Rotary Broaching Technical Help

Rotary broaching can produce an internal or external hex, square, or other form in a lathe, Swiss machine, or machining center. Results depend on more than the broach itself: the pilot hole, entry chamfer, holder condition, alignment, lubrication, cutting parameters, material condition, and form depth all affect tool life and part quality. This guide helps you identify the next adjustment and decide when a Rayco #7 broach is worth testing.

For an application recommendation, send Rayco the material and condition or hardness, form drawing and tolerance, broaching depth, machine and holder, pilot-hole dimensions, coolant, current RPM and feed, and parts per broach. Request a quote and application support.

How rotary broaching works

A rotary broach is held at a slight angle in a free-rotating holder. As the tool advances, its cutting edge engages the work progressively to form the profile. On a lathe, the rotating work drives the broach; on a mill, the holder rotates against a stationary workpiece. An internal form needs a prepared pilot hole; an external form needs a prepared outside diameter. Holder instructions and the part print govern the final setup.

Speeds and feeds: establish a controlled starting point

There is no single RPM or inches-per-revolution value that fits every broach. Form size, number of cutting corners, depth, work material, hardness, rigidity, lubrication, and holder speed limits change the load. Use the holder maker's rated speed and the broach maker's application advice as your starting limits, then document a short trial.

  1. Verify the holder rotates freely and is centered to the work. Check the actual work material and hardness, not just its grade name.

  2. Prepare the pilot and lead-in before tuning the program. An undersized pilot or missing chamfer can overwhelm a good broach.

  3. Start conservatively for hard alloys, small forms, long depths, or thin walls. Keep a consistent feed per revolution while evaluating the cut; adjust one variable at a time.

  4. Inspect the first parts for across-flats size, corner fill, witness marks, burrs, form straightness, and tool-edge condition. Record parts per broach before and after each change.

For CNC programming, feed in inches per minute = spindle RPM × feed in inches per revolution. For example, 600 RPM at 0.003 in/rev equals 1.8 in/min. This is a calculation example, not a Rayco setting recommendation. Ask us for a starting process based on your drawing and material rather than applying a generic chart to a hard-material job.

Part preparation and holder setup

Internal forms: Drill or bore a pilot at least as large as the form's minor diameter and, where the print permits, large enough to leave a manageable corner cut. Provide a lead-in chamfer that lets the broach enter progressively. For a blind hole, allow space beneath the finished form for chips and a clean bottom transition. Verify the print's permitted drill witness and depth before opening the pilot further.

Holder and machine: Confirm the correct shank and overall length, secure clamping, free bearing rotation, centerline alignment, runout, tool orientation where indexing matters, and rigid workholding. Use a lubricant or coolant appropriate to the material and machine. A worn or binding holder can cause premature failure even with a durable broach. Rayco offers rotary and push broaches, holders, and form gauges.

Rotary vs. push broaching

Rotary (wobble) broaching forms a profile through progressive engagement while the part or holder rotates. It often allows a hex or square to be made in the same machining setup. Push broaching drives a suitable form tool axially without the rotary holder's wobble action; it may fit a machine and geometry that can support the required force, stroke, and indexing strategy. The right method depends on the profile, depth, tolerance, material, machine, production volume, and allowable witness. Rayco manufactures both rotary and push styles, including custom forms. Share the drawing so we can help select the form and process.

Troubleshooting rotary broaching

Broach chips or fails early: Check pilot size, lead-in, depth, chip room, holder rotation and alignment, rigidity, material hardness, and feed. Inspect whether the edge is chipping or wearing before changing tool material.

Heavy burr, incomplete corners, or poor finish: Confirm pilot size and chamfer against the drawing, examine edge wear, lubrication, form depth, and actual feed. A larger pilot can reduce load only if the allowed drill witness and finished form still meet the print.

Form wanders, twists, or is off-center: Check holder and spindle alignment, runout, free rotation, workholding, and entry geometry. Verify the holder setup before changing speed.

Chips pack in a blind hole: Increase pilot depth or provide bottom relief where the drawing permits; inspect chip evacuation and broach depth.

Entry marks or a counterbored appearance: Check the chamfer, entry conditions, holder bearings, and whether the broach begins rotating or synchronizing as intended.

Tool life varies from bar to bar: Confirm hardness and material condition by lot, then compare setup, coolant, and edge condition using the same part count and inspection criteria.

Go gage gets tight at the bottom of hole: Sometimes a slight spiralling can occur in the rotary broaching process. This can cause a go gage to get tight towards the bottom. To fix this issue, stop your feed rate half way down, reverse spindle direction and continue to full depth. This should allow the go gage to go to full depth. Sometimes with very deep sockets, 2-3 times reversing spindle direction will be needed.

Changing to Rayco #7 cannot correct an undersized pilot, binding holder, or misalignment. Once the process is sound, it is a strong candidate when the cutting edge still wears or chips too soon in a difficult alloy.

Hard materials and the Rayco #7 option

Rayco #7 is Rayco's proprietary heat-treated high-speed-steel broach for demanding applications where the balance of toughness and wear resistance matters. Rayco has reported successful rotary-broaching applications in 17-4 H900, Custom 455, BioDur 108C, cobalt chrome, strain-hardened 316, titanium, Inconel 718, and other difficult alloys. It is intended for high-strength work, including material above 190 ksi, and for jobs where conventional broach tool life is inadequate. Rayco reports applications up to 50 HRC routinely and selected applications approaching 55 HRC; feasibility and tool life depend on the actual material condition, form, depth, holder, and setup. These are application examples, not a blanket hardness or parts-per-tool guarantee.

If a conventional HSS wears, microchips, or breaks before an economical part count, evaluate the failure mode first. Rayco's heat treatment is designed to retain toughness with high wear resistance; a trial of Rayco #7 against the current tool, under the same conditions and part-quality criteria, can show whether it improves your process. Read about Rayco's broach materials and hex forms.

Application examples and further reading

Individual case results depend on each customer's conditions. For sizes, materials, custom forms, and lead-time questions, see the Rayco FAQ and Products.

Get help with your specific part

Tell us what you are broaching and where the current process fails. Include a drawing, material and hardness, existing broach and holder, pilot and chamfer, RPM and feed, coolant, and current tool life. We can recommend a rotary or push approach and discuss a Rayco #7 evaluation. Request a quote or technical help.