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Secrets of Broaching on a CNC Mill

Nov 6, 2014   //   by Bob Warfield   //   Blog, Manufacturing, Products, Techniques  //  25 Comments

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

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


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 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.


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.


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Secrets of Broaching on a CNC Mill
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  • Yes, definitely interested! We often have to broach special couplings for pump shafts and such…

  • Speaking to Haas they said never do this. I have seen other machines do it. Their answer was even though the pressure may be similar to drilling or less, the bearings are not designed for load when they are stationary.

    • …and yet here in the video our client is using CNC Broach Tools on a Haas mill. And we have 100’s of clients using us on Haas and (ironically) a Haas factory outlet in Ohio demo’ing our tool on their machine. You spoke to somebody who doesn’t understand that you are only taking .001″ DOC….you could push that through with your hand….there is not the “crash” associated with traditional broaching. It’s a shaving action. This is answered on the CNC Broaching FAQ’s page of our website:

      the client on the video is Ruland Manufacturing, they are a very high end Manufacturing company, would they be “damaging” their machine? Call them and ask the MFG Engineer Scott Brooks that sent us this video, he will laugh….:) really

      • Im glad you replied. How do you go about indexing the broach and keeping everything in line? Which ever Haas personal I talked to shot down all my questions This is exactly what I wanted to do a year or two ago, but being shot down killed my enthusiasm. Ill surely be in touch soon, this will make my life MUCH easier.

        • Hi Steve,

          Bob, the Admin for CNC Cookbook, answers that a little lower in the thread and his site is a wealth of valuable info-

          “on a lathe, you may have the option of a C-Axis (spindle servo) to decide the index and accommodate multiple keyways. On a mill, you’ll need to use a 4th axis to index in most cases”

          – yeah the reaction of some of the machine company reps is confusing to us. My opinion is that it’s a newer process and that the practice is growing quicker than word of mouth is spreading. I’ve had Mazak reps tell me the ability to broach on the machining center helps them sell the machine because the more processes you can do in one location the more money/time you save. They say purchasers request the ability to broach on their machining center.

          Mazak distributors request quotes for “Machine packages” from us all of the time. I’ve found Haas reps to be lagging behind in comparison to DMG Mori, Mazak reps…

          But next time you go to a trade show IMTS or PMTS you’ll see the big machine companies demo’ing CNC broaching on their machines. They’ve been doing it since we were at IMTS in 2010…

          The proof for us is the Client testimonial video’s and letters we put up. Somebody taking the time to vouch for us is a big deal….

  • If using a Tormach for something like this, because of the slow speed. You are better off using an endmill to take away most of the stock, and then broaching in 2-3 passes.

    • Hi Brad,
      I definitely agree that taking out as much meat as possible with an end mill will save time and insert life. Smart thinking. we don’t recommend our product be ran at speeds slower than 150 IPM….

  • Please do! G-Wizard is my go to software and this would be a great addition to it functionality. Oh and thank you Bob for sharing this with us. I have a production part I needed to add a key way to and this is going to save me a ton of time!

    • Hi Eric,
      great talking to you today. Glad this saves you from having to vend out your part….fyi here is the testimonial of another client that had to vend out his part. cost him $15 per keyway + shipping to chicago and….again, the video above is from the client themselves, we just edited it. this recommendation from Mike at Derby Machine is unsolicited. We rely on 3rd parties to verify our product . we are not “marketing” but sharing our clients first person experience. And as it says in the video above, if they can do this and get this insert life, so can you.

      From Mike Turner
      Operations Manager at Derby Machine in Derby, Kansas
      To whom it may concern:
      We had a blind hole to id broach in 304 stainless and found it difficult to find a vendor who could broach our part to print. We were forced to ship our parts from Kansas to Chicago to get them broached. It cost us $15 per part with a $100 set-up fee plus shipping to and from Chicago. Our parts were continuously damaged in shipping so we began to ship them in ammo boxes for protection. While searching the internet we found a broaching tool at We were very skeptical but had nothing to lose and decided to give them a shot. When we received the tool and started setting up the machine John was very helpful in instructing us on how to install the tool correctly and worked with us to get our program, feeds, and speeds correct. This tool paid for itself several times over on the first run alone. We get well over 100 parts on a single insert point with a cycle time increase of only 2 minutes 14 seconds. We have ran over a thousand parts using very few inserts and never lost the tool itself. We have never scrapped any parts because of the broach. It holds the broach depth and width thru the whole run. I am extremely happy with the results and cost saving of this tool and would recommend it to anyone who has an id broach issue or would like to machine their part complete.

