Small scale Helical Broaching............

[BCjohnny]

One of the items I am about to make requires a part that has an internal Helical Spline.

I have yet to determine the exact details of the spline but ballpark figures are 14.5mm Dia, 18 splines (approx 1mm depth), 30deg lead and the broached length about 35mm.

The broaching, for various reasons, I would prefer to keep in-house. One reason is that the quantities will be small, maybe 10-15 parts per week.

I have sourced a firm to make the broach (push type) but don't want to go to the expense of creating a dedicated machine to carry out the operation (ie indexing the rotation to the broach). I have read that helical broaching can be carried out successfully by allowing the part/broach freedom to rotate, allowing it to follow the lead. This I can construct quite easily.

Does anyone have any thoughts/experience on this? The broach in itself is quite expensive so I don't want to go smashing them too often, if at all.

John.

"It's not always a case of learning more, but often of forgetting less"

 

[Pressed]

We had special acme threaded rod & nut parts (25mm dia x 150mm nut)that had alot of trouble mating as they should (I suspect the nut threads). We didn't do a thorough examination, not enough time and an overseas vendor, but we switched to a local vendor using his cnc mill. Parts fit, was comparable in cost, problem solved.

We only priced them one at a time and the prices were still similar to the old style, if you quote multiple pieces your shop should reduce thier price.

That spiral braoching might be the demise to the hair on top of your head.

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[ajack1]

John, have you considered spark erosion (EDM) if you drive the electrode from the spline this can be done, pre drilling the core diameter, or just under will help speed.

I have seen this technique used for tapping hardened steels, it works well. I have never seen or heard of spiral broaching, however that does not mean it cannot be done or that people do not do it.

Please keep us posted with how you get on.

 

[ornerynorsk]

Helical broaching is usually no sweat. The toolmaker will grind the broach in such a way as the tool will correctly find its own lead as it is pushed through. Yes you are correct in assuming to allow the part or broach to rotate freely. This can be accomplished with a good heavy duty thrust or angular contact bearing broach holder for your arbor press. Trying to machine or edm an operation such as this will only drive your per unit cost up and cycle times down.

 

[BCjohnny]

Cheers "ornerynorsk",

And without trying to sound like "I knew that", that is exactly what I was going to do, ie allow the component to rotate under a fixed broach, in a mandrel press, using a ball/roller type thrust bearing to reduce friction.

I have looked at the material removal rate and up tp 6 thou is ok in this application, but as it's not time dependent, I'll allow up to ten broach steps of 4 thou each. Hopefully this will put less stress, and create less wear, on the broach.

I was just not sure how doing it "freestyle" would affect broach wear/damage. And as the broach comes in at around £2k, breaking it would break my heart! Asking people who have "done it" is I find always the best to get good advice. Many, many thanks.

Thanks to the other replies but it does need to be broached, for the reasons stated.

If anyone else has anything to add/advice etc, and the names of other possible broach suppliers in the UK, please speak up.

John

"It's not always a case of learning more, but often of forgetting less"

 

[Warpspeed]

These same techniques are used to create the rifling in a gun barrel. An internet search will locate a LOT of information on many different methods.

The self rotating button broach is the usual mass production method these days, but it may not have an accurate or repeatable lead, especially if the spline has to accurately mate with something else.

The most accurate way to do it is still to cut each spline individually using an indexing head and a purpose built machine.