or inside it? Then make the change and look.

  1. Flip the arrow. Click Reverse, the pen button next to Loop, then Ok. Right-click Setup → Regenerate all toolpaths, then click the ▶ beside Setup and choose Verify. Where does the cut land compared with the plate's edge?
  2. Flip it back, then try None. Reverse the arrow again, set Compensation type to None, and Ok. Regenerate and Verify again.
  3. Put it back. Compensation type Computer, arrow pointing away from the plate, regenerate.
The same panel after clicking Reverse: the red arrow on the front edge now points into the plate, and the Reverse button's tooltip is showing

If a toolpath turns red, its outline got unpicked, usually by the × next to Loop. Open it, click inside Drive edges, and click the plate's bottom edge again with Toggle loop on.

Step 4: Step-Over and Step-Down

  1. Open Two axis rough, the pocket. Find the Step down and Step over sections and write down both values.
  2. Predict: if Step down were CAM Studio's default of 10 mm, how many layers would it take to reach the pocket's 6 mm floor?
  3. Look first. Hover over Two axis rough and click the eye button at the end of its row to show the toolpath, then zoom in on the pocket. The red spiral is the bit ramping down; the green lines stacked beside it are the bit moving down from one layer to the next.
  4. Set Step down to 10 and click Ok. Show the toolpath again: editing hides it. What happened to the stacked green lines? How many layers is the bit cutting now?
  5. Set Step down back to 1.5 and click Ok.

Step 5: Find the Onion Skin

Open Two axis profile again and find Heights. Write down the End height and what it's measured from. In your own words: what does that number do to the plate when the job finishes?

Step 6: Which Fix?

For each problem, name what's really wrong: the offset, the step-over, the step-down, or the toolpath type.

  1. A pocket comes out with thin uncut ridges between passes
  2. A bit snaps on its first pass into a thick panel
  3. A finished plate is about 6 mm smaller than drawn in both directions
  4. A 28 mm button hole is programmed as a Drill with a 6 mm bit
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Checkpoint

Before you move on, confirm you can answer these out loud, in your own words:

  • Why does the same drawn line need a different offset depending on whether you're cutting a part out or cutting a hole into it?
  • What's the difference between what step-over fixes and what step-down fixes?
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Reflection

Answer these on your own. Reflections are individual, even when the rest of the session was team work.

  1. How is today's offset problem the same problem as Unit 1's tolerance work, even though none of the vocabulary matches?
  2. CAM Studio's default step-down would snap the Genmitsu's bit, so the class works from a template with safe settings already in it. What else in a template like that is worth getting right once, so nobody gets it wrong by accident?
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Submit

Submit your sheet with your predictions and what you found, your Step 6 answers, and your reflection. Keep your template copy; Session 9 starts from it.

Activity Complete