How to Change Bits Mid-Job Without Ruining Your Project

Why You’d Need to Change Bits Mid-Job

Here’s the thing about CNC work: most interesting projects need more than one bit.

Maybe you’re doing a 3D carving that needs a big bit to hog out material fast, then a small ball nose to carve fine details. Or you’re cutting a sign where you rough out the pockets with an upcut bit, then switch to a V-bit for crisp lettering. Or you’re making a box that needs a straight bit for the joinery and a roundover bit for the edges.

Single-bit jobs are fine for learning, but once you start making things that people actually want, you’ll bump into this situation fast.

The good news: changing bits mid-job is completely safe and repeatable once you understand what’s happening. The bad news: if you mess it up, you’ll either ruin your workpiece or snap a bit. Sometimes both.

We’re going to walk through exactly how to do this without the anxiety.

The Real Problem: Z Changes, XY Doesn’t

When you swap bits, two things stay the same and one thing changes.

What stays the same:

• Your X zero (left/right position)
• Your Y zero (front/back position)
• Your material position on the bed

What changes:

• Your Z zero (height), because different bits have different lengths

Think about it: if you swap a 1-inch bit for a 2-inch bit, the tip of the new bit is now 1 inch higher than where the old bit was. Your CNC doesn’t know this happened. It’s still using the old Z zero position. If you tell it to cut at the surface of your material, it’s going to plunge an inch too high and cut nothing but air.

The entire challenge of mid-job bit changes is re-establishing that Z zero for each new bit while keeping X and Y untouched.

Your Two Options for Resetting Z

There are two common methods hobbyists use. Both work. One is significantly easier.

Method 1: Touch Plate (The Easy Way)

A touch plate is a small metal block with a wire attached. You connect the wire to your CNC, place the block on your workpiece (or spoilboard, depending on how you set up your job), and run a probing routine. The bit slowly lowers until it touches the plate, completing an electrical circuit. Your CNC now knows exactly where Z zero is for that bit.

Touch plates typically cost $20-40. If you’re planning to do more than a handful of multi-bit projects, just buy one. The time and stress savings are worth way more than the cost.

Pros:

• Fast (takes about 10 seconds per bit)
• Repeatable to within a thousandth of an inch
• Works the same every time, regardless of who’s using it
• No guessing, no “feel”

Cons:

• Costs money
• One more thing to plug in and keep track of
• Requires your CNC to support probing (most hobbyist machines do)

Method 2: Paper Method (The Free Way)

Place a piece of paper on your workpiece surface. Lower the bit manually (using jog controls) until it just barely pinches the paper. You should be able to slide the paper out with light resistance. When you feel that, you’re at Z zero (plus the thickness of the paper, which is negligible for most projects).

This is how people did it before affordable touch plates existed, and it still works fine if you develop a consistent feel for it.

Pros:

• Free (you already have paper)
• Works on any CNC, no probing support needed
• Simple to understand

Cons:

• Slower (especially when you’re learning)
• Requires developing a “feel” for the right amount of drag
• Easy to crash the bit if you’re not careful
• Inconsistent between different people or different bits

Software Setup: One Job, Multiple Files

Before you even think about changing bits, you need to set up your toolpaths correctly in your CAM software. The approach is the same whether you’re using VCarve, Easel, Carbide Create, or Fusion 360.

The basic workflow:

1. Design your project as normal
2. Create a toolpath for bit #1 (roughing, pockets, whatever comes first)
3. Create a separate toolpath for bit #2 (finishing, V-carving, etc.)
4. Make sure ALL toolpaths use the same XY zero position
5. Save each toolpath as a separate G-code file

Most CAM software makes this easy. In VCarve, for example, you just create multiple toolpaths and save them individually. In Easel, you set up different “cuts” with different bits and can choose to run them separately.

Critical point: Your XY zero must be identical for every toolpath. If you zero off the lower-left corner of your material for the first bit, you zero off that exact same corner for the second bit. If you zero to the center, zero to center for both. The software needs to think all the bits are starting from the same position.

