Archive for December, 2011

Keeping Your Spindle Healthy, Part 2

Monday, December 26th, 2011

This is the second in a three part series of our recommendations for preventing spindle damage and associated downtime. Last week we spoke about damage incurred when commissioning a new. This week we will look at some of the crashed spindles that come in for repair.

Spindle repair is very costly both in direct cash outlays as well as degradation of product quality and eventual loss of production. As one of the leading independent spindle repair shops we have seen a broad spectrum of preventable damage in our 26+ years of experience. So it is our hope that by sharing some of the failure modes that we’ve seen we can spare you the pain of a damaged spindle.

The topics that we will discuss in this series include damage that can happen from the following:

  1. New Start-Up
  2. Crashes
  3. Contamination

We will offer our insight from a repair shop point of view and invite you to send in any comments, anecdotes or corrections.

Crashes


A crash is the fastest way to end a spindles life and it is also the most avoidable. Hard crashes not only inflict expensive damage to the spindle, the damage often permeates several layers of machinery including ball screws, ways and actuators.

The first step in avoiding damage resulting from a crash is understanding the tool path in relationship to other objects. This is a job that becomes ever more difficult with today’s multi-axis machine centers. Fortunately there are a number of excellent software products that can help predict and hence avoid potential crashes. Time invested upfront can save many thousands of dollars in spindle repair.


A text book example of a perfectly Brinnelled bearing out of a Kitamura Mycenter spindle.

Even after a successful computer simulation most machine shops step through the program. The first catch word in this progression is SLOW. Even experienced programmers may miscalculate a tool path or load the wrong tool.

This inner bearing race came from a Kitamura spindle repair. It is an outstanding example of a perfectly Brinnelled bearing. After carefully checking the tool path program the operator promptly executed a high speed “Z” command and ran the spindle straight down hard and fast into the work piece. Of course this voided the warranty on a brand new Kitamura Mycenter and of course HST repaired the spindle and had the machine back up and running in a matter of days.

But what happened? Although a tool path program was run and checked out OK, the operator lacked familiarity with the new machines. He inadvertently gave the wrong command. This highlights a second point. Today’s machinists need sophisticated training with regular updates to keep pace with new technologies and processes. It is money well spent.


Cracked shaft out of a Franz Kessler spindle.

Closely associated with that step is properly securing the work piece. A well programmed tool path is of little value if the work piece is not secured and located on an accurate datum. The cracked shaft shown here is out of a Franz Kessler spindle used in a Vigel machining center. The spindle approached an improperly secured part at high speed. The mounting jig did not have provision for safety interlock sensors to alert the machine that the part is improperly positioned.

To help our customers avoid crashes HST offers powerful NCPlot software. If you are a recent customer this software may be available to you at no charge. If you’re not a customer, NCPlot is very reasonably priced and is an excellent training and actual programing tool. Click here for more information.

A slow speed version of a crash is excessive feed rate. This is more often found on grinding spindles than spindles used for cutting chips. Even if your process can accommodate hard feed rates be aware that you spindle bearings have a rated load. Exceeding that load rating does not necessary cause the bearings to fail right away. However, exceeding the load rating reduces rated bearing life (L-10) exponentially.

If needed, HST can often retrofit you spindle with higher load rated bearings. However, the trade-off is speed. Higher load rated bearings often use bigger balls and a fuller compliment in the race. These changes reduce the rated speed.

Our comprehensive evaluation will include photos of any forensic indication of a crash.

Take A Ways

  • Modern software can help simulate complex multi-axis tool paths before the machine actually cuts chips
  • Complex machining requires well trained operators
  • Provide accurate and secure work piece holding. (with sensor interlocks if possible)
  • Excessive feed rate damages bearings

NEXT WEEK

Contamination
The most common cause of spindle failure that we encounter is a result of contamination. High speed spindles are particularly susceptible to contamination. We’ll take a look at the problem and discuss why it is so prevalent as well as consider some ways to avoid or minimize the problem.


Keeping Your Spindle Healthy, Part 1

Monday, December 19th, 2011

This is a first in a three part series of our observations, musings and recommendations for preventing spindle damage and associated downtime. Spindle repair is very costly both in direct cash outlays as well as degradation of product quality and eventual loss of production. As one of the leading independent spindle repair shops we have seen a broad spectrum of preventable damage in our 26+ years of experience. So it is our hope that by sharing some of the failure modes that we’ve seen we can spare you the pain of a damaged spindle.

