Thread: BF-20 Mill Questions Fitting a Collet Chuck

I had the same issue with my HM-46, which is 1/2" whitworth. I bought some high tensile steel threaded m12 rod from a nut and bolt supplier and welded an m12 nut to the top. Then I did up one underneath against it for good measure. It's worked ok so far but you could successfully argue I am in general a dodgy person.

Originally Posted by Oldneweng

In what way do you think it is not in tolerance for a 1/2" Whit thread? Due to the rounded thread tips the size is somewhat less than 1/2". I recall a US YouTuber saying he had found this weird thread size. He measured it at 12 tpi and 31/64". He had no idea what it was.

We are basically talking different values and because of what I have briefly alluded too. Refer BS84 and BS1083. If mine was 31/64 then I’d accept it.

I’m not claiming to be an expert on this and my only understanding is from research as I like to find out from myself on most things rather than hearsay – so someone can correct me if I’m wrong. First point is that not all Whitworth threads have rounded crests – yes more typical in the past but there is also a “truncated Whitworth thread".

My draw bar is 12 TPI with flat crests therefore assuming this is in fact supposed to be a truncated Whitworth thread, the tolerance according to BS84 is 0.0074” and a truncation of 0.0123”. Assuming “free class” gives and upper value of 0.4862” and lower value of 0.4788”. The major diameter on mine measures 11.9mm or 0.469” around 15/32”.

The 31/64” value that you mentioned does of course fall within this range but mine does not. If 31/64” was a standard thread (not truncated) then it also wouldn’t fall within tolerance.

On a similar note on some vintage machinery that I needed replacement 1/4" BSF screws recently, the replacements didn’t come within correct tolerances either resulting in a sloppy fit.

Yes I’m fussy haha…

Originally Posted by Retromilling

That is not correct for NT tapers they hold after the drawbar is released and require a bang on the draw bar to release . If it just falls out after the drawbar is loosened it was never in properly and or too much oil or garbage on it . What you have been watching most likely is power drawbars that push the tooling out automatically .

That maybe the case but ISO/NT/CAT tapers are considered to be self releasing tapers as opposed to MT which is self holding. Sliding a clean MT tool into a clean taper will result in a self holding arrangement that will transfer the rotational torque to the tool without the need for a driving dog or lug. Of course it was really only designed for rotational loads such as drills etc and not side loads such as milling cutters, hence the need for a drawbar.

ISO tapers are considered self releasing. If you side a ISO taper collet into it's spindle, it will not grab and will fall out without the drawbar and will rotate in the spindle without the drive lugs. It may or may not fall out after releasing the drawbar but it's still considered a self releasing taper.

Simon

About 2005 when I was still teaching I repaired the drawbar for a H & F Mill belonging to the school.

The students had mashed the end by not following correct procedure with its removal.

I too had trouble with the ID thinking it was M12. another teacher (ex Fitter & Turner) checked it and said it was 1/2" Whitworth
I do recall that even the bolt shop did not carry this thread but referred me on to the Mower shop who had a junk box with the correct thread and appropriate length.

After that it was a simple matter to remove the mashed up section and center drill, vee and step and weld the components. To my knowledge it still runs today.

I never understood what an imperial thread was doing in a Chinese made milling machine.

To answer your question 1/2 Whitworth it might possibly be.

Grahame

We are basically talking different values and because of what I have briefly alluded too. Refer BS84 and BS1083. If mine was 31/64 then I’d accept it.

I am just wondering whether it is a Whit thread, but undersize. I agree it is not acceptable at the size you measured. I got a facing head and a drill chuck with the M14 thread in them with my mill. The facing head is a piece of junk and the drill chuck rarely gets used as have a keyless chuck.

I bought some high tensile steel threaded m12 rod from a nut and bolt supplier and welded an m12 nut to the top.

I used an M10 nut on the top of my drawbars to match the original one. The thick washer I made was too big (diam) to fit inside the spindle splines. It was not that big to start with. An M12 nut would not fit.

Dean

Not to long ago I made a M12 drawbar for a friend at work that has one of those mills

used a bit of threaded rod cut to length, machined a collar to thread on then used loctite and drilled and pinned the collar to make sure it wouldn't move. Machined a square drive directly on the threaded rod and bob is your uncle.

Not sure if I bothered to take a photo of it but may even still have the technical drawing I did for it somewhere. If you don't have a lathe and need one done let me know.

Cheers
Wayne

A self releasing drawbar is pushing it out for you . A manual system still requires a light hammer on the drawbar .

My mill has a MT3 taper (Arboga) held in place with a large threaded collar that screws onto the spindle, the collar is only knurled and does not have slots for a "C" spanner- I believe the reason being that you do not have to do it that tight to get the taper to hold, hand tight so it does not come loose during use is enough. When I first started using the mill I assumed it was the same as the iso taper fitted mill I had used at school and did lean on it with multigrips (just to be sure) and was planning to make a new collar with "C" spanner slots so I could do away with the multigrips, yes tooling was hard to remove and I just assumed this was part of the MT3 system. A toolmaker friend noticed this one day and explained that "hand tight" is all that is needed, pointing out that there are no "C" spanner slots on the collar for a reason, sure enough he was right, this worked with the added bonus that tooling came out easily. I only use hand tight now and I have never had a drill/collet chuck spin in the spindle taper.

How tight a collet needs to hold is also a function of how hard the milling job is , hard steels , deep cuts , high horse power all means you will need extra cutter security .