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Can't Draw my R-8 collets won't draw up tight?

Denny Graham

Aluminum
Joined
Sep 1, 2004
Location
Sandwich, IL
I've got an old 50's round ram Bridgeport with a J-head, small table
and knee that I've been using for about 30 years. About a decade or so
back, I got a bargain on a 2hp variable speed Bridgeport with the
intention of restoring it for use in my home shop one day, which would
give me much more travel then my old round ram mill.
So....this past winter I started work on the new mill. Had the motor
rewound for 220v 3ph (was hard wired for 440v 3ph). New Bijur pump,
metering units and plumbing. Fixed or made a couple dozen more things
that were missing, bent and nicked up from abuse by the previous owner.
Now here it is, summer and I'm just now getting time to actually make
some chips and see what other problems are going to pop up.
I'm well aware that 1/2 the machinists are of the belief that the
collet alignment screw is not necessary in a Bridgeport mill and the
other 1/2 believe that Bridgeport puts them in every machine they've
made for a reason. I fall in the second group and mine was missing it,
so I ordered a screw 'set', installed and adjusted it yesterday before
I even tried installing a collet in the mill.
Well, the first thing that I noticed was that the collets seated all
the way up flush into the spindle before the draw bar was threaded
into them. This seemed odd because in the old J-head on my round ram
the collets would initially stick out about 1/8" or so before they
were drawn up.
When I tried to tighten up the collet in the new mill the thread
bottoms out before it is drawn up tight. The first thing that came to
mind was that something was wrong with the draw bar. But I've got
three, one from my J-head, one that came with the mill and a spare that
I picked up somewhere along the line. All three are the same length
from the thread to the contact shoulder at the top. Next I thought that
the taper in the spindle may have been damaged along the way, from a
spinning collet due to the lack of the alignment screw and not being
tightened up correctly.
But the taper feels perfectly smooth, as it should be and appears never
to have been damaged.
So, I'm at a loss as to what's going on here. Over the years I've
worked on many Bridgport's and can't recall ever having a collet seated
all the way up flush with the nose of the spindle before it engaged
the draw bar and never had one that wouldn't draw up the tooling
tight in a good R-8 collet.
Any ideas??????

Denny Graham
Sandwich, IL
 
Is it possible someone reground the spindle ID at the collect taper to true it up and now the collects seat deeper and the draw bar needs a spacer to make it come tight.

That would be OK if the collects did not wear in to make a slight step so not pushing out easily ...Then you would have to add a bevel to the corner edge of the taper start.
 
Where did the collets come from? Last place I worked the boss bought a set of Chinese ones from enco and the whole set was completely unusable.

Other than that, I'd have to wonder if someone recut the spindle taper.
 
Hi Denny

As suggested above a quick test is a spacer on the head of the drawbar. I appreciate some Far Eastern collets are not as good as the regular European / US made ones but I've never found one that just "won't fit" because of production standards.

Just my two pence worth.

John :typing:
 
Spacer under the head of the draw bar. When the threads on the drawbar go away grind it back to the good threads and ditch the spacer. Get rid of that stupid collet key set screw.
 
I'd think that with some balls or a shop-made step rod, you could check the taper depth. The reground theory sounds likely and IMO the collets should never be flush. Have you checked it with a good old US made collet?
 
Not the collets, just a spacer as noted by Moonlight and Spencer.
Spacers are provided when a "2-life draw bar" is purchased.
3/16" or so for the small one and it wears instead of the draw bar shoulder.
John
 
Another thing is the length of the thread.. I think R-8s are 7/16 20.. but the depth or number of threads may differ with off brand collect . Perhaps good to turn the draw bad all the way tight out of the machine to see how many full turns can be made before shouldering to a stop. We used to be very careful when bar building (broach bars) looking for 7 to 11 turns for most SAE threads.
 
I noticed a similar issue last year and discovered that the key slot on the collet was milled slightly too short and the key was bottoming out. Further investigation found that there was quite a bit of variation in both groove depth and length between brands of R8 adapters. I had to readjust the pin to a more shallow depth to resolve the issue.
 
