Bar diameter for 1/2" BSW

[JimM]

Just got a Clarkson Autolock chuck for my mill but the drawbar end is threaded 1/2" BSW. Unfortunately I haven't got any 1/2" stock lying around but do have plenty of 12mm - will this be OK to use in this instance. Just a bit concerned as there will be a lot of force on the threads and don't want them shearing.

Thanks

Jim

 

[vlmarshall]

Ha, somebody answer this. I botched my answer... no coffee yet. ;D

Here's a thread chart.
https://ssl.perfora.net/metricshop.com/tech/thddat8.htm

 

[PhiberOptix]

Hi, JimM,

Well your only 1.4mm too small, but that said, this is not for some model that will be standing on a shelf doing nothing, this is for a drawbar that will hopefully be in daily use, for SAFETY's sake, I would do it right, and as you have gone to the expense of a new chuck, I would go a little further and buy a length of 1/2 inch bar, you raised the question yourself about been concerned over the stresses/torque etc, that would be enough for me to warrant the extra expense, if in doubt...... saftey first you only live once.

Best Regards
Andy

SAFETY FIRST

 

[b.lindsey]

I agree with Andy, that is a sizeable difference, especially when it will be under load. Most definitely a safety concern!

Bill

 

[JimM]

Thanks guys, you've confirmed what I'd pretty much decided myself.

Only really asked as hoped to knock up the drawbar today - I'll ring round a couple of stockists tomorrow and hopefully somebody will have some imperial sizes in. (Hate doing the mail order route for stock, costs almost as much for a 30cm length from a model supplies shop as a 6m length does from the steel stockist)

Cheers

Jim

 

[SandyC]

Hi Guy's,

Well I disagree with you: -

The difference in dia is only 0.0275" (0.7mm not 1.4mm) giving a radius difference of 0.0137".

1/2" whitworth thread form has a core dia of 0.3933" and a thread depth of 0.0534"

Using the 12mm bar means that the thread depth would be reduced by 0.0137" giving a final depth of 0.0397" which is just a shade under 75% thread engagement.

This should be more than sufficient for a safe drawbar since the majority of commercial ones are rarely better than around 80%.

The stress on the thread should not be excessive, since no draw bar should be done up to tight... just enough to prevent the taper from dissengaging..... generally around 1/2 turn - 3/4 turn past the hand tight spot.
Any more just over strains the drawbar, pulls the taper to tight, which just means more force required to break it free from the socket when the time comes to change to a different holder/tool, and puts unnecessary pressure on the spindle taper socket.

If you screw cut the 1/2" whit. thread on your 12mm OD bar then you can be sure it is at least concentric with the centre (using a die could possibly make the thread a bit off centre). Cut it to a depth that just allows the collet chuck to go on without any shake and you should be more than OK.

Hope this helps .

Best regards.

SandyC

 

[PhiberOptix]

Quote The difference in dia is only 0.0275" (0.7mm not 1.4mm) End Quote

Correct my math was/is wrong, I had my head thinking about Inch rather than half hence the difference.
Tho I myself would still go and get that 1/2 inch bar just to be on that right side of safe.

- JimM I have Pm'd you offering you some

Andy

 

[Tin Falcon]

12 mm = .4724 the published efective diameter for this thread is. .4466. this is not a v thread the top and bottom of the thread form are radiused. The way I read this is that there will be enough metal there I will have to agree with sandy on this one.
Another way of looking at this is the tap drill size is 10.5 mm the double depth of thread in mm is 1.346 that added to td size comes to 11.846 mm so I would give the 12mm a go .
Now folks this is not a USA 60 degree v thread form that would likely requr th 1/2 bar stock.
Tin

 

[deverett]

Another way to do it: Get a 1/2" BSW bolt, cut the head off and put a reduced thread on that end, say 3/8". Get your piece of 12mm bar, drill and tap for the 3/8", fit the 1/2" BSW modified bolt with locktite.

On the top end, put whatever convenient thread you like.

Dave
The Emerald Isle