Lathe Chuck Table

While getting my linuxcnc rettrofit lathe running again after moving shop. I wanted to turn close to the spindle. But my soft limits protect me from doing this. Of course you can ignore limits, but I was thinking about a "chuck table" with some more details on it like:
diameter, max rpm, z_limit, number of jaws and weight.

My concept so far...

Files:

• chucks.json – Stores your database of chucks, limits, and parameters.
  • current_chuck.json – Automatically created/updated to persist the currently active chuck across reboots.
    • chuck_manager.py – The Python handler script linked to your custom tab.
    
    Widgets:
    
    Widget TypeObject Name (objectName)Description
    QComboBoxchuck ComboBoxDropdown showing line numbers and chuck names (synced with the table)
    QPushButton activateChuckBtn  confirmation button to apply limits and save state.
    QTableWidget chuckTable 7-column interactive table to view, add, edit, and delete chuck profiles.
    QPushButton addChuckBtn Button to insert a new blank row into the table.
    QPushButton saveTableBtn Button to save table modifications permanently back to chucks.json.
    QPushButton deleteChuckBtn Button to remove the currently selected row.
    QLabel lblMaxRpm Displays the maximum allowed spindle RPM for the selected chuck.
    QLabel lblZLimit Displays the active Z-axis soft limit (ini.z.min_limit).
    QLabel lblDiameter Displays the chuck body diameter.
    QLabel lblJaws Displays the number of jaws.
    QLabel lblJawsDisplays the number of jaws.QLabellbl Weight Displays the chuck weight in kilograms.
    
    
    At the moment I have four chucks:
    
    {
       "Standard 3-Jaw Chuck (250mm)": {
          "max_rpm": 2400,
          "z_limit": -45.0,
          "diameter": 250,
          "jaws": 3,
          "weight_kg": 25.5
       },
       "centric 4-jaw Chuck (160mm)": {
          "max_rpm": 3000,
          "z_limit": -60.0,
          "diameter": 160,
          "jaws": 4,
          "weight_kg": 4.2
       },
       "Heavy 4-jaw Chuck (400mm)": {
          "max_rpm": 800,
          "z_limit": -30.0,
          "diameter": 400,
          "jaws": 4,
          "weight_kg": 40.0
       }
       "Power chuck (250mm)": {
          "max_rpm": 2400,
          "z_limit": -45.0,
          "diameter": 250,
          "jaws": 3,
          "weight_kg": 50.0
       }
    ​​​​​​   "Spindle": {
          "max_rpm": 3000,
          "z_limit": -110.0,
          "diameter": 60,
          "jaws": 0,
          "weight_kg": 0.0
       }
    }
    
    Next is writing and debugging the script.

I Cheated and used Claude in the end.If anyone wants to use soothing like this also here is a manual and the necessary files

OverviewThis UserTab lets a lathe operator pick the mounted chuck from a list and enforce that chuck's limits in HAL: a maximum spindle speed and a Z soft limit. It is built for Probe Basic Lathe (QtPyVCP) and tested on a Mesa 7i97 machine, but nothing in it is hardware-specific.

• Chuck table: add, edit, delete and save chucks (name, max RPM, Z limit, diameter, jaws, weight). Data lives in 

  chucks.json

   in the config directory.
• Dropdown + info line: select a chuck and see its data at a glance. The active chuck is highlighted green in the table.
• Activation with confirmation: one button writes both limits to HAL after a Yes/No dialog. If 

  halcmd

   fails, the chuck is not marked active and an error dialog appears.
• Plausibility check: rejects RPM <= 0, RPM above 

  [SPINDLE_0] MAX_FORWARD_VELOCITY

  , Z at or above 

  [AXIS_Z] MAX_LIMIT

  , and optionally Z below a configurable floor.
• Persistence: the active chuck is stored in 

  current_chuck.json

   and re-applied automatically at startup, so the display and HAL never disagree after a restart.
• Fail-safe RPM clamp: the spindle clamp is a HAL component with a conservative default, so the low limit holds even if the GUI never starts.

Requirements and file layoutYou need LinuxCNC 2.9 or newer (for the writable  HAL pins) and Probe Basic Lathe with UserTab support. Check the Z pin exists with .The folder name,  name and  name must match. The JSON files live in the config directory, not in the tab folder; the tab finds it through the  environment variable that LinuxCNC sets.Step 1: HALLinuxCNC has no writable HAL pin for the maximum spindle speed, so a  component is inserted into the speed command chain. The Z soft limit needs no HAL change: the tab writes the existing  pin.Find the line in your main HAL file that takes  (or ) to your output stage. Insert the clamp between them. Example for a chain that ends in a  component:Without a gearbox, connect  to whatever  fed before (a PWM generator, a scale component, a VFD pin). The clamp sits after the spindle override and before the gearbox, so it limits the real chuck speed and the override knob cannot push past it.The pins are named  and , without underscore. The  source declares them as  / , but  strips the trailing underscore.  aborts LinuxCNC startup with "parameter or pin not found".After a restart, check with .Step 2: install the UserTab files

1. Create 

   user_tabs/chuck_manager/

    in your config directory and copy 

   chuck_manager.py

    and 

   chuck_manager.ui

    into it.
2. Point Probe Basic at the folder in your INI, if it is not there yet:

   [DISPLAY]
   USER_TABS_PATH = user_tabs/

3. Optionally copy the example 

   chucks.json

    into the config directory. Without it the table starts empty and the file is created on the first "Save Changes".

