Boxford C3 Collet Drawbar

[Arc Tourist]

I need to chase the threads on the collet drawbar I acquired awhile back. Just want to confirm my measurements with other Boxford owners before I order a tap.
Am I correct in that the tap I need to order is a 5/8" x 26 tpi BSCY ( Cycle Thread ) ?

Thanks.

 

[Pete.]

I think it a bit over 5/8" let me go check...

EDIT: yep it's 0.640" 26TPI righht hand thread so a 5/8" tap at 0.625 is not going to chase it properly.

 

[Arc Tourist]

Thanks @Pete. , I measured an average of .625" on depth of thread across several collets, and .635" across the outside of the threads.

 

[Pete.]

Actually, I previously posted the collet specs here, it's 0.645":

 

[Agroshield]

I think I read somewhere that an old trick to make a reamer cut slightly oversize was to jam up one of the flutes with a chunk of hardwood.

The same dodge might work to make your tap cut oversize.

If it is just chasing them and you do not mind sacrificing a tap, grind the teeth off all but one flute, hold it in the toolpost, set the change wheels to 26tpi and 'screwchase' it.

 

[RichardM]

"If it is just chasing them and you do not mind sacrificing a tap, grind the teeth off all but one flute, hold it in the toolpost, set the change wheels to 26tpi and 'screwchase' it."

You could do the above with a 1/4 BSF tap without sacrificing its. Bottom tap - set one of the flutes cutting edge to centre height on the tool post and feed it in.

 

[Arc Tourist]

I think I read somewhere that an old trick to make a reamer cut slightly oversize was to jam up one of the flutes with a chunk of hardwood. ..................................

Have tried that before with small taps using toothpicks, but they were too soft.

 

[Arc Tourist]

"If it is just chasing them and you do not mind sacrificing a tap, grind the teeth off all but one flute, hold it in the toolpost, set the change wheels to 26tpi and 'screwchase' it."

You could do the above with a 1/4 BSF tap without sacrificing its. Bottom tap - set one of the flutes cutting edge to centre height on the tool post and feed it in.

I would prefer to hand tap it, as I could feel if the tap would not be cutting correctly.
Edit: Also, the collet draw bar is not long enough to fit through the headstock to be secured in the chuck.

 

[Pete.]

I suppose you could buy a carbon steel tap, slit it down the middle with a 1mm disc, anneal the shank and then use a tiny wedge pf material to spread the flutes 15 thou. If I were doing it though I would simply make a triangle point tap the correct size and pitch.

 

[Arc Tourist]

I suppose you could buy a carbon steel tap, slit it down the middle with a 1mm disc, anneal the shank and then use a tiny wedge pf material to spread the flutes 15 thou. If I were doing it though I would simply make a triangle point tap the correct size and pitch.

I just ordered a 5/8 x 26 tap & die set off the ebay, item # 274 813 470 999 , so will have three taps to work with. I like your idea, and do have a selection of slitting saws, but will try the hardwood wedge method first if the taps are undersize as you never know nowadays with import products; they may be spot on or not.

 

[Seadog]

I made a tap. Very rough, but did the job.

 

[MattH]

If you've got a spare collet then split it a few times with a cut off disc and then use that to clean up the threads.

 

[Seadog]

My drawbar was heat treated. Even though the tap was hardened and un-tempered, it still took a little effort to reform the bruised threads. A collet might not do it. But worth a try if you have a sacrificial one laying around.

 

[Agroshield]

You could do the above with a 1/4 BSF tap

1/4 BSF will be 55 degree thread vee-angle. I suspect that 3C collets will be 60 degree vee-angle. BSCY thread is unusual for a British thread in that it is 60 degrees.

...the collet draw bar is not long enough to fit through the headstock to be secured in the chuck.

You could use RichardM's idea and use a smaller BSCY tap, carefully held eccentric in a four jaw chuck or boring head so that one flute sweeps out the correct diameter. Weld the drawbar to the crosslide and chase the threads using the leadscrew/gearbox/change wheels. For such a short thread, it would not be too tedious to rotate the spindle by hand as that would give the feel you rightly say is needed for the operation.