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kNucKlbustr
05-23-2012, 06:33 PM
The drawbar stretched then snapped. Of course, when the project is due Fri.
Panicked and ordered a drawbar from ENCO and pd for 2 day shipping.

This is a universal 7/16-20, 20" drawbar. Should I oil harden the threads when it arrives? Or are these suppose to stretch?

In the meantime I did manage to repair the broke drawbar from a cutoff Grade 3 bolt, pocketed, 58% silver soldered.

Black_Moons
05-23-2012, 06:38 PM
Depends.
Do you want your drawbar to wear or do you want your collets/boring head/drill chuck/etc threads to wear? Drawbars are considered consumables.

As for stretching and snapping.. sure you didn't tighten it up too much? Or maybe it had a defect?

Bob Fisher
05-23-2012, 07:51 PM
In a pinch, I would use a length of 7/16-20 threaded rod with a nut welded to one end. All you really need to do is pull the collet. Never been able to break a drawbar myself though.Maybe in my younger years, but I didn't get drawn into this machining hobby till later Bob F.

Bob Fisher
05-23-2012, 07:51 PM
In a pinch, I would use a length of 7/16-20 threaded rod with a nut welded to one end. All you really need to do is pull the collet. Never been able to break a drawbar myself though.Maybe in my younger years, but I didn't get drawn into this machining hobby till later Bob F.

KiddZimaHater
05-23-2012, 07:53 PM
Don't harden your drawbar.
Its better to strip a $15 drawbar than a $40 collet, or $150 Jacobs chuck.
Next time order two drawbars, one for 'backup'

lakeside53
05-23-2012, 07:56 PM
The draw bars I get are 4140 and have rolled threads (verses "cut"). I have a bunch of cheap junk with damaged threads, but the quality bars (Kurt, Dorian) are fine.

wierdscience
05-23-2012, 08:05 PM
Chop them off and weld cutoff grade 8 bolts to them,they last for years and are a few points softer than the collets.A little wipe with never seize every few months never hurts either.

Some of the "US made" drawbars are absolute garbage,buyer beware.If it shows up with the head pinned on,you know you've been had.

lakeside53
05-23-2012, 08:08 PM
Not all... the Kurt's have a roll pin to back up an interference fit. The air draw bar is pretty brutal and they last.


A lot of problems with threads is because the engagement is insufficient. Many draw bars have spacers to set the engagement, and separate spacer for "dual-use" draw bars -you cut off the damaged threads and toss the spacer. But.. many don't - they just keep shortening the bar. I like 5-7 turns at least, preferably a few more.

Carld
05-23-2012, 09:03 PM
Many people over tighten the drawbar. I worked with a fellow that could twist them off and strip them easily. One time he had an endmill suck out and ruin the work so he decided the collet was not tight enough and always over tightened them to the point of damaging them and the collets.

If you understand what causes an endmill to suck out of a collet and learn how to prevent it you won't have to over tighten the drawbar.

Lew Hartswick
05-23-2012, 09:05 PM
Off hand, I'd say if youre breaking a draw bar, youre over torquing it.
...lew...

gnm109
05-23-2012, 09:44 PM
I made a drawbar for my Webb Mill from 4130 rod. I single-pointed the 7/16-20 thread for about 3" on my lathe. For the upper section, I got some 3/4 hex bar and drilled it for the drawbar. I milled a flat on one side of the drawbar rod and I used two 1/4" set screws to hold it together. It's worked fine for nearly three years now.

Perhaps the OP was tightening the drawbar up a bit too much. In a pinch, you can use pre-threaded rod as mentioned above. I wouldn't want one that was hardened, either. It's not good for your tools.

kNucKlbustr
05-23-2012, 10:42 PM
Off hand, I'd say if youre breaking a draw bar, youre over torquing it.
...lew...

Im using Tormach tools w/ their R8 and I believe they suggest 60ftlbs.

I started to click torque at 60. Now probably more likely at 45-50 or when my shoulder hurts.

With fixed one I finished. Center drill, drill bit, counter bore- 40 holes.

oldtiffie
05-24-2012, 05:18 AM
The OP:


The drawbar stretched then snapped. Of course, when the project is due Fri.
Panicked and ordered a drawbar from ENCO and pd for 2 day shipping.

This is a universal 7/16-20, 20" drawbar. Should I oil harden the threads when it arrives? Or are these suppose to stretch?