Bloody hell, you're tall. :D

PS:- Is that a hexagon bar or are there some strange vibrations happening to cause the marks on the bar. (Currently suffering from drawbar vibration on a non-Bridgeport machine)

Yeah, I like the weather up here. :-P

It's a hex bar, then a round down to the threaded portion. Probably pinned together. Probably came from Quality Tools.

Is your bar bent? (Not a personal question.)
 
That was one of the possibilities I posed to my wife last night when I came in from the shop
Buck.
Already measured for a spacer Spencer. The way the collet draws up into the spindle is
what’s bothering me. Tks
The collets Larry and John, are the ones that I’ve been using for 20 years in my J-head
round ram BP.
Exactly as I first posted Moonbeam, That is, 50% say that stupid collet key set screw is not needed, 50% and
and for 78 years, Bridgeport says to use them. Think I’ll go with what comes from the “horse mouth” rather
than that from the other end.
Haven’t had time Conrad, but I plan on checking the taper. Same collets, as stated, been using them for two
decades. Fit just fine in my old mill.
John and Ron, ????? what ever you’re talking about is a new one on me. A “2 life drawbar????
All three draw bars I’ve got here Buck, are exactly the same shoulder to end and thread length. I’m away of
The thread engagement and was carful not to tighten it when the thread bottomed out.

Was that a homemade drawbar Spencer. I’ve got three Bridgeport bars and they’’re all identical. The J-head
doesn’t need a spacer.
Tried a hand full of collets Steve. Adjusted the screw depth for the shallowest one.

Think the possibility that the taper was spun at one time and they sent the spindle out to
have it reground. That’s the only thing that makes any sense.
Thanks for the guesses guys.
Denny Graham
Sandwich, IL
 
QT} [that the taper was spun at one time and they sent the spindle out to
have it reground. That’s the only thing that makes any sense.] that is a good thing.
So now making draw tight?
I would set up a grinder and bevel the lead so the collects do not go into taper but just a little collect out.
 
Well Buck, that's probably a bit out of my league. Not equiped to
do any thing that sophisticated. This is the home shop of an old
retired dude.
I'd put my money on that answere though. The machine didn't have
the alignment screw and that habit was probably the reason
in the first place that they needed to send it out to be reground.
My question now is...just how far up into the spindle will the collet
pull up before it bottoms out.

Right now I've got to stand on my head to see into it. Gonna have to
rotate the head around this week so I can take a serious look at it
and try to take some measurements.
The other thought that crossed my mind was that they reground it to
take a different taper for some other sort of collet. Don't have enough
experience to know what other taper it could be modified to take.
But...that's just a wild thought. 99% chance that they had just spun
a collet and tore up the original taper.
dg
 
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Just clamp a carbide cutter in the vice and turn the spindle a bit shorter, then the collet will stick out however far you want. If you are all worried about it just get another spindle, not hard to find used. Just because you have not heard about a double life draw bar does not mean they don't exist. as far as the stupid screw goes all it is good for is if you have debris in the threads and insist on doing up the draw bar anyway. If you put enough power through the quill to slip a properly tightened collet that little piss ant set screw will shear instantly. When it does it will do some damage to the taper. I have seen it happen. The screw has been out of my BP since 1978 with no problems.
 
Collet spun in the spindle bore in a Bridgeport Vary-speed mill?
Not likely as the spindle will stall first. The motor will continue to spin and burn a spot on the belt.
Use a mirror and a flashlight to inspect the spindle bore. Pay close attention to the key location.
Inspect for damage in this area visually or with your finger.
When the key and drawbar are removed the collet should slide nearly home.

Shop had eight Bridgeports in addition to a lot of other larger machines.
I did the maintenance for the shop and inventoried $1,800 or so in spares. The shop ran 2-3 shifts and I did a lot of rebuilds.
No collet keys, no problems.

Two life drawbar simply had two sets of threads on the end. When the first length of threads was past prime lop it off and remove the spacer to engage the new second length of threads.
Photo shows three lengths of drawbars. The stub hex on the bottom fits the J-head you have. The upper bar is for the Series I vari-speed and the middle I think is for the right angle head.

Does the quill retract freely to the up position or does it drag the last three inches?
John

RL35-2193502.jpgTwoLifeDrawbar-e1335454917480.jpg
 








 
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