      Mike Turner
      Operations Mgr.
      Derby Machine

  • All of the negative comments I have read here about this process with this tool from “CNC BroachTools” are from individuals who took the word of people who have no evidence to back up their claims of damage done “by this process with this tooling”. Then nothing but positive comments from people who actually use this and know the huge cost savings of this tool. The negative comments are so absurd. If people would just use common sense, “A hard to find commodity”, they would realize we are not in any way talking about broaching these keyways in one pass. Common sense will tell you, If you understand the process “with this tool, the force on the spindle generated by this tool, if programmed correctly, is no more, maybe even less than the force of a tool change. Which by the way, the spindle is in the oriented position and not rotating.
    If everybody took the word of others and didn’t try things for themselves, we would never advance.
    As “Nicolo Machiavelli” said:
    “It must be considered that there is nothing more difficult to carry out nor more doubtful of success nor more dangerous to handle than to initiate a new order of things; for the reformer has enemies in all those who profit by the old order, and only lukewarm defenders in all those who would profit by the new order; this lukewarmness arising partly from the incredulity of mankind who does not truly believe in anything new until they actually have experience of it.”


  • How do you get that spindle to index in the right position? and how do you get that going on say more than one spline without a rotary table. BTW, how come no one makes an offset hole to lighten the load on the broach?

    • John, on a lathe, you may have the option of a C-Axis (spindle servo) to decide the index and accommodate multiple keyways. On a mill, you’ll need to use a 4th axis to index in most cases.

  • We have had great success using CNC Broach tools in our Mazak Nexus 250 lathe. That being said, I would have no reservation using them in our HAAS VF-2 mills given the opportunity.

    The statement that “this should never be done” appears a little reactionary.

    One could build confidence in a particular decision by considering:

    1. the specific broach cutting force vs maximum static load of the spindle bearings

    2. review the blogs of people with successful results

    3. review the blogs of people that have had spindle bearings damaged as the result of this process (I am not saying it does not exist, but I have not found a blog like this to date)

  • I have been reading these broaching blogs for some time now…find it very interesting the strong opinions being voiced. These broaches have been in our shop for about 3 years. Different sizes, custom shapes, even non recommended processes. There is absolutely no wear on any spindle, lathe or mill that we have experienced. Tools are worth every penny and the support is even better.

  • The November 2014 MMSOnline has an article on p85 that talks about how Makino certified their HMC’s for broaching operations. The broaches being used there are multi-tooth and will be much harder on spindle bearings than these tools, yet Makino gave the go ahead.

  • […] the process was fine and I've used the tools now, I think it's completely safe to CNC mill broach Secrets of Broaching on a CNC Mill – CNCCookbook CNCCookbook here a really cool video: CNC Broaching Blind Internal Keyway – YouTube Reply With […]

  • John Gardner, you’ll be getting a call from me tomorrow! This is exactly what I need.

  • I just purchased one of these linear broaches and now I need help programming my HAAS.

    This is what needs to happen.

    1. I need to change tools to the broach. (T99)
    2. Orient the spindle to 180deg
    3. Rapid to an X,Y starting position
    4. Plunge at 150ipm
    5. Move away from keyway X,Y Position
    6. Rapid retract.

    I know what I want the machine to do. But I have never used the M19 feature on my VF2SS
    If anyone has done this I could use a little direction.

  • new Keyway Broach client testimonial video documenting CNC Broaching of blind internal keyway on CNC Fellows and the client getting 130 keyways per cutting edge in 8620

  • These tools are great! They leave a good finish and save us a ton of time. Anyone who is a non-believer either doesn’t know how to properly set-up/program their machine or simply hasn’t tried one. We cut 4140PHT with no problems at all. Saves us from a second set-up or farming out the parts.

  • CNC Broach Tool client input on a common question, thanks Joe!!:

    “We were having trouble indicating the broach square in our Haas Minimill. Here’s the solution:
    When orientating the spindle to square up the broach we use an M19 code. We needed to move the position to indicate the broach square. When you use a whole degree you can do M19 PXXX when you need to go less than a whole degree you use an R example M19 R45.05 If you don’t use the R code the spindle will quiver and make indicating difficult. When we got this figured out we had very good success. Our parts are holding within .001 ”
    Drifton Precision Machining

  • This was an awesome and helpful article.Thank you very much to everyone who contributed their advice and knowladge.

  • Here is a link to a Haas Mill program supplied by a client Cutting Edge Mfg in Arizona. You have to scroll down to the bottom of the blog post.

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