In the meantime I did manage to repair the broke drawbar from a cutoff Grade 3 bolt, pocketed, 58% silver soldered.

No-where does it say what he was was pulling - R8 or MT collets or an MT adaptor for say ER-32 collet adaptor.

Some of my draw bars - dictated by what they are pulling on/up to - are M12 or 1/2 - UNC adaptors or the MT3 shank of a drill chuck etc. - and they are different lengths for my HF-45 and Sieg mills.

I just use high quality high tension "all-thread" bar cut to length with a "nyloc" nut and a plain high pension nut as a jamb nut with a high tension plain washer under it.

I have never had a taper slip in a quill taper nor have I stripped a draw-bar.

dian
05-24-2012, 05:18 AM
for drawbars i use a threaded rod with a lose long nut. thus i tighten the nut and there is no wear in the toolholder.

carld, what exactly causes an endmill to suck out of the collet besides low clamping force?

willmac
05-24-2012, 06:39 AM
I have made 4 different drawbars for my Thiel mill, all to basically the same design. The reason for making this number is that you need drawbars for both the horizontal and vertical spindles and I buy good but used 40 taper tooling which may have either metric or UNC threads in a couple of different lengths.

The original drawbars are basically one piece, which are fine, but use a lot of steel and would be a pain to make multiple copies of. The design that I came up with has replaceable ends top and bottom. These are Loctited to a central section and then fitted with roll pins. Loctite on its own might be sufficient, but the roll pins are added insurance. This design makes it really simple to make up a new drawbar.

The threads are all screwcut and accurately made. I have never had any problem with these wearing or stripping and they are not hardened. I don't see why you could not have the same approach with a Bridgeport type drawbar.

Note that these drawbars have a tool extraction feature, which is why the have the collar at the bottom and the cone nut on top. No need to hit the drawbar to release tools. This one is for the vertical spindle.


http://i122.photobucket.com/albums/o248/moorbrook1860/P1000795.jpg

mike4
05-24-2012, 07:45 AM
Off hand, I'd say if youre breaking a draw bar, youre over torquing it.
...lew...
+1 on that as I have made a couple to match some odd threaded tooling i purchased once and they were made from 1/2" mild steel bar cleaned up and threaded as required , mine are only tightened enough to hold the tool holder in the spindle without any movement .
To tighten a drawbar so that it stretches is a sign of concern for me.
I would investigate the internals of the spindle for foreign material or damage if your tooling is not seating correctly with normal torque levels.
Michael

John Stevenson
05-24-2012, 08:08 AM
Hard to post this without being accused of selling but I make drawbars for Bridgeports in the UK.

The thread is made form EN16T machines as is and professionally blacked.
The head is 3/4" AF hex cyanide hardened and blacked.

Never had one returned broken.

http://www.stevenson-engineers.co.uk/files/drawbars.jpg



http://www.stevenson-engineers.co.uk/files/drawbar1.jpg

Step speed, varispeed and drawbars for the horizontal attachment, all double life bars.

DFMiller
05-24-2012, 08:41 AM
I don't see that as selling.
Revealing trade secrets is another thing.
You should be banished for sharing such secrets.
Dave

oldtiffie
05-24-2012, 08:46 AM
The drawbar stretched then snapped. Of course, when the project is due Fri.
Panicked and ordered a drawbar from ENCO and pd for 2 day shipping.

This is a universal 7/16-20, 20" drawbar. Should I oil harden the threads when it arrives? Or are these suppose to stretch?

In the meantime I did manage to repair the broke drawbar from a cutoff Grade 3 bolt, pocketed, 58% silver soldered.

As that seems to be a pro-made draw-bar and if it were me, I'd leave well enough alone and us it "as supplied".

Carld
05-24-2012, 09:16 AM
dian, an endmill can get sucked out by to fast feed in relation to the rpm, especially with a two flute. The heavy feed will cause the spiral and leading cutter face to dig into the work with each pass and will make a stair step deeper cut along the work. Also, a worn collet will tend to let the endmill suck out. The endmill starts wobbling in the collet and starts working it's way either in or out as it cuts.

