Fit a 15-inch touchscreen at arm's length and a knee-mill CNC retrofit reads comfortably: standard 16-pixel text on that panel has 3.3 mm capitals, good to 0.8 m. Mount the panel and its control computer off the mill, and drive the quill as the CNC Z axis with the knee for positioning only.
A knee-mill retrofit runs every session through one screen: work offsets, the tool table, the program listing, the alarms. The panel question is easier than it looks. Fit a 15-inch touchscreen at arm's length, right where you stand, and hang it on an arm or bracket bolted to a post or wall, off the mill casting. A 17-inch buys nothing at that distance; it pays only when the operator reads from past 0.8 m. This article runs the arithmetic, compares the common sizes, and settles the two decisions that sit around the screen: where the computer lives, and which axis becomes Z.
The answer: 15 inches at arm's length
For a single-operator retrofit, a 15-inch touch monitor is the right panel, mounted at the position you already stand in. Two numbers decide it: the arm's-length gap between your eyes and the panel, and the size of the text you draw on it.
At that distance the comfortable-reading figure, 20 ft of viewing distance per inch of capital letter height (the safer version of the FHWA highway-sign planning figure), asks for capitals about 2.5 mm tall. Standard 16-pixel body text on a 15-inch panel carries 3.3 mm capitals. That is a third of margin, and the same text still meets the comfortable figure out to 0.8 m.
A touchscreen also earns its place on the operator panel: soft keys drawn on screen replace rows of pushbuttons, and the layout changes per screen. The distance arithmetic is the same one behind our screen size and viewing distance guide, scaled from a lobby sign to a panel you can touch.
What decides the size
Viewing distance decides the minimum text size, and text size decides how much screen you need. Work the chain in that order:
- Measure the standing position. At a mill you work at the controls, so a panel on an arm sits about 0.5 to 0.6 m from your eyes.
- Apply the legibility figures. Road-sign practice takes 30 ft of viewing distance per inch of capital height as the minimum and 20 ft per inch for fast reading, with line spacing at 1.7 times the capital height.
- Convert to pixels. HMI projects draw text in pixels, so a character's physical size is its pixel height times the panel's pixel pitch, and the pitch shrinks as resolution climbs on a given diagonal.
An HMI selection guide bands panels by operator distance: up to 0.5 m, a 4 to 7 inch panel; 0.5 to 1 m, 8 to 10 inch. Those bands size a simple button screen. A mill screen carries four windows at once, position readout, program listing, tool path and alarm banner, so take the top of the band and a step up: 15 inches at an arm's-length stand.
Example: a 15-inch panel at arm's length
Inputs. Standing distance 0.6 m, arm's length at the controls. Panel: the standard 15-inch 4:3 touch monitor, 1024 × 768 pixels, 304 mm viewable width. Text: 16-pixel body cells, with capitals at about seven tenths of the cell height in most fonts. Figures: 20 ft per inch comfortable, 30 ft per inch minimum, FHWA-based.
Rule. Capital height in millimetres equals cell pixels times pixel pitch times 0.7, where pitch is viewable width divided by horizontal pixels. Comfortable reading distance in feet equals capital height in inches times 20.
Arithmetic. Pitch: 304 divided by 1024 = 0.297 mm per pixel. Capitals: 16 x 0.297 x 0.7 = 3.3 mm, or 0.131 in. Requirement: 0.6 m = 1.97 ft, so the comfortable figure asks 1.97 divided by 20 = 0.098 in = 2.5 mm and the minimum asks 1.97 divided by 30 = 1.7 mm. Margin: 3.3 divided by 2.5 = 1.3. Range of the same text: comfortable to 0.131 x 20 = 2.62 ft = 0.80 m; minimum to 0.131 x 30 = 3.93 ft = 1.20 m.
Result. Standard text on the 15-inch reads comfortably at the arm's-length stand with a third of margin, and it holds the comfortable figure to 0.8 m exactly, because a 0.131 in capital at 20 ft per inch lands on 2.62 ft.
Decision. Fit the 15-inch for a stand-at-the-controls retrofit. Step to the 17-inch, whose larger pixels give the same 16-pixel text 3.8 mm capitals and a 0.9 m comfortable range, when the operator reads from past 0.8 m.
The sizes compared
Ranges below are computed the way the example computes them: 16-pixel text, capitals at seven tenths of the cell, the 20 ft per inch comfortable figure.
| Panel | Resolution | Viewable width | 16 px capitals | Comfortable range | Fit it when |
|---|---|---|---|---|---|
| 12 inch | 1024 × 768 | 246 mm | 2.7 mm | 0.65 m | It lives on the splash guard and you lean in to read |
| 15 inch | 1024 × 768 | 304 mm | 3.3 mm | 0.80 m | One operator at the controls; the retrofit default |
| 17 inch | 1024 × 768 | 345 mm | 3.8 mm | 0.90 m | The operator stands back past 0.8 m |
| 19 inch | 1280 × 1024 | 376 mm | 3.3 mm | 0.80 m | Same text size as the 15-inch, two thirds more screen; pick it for content or two people |
| 21.5 inch | 1920 × 1080 | 477 mm | 2.8 mm | 0.67 m | A full desktop of windows, program, tool path, CAM; raise the font one step |
A higher resolution on the same diagonal buys window space and costs text size; raise the font one step and the 21.5-inch still carries more windows than the 19-inch.
