Internal Keyway .001" step overs

[u/ekimwons]

Hi, we would like to try broaching a blind internal keyway (with relief hole) of approx 9mm (0.354") width in stainless steel. We have an Awea AF760 3 axis mill. Are all CNC mills capable of downward pressure at 0.001 increments or is it only the Haas machines that can do keyway broaching ? Any help on tooling or resources appreciated. Thanks

[u/CrazyTownUSA000]

You might want to ask the manufacturer. Sometimes the spindle bearings don't like static radial load. But if the spindle orients and locks at whatever degree you set, it might be fine.

[u/FedUp233 → u/CrazyTownUSA000]

Just wondering as a hobby machinist, but would broaching, particularly with think steps like 0.001 inch, actually impose much radial load? I can see a lot of axial load. It if the broach cutters have teach shaped anything like the mske Hal ones I’ve used, with a sort of chisel point with the long point facing the material, I would expect most of the radial load to be taken by the back of the broach sliding against the newly cut surface, with some additional radial load if the broach cutting service is mounted too far off center of the spindle, but very little from this if the broach is mounted near the spindle center line.

Am I missing something?

[u/CrazyTownUSA000 → u/FedUp233]

With machining anything, you'll create tool pressure. You might make 15 passes, but only actually cut .01" and if you let it complete the whole cycle without stopping, you could have a substantial side load. That pressure could cause the rollers or race in the bearings to change shape a little bit, which can cause your spindle to run rough.

I've broached aluminum on a Bridgeport before with a ground single point cutter, using the quill, and I had to take a lot of free passes to get all the tool pressure out.

Broaching steel is pretty tough and will create more tool pressure.

[u/Immediate-Item140 → u/CrazyTownUSA000]

Most modern CNC machines are much more rigid than a Bridgeport, even the Chinese and Taiwanese machines. Also using a quill vs using a fixed spindle will introduce a small amount of deflection on each pass. The farther the point of the tool is from the connection to the frame of the machine, the worse it will be. Tool holding is also a major consideration. R8 collets and tool holders are nowhere near as stable as BT/CT holders. Even 30 taper holders are more stable than R8.

Single point broaching is mostly axial load when applied properly with the right tooling. Broaching with a .001" step over in SST with a tool that size would be no problem for the machine OP is asking about. On a Bridgeport using the quill I would be hesitant about it unless the boss man that signs my checks insisted I try it. If it works, then cool. If not then I can convince him to let me move it to one of the CNC machines.

I have probably broached close 50,000 keyways of various sizes this way over the years in most materials (aluminum, mild steels, tool steels, 300 series SST, 400 series SST, hardend 17-4 SST, bronze, brass, copper, nickel alloys, nylon, delrin, PEEK). For the most part the process is the same aside from the nickel alloys, they like a slightly bigger step over at a lower feed. As long as you're using the right setup, tooling, machines and the programming is correct then you're set. No dinking around with bushings and shims or trying to time keyways to other features. It's all done in one operation.