Shop Doc – Coming Up Short

Dear Shop Doc,

I’ve been struggling with part length issues on one of my Brown & Sharpe #2 Ultramatics. The variance does not appear to be related to tool movement or collet tension as the length varies, sometimes it’s long and sometimes it’s short. The tolerance is reasonable (+/- .005). I’ve checked the regular stuff; rolls, pins, feed finger tension. Do I need to design every job to feed long and face off?

Coming up short (and long)

Dear Short and Long,

The Brown & Sharpe Ultramatic should be able to hold +/- .0025 when working properly. So no, you should not need to face every part to hold .010 total. Length variance can be particularly difficult to troubleshoot. Check one item at a time. Let’s start with the most obvious but common problems.

The feed stroke should be set to over travel by .250 maximum. If not set correctly, it cannot only exacerbate other problems, it can cause variance as the bar weight changes from a full bar to two feet of bar.

Check your trip dogs next. Make sure the feed is finished before the stop is moving. This sounds basic, but if the stop indexes away just as the collet is closing there can be variation, and it can look like the feed operation is done when it’s not.

Next, check the items involved in the lead cam actuating mechanism for the turret. First, check the lead cam shaft. If there’s any perceptible movement when you take the weight off the lead cam shaft you may have worn bearings or a bad shaft. This can cause variation because the weight of the bar changes over time as the bar gets shorter. Next, check the linkage in the lead lever mechanism. There are several fulcrum points to this lever and wear here can cause variation. Finally, check the turret rack and lever. These teeth can become worn and cause length variation. This area can also become full of chips (brass and aluminum are especially pervasive), and the chip movement can cause variation. The abrasive nature of the chips can wear the teeth away and cause huge variations. Keeping this area clear of chips should be a regular maintenance item. I have seen machines where the rack teeth look like needle points.

The withdrawal cam on the adjustment plate in the back of the machine must be kept adjusted. The proper adjustment is .002 max clearance. Brown & Sharpe recommends .001. I’ve seen machines at .015 and have often wondered how they made any good parts on them.

The last thing to check is the spindle. The only thing you need to look at here are the thrust bearings. There should be little or no endplay on the spindle. Check this with an indicator on the spindle end and then manipulate the clutches back and forth (you might as well check and adjust the clutch tension while you’re at it). There should be little or no movement in the spindle. The thrust bearings can be adjusted with the nut near the back end of the spindle.

There are several models of upgrades for Brownies available commercially, all of which eliminate any variance issues resulting from the lead cam shaft, the lever and rack, or the withdrawal cam. The withdrawal cam adjustment should be a maintenance item checked regularly along with keeping chips from the rack area.

Greg Knight and George Morris
AMT Machine Systems

Greg Knight is the Vice President of Machine Tool Automation and George Morris is an Application Engineering Manager with AMT Machine Systems in Columbus, Ohio.

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2 thoughts on “Shop Doc – Coming Up Short

  1. avatarbruce pallesen

    I’ve seen this before, & many related problems even on short parts. including concentricity issues all for the lack of proper bushing in the back of the feed tube. i.e. A-hole bushing, your old beat up B&S,or even your good as new one ! will be much better if you follow all the rules!!!

     

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