Z Zero: Top of Material vs Top of Spoilboard

You have two options for where Z zero lives:

Top of material (most common for hobbyists):

• Pro: Your toolpaths reference the actual surface you’re cutting
• Pro: Material thickness variations don’t matter as much
• Con: You have to re-zero Z on the material surface for every bit change

Top of spoilboard:

• Pro: Z zero never changes between bits (as long as your material thickness is consistent)
• Pro: Faster bit changes
• Con: You must enter exact material thickness in your CAM software
• Con: Material thickness variations will throw off your cuts

Most hobbyists use top-of-material because it’s more forgiving. If your plywood is actually 0.72” instead of 0.75”, it doesn’t matter since you’re zeroing directly to its surface.

Step-by-Step: Changing a Bit Without Losing Zero

Let’s walk through an actual bit change. We’ll assume you’re using a touch plate and zeroing to top of material (the most common setup).

Before you start the job:

1. Load your first G-code file (bit #1)
2. Install bit #1 in your router
3. Jog to your chosen XY zero position (we’ll use lower-left corner of material)
4. Set XY zero in your controller
5. Place touch plate on material surface
6. Run Z-probe routine to set Z zero
7. Remove touch plate
8. Start the first toolpath

When it’s time to change bits:

The first toolpath will finish and your spindle will stop. Most controllers will pause and prompt you for a tool change, or you can set up your CAM software to insert an M0 (pause) command between toolpaths.

1. Don’t turn anything off. Your CNC controller needs to remember your work coordinates.
2. Jog the Z axis up to give yourself room to work (5-6 inches is usually plenty)
3. Stop the router/spindle (but don’t turn off the CNC controller)
4. Swap the bit (use two wrenches, don’t overtighten)
5. Place the touch plate on the material surface at the same spot you used before (this doesn’t have to be exact, just on the material)
6. Run the Z-probe routine again
7. Remove the touch plate
8. Load your second G-code file
9. Press start

The second toolpath will run perfectly aligned with the first because XY never moved and Z is now correct for the new bit.

Common Mistakes (and How to Avoid Them)

Mistake #1: Picking an XY Zero That Gets Cut

If you zero to a corner of your material and then your first toolpath cuts away that corner, you’ve just destroyed your reference point. When you go to run the second toolpath, it won’t align because the physical corner you zeroed to no longer exists.

Fix: Choose an XY zero location that won’t be touched by any toolpath in your job. A corner that’s outside your cutting area, or a spot on the spoilboard, or the center of your material (if you’re cutting around the edges). Scout your toolpaths in your CAM software preview to make sure your zero point is safe.

Mistake #2: Re-Zeroing XY When You Meant to Zero Only Z

This is an easy one to mess up if you’re rushing or distracted. You swap bits, grab your controller, and hit “Zero All Axes” instead of “Zero Z.” Congratulations, you just shifted your coordinate system and your next toolpath is going to cut in the wrong place.

Fix: Slow down. Read what you’re clicking. Most controllers have separate buttons for “Zero X,” “Zero Y,” and “Zero Z.” Only touch the Z button during a bit change.

Mistake #3: Turning Off the CNC Between Toolpaths

Your work coordinates live in your controller’s memory. When you power cycle the machine, that memory gets wiped. Your CNC will still know where it physically is (if it has homing switches), but it won’t remember where you told it XY zero was.

Fix: Leave the CNC controller on for the entire job. If you need to stop the router between toolpaths to save electricity or reduce noise, that’s fine. Just don’t power down the controller itself.

Mistake #4: Not Accounting for Bit Length Differences

We already covered this conceptually, but it’s worth repeating because it’s the most common failure mode: different bits are different lengths. A stubby 1.5-inch bit and a long 3-inch bit will have their tips in very different Z positions even if they’re both installed in the collet the same way.

This is why you re-probe Z for every single bit. No exceptions.

Mistake #5: Changing Feeds and Speeds Between Bits Without Updating G-Code

This one catches people who are trying to optimize as they go. You run your roughing pass, realize it was too aggressive, and decide to slow down the feed rate for the finishing bit. You change the setting in your CAM software, but you forget to regenerate and reload the G-code file.

Fix: If you change any toolpath settings (feeds, speeds, depth of cut, anything), you must re-save the G-code file and reload it into your controller. The controller runs whatever’s in the file, not whatever’s currently in your CAM software.

When to Use Multiple Bits vs Sticking with One

Not every project needs a bit change. In fact, most simple projects are better off using a single bit if you can.

Good reasons to use multiple bits:

• Speed. A big bit roughing out material is way faster than a small bit trying to do the same work.
• Quality. A V-bit or ball nose can carve details that a straight bit can’t touch.
• Specific features. Some cuts just require specific bit geometry (roundovers, chamfers, keyhole slots, etc.).

