Rotary Broaching question?

[Chitownmachine]

So... maybe you guys can clear this up for me. Why does the piece have to spin (lathe) or broach spin(mill) in order for the rotary broach to work? Ive broached at work with a standard hex broach before, but that was just in a press and that defiantly wasn't spinning. Couldn't you just press in the rotary broach tool for a blind hole? Thanks!

 

[rhankey]

A rotary broach is fat at the end, and tapers towards the shank (opposite of a normal broach). All the cutting occurs at the tip (rather than with a normal broach that looks more like a saw blade). The broach is mounted in a special tool holder that allows the broach to spin freely, but holds it such that it oscillates or wobbles as it turns, which is why the broach needs to taper in towards the shank. This wobbling action causes the leading edge of the broach to move up and down relative to the work piece, and thus nibbles away at the work piece. Rotary broaching is limited in depth relative to the diameter of the broach. But rather than using a press, you can do the broaching on a lathe, drill or mill.

 

[rkepler]

There's a good writeup here:

https://en.wikipedia.org/wiki/Broaching_(metalworking)#Rotary_broaching

 

[Ken I]

See thread

http://www.homemodelenginemachinist.com/index.php?topic=14871.0

Regards,

Ken

 

[ruzzie]

This show one in a milling set up but works the same in a lathe set up

http://www.youtube.com/watch?feature=player_detailpage&v=J2OAISkHHbI

 

[Ken I]

Since this subject has raised its head again, I have posted a set of plans for a rotary broaching tool in the downloads section.

In *.dwg, *.dxf & *.pdf in a *.zip file.

It is an adjustable (floating) type on the MT3 shank with a sketch of an alternative square / guide angle shank for toolpost mounting.

You can play with the mounting type as you please.

If you intend to use one length of broach (fixed centre) or come in from the toolpost and find centre, then the "float" option can be eliminated also.

Personally I've always found the float to be an easy option - you loosen the bolts - drive it into your predrilled & chamfered hole - hold it there and tighten the bolts - its set - simple.

Ken

PS - Ruzzie - excellent little graphic you rustled up there.

 

[Chitownmachine]

Awesome!!! Thanks for all the info guys! Time to put it to some use

 

[crankshafter]

Hi all
If you like to make your own rotarybroatch why not make one like Mike did.
Here is the link to his site:http://mikesworkshop.weebly.com/rotary-broaching.html
He have a good build log to ,so just follow it and you have your own rotarybroatch in no time :
CS

 

[polygon]

Chitown, the key (as noted by rhankey) is that the broach is held at an angle. Interestingly, the rotary broach tool will also work in a mill, where the part is stationary. In the mill, the rotary broach holder is turning and moving the broach in an orbital motion while keeing the form on center. As the broach is fed through the hole, eventually the full form is cut.

[ame]http://www.youtube.com/watch?v=uTsPXFy7Xx0[/ame]

 

[coopertje]

Hi Ken,

I am planning to build the broaching tool you have build according to your plans. I will post a build tread later when I start to make parts. At the moment I am collecting the material, bearings and studying your drawings. It seems that there is one dimension missing (or maybe not). At the front of the holder there is a trust bearing of 32mm (55202). For the outer ring you have drawn the diameter a little bigger then 32mm but not mention how much bigger. I guess its to give the outer bearing ring a little free space to do its job. It would be nice to learn your comments on this before starting machining the part.

Thanks in advance, regards Jeroen

 

[coopertje]

Mmmm.. just realized that the inner part of the bearing is steady and the outer ring will be rotating with the broaching tool. Thats why it needs some clearance and this dimension is not critical....must be the jet-lag :

Regards Jeroen

 

[Ken I]

Jeroen,
Correct - that is clearance only - in fact it is probably not needed as thrust bearings are normally different end to end - one end is size to shaft but undersize to bore - the other end is size to bore but oversize to shaft - so it has its own built in clearance. (It therefore also has a right and wrong way around.)

A further suggestion from me - make the shaft clearance bore a little bigger than the thrust bearing clearance bore to leave a lip that you can drive the bearing out with a drift if it ever needs replacing.

I drew it size for size and once assembled cannot be removed - not a good idea.

Good luck with your project - my only caution would be to get the 1° angle as close to spot on as you can - the reason being the clearance on the broach needs to be a little more - like 1¼° - as little over 1° as possible to help prevent corckscrewing - obviously if your tool is 1¼° you'll have no clearance and if your tool is 1½° you'll break the broach.

