# Question on drawbars for Bridgeport clone



## Pmedic828 (Aug 14, 2013)

I understand the basic concept of a power drawbar - the device pulls the collet into the head and keeps tension on the threaded rod until whatever tool that is connected to the head needs to be changed.  I can also understand a device like a solenoid compressing washers creating space for the collet or tool to be removed from the head or a piston pushing or pulling the top nut drawing the collet into the mill head.  What I don't understand is how does the collet slip off of the drawbar and another tool re-mounted.  My mill has an R8 taper drawbar - I believe it is 7/16"; that a nut screws down on a threaded rod that travels thru the mill head connecting the collet or tool.  How does the tools or collets slip off of the threaded rod without turning or unscrewing?  They seem just to fall out, and a replacement is held some way and reconnected.  
Can someone send me a picture of how the collet is held to the threaded rod or explain in simple terms how the collet is held by the drawbar without screwing the drawbar into the nut on top and the collet in the tool head.  Thanks for your time.


----------



## Richard King (Aug 15, 2013)

With the collet out of the spindle and machine shut off, stick your finger up in the the hole and fee for a small key on a dog point sets screw or a hole and a broken key.  The collet has a straight key way in it and when you put it in there you are suppose to turn the collet until the set screw. (on a dog point set screw the reads turned off the bottom of the sets screw)  lines up with the  keyway in the collet.   Some times the set scre get broken off and the collet just pulls up in the taper and gets tight.  I hope you know when you loosen the drawbar you need to lossen it 2 or 3 turns and tap the top with a lead or brass hammer and it opens the collet and end mill falls out.  Usually one has a glove on or wraps the end mill (EM) with a rag, so you don't cut your finders on the cutting edge of the EM .  You will have to look on here to find the spindle design and http://hqtinc.com/quillhousingassembly.aspx
I have seen some of the imports don't have one, so don't be surprised you don't find one.
If you find it broken or backed off ask them at High Quality Tool how to replace it.  I know how, but I'm tired of typing.
Rich
PS.  If they don't know, PM me and I'll come back on tonight and tell you how...  Rich


----------



## Ray C (Aug 15, 2013)

In addition to what Rich said, most spindles including R8 have a taper.  When you insert a collet or endmill holder, just as Rich said, there's a pin in the spindle which rides inside a slot on the side of the collet or endmill holder (AKA toolholder) to keep it from spinning.  The drawbar pulls up on the inserted toolholder and the taper aligns it and in the case of a collet, closes down on the fingers of the collet.  It's a very simple process.

One thing to mention is that you should not overtighten a drawbar.  Make fingertip contact and use wrenches to pull out the slack and give it a light snug but nothing more than that is needed.  I've heard horror stories of people trying to get tooling unstuck from a spindle.  -And like Rich said, when you loosen it, unthread the drawbar a couple turns and give it a light tap to push the tool out of the taper.   I welded a little block of metal to the designated drawbar wrench to serve as a little tapping hammer.  One sharp little tap is all it should take to loosen the taper bond.  Also, as you unscrew a drawbar, don't let a tool drop out and hit the bed or it will make a good ding and if it's carbide tooling, might crack/chip the tool.


Ray


----------



## kd4gij (Aug 15, 2013)

A power drawbar for a bridgeport and clones is an impact wrench that spins the stock drawbar to loosen or tighten the collet.


----------



## Pmedic828 (Aug 15, 2013)

Thank you for answering my post - maybe i could be more specific.  I understand that the R8 collet has threads in it that screw into a long rod that passes thru the quill to the top of the machine.  On this rod is a nut that is loosened to allow the the collet or mill adapter to hang down so it can be unscrewed.  My milling adapters have a groove in them but my machine doesn't have a set screw or piece sticking out that locks the collet or milling adapter in place.  What i don't understand is that when a power drawbar unscrews the nut from the top of the mill, the threaded rod and the milling adapter falls out of the mount since the threaded rod is also screwed into the mill adapter.  Is there something that keeps the threaded rod from dropping out.  When watching a power drawbar work, you activate the power on top of the mill to release the nut on the threaded rod.  That, in turn will drop the milling adapter but there is a threaded rod still attached on the collet or the milling adapter.  Why doesn't the nut spin off the top and completely be free of the threaded rod and if the nut loosens, the threaded rod drops down with the mill adapter still attached.  I have to still unscrew the adapter off the threaded rod.  The power drawbar seems to release the adapter or collet free from the rod.  That is what i don't understand.


----------



## ScrapMetal (Aug 15, 2013)

Does this http://bridgeport.askmisterscience.com/drawbar.htm help to explain a bit?

-Ron


----------



## Richard King (Aug 15, 2013)

http://www.kurtworkholding.com/documents/manuals/online_automatic_power_drawbar.pdf

http://www.kurtworkholding.com/documents/manuals/online_mechanical_power_drawbar.pdf


----------



## kd4gij (Aug 16, 2013)

The nut on the top of the drawbar should be pinned to the threaded rod. It sounds like you may have a home made drawbar in your mill with a nut thread on and not pined.


----------