Mount the panel and computer off the mill
A knee mill shakes on every cut. Hang the panel on an arm or post bracket a step away from the casting and it stops sharing the machine's vibration while landing at the arm's-length reading distance. Match the bracket to the panel by VESA pattern and load rating; brackets publish both, and ours, for instance, is listed for VESA 75 and 100, for panels from 14 inches up to 30 lb, about 12 mm off the wall (mounting and power accessories).
Put the control computer off the machine too, in the cabinet beside the panel or on a wall plate behind it. A spinning hard drive is the component that suffers most on a shaking machine, so fit solid-state storage and strain-relieve every cable that crosses from the cabinet to the mill. If the enclosure door is the natural spot, a flush cutout with a backing plate works; mounting a touch panel in a wall cutout shows that job.
The panel pairs with the computer one of two ways: a touch monitor driven by the video output of a mini-ITX-class x86 computer, or a panel PC that folds the computer into the display. Touch input reaches the PC as a standard USB device; the details of driving a computer's desktop from a touch panel are in using an HMI panel as a PC monitor.
A standalone signage player is not this screen. Our panel-mount Android displays play their own pictures and video from storage, and the HDMI port is an output, so there is no video input for the control computer to drive. Where they fit a shop is as a second screen by the cell, carrying job queues, setup sheets or maintenance notices; see the shop-floor screens and the panel-mount display range.
Keep the hardware emergency stop and feed hold as physical buttons within reach. The touchscreen carries soft keys and screens; it never carries the emergency functions.
Quill for Z, knee for positioning
Drive the quill as the CNC Z axis. The knee's moving mass is the whole stack: knee casting, saddle, table and the workpiece. Peck drilling accelerates that stack up and down at every retract, and the inertia is enormous; the knee was never intended for that kind of active use, and a servo big enough to force it is slow on the cycle. The quill moves the spindle head alone, so the motor is smaller and the peck cycles behave.
A motorized knee still has a place: position it between setups, clamp it before cutting, and leave every cutting move to the quill. Some knee-type mills take servo Z on the knee as built, several European designs among them, many with no quill at all; a converted manual knee mill is not built that way.
Before the servo goes on, make sure the quill slides freely through its travel. Sticky quill travel is a known service item on knee mills, and a sticky bore turns every Z move into a fight between the servo and the spindle.
When to pick something else
Step to the 17-inch when the operator reads from past 0.8 m; the example puts the 15-inch's threshold at exactly 0.8 m. Pick the 19-inch when two people share the panel or you want program and tool path side by side; its text size matches the 15-inch, so the distance rule stays. Take the 21.5-inch only with a font bump, for a screen that doubles as a CAM desktop.
Touch type follows the hands. Resistive registers pressure, so it works through gloves and tolerates mist; capacitive needs a bare or conductive fingertip and gives the sharper image. If you add a protective cover over the panel, small text can soften; the anti-glare cover sharpness fix covers that case. In a garage with the door open, an office monitor's 250 to 300 nits washes out in direct sun; shade the spot or order a high-brightness panel.
What to ask for
Put this list in front of the supplier:
- 15-inch class touch monitor, 1024 × 768 or better, with the viewable width printed in the datasheet so you can run the example arithmetic for your own standing distance.
- Resistive touch if the shop runs gloves; capacitive if hands are bare.
- VESA 75 or 100 mounting, or a panel-mount bezel for a flush cutout, with a load rating at or above the panel's weight.
- A control computer, mini-ITX or panel PC, with solid-state storage, mounted off the mill with cables strain-relieved.
- One DC feed for cabinet, panel and computer, so nothing runs off a wall strip near the chips.
Then test before it goes on the machine: power the panel with the control computer, open the screens, and check startup, touch response and text at your standing distance; choosing an HMI panel: test startup and runtime first gives the short version.
Recommendation: Fit a 15-inch resistive touch monitor at 1024 × 768 on a VESA 100 arm at the arm's-length stand, with the mini-ITX computer and solid-state drive in the cabinet beside it; step to 17-inch only when the operator reads from past 0.8 m, and drive the quill as Z with the knee for positioning only.
Questions
Should I motorize the knee at all?
Yes, if you want powered positioning between setups: raise or lower the knee to reset head height, then clamp it before cutting. Keep every cutting Z move on the quill. The knee only becomes a problem when it must accelerate its whole stack of castings, table and workpiece at every peck; a slow positioning move does not ask that of it.
Capacitive or resistive touch on a mill?
Resistive if you machine in gloves or near coolant mist: it closes under pressure, so a gloved fingertip registers. Capacitive if you work bare-handed and want the sharper image; a dry cotton glove will not register on it. Either way, mount the panel outside the splash zone and wipe it at shift change.
How far can the panel sit from the control computer?
Keep them within one monitor cable: about 5 m for HDMI or DVI, while VGA at 1024 × 768 stays stable at 15 m or more over a good cable. Past that, use a video extender, or move the computer into the cabinet behind the panel, which is the cleaner answer in a one-operator shop.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.