What causes Haas alarm 28?

Carousel index motor failure or gear train binding preventing proper tool pocket positioning at the pickup station. Tool changer arm proximity switches misaligned, failed, or covered with debris causing false position feedback to IOCON. Insufficient air pressure to pneumatic cylinders operating the tool changer arm or drawbar causing slow or incomplete actuation. Drawbar not fully releasing or seating the tool due to worn springs, contaminated collet taper, or failed drawbar belleville washers. Broken or stretched tool changer timing belt causing positional errors between actual and expected carousel location.

How do I fix Haas alarm 28?

1. Step 1: Press RESET and observe tool changer position, verify carousel is aligned with arm and no tools are obstructing the path 2. Step 2: Enter Manual Tool Change mode from Settings 23 and manually jog carousel and arm through the cycle to identify binding or failed motion 3. Step 3: Access Diagnostics Inputs page and monitor all tool changer proximity switch states through the complete cycle to find switches not changing state 4. Step 4: Verify air pressure at the pneumatic manifold reads minimum 80 PSI and check all air cylinder actuation speeds for sluggish movement 5. Step 5: Remove spindle tool and inspect drawbar travel, collet taper cleanliness, and retention knob condition for mechanical issues preventing proper tool seating 6. Step 6: Check carousel index encoder feedback on IOCON diagnostics and verify belt tension and motor operation, replace carousel motor or encoder if position counts are erratic

Haas codes

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Haas Controller

Overtravel/Limit · Basic · 1-3 hours

Alarm 28

Overtravel Positive B

Alarm 28 Tool Changer Fault on Haas NGC systems indicates the automatic tool changer (ATC) has encountered a mechanical or electrical failure during tool change operations. This alarm triggers when the tool changer carousel fails to rotate properly, tools jam in the spindle or carousel, or position feedback sensors detect incorrect tool positioning.

HaasOvertravel/LimitBasic1-3 hours

28

Overtravel Positive B

Tool Changer Fault

This alarm is triggered when the MOCON board detects the automatic tool changer has failed to complete its programmed sequence within the expected time window or a mechanical safety interlock has opened. The IOCON board monitors proximity switches for tool changer arm position, carousel position, and drawbar status, feeding signals to MOCON for sequencing validation. Machine motion is halted and spindle is locked out until the fault condition is cleared and the tool changer cycle is successfully completed or reset.

Root Cause Summary

Safety

Do not attempt to force tool changer arm manually if resistance is strong, risk of injury
Ensure spindle is stopped and tool is secure before diagnosing tool changer
Tool changer arm may be spring loaded, use caution when manually positioning arm

Causes and Fixes

1
Check Carousel Position Display
Press CURRENT COMMANDS and verify carousel position matches the displayed tool number. Look for erratic position readings.
Parameter 37 shows current carousel position - compare with physical position
2
Inspect Drive Belt and Motor
Open electrical cabinet and visually inspect carousel drive belt for breaks, stretching, or slippage. Check motor coupling alignment.
Lock out power before opening electrical cabinet
3
Manual Carousel Test
Use MANUAL mode and jog carousel using ATC JOG function. Listen for unusual noises or binding during rotation.
Each position should index smoothly with consistent timing
4
Verify Carousel Parameters
Check Parameter 37 (Carousel Position) and Parameter 38 (Tool in Spindle) for correct values matching physical setup.

DIY Feasiblemedium confidence

Call a technician if:

·Hydraulic system repairs needed
·Encoder replacement required
·Drive motor failure confirmed
·Multiple sensors malfunctioning

Prevention

Regular carousel lubrication per PM schedule
Keep tool holders clean and properly torqued
Monitor hydraulic fluid level monthly
Clean position sensors during routine maintenance

Common Mistakes

Forcing jammed tools instead of identifying root cause
Ignoring hydraulic pressure warnings
Using incorrect tool holder pull studs
Running ATC cycles with damaged tools installed

Related Alarms

27 29 31 33

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