lapping plate basics

[petertha]

Thus far for finishing flats I have been an amateur wet-o-dry paper attached to glass plate kind of guy. I've been Googling lapping plates more out of curiosity, plus I will be receiving some lapping compounds for my cylinder trials & experiments shortly anyway.

I think I understand the basics - cast iron is preferred because its inert + can be trued + re-surfaced + has a surface type that accepts the lapping compound + relief groves can me machined in etc. Here is smaller example maybe catering to woodworking world.
http://www.leevalley.com/en/wood/page.aspx?cat=1,43072&p=59752

What I don't understand is:

- why the relatively deep recess grooves? I would think after only a few lapping circuits, 99% of the compound slurry will just reside in the valley not up on the flats where the cutting action is? Is there maybe a suction principle with the work piece that slurry gets sucked out & re-introduced to the top flats? If the plate had no grooves, wouldn't the excess compound just be pushed off on the side of the motion footprint?

- when vendors talk about oil/petroleum based lapping compound, either pre-mix or adding oil to dry grit, I can visualize the cleaning & re-charging with petroleum based solvent & you are good to go. So is water as a slurry carrier a no-no on the cast iron for obvious (rust) reasons? Maybe water based lapping is only for glass plates & ceramics surfacing for example?

- I also hear diamond compound is kind of specialized & be careful, because its so hard & sharp it can embed in both lap & part even when they are both steel & not easily removed. I've read where ultrasonic cleaners are required. So is diamond a non-no for CI lapping plate?

-because over time a (baseline quality 0.001") plate would wear into a dish or undulation, could a home shop guy ever hope to re-surface it to good original condition himself?

- what about this business of charging a lapping plate? I've heard you should roll it on. I can understand this mechanism for a softer lapping tool like brass or copper, but what kind of roller could be expected to 'press' this into hard CI?

- if you lapped yourself a nice flat surface A, now you simply want a lapped squared surface B, is there something like a 'fence' or datum that can be attached? I've only seen this truing action on scraping operations & even that is a different procedure. So is this getting outside the scope of a lapping plate?

 

[Cogsy]

Without reading your post

..."snip"...

Perhaps you could enlighten me.

All the information is in his post - the one you didn't take the time to read. He is after information on the finer points of using and understanding a lapping plate, not creating one.

 

[goldstar31]

All the information is in his post - the one you didn't take the time to read. He is after information on the finer points of using and understanding a lapping plate, not creating one.

OK- deleted!


You have time to criticise. So ?


Norman

 

[deverett]

Interesting questions, Petertha.

I was given two lapping plates a long time ago - a rectangular one ex Brown & Sharpe with the grooves, and a circular one without the grooves that was being used by contractors on an oil rig that I worked on for lapping valves (I seem to remember).

I have never used the rectangular one because I share the same concerns as you about the deep grooves. The circular one I use with the lapping paste that I was given with it (a little goes a long way!) I haven't used it too many times, but there doesn't seem to be any surface wear, although I do try and use as much of the surface as possible in an effort to minimise wear.

Here's an outline design for a lapping machine using a circular lapping plate View attachment lapping machine.pdf. This would even out any wear on the surface. Looks simple enough, but I've got into more than enough projects to complete without taking on any more at the moment.

Dave
The Emerald Isle

 

[SmithDoor]

FYI cast iron weight for a weight set work great too.
Low cost grit for the a local lapidary store

Dave

Thus far for finishing flats I have been an amateur wet-o-dry paper attached to glass plate kind of guy. I've been Googling lapping plates more out of curiosity, plus I will be receiving some lapping compounds for my cylinder trials & experiments shortly anyway.

