Thread Equivalents for a Draw Bar

[joco-nz]

Having taken delivery of the last of the new tools (BF20LV) I'm looking at the drawbar with the expectation of getting some tooling.

The manual says its M10, but the JT6 drill chuck arbor that came with the tool has these numbers on it:
MT3-JT6-1/2-12

I have tested the arbor and it screws on to the draw bar just fine. I also measured the internals of the arbor and the diameter between the top of the threads is a tad over 11mm. So the draw bar can't be M10 can it.

From some reading around the net it looks like the 1/2-12 will mean 1/2 inch and the 12 refers to a BSW tpi of 12.
This looks to be VERY close to an M12 thread.

My question is can I get away with using an M12 thread. For example an M12 threaded MT3 ER32 collet chuck? Or am I going to have to either source correctly threaded tools or pull out the existing draw rod (which is fixed in the mill head some how as it "self ejects" the tool as you unscrew) and make a new one with a more standard thread.

Thanks,
James.

 

[Wizard69]

1/2"-12 is an old standard size in the USA which is more or less out of circulation now. 1/2"-12 is also an old Whitworth thread. Then you have close metric sizes to contend with.

At best you would buy a set of thread pitch Gage's if expect to work on older machinery. In that respect it is always a good idea to state the model and other information for the machine. Beyond that it isn't uncommon on used machines to find a custom draw bar that has replaced what was original. Also some metric and Unified screws can be extremely close in size and pitch, M5 and 10-32 come to mind, sometimes you need to measure or use other devices to get the right size.

 

[joco-nz]

The new tool being referred to. The draw bar is as supplied from the supplier.

Guess I am going to have to pull the draw bar out and test it on an M12 nut to see if it works or binds up.

 

[grapegro]

Hello James, On my BL 17, I found that the drawbar was in fact an odd size. Cannot remember the exact size difference, but your comments recall my problem. I withdrew the original drawbar and made one to suit existing tools which I had, namely 3 MT 12 metric. While the manual states a 2MT arbor, Machinery House seem to have altered this to their own species to entice users to buy their own tools. Mine was made by using a long 12 m High tensile bolt, fitting a collar onto the bolt, and milling a square on the top to enable the releasing action to take place. Good luck, Norm

 

[bazmak]

I thought the draw bars were M12 and 7/16 bsw bothe near enough to be interchangeable on most chinese machines

 

[Blogwitch]

You will find this on MT spindled machines.
Unless you are very careful, or unlucky, tooling for MT3 can come with either thread.
The easiest way is to either make sure the tooling you buy has the required thread (sometimes you cannot get what you want with the correct thread) or just have two drawbars, which is what I resorted to when I had an MT3 milling machine.
I have just bought some MT3 blank arbors for making tooling for my lathe, and they only come with imperial sized threads, but because I am going to be using screw in tangs, I got those to fit, they were 3/8" Whitworth.

John

 

[joco-nz]

From what I understand from looking about the net the draw bar is a "captured" one. So its not just a case of pulling it out. But I think some people have also just used some threaded rod cut to the correct length with appropriate nuts in place.

Anyway ... I think I smell one of my first real turning projects. :thumbup:

 

[goldstar31]

I agree with John( Blogwich) about the two necessary drawbars( for me anyway)
So I tapped rod and added square ends welded on. Saves a lot of unnecessary turning!

Cheers

N

Digressing, but feel a further note or two are merited. I have draw bars for both Myfords and also draw bars or whatever they are called so that I can hand tap and screwcut large threads

 

[Journeyman]

Just don't do what I did when I was proud owner of new mill similar to yours but smaller. I carefully read the very "Chinglish" manual and it said to release the 2MT taper slacken the drawbar and hit it with a hammer! Not knowing any better I did this and promptly broke the square end of the drawbar which had probably been over hardened. The captive drawbar just needs undoing and it will push against the hollow nut and push the tooling out - easy! I too find I need two drawbars one 3/8" whit and the other M10. Strangely the machine (WM 14) is metric but was supplied with an imperial drawbar. Changing drawbars is a matter of a couple of minutes work with two spanners so no great difficulty.
Cheers John

 

[joco-nz]

Cheers guys. After a little reading and some pics texted to me (thanks Bruce) figured out how easy it is to remove the bar. Definitely going to make a metric one for the ER32 collet chuck Bruce is kindly offering to sell me.

On the material front to make this I don't think I need anything other than mild steel round stock as the starting blank? I can use a file to shape the square section on the top to fit the tool supplied with the mill.

