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 weâ€™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:
- New Start-Up
We will offer our insight from a repair shop point of view and invite you to send in any comments, anecdotes or corrections.
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 todayâ€™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 â€śZâ€ť 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. Todayâ€™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 youâ€™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
The most common cause of spindle failure that we encounter is a result of contamination. High speed spindles are particularly susceptible to contamination. Weâ€™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.