Dear Shop Doc,
I have a part that has an internal hexagon that needs to be put into the part in relation to milled features. Is there some way that a wobble broach can be oriented to the C-axis on my CNC Swiss?
-Vexed Hex
Dear Vexed,
On a full featured CNC Swiss there is a unique solution to this issue. As you know, rotary broaching holders offer no way of orienting the polygon shaped broaches to the work. The method that follows will also allow you to broach faster and will never “spiral” on a deep broached feature.
If your CNC Swiss has a Fanuc control equipped with the polygon cutting option, you should be able to use an adjustable angle live drill unit to wobble broach the hexagon shape while holding angular relationship to other live tool features on the work. Here’s how; mount an off-the-shelf rotary broaching bit into the angular drill unit and set the angle to 1 degree. This puts the broach in the same attitude as it would be if it were sitting in an ordinary rotary broach holder. If you have a CNC lathe or Swiss with a programmable B-axis, simply command the live tool B-axis to a 1 degree angle.
Use the G51.2 polygon cutting command to orient and synchronize the live tool spindle to the work spindle. Ordinarily this command is used for cutting external polygons on the work using a polygon attachment and cutter, but it works just fine for wobble broaching.
Example of the command when used for broaching: G51.2 P1 Q-1 R45.0;
The P value equals the ratio of the work spindle to the tool spindle. Q equals the ratio of the live tool spindle to the work spindle. The sign of the value determines the spindle rotation direction of the live tool. A negative value is usually the Vexed Hex counter-clockwise direction, which would match a clockwise direction on the opposing work spindle.
If the live angle tool attachment has a gear ratio to the commanded speed then you would use P and Q to compensate for that ratio. For example, if the live tool spins at 4,000 rpm when you program 2,000 then you would program values of P1 Q-2.
The R value sets the angular relationship of the live spindle to the work spindle. This allows you to adjust the orientation of the broach in relation to the C-axis of the main spindle. The value range is from 0 degrees to 359.999 degrees. I prefer to program a macro variable instead of a numeric value so that the orientation can be adjusted without editing the program. For example—G51.2 P1 Q-1 R#510: Variable 510 can now be used as an offset to adjust the orientation of the broach to the work.
Once you have commanded the polygon turning function G51.2, program the broaching operation the same exact way you would if you were using a conventional rotary broaching tool. In most cases you can broach at a much higher rpm using this method than you can with a rotary broach holder. You are only limited by the maximum speed of the main or tool spindle. Cancel polygon mode by commanding G50.2.
-Dan Murphy
Tsugami REM Sales
4 Comments
If a secondary operation is acceptable rotary swaging can be a very good method for creating especially a blind internal hexagon, with or without a helix.
My brother Pat accomplished that task 40 years ago on a Brown & Sharpe OG before there was CNC Machines
This has worked for me on several occasions, but the first part you run is a throw away because the broach isn’t “parked” yet.
Use G32 instead of G1 for your in-feed and then orient your spindle while the broach is still at the bottom of the hole and pull the broach out of the hole with the spindle stopped and oriented, then adjust your Z axis starting positon to adjust the radial orientation of the broached feature.
Example:
M3 S1000
G00 Z0.2 (ADJUST HERE FOR RADIAL ORIENTATION)
G32 Z-0.3 F.001
M50
G28 H0.0
G00 C0.0
Z-0.2
Radial tolerances are commonly +/- 1/2 Degree
It’s a little bit hair-ball, but a hell of a lot cheaper than a machine with a B-axis. Not recommended for a turret mounted position where centrifugal force could rotate the broach.
And check your parts more frequently than you normally would!
Slater is correct (the other guys could be also, just not sure). You could put a spring loaded locater on the broach holder and the mating ‘hole’ on the collet to time them. That is trickey to set up but it would work without spending a lot of money. it just requires a bit of finesse.
Don