To finish it I properly should either blacken or blue the rod?. Using gun blue chemicals seems the less fussy since its a cold process. The whole heating to red and dunking in motor oil multiple times seems a bit more than I want to get into at this stage. Unless someone has another, easier method?

 

[goldstar31]

From what I understand from looking about the net the draw bar is a "captured" one. So its not just a case of pulling it out. But I think some people have also just used some threaded rod cut to the correct length with appropriate nuts in place.

Anyway ... I think I smell one of my first real turning projects. :thumbup:

As a first project or two, I'd be looking at Chris Heapy's Model Engineering Support Pages. Possibly I've got to repeating myself but everyone else seems to be resurrecting things. At least, I am giving the various authors due acknowledgement

Norman

 

[joco-nz]

Norman - this site? http://easyweb.easynet.co.uk/~chrish/cancelled account/homepge2.htm

The" cancelled account" words in the url were a little off putting and made me wonder if this was an archive of a currently active site.

Cheers,
J.

 

[joco-nz]

I had a play in the CAD software last night and modelled up the draw bar I need to make. Please forgive the slightly odd ball dimension on the M12 thread, that's the CAD doing screwy crap.

From a construction point of view I guess I could either do this from a single blank of say 20mm round stock or approach it as a two part with a 12mm shaft and a smaller piece of 20mm turned to size and a close fit and perhaps braze them in place. From a turning practice perspective starting with the 20mm stock and using tail support and probably a following steady could be good learning.

Cheers,
J.

 

[goldstar31]

Norman - this site? http://easyweb.easynet.co.uk/~chrish/cancelled account/homepge2.htm

The" cancelled account" words in the url were a little off putting and made me wonder if this was an archive of a currently active site.

Cheers,
J.

This is a 'resurrected site' done, I believe by a Japanese gentleman after Mr Heapy moved to star gazing!

I've a lot of time for the information, made several bits and heeded a lot of advice there.

Cheers N!

 

[Cogsy]

If you turn down a 20mm bar to 12mm you'll be removing something like 2/3 of the mass of the bar. I'd go for a built up one myself. There'll be countless other jobs to do with your new lathe so I wouldn't go chasing a horribly boring exercise like this one.

 

[goldstar31]

If you turn down a 20mm bar to 12mm you'll be removing something like 2/3 of the mass of the bar. I'd go for a built up and one myself. There'll be countless other jobs to do with your new lathe so I wouldn't go chasing a horribly boring exercise like this one.

Cogsy is quite right. A bit of cheap studding with a bit of square 'stuck on' or a bit of round and threaded appropriately with a bit of square.

Cheers

N

 

[joco-nz]

Yeah - pretty much my approach 2.

 

[Blogwitch]

J,

One of the main problems with MT tapers is that it is self locking due to the angle of the taper, notice the R8 taper, it is much larger, and so self releasing.

Where people go wrong is that they think the drawbar should be given the white knuckle treatment and then have trouble releasing the taper. Once done up finger tight, it only needs a few more flats to make sure the taper doesn't release while in use, it doesn't need to be over tight.
Get yourself a soft hammer, preferably lead (I make my own for other jobs) and a short sharp wack should have the taper released very easily.
Another point is that if you are not going to use the machine for a while, take the toolholder out as temperature changes or a tiny bit of corrosion can cause the MT taper to stick like it was solid. Remember, even though you think you won't get corrosion, fingerprints are usually acidic and can cause things to rust up.
Some machines have a couple of holes around the top of the machine spindle, these are usually for using a plate and a couple of bolts to press down on the drawbar more gently than belting it with a hammer and are used to release the taper. Others use a 'C' plate that sits in a groove in the nut at the top of the drawbar which does a similar sort of thing, as you screw the drawbar out, it automatically pushes down on the drawbar to release the taper.

John

 

[joco-nz]

J,

Others use a 'C' plate that sits in a groove in the nut at the top of the drawbar which does a similar sort of thing, as you screw the drawbar out, it automatically pushes down on the drawbar to release the taper.

John

John - I'm not sure I have a 'C' plate as such but the the 17mm section near the top of the bar is trapped under a screwed down cover. So as I unscrew the bar from the taper the bar can not rise up and so pushes the taper out. No need to "tap" anything so long as you don't try and do the "white knuckle" treatment on tightening things up.

The other tips re skin grease is a good reminder.

Cheers,
James.

 

[Blogwitch]

Sorry J, looks like I had read Journeyman's post just before yours, so got the two mixed up.

But anyway, as long as you got a bit of info out of it, no damage done.

As stated by other people, just a bit of threaded rod will do for another drawbar as not much pressure is required. I actually did what you wanted to do, make up a new drawbar out of some hex bar I had, but that was just me going overboard.

John