Wobble broach that doesn't Wobble.

[DanClarke]

Hi folks,
I am trying to put 12mm AF hex holes into the centre of 34mm round threaded discs sitting in the lathe chuck.
An expensive wobble-broach bit was purchased along with a holder for said "bit", and that meant also buying a bigger chuck for the tailstock of the lathe to hold the wobble-bit holder itself.
The money expended on new shiny things to accomplish this task is now >£100.
Only issue is that the bit just doesn't wobble and doesn't cut the hex keyway.
Now I understand that "some" wobble-broach holders have the required 1° offset inbuilt into their structure, and some don't.
I can only assume that mine does not. So how do I get the tailstock to skew by 1° off true?
And IF I could do that, as the business-end of the bit is many CM's away from the root of the tailstock will it not now be making a hex-hole wayyyyyy off-centre of the disc?
I am stumped. Can anyone shed some light on this problem please?
Thanks in advance.

 

[Agroshield]

How many holes total?
Material?
Depth of hexagonal hole?
Through hole or blind hole?

Please could you link to or show pictures of the items you have bought as well as how you have it all set up in the lathe.

 

[MattF]

What broach, & what lathe?

 

[Seadog]

Surely, if it does have the 1° offset it's not a wobble broach. Offsetting the tailstock will not achieve what you want.
The length of the tool is also important.

 

[Kram]

Set it up in tool post not tailstock. That will allow you to correct the position while maintaining the skew angle.

 

[DanClarke]

How many holes total?
Material?
Depth of hexagonal hole?
Through hole or blind hole?

Please could you link to or show pictures of the items you have bought as well as how you have it all set up in the lathe.

Thanks for replying.
I need to do this about 6 times.
Currently it is T6085 Ali, but I may have to go to mild steel.
It needs to be as deep as I can make it, the broach spec says "up to 1".
The bar I am making the hole(s) in is 3" long with the required 24TPI thread on the outside and with a clearance hole all the way through it. I need to cut a hex hole/keyway, part off a section, rinse, repeat. So it's a blind hole that becomes sighted.
It is this broach holder https://www.ebay.co.uk/itm/363405325040
And the broach is this one: https://www.ebay.co.uk/itm/113854767132
I put the holder in my tailstock mounted chuck and I can see the bit doesn't wobble. I am just about to put a dial indicator on a drill shank held in the holder to check if it has any "wobble" or if it is (my suspicion) just a true bearing supported spinning holder. Pictures to follow.
Just checked the wobble-bit holder, and it has <2 thou of run-out, so there's no "Auto-Wobble" in that. Some have it built in, some don't...this one don't.

 

[DanClarke]

Set it up in tool post not tailstock. That will allow you to correct the position while maintaining the skew angle.

Ok, this means I may need an MT2 capable AX toolholder. More toys/expense. :-)

 

[Brad93]

That looks like junk that broach holder.

 

[DanClarke]

Set it up in tool post not tailstock. That will allow you to correct the position while maintaining the skew angle.

The shaft of the wobble-bit holder is too big to go in a tool-holder unfortunately.

 

[DanClarke]

That looks like junk that broach holder.

Yup, it appears to be junk.

 

[DanClarke]

What broach, & what lathe?

Boxford AUD Mk2 and THIS broach.

 

[Seadog]

 

[DanClarke]

Surely, if it does have the 1° offset it's not a wobble broach. Offsetting the tailstock will not achieve what you want.
The length of the tool is also important.

Well it was advertised as such and I am a numpty/noobie to the topic.

 

[Seadog]

Who supplied it?

 

[DanClarke]

Yes, I know the theory, it's the practice that is failing. :-)
I had expected a "wobble broach holder" to errr.... "Wobble" a bit. But no.

 

[DanClarke]

Who supplied it?

I bought it off evil-bay. My bad.

 

[iNf]

This might give you some ideas/ info
http://mikesworkshop.weebly.com/rotary-broaching.html

 

[Seadog]

Yes, I know the theory, it's the practice that is failing. :-)
I had expected a "wobble broach holder" to errr.... "Wobble" a bit. But no.

It's not a huge amount of wobble, but should be detectable.
Have you a pillar drill that you could try it on?
Alternatively, hold the bit in a vice using soft jaws.
Rotating the so-called wobble broach body will give a magnified movement if it is in fact made correctly.

 

[DanClarke]

It's not a huge amount of wobble, but should be detectable.
Have you a pillar drill that you could try it on?
Alternatively, hold the bit in a vice using soft jaws.
Rotating the so-called wobble broach body will give a magnified movement if it is in fact made correctly.

Tried that with a run-out gauge and there's <2-thou of movement on a shaft inserted in the holder around the 360-degree rotation of the broach holding element. Nothing visually detectable as "wobble".
I just have to accept I have the wrong tool for the job and get/make a better one.

 

[Seadog]

To clarify what we're talking about, the "wobble" will show as the shaft centre moving around in a defined circle when viewed from the end (using the vice method) as you spin it.

I started making a broach with the idea that I could have the bearing concentric to the tool axis and then offset in a tool post. It was only once I bored, and ruined, the blank arbour that it dawned on me that I hadn't appreciated exactly how it functioned.