In one of my previous articles, I described installing a bed probe on my Graber i3. This upgrade allowed me to move beyond purely manual adjustment and start measuring the bed surface with Klipper.
However, I barely discussed why I chose that particular type of probe.
Today, there are many ready-made solutions for automatic bed probing. Among the best-known are BLTouch and its cheaper alternatives, such as 3DTouch.
For my printer, though, I chose a simple inductive sensor.
And I had several reasons for doing so.
Why Do You Need a Bed Probe?
Even if a 3D printer's bed is adjusted manually, its surface isn't necessarily perfectly parallel to the plane in which the printhead moves.
The bed itself may also have slight irregularities.
This becomes especially noticeable when printing the first layer: in one area, the nozzle is too close to the surface, while in another, it's too far away.
Klipper can measure the surface at several points and use the resulting data to compensate for these deviations.
But to do that, it needs some way to determine the distance to the bed.
And there are different approaches to this.
BLTouch and 3DTouch — How They Work
BLTouch is a probe with a retractable pin that physically touches the bed surface.
Before taking a measurement, the pin extends and the printhead moves downward. When the pin contacts the surface, the probe generates a trigger signal.
After the measurement, the pin retracts.
3DTouch is a common name for cheaper probes with a similar design.
The main advantage of this approach is that the probe measures the position of the actual printing surface.
The material the bed is made from hardly matters.
Glass, metal, PEI, or another coating — as long as the pin can properly touch the surface, the probe can take a measurement.
It's a genuinely versatile approach.
But it also has some characteristics I don't particularly like.
A Moving Mechanism — A Potential Weak Point
The defining feature of BLTouch and 3DTouch is the retractable probe pin.
For the sensor to work correctly, the pin must move freely, extend properly, and retract after each measurement.
That mechanism doesn't automatically mean poor reliability.
Genuine BLTouch probes are widely used in 3D printers and can operate quite consistently when installed correctly.
However, a moving part still adds another component that the accuracy of the measurements depends on.
This is particularly relevant for inexpensive alternatives.
Judging by user reviews, some 3DTouch probes have issues with measurement repeatability, triggering, or the retractable pin mechanism.
Of course, that doesn't mean all 3DTouch probes are unreliable.
But personally, I wasn't particularly confident in that mechanism.
If the same task can be handled by a sensor without moving parts, why not use that instead?
An Inductive Sensor — A Different Operating Principle
An inductive sensor works without physically touching the surface.
Inside the sensor is a coil that generates an alternating electromagnetic field.
When a suitable metal object enters the sensing area, the electrical characteristics of the system change, causing the sensor to produce a trigger signal.
There's no retractable pin involved.
The sensor is simply mounted next to the printhead and detects when a metal surface approaches.
That simplicity is exactly what appealed to me.
There's no mechanism that needs to extend before every measurement, no need to touch the bed, and no moving probe pin that could potentially be damaged.
Of course, this approach has its limitations.
The Main Drawback — Dependence on Bed Material
Unlike BLTouch, an inductive sensor can't work equally well with every surface.
It responds to metal, and its triggering distance depends on the type of metal.
For example, the same sensor may detect steel and aluminum at different distances.
The size of the metal object, the sensor's design, and the measurement conditions also affect the result.
If you're printing on thick glass with a metal bed underneath, a typical inductive sensor may not be suitable because the metal is too far away.
That's why this type of sensor needs to be chosen with the printer's specific construction in mind.
In my case, however, I wasn't planning to use a nonmetallic base for the printing surface.
In fact, I intend to switch to a removable magnetic build plate in the future.
What About a Magnetic PEI Build Plate?
At first, this might raise a question: PEI is plastic, while an inductive sensor detects metal.
But a typical magnetic PEI build plate isn't made entirely of polymer.
It's usually a flexible sheet of spring steel with a PEI coating.
A magnetic base attached to the heated bed holds the steel sheet in place.
PEI itself is nonmetallic, but there's steel underneath it.
That's what the inductive sensor detects.
So switching to a magnetic PEI build plate is entirely compatible with my chosen probing method.
Of course, the coating thickness and the sensing distance of the specific sensor need to be taken into account.
Also, replacing the build plate with another one may change the position of the printing surface relative to the metal detected by the sensor.
That's why it's important to check the Z-offset after making such changes.
But I don't see any fundamental obstacles to using an inductive sensor with this type of bed.
What About Measurement Accuracy?
It's important to understand that none of these sensors can guarantee a perfect first layer on its own.
BLTouch and 3DTouch determine the surface position through mechanical contact.
An inductive sensor measures the distance to a metal layer.
These are different measurement methods, each with its own potential sources of error.
With mechanical probes, the result depends partly on the condition of the pin and how consistently it triggers.
With inductive sensors, it depends on the metal, the distance to it, and the sensor's own characteristics.
Some inductive sensors are also sensitive to temperature changes.
This means measurements taken with a cold bed may differ slightly from those taken after the bed has heated up.
When using an inductive sensor, it's therefore important to check measurement repeatability and, where possible, take measurements under similar temperature conditions.
In my case, the installed sensor produced sufficiently consistent results to use it for bed calibration.
And that was what mattered most to me.
Cost Matters Too
There's another factor I didn't want to ignore: price.
A genuine BLTouch costs noticeably more than a typical inductive sensor.
For an old Graber i3, that purchase didn't seem particularly justified.
Of course, I could have bought a cheaper 3DTouch.
But then there's the question of the quality of the particular unit and how reliably it works.
Meanwhile, I already had an inductive sensor available.
So I didn't need to buy anything at all.
I could simply install the sensor I had, connect it to the printer, and test the result in practice.
That's exactly what I did.
And since I was satisfied with the result, I had no reason to switch to another solution.
Only for the Old Graber i3?
Interestingly, my choice isn't just about saving money on an old printer.
I'm also currently working on my own CoreXY printer project.
And I'm planning to use an inductive sensor in its design from the beginning.
The reasons are much the same: simple construction, no moving probe pin, and compatibility with a removable metal build plate.
Of course, when designing a new printer, you can plan for almost any type of probe.
But for now, I don't see a reason to switch to BLTouch or 3DTouch.
If an inductive sensor provides the required measurement repeatability and works with the chosen build surface, that's enough for me.
Does That Mean BLTouch Is Worse?
No.
BLTouch and similar contact probes have an important advantage: they can measure the actual printing surface regardless of whether there's a metal base underneath.
For a printer with a glass bed or other nonmetallic surfaces, that can be the deciding factor.
Genuine BLTouch is also a widely used and well-supported solution.
So I don't consider it a bad probe.
It's simply that, for my printer, its advantages didn't justify the additional cost.
And the potential weaknesses of the retractable pin mechanism pushed me toward a simpler solution instead.
Conclusion
When upgrading my Graber i3, I try not to replace components simply because newer or more popular alternatives exist.
Every modification should solve a specific problem.
The bed probe was no exception.
I already had an inductive sensor, it required no additional spending, and it had no moving mechanism.
At the same time, it provided the measurement repeatability I needed and suited my approach to the bed design.
It does have limitations related to surface material and temperature stability, but for my printer, they weren't reasons to reject it.
As a result, I ended up with a working bed probing system without buying a BLTouch or 3DTouch.
And for now, I see no reason to change anything.
What's Next
I had gradually sorted out the electronics and the bed probe, but there was still another important part of the old Graber i3 to deal with: its mechanics.
Even a properly configured Klipper setup and modern stepper drivers can't eliminate backlash, misalignment, or insufficient structural rigidity.
So the next stage was working on the printer's mechanical components.
That's what the next article will be about.