← AskMachinist · Guides · Calculator · 🔎 Search
How to Cut a T-Slot
Guide · Updated October 28, 2026
A T-slot is cut in two operations: an end mill cuts the narrow neck slot to full depth, then a T-slot cutter enters from the end of the part and cuts the wide undercut at the bottom. The T-slot cutter is fragile and packs with chips, so slow down and clear chips often.
Check slot widths with blocks
Check the neck width with gage blocks or pin gages and confirm the head depth before the finish pass. The free Gage Block Calculator builds the stack.
Step by step
- Read the slot dimensions: Note neck width, neck depth, head (undercut) width and head height for the bolt size.
- Mill the neck slot: Use an end mill at neck width; cut to the full depth of neck plus head so the T-cutter has room.
- Pick the T-slot cutter: Its diameter matches the head width, its thickness the head height, and its neck must pass through the slot.
- Set the depth: Touch the cutter bottom to the slot floor, or set from the top surface: head bottom depth = total slot depth.
- Enter from the end: Start the cut off the part and feed in along the slot; the cutter cannot plunge.
- Run slow and clear chips: About half the feed of a similar end mill, with plenty of coolant or air blast.
- Check and clean: Blow out the slot, check widths and try the T-bolt.
Worked example
A slot for a 3/8 in T-bolt: neck 0.437 in wide, head 0.750 in wide by 0.312 in tall, total depth 0.750 in. Mill the neck with a 7/16 in end mill to 0.750 in deep. A 3/4 in × 5/16 in T-slot cutter (2-flute HSS at 60 SFM) runs 3.82 × 60 ÷ 0.750 = 306 RPM. At 0.001 in per tooth, feed = 306 × 2 × 0.001 = 0.6 IPM. The top of the head undercut is 0.750 − 0.312 = 0.438 in below the surface.
Why the neck goes full depth
If the neck stops short, the T-slot cutter has to cut its own clearance with its weak neck, and it will snap. Always clear the full depth first.
Common problems
A broken cutter is usually chip packing or too much feed; slow down and clear chips. A tapered slot means the head or vise is not trammed. Rough walls mean a dull cutter or chatter from too long a reach.
Tips
- Similar cutter: cut a dovetail uses the same end-entry approach.
- Slots: cut a keyway.
- Setup: tram the mill first.
FAQ
Do I cut the neck first?
Yes, to full depth, then the T-slot cutter.
Can a T-slot cutter plunge?
No, it must enter from the end of the part or a drilled clearance.
What feed for a T-slot cutter?
About half that of an end mill of similar diameter; keep chip load light.
Why did my cutter break?
Chip packing or too much feed, or the neck slot was not deep enough.
How do I measure the undercut?
Use a caliper with thin jaws, a T-bolt fit check or gage blocks against the slot wall.
Related guides and calculators
- T-slot bolt dimensions (DIN 787)
- Clamping washer dimensions (DIN 6340)
- T-slot stud dimensions (DIN 6379)
- T-slot nut dimensions (DIN 508)
- How to Cut a Dovetail
- How to Cut a Keyway
- How to Use a Slitting Saw
- Milling Speeds and Feeds
- How to Tram a Mill
- How to Cut a Square on a Mill
- How to Cut a Woodruff Keyseat
- All free machinist calculators
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.