View Full Version : R 8 collet release Bridgeport mill

01-31-2003, 06:04 PM
I have designed ,built and tested an R 8 collet release for my Bridgeport J head mill that eliminates the hammering normally used to free the collet. The reason I did this was to save the life of the pricy bearings I assume could be damaged by the hammer impact caused by hitting the drawbar.
This design is easy to make using easily available materials.
A picture and text is posted in the www.metalworking.com (http://www.metalworking.com) archives titled "final R 8 collet release". I hope soon to be able to post similar stuff in this forum.
Basically what it is, is a collar (can be made from a collar) that fits loosly around the BP spindle and acts as a spacer.Then a 1/8" thick washer ( made from a standard steel washer) is bored ot at a 9 degree on a side dia to 1.217" . This fits at the very end of the collet (in the part that sticks out of the spindle end. Insert endmill and tighten drawrod. When done milling, unscrew drawbar a couple of turns and insert two (one on each side in a simple frame) the hex screwdriver bits in a space about 1/8" between the spacer and the washer. Press down and it levers out the collet with no load on the bearings. I have used it dozens of times sucesfully.
I am not sure how other R 8 machines are built , but may work with them.

Spin Doctor
01-31-2003, 06:43 PM
From most of the experience that I've had with BPs in the past thirty years I can only recall acouple of times that we had a R8 stuck in the spindle. I think that people do far more damage to the spindle bearings with heavy interprupted cuts than any else. If the collet is stuck I suppose that you could Brinnel the bearing races but the drawbar is flexible enough that I would think that that would relieve a lot of the stree on the bearings. The bearing set-up is a little on the weak side as far as I'm concerned. When rebuilding one head we used a triple bearing set-up with the bottom two in tandem back to back with the rear bearing. Of course when doing this you have to make damn sure that new bearing spacers are absolutely parallel and your preload amount is reasonable. Rigidity improved imensely. This not something that I would advacate some one with a homeshop try unless they've got a really good surface grinder

02-03-2003, 07:42 AM
Hi spin doctor. I posted this article thinking it was the greatest thing since scliced bread. I wish you would tell me and maybe other R 8 collet users how to keep the collets from sticking. You can't lube em because its the friction that gives the torque to turn end mills. A recent ad for drawbars included a hammer with it. Wonder what they thought you needed a hammer for. My experence with R 8 collets is limited to my Bridgeport.
Could some of you other R 8 users tell me if my solution has any merit or not.

Forrest Addy
02-03-2003, 10:38 AM
R-8 tooling will stick slightly and it might take a sharp tap with the wrench on the drawbolt to release it. Just a little shock is all that's required. The spindle has a fair amount of mass and the bearings do have reasonable resistance to brinelling. A light sharp axial tap won't hurt the machine.

I've seen guys banging away, cussing the stupid collet, when all they had to do was be sure the draw bolt was spun out a thread or two. The threads have some axial clearance. Barely loose doesn't work when the shock is transmitted through the draw bolt head to the spindle spline instead of down through the long body diameter of the drawbolt to the threads of the collet.

Using the gadget described in your link seems time-consuming and cumbersome. The concern expressed for the spindle bearings is laudible but not really necessary.

If you've ever seen a Bridgport worked to capacity by a hot machinist you'd think the machine wouldn't last the day when in fact they frequently last 10 years in continuous production, amortizing their purchase price in six months. For that matter, I've seen Bridgports thrown completely out of whack by wrecks severe enough to require re-grinding the spindle taper and survive needing only re-tramming to be put back into service.

The turret mill is a light, tough design proven over 60 years. It'll take more than frequent knocks on the spindle to cause a problem.

[This message has been edited by Forrest Addy (edited 02-03-2003).]

Spin Doctor
02-04-2003, 06:56 PM
Tha last time I saw a R-8 really stuck wes when one of those R-8 taper slotting heads literally broke. Other than that most times they hang up has nothing to do with the taper. It's really to short to have that much friction holding it. Where they hang up is in the straight portion of the spindle either on the key or because of crap up in the bore.
But hey if your gizmo works for you, fine. But I don't think I would of put the design or even the idea on a web site. If it's that slick go into business with it, But the Kurt Power Drawbar and others have beaten you to the market place