The database is one object per chuck, keyed by its name:The values in the example are placeholders. Take max RPM from the chuck's rating plate or datasheet, and measure the Z limit on your machine. Z is in machine coordinates, so its sign depends on your machine's travel and home position.Step 3: configure and testFour constants at the top of  control the behaviour:ConstantDefaultMeaningRe-apply the saved chuck to HAL at startup without a dialog.  only logs a warning and marks nothing active.Pin for the Z soft limit.Pin for the RPM clamp from Step 1.Lowest Z limit a chuck may have, in mm.  disables the check. Set it a few tenths of a mm inside the physical limit switch once you have measured it.The chuck limit can only make the machine more restrictive. LinuxCNC still enforces  and  from the INI, so a chuck Z limit below them has no effect. Set the INI values to the real travel and let the chucks narrow it.Test before cutting:

1. Start LinuxCNC and open the tab. The dropdown and the table must show your chucks.
2. Activate a chuck. Then run 

   halcmd show pin ini.z.min_limit chuck-rpm-limit

    and check both values arrived.
3. In MDI, command an 

   S

    value above the chuck's limit. 

   chuck-rpm-limit.out

    must stay at the limit while 

   .in

    shows the commanded value.
4. Jog Z slowly toward the chuck. Motion must stop at the chuck's Z limit.
5. Restart LinuxCNC. The same chuck must be green again and step 2 must show the same values.

How it worksEditing and activating are separate: changing the table never touches HAL, only "Activate Chuck" does.

• Edit: type into the table, use "Add Chuck" / "Remove Chuck", then "Save Changes". Save validates all numbers and rewrites 

  chucks.json

  . A row with invalid data aborts the save with a message.
• Select: pick a chuck in the dropdown or click its row. The info line shows max RPM, Z limit, diameter, jaws and weight. Nothing is applied yet.
• Activate: "Activate Chuck" runs this sequence:
  1. Plausibility check against the INI and 

     Z_LIMIT_FLOOR

     . On failure: error dialog, nothing applied.
  2. Confirmation dialog showing the two limits.

  3. halcmd setp

      for the Z pin, then the RPM pin. Each return code is checked.
  4. Only if both succeeded: green row and 

     current_chuck.json

      written.
• Startup: the tab loads 

  chucks.json

  , selects the saved chuck and, with 

  AUTO_APPLY_ON_START

  , re-runs steps 1, 3 and 4 without the dialog.

The dropdown items carry the chuck name as item data. Matching is exact, so "Collet" never selects "Collet ER32".Safety notes and limitationsThis is a software convenience layer, not a safety function. It reduces operator error; it does not replace hardware protection.

• It clamps the command, not the spindle. A drive fault, a wrong analog scale or a runaway VFD can still overspeed the chuck. Real overspeed protection needs independent monitoring of the encoder speed, or a drive-side speed limit.
• The clamp is silent. 

  S2000

   with a 1500 RPM chuck runs at 1500 RPM without a warning. Surface speed and feed per rev change accordingly.
• The operator must select the right chuck. The system cannot detect which chuck is mounted.
• The fail-safe default only helps if it is low. Set 

  chuck-rpm-limit.max

   in the HAL file to your weakest chuck.
• Limits apply until the next activation or restart. Changing a chuck's row and saving does not re-apply it; activate again.
• Keep hardware limit switches and E-stop in the drive enable chain. If LinuxCNC or the GUI hangs, only hardware stops the machine.

Use at your own risk, and test every chuck with a low commanded speed first.Pitfalls we hitThese cost the most time during development and apply to any QtPyVCP tab.SymptomCauseFixDropdown stays empty although the table loadsPyQt gives  a , so an empty combo box is falsy.  returns before the first item is added.Test widgets with  / , never by truthiness. behaves the same.LinuxCNC aborts: "parameter or pin 'chuck-rpm-limit.min_' not found" strips a trailing underscore from pin names.Use  / .Activation seems to work but the spindle is not limited does not exist; ignored the error.Clamp in HAL (Step 1) and check the  return code.A button does nothingIts objectName in the  (e.g.  after copy/paste) differs from the name in the code.Keep the names in the table below. on a LED pin failsQtPyVCP turns  in a widget name into  in the pin name: widget  becomes .Look up pin names with .Object names the code expects in the :WidgetobjectNameQComboBoxQPushButton, , , QTableWidgetQLabel, , , , If  is missing from the , the code creates one and inserts it into the layout named .