Centerless Grinding (SPC) - My process is extremely out of control

C

CMfgT

#1
I know there is a post about Centerless grinding and SPC, but I didn't want to hijack the thread. I attached my data in an excel file for other to view if they can or would be willing. Basically my process is extremly out of control. The work order had 77pcs and I had them collect 30 pieces so they SHOULD of collect roughly around every other piece. I did an xbar and R chart with a subgroup of 3 and the chart was out of control everywhere, and did an I-MR chart and gain out of control everywhere. I know at part 10 they dressed the wheel. The range of the data was only .0004”, so what should I do? So may special causes I guess I don’t know where to look.

Thanks
 

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J

justncredible

#2
Re: Centerless Grinding (SPC)

The other thread he had a better idea of what he was looking for, which was the wheel dress process.

What are you trying to get information on?
 
C

CMfgT

#3
Re: Centerless Grinding (SPC)

Basically I am starting SPC from SCRATCH in our centerless grinding department. I just want to know what to do to get started. We collected 30 checks to satisfy our customers requirements and I ran the numbers and they were out of control. I just want to know where to go from here? Or what I should do to get started.

Thanks
 
D

Duke Okes

#4
Re: Centerless Grinding (SPC)

I didn't look at the data but there's a good chance you need to use control limits that are at an angle (e.g., starting at one point just after wheel dress and going up until just before wheel dress, then returning back to original point just after dress, etc.). Sawtooth form.

You'll need to pull a lot more data to see this. Less time between samples and many more samples.

In effect the control limits will depend on which sample number (after wheel dress) the data represents.
 

Stijloor

Staff member
Super Moderator
#5
Re: Centerless Grinding (SPC)

I know there is a post about Centerless grinding and SPC, but I didn't want to hijack the thread. I attached my data in an excel file for other to view if they can or would be willing. Basically my process is extremly out of control. The work order had 77pcs and I had them collect 30 pieces so they SHOULD of collect roughly around every other piece. I did an xbar and R chart with a subgroup of 3 and the chart was out of control everywhere, and did an I-MR chart and gain out of control everywhere. I know at part 10 they dressed the wheel. The range of the data was only .0004”, so what should I do? So may special causes I guess I don’t know where to look.

Thanks
Let me throw out some thoughts....
  • Is your measuring system stable (GR&R)?
  • Could the location of measurement have some impact?
  • Have you experienced form (roundness and or cylindricity) variation which is very common in centerless grinding processes?
  • Is there any historical data to look at and possibly assess the distribution of the data?
I am sure that my Fellow Covers will add to the list....

Stijloor.
 

Miner

Forum Moderator
Staff member
Admin
#6
Re: Centerless Grinding (SPC)

This chart is a classic example of a gage that has inadequate resolution to see the process variation. The true process variation is probably stable with the one exception where the wheel was dressed.

If SPC is a requirement for this process, you will need a gage with better resolution.

You did not state the tolerances that the process must hold. If the process capability is high, you may not need SPC.
 
P

Pudge 72

#7
Re: Centerless Grinding (SPC)

Also, is this a thrufeed operation or an infeed?
Is it a profile grind?
My point to those questions is - are you using the entire surface of the wheel or just a portion? What is your backtaper requirement? How much stock are you hogging off?
I used to grind on Cinncinatti Centerless grinders. There are so many variables that I don't think we are getting a true snapshot of what you are trying to convey and what the point is. Applying SPC to Centerless grinding is not just a matter of is it in control or isn't it - it really is about controlling the variables as it applies to the specific operation that you are trying to complete. We used to hold -0.0002" point diameter with 0.0005" backtaper on a .125 drill bit all day on a thrufeed operation. But, change that to a infeed operation or switchover to a different diameter and it's a whole new ballgame. Also, what size blades are you using - are they carbide or HSS?
I am not trying to tell you how to grind, I am just trying to stress that you have to have controls in place before applying a cover-all that the end measurement is producing results that are out of control - you have to be able to have linear correlation with the set up components and results that you are seeing in order to make a change to control the entire process.
Also, I believe this was asked by Miner, what is your tolerance on the piece that you are grinding? Your process may be as robust as it needs to for what you are running - don't try to make it more difficult than it is and waste money trying to figure it out.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#8
Re: Centerless Grinding (SPC)

This chart is a classic example of a gage that has inadequate resolution to see the process variation. The true process variation is probably stable with the one exception where the wheel was dressed.

If SPC is a requirement for this process, you will need a gage with better resolution.

You did not state the tolerances that the process must hold. If the process capability is high, you may not need SPC.
What Miner has posted is a thoughtful post. (my fellow cove members: when the OP has the forethought to post their data, we should look at it. It usually tells the story so we can provide helpful answers or at least set us up to ask insightful questions)

this data certainly tells a story.
let's start with the gage resolution: the variation in the data is only .0004 (that's at the 4th decimal place). Given this the control chart can only look out of control because the data is "chunky" - very few possible results given the gage resolution and the variation of the data. It behaves like ordinal data. IF the parts were to vary .004 then the resolution would be sufficient.
Getting a gage with better resolution will be expensive tho - this is a very small dimensional difference, your average caliper won't do it and a CMM will have potential fixturing issues.

This leads to my next point, spec limits. Miner is again correct to ask this question. if the spec limits are very large compared to the range of data provided we have awhole different situation. if teh OP could provide that we will have more to work with.

Next, is the concept of rational subgrouping. taking 30 random parts then subgrouping wthout regard to how the parts were taken (you shoud lalways specify and know how the parts are taken; this is essential to proper informative usefull charts) Subgroups are typically sequential parts or values. (there are times when they are not but this is a delibrate decision in order to create a rational subgroup) . Additionally, your first set of data shodul span most of teh variation in the process BEFORE you begin calculating limits. SPC is specific sampling, not random sampling.

My recommendation would be to perfrom a gage R&R (30 pieces that span the full range of variation measured twice) Get yourself a copy of Donald Wheeler's books on measurement error adn go out to his website (SPC Press) and download some of his articles on measurement error and it's effect on SPC. My experience is that he is right on the point.

Also buy two books on SPC: Wheeler's "understanding statistical process control" and Wise's "innovative contro lcharting". then read them. These books will be invaluable to your understanding and the deployment of a successful program.
 
P

Pudge 72

#9
Re: Centerless Grinding (SPC)

Bev -

I completely agree with your assessment. However, when analyzing data, the process needs to be understood. That is not to say that just because you have never run a grinder you can't understand what is going on. But, for those of us that have been in this particular trench - I think we can assist those that haven't by informing and providing information as it relates to the process. Having run a grinder for 7 years and looking at this data, my initial reaction was - not enough information about what is going on to provide an insight that would provide a value added solution to a problem. I can throw theories around all day about Centerless grinding and what may or may not be happening to these parts and what to adjust if anything to fix the problem, but until you have done it, or are familiar with the machines, the parts, and the tolerances - there really is no point.
The subgrouping will depend on whether or not this is infeed or thrufeed - thrufeed could be considered 1 set-up with limited variables ; infeed depends on human touch etc.
We need to understand the process to evaluate the data - trust me.
 
J

justncredible

#10
Re: Centerless Grinding (SPC)

Pudge, he is not asking for help fixxing something he is asking how to set up and do SPC. Bev is right he needs to read books, and take a few classes. He needs basics of SPC. Yet even basics will not help him design a working SPC program. He will need to hire someone with a clue or risk a pissed customer.
 
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