Double sided lapping stock removal rates

[dgallup]

We buy a fine blanked part that we double side lap. Normally when we do the double sided lapping we get equal stock removal rates from both sides. Our supplier sent some parts that have a thin ridge around the OD on one side. It is my contention that the ridge will wear down faster in the double sided lapping than the other side because the contact area with the lapping plate is much smaller and the contact pressure is higher. Other people say the stock removal rate will be the same on both sides regardless of the contact area. Our process engineer is on vacation & I can't afford to scrap any parts running trials. It's important to know because we need to clean up both surfaces in the lapping process. We are talking just a few microns here.

 

[ajack1]

Personally I have no idea and my guess is you will get both points of view on here as well. If you cannot afford to scrap any parts it would take a big leap of faith to base your decision on any free advice you get from here.

Could you not just turn up some dummy parts that roughly mimic the actual parts and experiment with them?

 

[unclesyd]

Your problem can easily be resolved by going to Diamond Lapping
where you get no discernible edge at 10X. With a properly dressed wheel you should be able to get no edge at 50X. Diamond lapping will also give you better parallelism, surface finish, and production. More so than conventional compounds when lapping with Diamond your surface will get dull the closer it comes to being flat and smooth. Don't fall into the trap where shinny is smooth and flat. If you need a shiny surface using Diamond you will have to polish.
Remember that big equipment can do small jobs, but small equipment will not do large jobs.

We use both Engis and Lapmaster for our in house processing.

http://www.engis.com/

http://www.lapmaster.com/

Depending on the number parts you might want to look at double
disk grinding your blank to size..

http://tciprecision.com/machine-ready-blanks

 

[unclesyd]

I should have added that it is almost impossible not to have rounded edges, including penetrations. There has been major improvement in minimizing rounding with conventional compounds by using conditioning rings on the lap. This not only mitigates some of the rounding, but also improves finish and flatness. It will not eliminate it.
Our use of Diamond Lapping has made tremendous improvement in reworking some very high dollar parts. It was an interesting project in that we we able to reclaim some parts that had been tossed due to rounding some very small holes where the original was lapped on conventional wheel. During subsequent manufacturing of parts were able to leave some extra metal on the lapped face and then lap on the Diamond Lap. The small additional thickness and using Diamond Lapping on the complete process allowed us to recycle the parts 4 to 6 times. The holes were rounded in operation by corrosion.

 

[dgallup]

We are diamond lapping on Engis equipment. My question has to do with the rate of stock removal with different contact areas on either side of the parts. The edge is in the fine blanked part going into the double side diamond lap.

 

[unclesyd]

Sorry that I missed on the point of your post.

The only thing that I can recall is that our spinneret manufacturing group tried it several years ago and it wasn't purchased as our fiber business undergoing on a big decline and he quantity of spinnerets nested parallel the fiber production.
I made several calls to some people who might or should have a double lap machine, neither had one. No one could offer any information to answer you OP even though both people had been to presentations of these machines and he said on these machines even if their was any difference in metal removal rate it could be compensated for by the in place measuring system. Looking at some literature it looks like they control the potential for uneven metal removal by operation of the wheels through speed, wheel directional rotation and feed pressure of the upper wheel.
At first glance one would assume that the lower wheel would remove more metal and wear faster, but the literature and company brochures say this isn't true.

All my contacts at Engis and Stahli have long since left. Since you have Engis equipment I would give their technical people a call and request this information. This is one of those cases where you will have to get past the front desk. You Engis rep should also be able to help.

 

[racookpe1978]

From your reference plane (which is where ??) which side of the part differs from spec?

How do you stop the polishing/lapping process? (By time of lapping, by measurement of thickness removed, by observation of the two finished surfaces? By thickness of final part - after both sides have been lapped? See, even if the small raised "ring" is removed, that surface of the part on the lower surface (the face of the part inside the raised ring) would not have been touched by a "perfectly flat" lapping surface, and so you would still need to continue lapping to get the surface finish you want.

But while that happens, the far side (the good side with no raised ring) would seem to get too much material removed since it continues to be lapped the whole time.

 

[dgallup]

The lapping process is stopped when the overall thickness of the finished part is reached.

 

[unclesyd]

racookpe1978,

I know of two ways to easily determine the end point of the lapping and are quite sure they are others.

The first is to measure while lapping. The second one is to set a limit on the in-feed of the wheels to the required size. These methods a generally used with lapping not grinding tough fine grinding is closing the gap with lapping.

One thing to remember is a Diamond lap is correctly used you get very little wear that will require redressing to Diamond Abrasive carrier.