The topics that we will discuss in this series include damage that can happen from the following:

We will offer our insight from a repair shop point of view and invite you to send in any comments, anecdotes or your own observations and suggestions.

New Start Up

Starting up a new machine or one just repaired is perhaps the time when a spindle is most susceptible to damage. We receive a surprising number of damaged spindles that have very little run-time or no run-time at all. The specific reason for the damage runs the full gamut including, crashes, lubrication failure, bearing failure, overheating and many others. However, the vast majority of these repairs share one of three common root causes:

  • Rush into service
  • Unqualified installation personnel
  • Failure to follow start-up procedures.

Rush Into Service


Inner race from a Heald spindle repair. The inner race heated up and expanded.

Even though most qualified repair shops will completely run-in a spindle before shipping (Why is this important) once installed it is still important to allow your spindle to warm-up before going to full speed.

Cranking a spindle up to full speed heats up the rotating components faster than the housing. These components including the inner bearing races will expand faster than the outer bearing race. When there is uneven heating and expansion, bearing preload can change resulting in excessive loading. This excessive loading exacerbates the heat buildup and can result in premature failure. When starting up a “cold” spindle it is always advisable to bring it up to speed gradually before cutting chips at full speed.


Failure to purge air from the actuator lines caused this drawbar actuator on a Camau spindle to fail shortly after an expensive repair.

This bearing race (right) is out of a Heald spindle and shows the inner race severely damaged. It was in service for less than a month. The outer race showed very little damage. The reason for failure was that the inner race heated up so hot so fast that it got “soft” and expanded into the ceramic balls Brinelling the race. The outer race that is in contact with the housing was much cooler and harder; it displayed no indication of Brinelling.

Another issue that we see is failure to purge hydraulic lines. During breakdown and repair hydraulic lines and actuators will have been drained of fluid. This allows air to enter. The air is rather springy and soft compared to the hydraulic fluid. This springiness can manifest itself in different negative ways. One way is during a drawbar actuation the air may compress and not have enough force to eject a tool.

With some high speed spindle designs the actuator assembly needs to completely pull away or disengage from the rotating shaft. But when not properly purged, air in the actuator or lines may cause the actuator to spring back and rub against the shaft. The actuator system shown on the left is from a Comau spindle repair. After a very expensive repair the customer ran the machine for only a few hours before very serious damage resulted on the actuator and the drawbar. After a tool change operation the air in the actuator caused the actuator to spring back into contact with the rotating shaft.

Always take time to purge air from all hydraulic lines and actuators. This may require many actuation cycles. Check your owner’s manual for best practices.

Unqualified Installation Personnel


One wrong connection can ruin an expensive repair.

Although a good maintenance mechanic is typically well trained and very competent he may not be the right choice for re-installing your just repaired spindle. The knowledge base required for day to day maintenance of machinery and plant operations may not cover the very special knowledge base for properly installing and commissioning a high end CNC spindle.

Some of the errors we have experienced over the years:

  • Cover removed that captured and preloaded bearings
  • Incorrect hook-up of hydraulic and cooling lines.
  • Incorrectly alignment
  • Incorrect Indexing
  • Incorrect setting of sensors and other feedback devices.
  • The list goes on

Although HST does not perform field installations we maintain close relationships with a number of professionals that specialize in setting up and troubleshooting CNC machines. So if you’re in trouble, give us a call. We may be able to recommend a qualified service technician.

Failure to Follow Start-up Procedures.

Specific start-up procedures are typically in place to avoid or address a specific issue. Failure to follow these procedures leaves you susceptible to failure related to these issues. The two examples cited above, overheating and entrained air, are but a few. One important procedure in particular is indexing your machine. Does your machine accurately know where the work head is in reference to the work piece?

The OEM will provide instructions for commissioning a new spindle. However, sometimes a particular spindle may have certain nuances that may not be covered in the O & M manual. When that is the case HST will include spindle specific instructions with the spindle. However, in every case the OEM instructions take president.

Take A Ways

  • Follow OEM guidelines for starting up a new spindle
  • Proceed with caution when commissioning a new or recently repaired spindle
  • Use qualified personnel to install a new spindle, align and index your machine
  • Cycle all actuators many times to purge air. Watch for “springy” action
  • Check sensors to make sure they are properly positioned
  • Do not operate spindle at full speed without warm-up
  • Follow any special procedures provided by the spindle repair house

NEXT WEEK

Crashes
A crash is the fastest way to end a spindles life and it is also the most avoidable. We’ll discuss some of the things we’ve seen and how we are trying to help our customers avoid that problem.