Grinding Problems After Shutdown 1

[1plus1]

We run production parts on a Blohm Grinder with tolerances usually around + or - .001. After a shut down of 48 hours over the weekend, the first part off the grinder is usually scrap. It is usually .002 or .003 under tolerance. Anyone have any ideas of what is happening? Wheel balance? We have started changing offsets on the first part and then working it back in tolerance, but this wastes time. We run the grinders three shifts 5 days a week and shut down over the weekend. We don't want to keep grinders on over the weekend. Anyone have similar problem? Any solutions?

 

[jbel]

Do you warm the machine up upon startup after the weekend???

In order to hold close tolerances like these, the ideal situation is to have your machine as close to a steady state condition as possible in order to diminish or eliminate frame growth. All those hydraulics and electric motors heat the frame up and cause it to expand. It sounds like your machine is growing as it warms up and causing your parts to go undersize.

If the machine historically cuts undersize after a long shutdown, couldn't you just back it off a few thousandths for the first several pieces and creep up on your numbers??

 

[MechEng13]

jbel,

Star for you, I was thinking along the same lines.

Can't add anymore to what you said, it covers the same things I would try.




Alan M. Etzkorn
Reliability Engineer
Wabash National Corp.

http://www.wabashnational.com

 

[Barry1961]

Back in a previous life I worked maintenance in a bearing plant. It was SOP to get the finish grinding machines started a couple of hours before 1st shift came in Monday morning. If I remember correctly they started up anything with hydraulics early.

Barry1961

 

[rmw]

In that .002-.003 is in the range of the diameter of a human hair, I agree that the machine tolerances are affected by temperature changes and stabilize only after all machine parts and lubricants come up to a sustained operating temperature.

If you have known defects available, you should just run a piece of known scrap as the first piece to get the machine up to temperature instead of making a piece of scrap with the first piece.

Short of that, reset the machine for the first piece, then adjust to compensate for the temperature change.

rmw

 

[1plus1]

Thanks for the help. Our team has discussed loading a scrap piece at startup just to see where we are at and then adjust as needed. We are backing off on the offsets now and adjusting to bring it back into tolerance. But this wastes almost an hour, but better than a scrapping a part. Once the machine is up and running it holds these tight tolerances all week long with very few offset changes. Having someone come in early to warmup the machine is not feasible right now.

 

[Tmoose]

Pay someone an extra hour. ($50 ? )
If using a mag chuck put a 0.0025 inch shim under the part the rest of the week.
Schedule that grinder for 1 hour less/week. 119 instead of in 120 = 0.8% less production.
Stagger that person's (or 2 people's) shift. (~ free ?)
Forward ( to get machine warming up
Buy some heat blankets and maybe a timer ? ( $ 100s )
Leave the coolant on all weekend, with a heat blanket on the sump.