Bad reasons to use multiple bits:

• Because it seems more professional
• Because a tutorial you watched did it that way
• Because you want to try out your new bits

If a single bit can do the job in a reasonable time with acceptable quality, use a single bit. Every bit change is an opportunity for something to go wrong. Keep it simple until the project demands otherwise.

For reference, here are some common multi-bit workflows that make sense:

• 3D carving: Roughing bit (1/4” or 1/8” end mill) to clear material, then ball nose (1/16” or 1/8”) for detail
• Sign making: Upcut or straight bit for pockets, V-bit for lettering and detail lines
• Furniture parts: Straight bit for joinery cuts, roundover or chamfer bit for edges
• Inlays: Straight bit to cut the pocket, V-bit to cut the inlay piece (with a taper so it fits snug)

Advanced Tip: The Reference Pin Method

Some experienced users drill a small hole in their spoilboard and use a precision dowel pin as a permanent XY zero reference. They zero to the center of that pin for every project.

Pros:

• Your XY zero is always in the exact same place
• You can remove and replace material without losing position
• Useful for repeated production runs

Cons:

• You have to design around it (your material has to be positioned relative to the pin)
• Requires a bit more setup thinking for each project
• Not worth the hassle for one-off projects

This is a “nice-to-have” optimization, not a requirement. But if you’re making multiples of the same project, it’s worth considering.

Multi-Bit Projects to Practice On

If you’re new to bit changes and want to practice without the pressure of ruining an expensive piece, here are some forgiving starter projects:

Two-bit sign: Design a simple rectangular sign with text. Use a 1/4” straight bit to cut the outline and a 60° V-bit for the lettering. If you mess up the alignment, the worst that happens is the text is slightly off-center.

Shallow relief carving: Find a simple clipart relief file (lots of free ones online). Rough it out with a 1/8” end mill, detail it with a 1/16” ball nose. Work with soft wood like pine or basswood so you’re not fighting the material.

Coaster with inlay: Cut a shallow pocket with a 1/8” end mill, then switch to a small V-bit to cut a decorative pattern in the pocket floor. Small project, low material cost, good practice.

The key is to pick projects where a small mistake is visible but not catastrophic. You want feedback without punishment.

Software-Specific Notes

Different CAM programs handle multi-bit workflows slightly differently. Here are quick tips for the most common hobbyist options:

VCarve / Aspire: Set up all your toolpaths in one project file. When you’re ready to save, save each toolpath individually (there’s a “save selected toolpath” option). This gives you separate G-code files for each bit.

Easel (Inventables): Easel can handle tool changes, but the free version has limitations. Set up different “cuts” for each bit and Easel will pause between them for you to swap bits. Pro version gives you more control.

Carbide Create: Similar to VCarve. Set up separate toolpaths and save them individually. The workflow is straightforward.

Fusion 360: This one’s more complex because it’s a full CAM package. You can set up tool changes within a single operation or split into separate operations. The post-processor you’re using determines how it outputs the G-code. Check your machine manufacturer’s documentation for the recommended approach.

If you’re not sure how your specific software handles this, search “[your software name] tool change tutorial” and you’ll find plenty of walkthroughs.

The Bottom Line

Changing bits mid-job sounds scarier than it is. Once you’ve done it successfully a couple times, it becomes routine.

The key things to remember:

• XY zero stays the same, only Z changes
• Get a touch plate (seriously, just get one)
• Set up your toolpaths correctly in CAM software from the start
• Don’t power cycle the machine until the whole job is done
• Pick a safe XY zero location that won’t get cut

Start with simple two-bit projects to build confidence. By the time you’re ready to tackle a complex multi-bit carving, the bit-change part will be the easy part.

We’ve ruined our share of workpieces learning this stuff. The mistakes we’ve outlined above? We’ve made every single one. Some of them multiple times. You’ll probably make a few of them too. That’s fine. CNC is a learning process, and bit changes are one of those skills that click suddenly after a few frustrating attempts.

When it clicks, a whole new category of projects opens up. Projects that actually look professional instead of “obviously made on a hobby CNC.” That’s worth the learning curve.

Related: Our feeds and speeds guide covers the cutting parameters you’ll need for each bit, and the best bits for beginners guide helps you choose which bits are worth investing in for multi-tool projects.