View Full Version : Wacky quill balance spring

01-09-2014, 10:09 AM
Ok, wacky to me

I'm working on a late model Taiwan BP clone head. All done except the quill balance spring adjustment. How hard can that be?

On all the BP's I've worked on or with, and the three I have access to locally, the balance spring is simply set so that it supports the weight of the quill/tooling, and at any point in the 5 inch stoke it just stays where you place it. You don't want it pop up or drop down particularly when using the power freed and trip. Dropping on power feed limit trip dumps your tooling into whatever is below (or keeps drilling!), and popping up can trash your finish (or worse).

With this clone (10x50, 3hp) no matter where I set the "balance point', raising the quill above that will tend to let it drop down, and below that will make it spring up. It will balance the quill at about 1/4 turn rotation of the spring holder from the "no tension point"; one more hole will make it rise, one less will make it drop. Seems way too sensitive to me.

I don't think I can put in a BP balance spring as the shaft attachment is different (better in the clone), and the existing spring is clean and looks like new. I've done a quick search and I see some similar comments but I can't tell if they response were the result of maladjustment. Some suggest "dragging the quill lock" but I don't need to do that on the other machines.

A few of you have BPs and clones out there - what do you experience with your balance spring and quill movement? Does it stay where you put it, or tend to drop or spring up?

01-09-2014, 02:03 PM
Its only acting exactly the way any spring will act, more preload equals greater static load the spring will suuport.

When you raise or lower the quill you are only increasing or decreasing the preload.

You may need a different spring, designed such that it has more total movement. (ie. 5 turns total working range vs 10 turns working range).

This will provide you with less of a preload increase or decrease per turn of movement.


01-09-2014, 07:20 PM
I have found only Bridgeport`s will balance. The Chinese and Taiwanese clones will not balance out but are designed to spring up like a drill press and I hate that. Like you I tried every thing and got into some heated arguments about it . Before we found out they are designed that way and it sucks.

01-09-2014, 08:12 PM
I was hoping not to hear that! I agree. No only sucks, but is stupid and potentially tool destroying when boring to the set depth with the auto-feed trip.

I need to find another spring and make it work. "Maybe" I can modify the end of a BP spring and get it to hook on.

I'd like to hear from any others owners of Taiwan or Chinese BP. Maybe one of those models doesn't do this and has a different spring I can get to fit.

01-09-2014, 08:44 PM
When I had my Millport Taiwan BP clone the quill balanced and I even used a BP spring as a replacement. It is a pain in the neck to set. I added additional adjusting holes on the spring housing to fine tune.

Bill Pace
01-09-2014, 09:45 PM
My Enco clone is pretty much right on all through the 5", delivered that way. If my single handle quill is on the back side it will try to creep up a little, suppose from that little extra weigh of the handle, but when I move the handle to the front it stays. I think if the 3 spoke was still on it it would stay put.

Don Young
01-09-2014, 10:00 PM
A flat spring in a cage does not act exactly like a common spring in that its force remains more nearly constant as it is wound. If the spring is not correct you can not make it work properly by making it tighter or looser. You run into the same thing with garage door springs, they have to be matched to the weight of the door. You could probably make it work by adding a supplemental spring or counterbalance and then adjusting each one until it stays in balance.

01-09-2014, 11:19 PM
The spring seems stiffer and wider than those I remember in BP's. Somehow BP made it work just fine with a single spring. I'm going to pull one out of my BP tomorrow and compare.

There's several hundred thousand BP's and likely several times the number of clones. Many of the clones likely simply copied BP and got it right; this is a head from the early 2000's when they were starting to "improve" on the design. Some things they did get better.

01-11-2014, 02:08 AM
A few months ago I replaced the clock spring on my variable speed Bridgeport. I believed it was original and thought it had gotten weak over the years. After installing the new spring I had the same problem as before...the quill would creep down about an inch and stay there.

After increasing the preload (prewind???) it finally hit me. I use a Bison R8 shanked ER40 chuck. The additional weight is just a bit too much for the clock spring. Oh well, at least the new spring is shiny...

Weston Bye
01-11-2014, 07:54 AM
Waiting for a report from Blidgeport......

01-11-2014, 12:10 PM
After increasing the preload (prewind???) it finally hit me. I use a Bison R8 shanked ER40 chuck. The additional weight is just a bit too much for the clock spring. Oh well, at least the new spring is shiny...

it is normal (not that we tend to do it) to change the preload based on the tooling weight. I have a honking boring head that needs a little more when mounted.