I think I understand the basics - cast iron is preferred because its inert + can be trued + re-surfaced + has a surface type that accepts the lapping compound + relief groves can me machined in etc. Here is smaller example maybe catering to woodworking world.
http://www.leevalley.com/en/wood/page.aspx?cat=1,43072&p=59752

What I don't understand is:

- why the relatively deep recess grooves? I would think after only a few lapping circuits, 99% of the compound slurry will just reside in the valley not up on the flats where the cutting action is? Is there maybe a suction principle with the work piece that slurry gets sucked out & re-introduced to the top flats? If the plate had no grooves, wouldn't the excess compound just be pushed off on the side of the motion footprint?

- when vendors talk about oil/petroleum based lapping compound, either pre-mix or adding oil to dry grit, I can visualize the cleaning & re-charging with petroleum based solvent & you are good to go. So is water as a slurry carrier a no-no on the cast iron for obvious (rust) reasons? Maybe water based lapping is only for glass plates & ceramics surfacing for example?

- I also hear diamond compound is kind of specialized & be careful, because its so hard & sharp it can embed in both lap & part even when they are both steel & not easily removed. I've read where ultrasonic cleaners are required. So is diamond a non-no for CI lapping plate?

-because over time a (baseline quality 0.001") plate would wear into a dish or undulation, could a home shop guy ever hope to re-surface it to good original condition himself?

- what about this business of charging a lapping plate? I've heard you should roll it on. I can understand this mechanism for a softer lapping tool like brass or copper, but what kind of roller could be expected to 'press' this into hard CI?

- if you lapped yourself a nice flat surface A, now you simply want a lapped squared surface B, is there something like a 'fence' or datum that can be attached? I've only seen this truing action on scraping operations & even that is a different procedure. So is this getting outside the scope of a lapping plate?

 

[petertha]

Hmm, don't know how I missed this web site, but some lapping goodies, info & partial answers..
http://www.americanlap.com/how_to_page.htm

FLAT LAPPING A flat surface lapping plate or rotation table is used to lap flat parts. Most production parts are lapped on rotary table machines. Hand lapping can be done on a flat lapping plate. These plates usually have slots or grooves machined in the surface to assist in compound and lubricant distribution. Plates without grooves lap much slower than plates with grooves.


CLEANING
SOLVENT BRUSH - Solvent brush cleaning is the easiest. Kerosene and nylon brushes work well to loosen the swarf and wash it away. Sometimes a clean rinse follows the brushing.

SOLVENT VIBRATION - Solvent vibration is also available especially if diamond abrasive is used. The vibration will loosen embedded abrasive grains.

 

[Wizard69]

Here is my take on lapping plates. The grooves are there to equalize grit distribution to help equalize material removal. I take this idea from armature telescope making where concave mirrors are made by rubbing (lapping) two surfaces together. To put it another way lapping is a way to a flat surface as the tendency is that the outside circumference wears faster than the center. You can often see this happen when lapping a finely machined surface, the center often displays tool marks far longer that outside.

So what the grooves are doing for you is providing a way to clear scarf from the center of the part being lapped to equalize wear. Mirror making is a fine art and they create a number of laps for their hand done telescope mirrors. All of those Lapps have carefully designed groove cut into the laps surface. Obviously optical work is done to a higher degree of finish than most metal working. Technique has a dramatic impact on the quality of surfaces generated by mirror grinding.

On a side note ways are flattened by scrapping not lapping. I really don't see lapping as a way to a flat surface but rather a way to polish up what is already a flat surface. Really bad lapping technique can have a negative impact on the flatness of the part being lapped.

In any event that is my take on the grooves, you really want them there to equalize wear.

 

[bb218]

I think the reason the grooves are so deep in the plate is so you can resurface the plate without re-cutting the grooves. Over time the plates get worn in the middle and need re ground with a surface grinder.
Flat lapping plates are to lap parts flat. You need to lap in a figure 8 pattern to keep the surface flat. I have used both the rotary lap shown and have hand lapped. The rotary laps are rotating the parts as the table rotates around. I have hand lapped injector cups flat to within 2 light bands, to check these you need to polish the surface with a special ultra fine stone or mylar paper then check using a quartz optical flat.

Mike