Friday, March 16, 2012

Why don't my new linear scales seem to be accurate?

Recently we were asked this question;
"I purchased a new Digital Readout Package from your salesman and had it professionally installed by your technician.  I thought I purchased the most accurate DRO available but apparently your salesman was wrong.  The very first part that I made using the new DRO was rejected by inspection.  Our inspector said that the part is too large by more than 0.002” and blamed the new DRO.  I ran some tests of my own and found that he is correct; the table is in fact traveling farther than the display says it is. I am very frustrated with our purchase decision.  What’s going on!?" 

Dear Customer,
          The DRO you purchased is in fact accurate to within 0.0004” over the length of the table and your part is in fact moving 0.002” too far as described.  I understand this statement may sound contradictory so let me explain.  The very common error you are experiencing is called pitch error which is caused by your table traveling on an arc instead of a straight line.  This arc occurs when the unsupported table sags as it is traveled from end to end.  Since the part you are machining is clamped a few inches above the table, it will travel a longer path than the scale resulting in a part that is too large.  The proper corrective action to be taken in this case is to have your table ways professionally scraped.   You can also have the Digital Readout laser calibrated if you always machine at the same level above the table.

For a more technical explanation of pitch and yaw error, continue reading this document


Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518
   

Thursday, March 15, 2012

Annual Encoder Certifications

Dear A Tech
“I have a Heidenhain ROD-880 that was purchased new, about a year ago. My QA department says that I have to have it calibrated every year in order for me to use it in production. Where can I send it for annual calibration and how long will I be without it?” Thanks, Terry.
Dear Terry...
Your situation is not uncommon to us. When it comes to calibrating or certifying the accuracy of a HEIDENHAIN gage or encoder you have two options. Your first option is to send it back to HEIDENHAIN in Germany where it will be tested against a standard and returned to you with a calibration chart just like the one that originally came with the encoder which will show the positioning accuracy of the encoder over its full travel. Or, your encoder can be calibrated and certified here at our facility in Southern California where we will analyze the mechanical and electrical characteristics of the encoder, make adjustments as necessary, and then provide a certificate based on our findings. Before you can make a decision however, you must understand the process of each option as they are very different.

Click Here to download my full response.

Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518

Tuesday, March 6, 2012

Repeatability Problems

Dear A Tech...
"I recently bought a three year old, gently used CNC mill and discovered it's not repeatable and I'm having problems maintaining min. tolerances. Can a technician call me about fixing this? I'm getting conflicting reports from three service guys!" Thanks, Desperate.

Dear Desperate,
Position errors are very common and are typically caused by worn-out ballscrews being used as positioning devices, physical deviations in the machine being transferred to the finished part, or by encoder feed back errors.
Our technicians are routinely called out on these "mystery" types of problems. Experience has trained us to first analyze the type of error being reported in order to reduce the possibility of chasing erroneous causes. You stated in your email that your machine is not repeatable. Non repeatability can be caused by your encoder missing counts, ballscrew growth, or a control component failure. Since today’s controls use stable solid state electronics with no moving parts, failures are rare. So, let’s focus on the first two causes: encoder miscounts and ballscrew growth.

Click Here to download my full response.

Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518

Thursday, March 1, 2012

Four Steps to Understanding Scales

Weekly we receive calls and emails from customers asking, "What’s the difference between linear scales?" The answer to this question is critical to your expected manufacturing quality outcome in addition to the overall cost of ownership. A Tech Authority is positioned to offer you the best scale solution to match your machining process, budget, and tolerances. Here are four easy steps to understand and help you make an educated decision.

Step 1 - Understand Signal Generation
Step 2 - Understand Resolution
Step 3 - Understand Accuracy
Step 4 - Understand Incremental & Absolute

Click Here to download my full article.

Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518

Friday, February 24, 2012

Machine Tool Calibration Methods

Dear A Tech,
“So I just had my machine laser calibrated, but the finished parts are marginally acceptable! Why does the laser chart say one thing and my CMM in inspection says something else?”

Dear Customer,
We frequently hear of this situation and the source of the conflicting results is in the difference between calibrating the machine and calibrating the process. Let me explain… Temperature is the single largest enemy of any machine tool calibration practice. The lack of proper thermal consideration in the calibration process can result in the introduction of massive errors into the machine tool that weren’t there before. And the worst part is that the resulting laser plots will look good, giving no indication that anything is wrong. Simply by analyzing the resulting plot, I have personally identified and recalibrated countless machines that were originally calibrated using the wrong method. You see, there are two primary methods of machine tool calibration; calibrate the machine or calibrate the process.

Click Here to download my full response.

Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518

Saturday, February 18, 2012

Confused About Digital Readouts

Dear A Tech...
"I have a very old, but good condition Bridgeport mill. However, the digital readout on it hasn't worked for years. I'm considering my options. Should I buy a new readout, or try to have this one repaired? I see complete DRO kits advertised in trade magazines for as low as $599 and some for over $1200, as seen on your website. If my unit is not repairable, what's the difference between DROs (besides $600!), and which one should I buy? " Thanks, Confused.

Dear Confused,
Your situation is not uncommon; we regularly hear this from customers and help them understand the differences and benefits of various digital readout packages on the market. Basically, as you stated, you have two options. We can repair your existing digital readout if parts are still available, or you can replace it with a new one. There are advantages and disadvantages of each option so, let’s first discuss option one.

Click Here to download my full response.

Craig D’Ambrosio
General Manager
A Tech Authority, Inc.
www.atechauthority.com
craig@atechauthority.com
(909) 614-4518