Are VMC manufacturer accuracy claims valid?

[rlevin]

Greetings,
I am shopping for a new vmc in the 30 inch range. Below is a short summary of position/repeatability claims for a few machines.

<table border=1 cellspacing=0 cellpadding=2>
   <tbody>
      <tr>
         <td>Make</td>
         <td>Model</td>
         <td>Accuracy<br>(0.001")</td>
         <td>Repeatability<br>(0.001")</td>
      </tr>
       <tr>
         <td>Monarch</td>
         <td>VMC 30</td>
         <td>0.10</td>
         <td>0.06</td>
      </tr>
       <tr>
         <td>Chevalier</td>
         <td>QP2033L</td>
         <td>0.16</td>
         <td>0.10</td>
      </tr>
       <tr>
         <td>Haas</td>
         <td>VF2</td>
         <td>0.20</td>
         <td>0.10</td>
      </tr>
       <tr>
         <td>Lagun</td>
         <td>VMC 3516</td>
         <td>0.20</td>
         <td>0.10</td>
      </tr>
       <tr>
         <td>Hurco</td>
         <td>VMX30</td>
         <td>0.20</td>
         <td>0.10</td>
      </tr>
       <tr>
         <td>Dynapath</td>
         <td>DMC E3</td>
         <td>0.40</td>
         <td>0.20</td>
      </tr>
          <tr>
         <td>Clausing-Kondia</td>
         <td>S-1050</td>
         <td>0.40</td>
         <td>0.20</td>
      </tr>
   </tbody>
</table>

I don't yet have prices for all of these, but with the data I have so far they all seem to be about the same class of machine. So, do you think the accuracy claims can be compared meaningfully? Do they really tell us which machine will make a more accurate part under the same conditions?

Thank you for your opinions.

[rlevin]

Oops. How can I display an html table in a post???

[Genguy]

Originally Posted by rlevin Oops. How can I display an html table in a post???

Take a screen shot (alt+print screen), save as .jpg and crop it, then have ImageShack host it for you like this...



I'm sure there must be other ways. That is the first that came to mind.

[rlevin]

Thank you Genguy.

[fpworks]

The accuracy/repeatability ratings of a machine at 68°F itself is only the very beginning of your comparison.

You asked about accuracy/repeatability "under the same conditions." What are your conditions?

[Caprirs]

Those numbers are just positioning. They are not accurate under cutting loads as the rigidity of the machine makes a huge difference. Most any machine these days can be made to position accurately and repeatably with no loads.

How the machine behaves under different conditions is the true measure of its performance. As machines change in temperature, some change dramatically while others only a thousandth. Some have temp compensation built into the control.

[JPMach]

I would add machine weight to your table of accuracies.

The accuracies are just posistioning. Once you take into account cutter deflection and thermal growth it's hard to obtain those accuracies. But yes a machine with a higher posistional accuracy will have an much easier time machining a larger tolerance.

If your tolerances are only +/- .005" probably just about any mill will do. If you are going to be working to tenths every day then you need a very precise machine along with a good cmm to check the parts.

JP

[Geof]

Originally Posted by JPMach I would add machine weight to your table of accuracies......JP

I think this is a more relevant basis for comparison. Go for the heaviest machine for the money. Chances are it will have less deflection under load so you will come closer to the theoretical accuracies and it will certainly be more resistant to chatter.

[CNCADEPT]

In todays industry, it is important to look at the company backing up its product regarding service and technical support. I believe Haas automation offers the quickest response of any machine tool manufacturer in the world when it comes to solving any issue that may arise. We offer extensive technical assistance to the newcomer that has limited knowlegde (programming and set up guidance). Regarding the positioning claims, I do not understand what your units of measurmnet are above. But Haas offers .0001"
(.025mm) repeatability and .0002" (.051mm) positioning. We also have Electronic Thermal compensation on our machines which makes up for your environmental changes throughout the day. Cutting loads will make a diference when machining, but of course if you want accuracy, then you make a finish pass which the idea behind it is to have no load while cutting andf therefore get the requiered accuracy. Feel free to email me or call if you need furhter assistance and or guidance regarding your future purchase.
Regards,

Milton Ramirez
Applications Engineer
Haas Automation, Inc.
mramirez@haascnc.com
800-331-6746

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[rlevin]

Here is the table including, and sorted by, machine weights.
Consistent with some of your comments,
the accuracy improves with increasing weight, approximately.

[NC Cams]

I'd contend that you can't buy from a spec table. The ACTUAL ACHIEVEABLE accuracy of a machine is a function of many things. Fixturing, cutters, coolants and speed of operation are but a few things that will determine how accurate you can get one to run.

Example: we're running cam masters on the neighbor's VMC - the brand is in your list. The masters are all run with the same code structure but the profiles/shapes differ from piece to piece.

Yet, each and every time we run them, there are small subtle differences that show up in fits and finishes. We're getting better with reach pass but still, simple tool wear (we're cutting steel) and temp changes in the machine will show up in part sizing (we're shooting for 0.0001"s in tolerances).

Since thermal growth in a machine is not considtant nor necessarily predictable, I'd contend that a "temp comp" factor that a machine may have is NOT something you can stake your reputation upon - life is NOT that simple.

Theoretcialy, the machine we're using can NOT support the sort of tolearnces we're trying for - their literature clearly says so. Yet, we do it.

How? Jigs, fixtures and iterative tweaks in the tool path/cutter feeds/speeds to find what the machine "likes" to run at. Thankfully, even though we have CNC machines which can automate things for the machinist, there still is a segment of the profession that requires art and skill to achieve exemplary results in a finish machined part.

The FIRST thing I'd look for is service. Then I'd look for an agent who's sold some machines AND who has customers you can go see them run. THE CUSTOMERS will tell you if the machines REALLY perform as advertised. They'll also tell you if the dealer is worth a damn or not.

The last thing you want to buy is a machine is one from an agent/broker who is merely shuffling the papers and not servicing/supporting the machine. There are any number of horror stories from guys who bought "vaporware" machines and then they didn't work as promised or all of a sudden you couldn't get them serviced if the OEM vacated the market - lots of "pacific rim" suppliers did that at one time or another.

HINT: if you want a good simple test for some of the issues you're looking at, have them cut a perfectly round circular disk AS FAST AS POSSIBLE in both steel and aluminum - do it for several sizes. Also do several for ID holes of several sizes.

This is a good fast check of repreatablity and for follower error. It will show you if they have axis tuning and motion control figured out under constantly varying speeds and feed.

If the circle isn't round (should be so with 0.0003" or better on DIA), they can't reliably repeat or hold tolearance, no matter what their spec sheet claims.