I would suggest you clock it to the trig values for 1° over a known length.

Regards,
Ken

 

[coopertje]

Hi Ken,

Thanks for the tip increasing the 15mm to lets say 17mm to be able to remove th ebearing, did not think of that. It seems that the bearing dimensions are a little different in South Africa.... When I look at the datasheets of SKF I have:

6001 -> 28x12 8mm thick (your drawing state a diameter of 32mm). I will use the 6201 instead, this has 32x12 and 10mm thick

For the 51202 the thickness stated by SKF is 12mm, were your is most likely 15mm (your seat is 14.5mm deep, leaving 0,5mm clearance).

Nothing very important, just something to keep in mind for other people who will build this great tool too.

I am planning to make the 1degree angle by using a digital angle meter. If it should come out a little more, I think before making the first cut I can check the relieve by mounting a straight bar with the same diameter as the broach and visually check the clearance on the broaching tool. If too little I regrind the broaching tool to give it a little more relieve.

Best regards Jeroen

 

[Ken I]

Jeroen,
Bearings are the same in SA - its me that screwed up - the bore is supposed to be 28mm - during the drawing I cropped the same lines as the 32 thrust bearing.Similar with the thrust.

Attatched corrected drawings.

Note to moderator - can you replace the *.zip file in the downloads section with this corrected file.

Edit
I ammended the drawings further - I see three people uploaded prior to this - please repeat.

Regards,
Ken

View attachment WOBBLY.zip

 

[Ken I]

The *.zip file in the downloads section has been replaced with the corrected version.

FYI

Ken

 

[coopertje]

Thanks for the update Ken, now the dimensions make sence and I am glad to learn the bearing specifications are the same all over the world!

With a little bit of luck I get the bearings in house today, eager to make some chips!

Regards Jeroen

 

[coopertje]

Hi Ken,

Got the bearings when I arrived home from work today. I measured the inner diameter of the 55202 inner ring and its 17.2mm :'( So with the change from 15 to 17mm it will still be impossible to get the inner ring out. I will make it 20mm. Its strange that they use so much clearance on the inner ring, the outer diameter of the outer ring is 32mm meaning there is no clearance at all ??? I think its good to have the clearance in the housing as you mentioned in your drawing.

Sorry for the trouble caused, regards Jeroen

 

[Ken I]

Jeroen,
The clearance varies from manufacturer to manufacturer - it is seldom quoted in the specs.

That is a lot of clearance but the size of the hole under it is of no consequence just make it 19mm - that will be fine.

Ken

 

[coopertje]

Ken, I have one more doubt. Its about the final alignment of the broach holder. Since there is an angle of 1 degree this will move the center line at the end of the holder (41mm length) by 0.7mm. When the broach holder is mounted together with the broaching tool this deviation from center line will increase even more.

Should I mount the holder in the tailstock including broaching tool in the tailstock of the lathe and then adjust it such that the center of the broaching tip is in line with the center of the lathe nose? Or is there is different procedure?

Thanks for your help!

Regards Jeroen

 

[Ken I]

Jeroen,
If you look at the shank drawings you will notice the tapped holes are offset slightly to allow for that.

Yes the face of the broach must be on the tailstock centreline.

If you make it "fixed" - no floating adjustment - then all your broaches must be the same length. Also resharpening them will throw them off slightly (and reduce their size a little) within reason this is not a problem.

That design is based on the design used by the automatic lathe manufacturer "Index" of Germany - they use the "float" feature.

As I have stated earlier I prefer this method - you drill and chamfer the hole you are going to broach - bring the broach to bear against a rotating part with the bolts loose - stop spindle whilst maintianing pressure - tighten bolts - that's it.

When using a fixed design (particularly from the toolpost) if you don't get your alignment right, there is a risk of snapping the broach.

A further tip - the drilled hole needs to be just a little larger than the size of the broach - there is a very large change in force from just bigger to just smaller - imagine an 8.00mm AF hex into an 8.02 hole - produces 6 swarf strands which curl over into the bottom of the hole (normally left in the hole - alternatively they break off if you employ an undercut or you can reapply the drill to remove them) if instead the hole is 7.98mm the broach is going to try and form a "tube" of swarf turned inside out which takes a hell of a lot more force.

When I ran an auto shop we used to grind the drills "odd leg" - one cutting edge slightly longer than the other which causes the drill to drill a little bigger.

Regards,
Ken