This is a little of the topic but of interest to me at least.

daved20319
05-24-2012, 11:52 AM
Note that these drawbars have a tool extraction feature, which is why the have the collar at the bottom and the cone nut on top. No need to hit the drawbar to release tools. This one is for the vertical spindle.

http://i122.photobucket.com/albums/o248/moorbrook1860/P1000795.jpg

Hey, Bill, would you mind expanding on this a bit? And would it work on a smaller mill like the Sieg XS-3? I'm really enjoying my mill, and get more comfortable with it every time I use it, but whacking that drawbar with a hammer to released the collet still feels wrong. Besides, my drawbar is kind of a sloppy fit, so I've been considering making a replacement. I'm glad this post came up, based on what I've been reading here, I'd have picked the wrong material for sure. Thanks.

Dave

corbin
05-24-2012, 12:51 PM
Many people over tighten the drawbar. I worked with a fellow that could twist them off and strip them easily. One time he had an endmill suck out and ruin the work so he decided the collet was not tight enough and always over tightened them to the point of damaging them and the collets.

If you understand what causes an endmill to suck out of a collet and learn how to prevent it you won't have to over tighten the drawbar.

I've had endmills suck out of a collet. What causes this to happen? I've since switched to using an endmill holder with a lock screw on the side, but sometimes I still have to use collets with certain bits.

corbin

Alistair Hosie
05-24-2012, 01:24 PM
I too would not harden something that is clearly designed not to be hardened.Leave it well alone before you strip something . In fact why not fit a brass end piece and thread that to to the thread in your new drawbar to save your new threads from being stripped.In effect a sacrificial end piece in brass if your concerned with damage.Alistair

Bob Fisher
05-24-2012, 01:53 PM
I have had an end mill suck out of the collet, 3/4 in shank too, switched collets and all ok after. Probably a cheap Chinese collet. Don't know what torque I apply when I tighten the drawbar, I think I will try a clicker torque wrench to get an idea of what I have been doing. Bob F.

willmac
05-24-2012, 02:47 PM
Hey, Bill, would you mind expanding on this a bit? And would it work on a smaller mill like the Sieg XS-3? I'm really enjoying my mill, and get more comfortable with it every time I use it, but whacking that drawbar with a hammer to released the collet still feels wrong.
Dave

Dave - I doubt if this feature would work on most R8 machines. It requires the mill spindle to have a register at the top of the spindle taper, above the toolholder that the collar at the left of my photo can engage. When you loosen the drawbar, it will turn for maybe a couple of turns until that collar engages, and then further rotation forces the tool holder out of the spindle. This places zero load on the spindle bearings and requires no hammering.

The vertical spindle drawbars go into the spindle from the bottom and are secured by the cone nut at the top. They are quite quick to swap.

dian
05-24-2012, 02:53 PM
overtightening drawbar:

the specified torque for a 16 x 2 mm thread (8.8) is 200 nm. my wrench is 30 cm. how can you overtighten that? applying 670 nm over your head? nobody can do that.

its hard enough to tighten the collet nut properly and that is at breast height. er32: 100 foot-pounds, er40: 130.

willmac, how does this bar eject the toolholder?

edit: o.k., i see above post. what taper does the thiel have. or, on what mills can you use that feature?

willmac
05-24-2012, 06:00 PM
Dian -

Thiel 159 has 40 taper in both horizontal and vertical spindles. The toolholder ejection feature is found in other German mills, but I can't say how widespread beyond that. I seem to remember that some Cincinnati mills had a similar arrangement, but I may be remembering wrong.

goose
05-26-2012, 03:52 PM
Inspiration. It's funny, turns a cloudy day sunny.


Inspiration:


http://i122.photobucket.com/albums/o248/moorbrook1860/P1000795.jpg




The old is just a dog poop 7/16 threaded rod and loose nut.
The new is below, a knurled top piece with an 11/16 hex integrated into the design. I used O1 tool steel (unhardened) for the rod, I think the better steel is less likely to stretch under tension or cross thread in a collet.
http://www.hammerscale.com/images-global/Drawbar1.jpg




Here it is assembled with Loctite and fresh out of the black oxide solution.
http://www.hammerscale.com/images-global/Drawbar5.jpg


And now installed on the mill, ready for action.
http://www.hammerscale.com/images-global/Drawbar4.jpg

Shuswap Pat
05-26-2012, 05:19 PM
I have used Gr8 or B7 redirod. Just D&T the end of your broken drawbar, and locktite a piece of redi rod - it is tough material, rolled threads, cost effective, hard to beat.

Patrick

willmac
05-26-2012, 05:46 PM
Goose -

That looks very good. You will appreciate it every time you swap tools, which if you are like me is frequently.