30 Potential Causes of Part Length Variation On Screw Machine Parts

August 14, 2015

 There are many different ways that part length can vary when using a cut-off tool on Multiple Spindle Automatic Screw Machines.

Cut off Tool and Holders for Acme Gridley Multispindle Screw Machine

Here are some of the major reasons grouped into a rough classification by where the cause exists.

The Cut-off Tool itself

  • Tool is dull
  • Tool is improperly ground (point angle too large)
  • Tool loose / improperly inserted into holder
  • Tool Blade too thin
  • Cut-off tool hitting while in high speed
  • Cut off tool being hit by die head or chasers

The Cut-off Tool Holder

  • Tool holder itself is loose
  • Tool holder is hitting work spindle
  • Tool holder is hitting tool post
  • Tool holder is warped or bent
  • Tool holder is worn

The Work Spindle

  • Spindle has end play
  • Spindle has worn bearings
  • Spindle carrier has end play
  • Index Lock Pin Spring is broken
  • Finger holder not adjusted properly
  • Broken pins or fingers in finger holder
  • Feed tubes bent or beat up
  • Wrong stock feed cam- Overfeeding stock will cause bounce back from stock stop resulting in short part
  • Incorrect collet tension

The Cross Slide

  • Cross slide play
  • Cross slide loose
  • Cam is loose
  • Cut off cam too large causes too much feed
  • Cam Drum is loose

Other Tools

  • Stock pushed back into collet by drill (dull drill pushing stock rather than cutting chip)
  • Stock pushed back into collet by reamer
  • Face-off tool is loose
  • Face-off tool is dull
  • Face-off tool holder is loose
  • Die head pulling stock out of collet making part long

Part length can occasionally go awry when using cut-off tools on automatic screw machines.

This post lists  over 30 reasons that I can think of- what did we miss?

Photo Credit- Acme Gridley Multiple Spindle Bar Machine Manual First Edition 1961 page C11.

 

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The First Tool I Look At

April 20, 2010

Listen! There is a reason they have canaries in the coal mine.

As a steel company Metallurgist and Quality Director, I was the guy who got the call to visit a shop because the material we sent wasn’t machining right.

“This stuff won’t drill! Help!”

“This stuff is killing my OD form tool. Can you check the steel?”

“This steel you sent is acting crazy. It machines fine on one machine, but not on the other one.”

As the fellow responsible for the processes and quality system that produced the bars, and having visited my hot mill upstream suppliers, I was always confident that I had provided conforming product.

But how could I make sense of the problems reported?

My solution was to always look at the wrong tool- first.

If they complained about the drill, I asked them to show me the cut off tool.

If they complained about the rough, finish form, or shave tool, I asked them to show me the cut off tool first.

They said “Hey Mr. Free you aren’t paying attention. I said the drill is giving me trouble, not the cut off tool.”

To which I  cheerfully replied “Yes?”

After letting that sink in for a bit I would ask the following appreciative inquiry type of question to lead their thinking:

If the cutoff tool sees every aspect of the steel provided-  the very surface of the outside diameter (OD), the sub-surface, the mid-radius, the core, and it does not have any abnormal issues resulting from this material, what is there about this material that you think would allow it to affect this one tool, but not the cutoff?”

Then we focused on the aspects of the operation that inevitably were found to be the cause.

How does the steel know to only interfere with the drill, lets say? Or the the finish form? While leaving the cut off tool unscathed?

While there can be material conditions that are specific to a certain zone in the steel and thus would manifest on a particular tool, that conditon would also have an impact on the cut off.

 If the cut off  tool is A-OK, it’s probably not the steel.

This is the tool that will tell the tale.

It may not look like a canary, but a cutoff tool can sing a song about your process, if you can listen with your eyes.

Canary.

Cutoff.

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