3 months by cncdivi

We’ve written about CNC broaching tools before.  It’s a common operation for cutting keyways and such and one that can certainly be automated via CNC.

Here’s a Monel socket that required broaching right through the helical threads:

A stainless steel container with cnc tools.

That Monel can be some nasty stuff!

As we all know, that Monel can be some nasty stuff.  Here’s the tooling from CNCBroachTools that did that job:


CNC Broaching Tools…

It’s indexable tooling and the broaching inserts are easy to replace.  Here’s what a typical CNC broaching operation in a mill looks like:

I’m thinking of adding feeds and speeds for this kind of tooling to G-Wizard Calculator and perhaps a Conversational Wizard to generate the g-code to G-Wizard Editor.  Let me know in the comments if you’d be interested in this sort of thing.

CNC broaching in this way can be done on either a mill or a lathe.  Here’s a broach on a lathe:

CNC Broaching on a Lathe...

Postscript: Can CNC Broaching Damage your spindle bearings?

I want to summarize the broaching vs spindle bearings issue a bit. There are no end of threads in various places going back and forth on this Holy War. Here are some examples for those who are interested:





Plenty of people show up to these parties from two camps:

– We’ve done it for years and it works great. Here’s a good quote along those lines:

“Forget the bearings its all about the width and depth of cut. If the bits sharp and narrow the z axis on a non counterbalanced machine may be doing little more than letting gravity make the cut! A 1/8″ wide sharp cutter in a bridgeport spindle will easily take a thou per pass with just minimal pressure on the quill handle. Like less than drilling with a 3/8” drill bit pressure.

Yes static bearing loadings are significantly less than dynamic, but even for precision spindle bearings, on a 40 taper machine, there static load rating is in the tonnes range. Remember shaping is a delicate single lip cutting aplication, not a 50 ton broach press cutting method.”

– You should never do this. Spindle bearings are easily damaged when not spinning. You’ll get false Brinelling, my XYZ repairman told us never to do this, etc., etc.

You’ve got to decide which category you fall into on this. Some things to consider if you want to be on the safer side:

– Use of this kind of process on a lathe seems far more accepted.  You could always choose to do your broaching on your CNC lathe exclusively.

– The main issue is doing it while the mill spindle is locked. There is tooling available that lets the spindle revolve about 50 rpm so the load is spread and false Brinelling is eliminated. Here is an example from Benz called the LinS Linear Broaching Tool:


The LinS uses the familiar stop pin arrangement we see on tapping heads, right angle heads, spindle speeders and the like.  There are also those talking about clamping the broach to the spindle so no force is applied to the bearings at all.  The difficulty there is you lose the ability to tool change unless something clever is going on.

Personally, if I was only going to do a bit of this, perhaps for one or the occasional job, I would just go ahead and keep the depth of cut light.  Like the quote says, we’re not talking that much pressure.  If I was going to do it often, a tool like the LinS is probably a lot cheaper than new spindle bearings.

And keep in mind that while forces are not so bad when all is working well, if you crash the broach they’ll be quite a lot higher.

Mount the Broaching (Shaper) Tool rigidly

You could also choose to mount the broaching tool rigidly to the head and isolated from the spindle bearings to avoid these kinds of issues altogether:



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Recently updated on April 29th, 2024 at 12:05 pm