Beginner's questions on hand lapping

[compact8]

So I have made a hole plate for my rotary table and I want to remove the machining marks on the surface. This will serve no functional purposes but I just want to make it look pretty and get some experience on hand lapping in the process.

The plate is 150 mm in diameter , 10 mm thick. material is hot rolled steel. I have watched a couple of videos on youtube so I have some basic ideas on how it is supposed to be done but I still have some questions :

1) Instead of rubbing the plate on a bigger flat surface which I don't have, is it possible to rub a smaller surface on the plate ? I am thinking of using a 6061 aluminum block with grooves cut on the surface.
2) What's the difference between lapping paste and liquid ? Is one better than the other ? It seems that paste is graded in grit ( 1000, 1200..... etc ) and liquid in microns. How are the two units interconverted ?
3) Paste or liquid, I will likely end up in having some residual left in the screw holes on the plate. Will any harm be done in future use of the plate other than getting polished screws ?

Thanks in advace.

 

[Alexander McGilton]

The Machinery's handbook has a good chapter on lapping. Should answer most of your questions.
The paste form is almost never used as is in its packaged form. In charge lapping it is intended to be spread thin across the lap, pressed and embedding the grit by stamper or roller of hard material, then washed away by solvent. This leaves only the minimal grit needed, and embedded in the lap, the voids between grits leaves room for accumulated lapped material.


If all you are after is to improve surface finish then wet/dry extra fin grit sand paper is the way to go. Only when you are working to sub tenths or microns flatness dose the uneven thickness of sandpaper become an issue.

 

[homebrewed]

While an aluminum lap will work, it will turn your lapping compound black in no time as the aluminum is abraded. From there, your hands, face, shirt, etc. Tough plastics like HDPE actually will wear less, and, in the end, be less messy. Plastic cutting boards commonly are made from it. One thing to beware of though is if you decide to switch to a finer grit. The coarser grit will be embedded in the plastic. You'd either need to use a fresh piece or mill off the surface to get down to fresh stuff.

I happen to have a TAP Plastic store near me and they often have HDPE pieces in their scrap bin for not a lot of money. You may have a similar store near you. Beware though, they're another machinist's candy shop

If you use sandpaper I don't recommend using adhesive-backed stuff. In addition to becoming difficult to remove without leaving residue on the support surface, the adhesive is sort of soft and squishy. The end result is that the edges of your workpiece can get rounded off.

Check the machining marks as you go to monitor how uniform your surface is and stop if you see things going awry. After all, as you said it's more for appearance so don't shoot yourself in the foot.

 

[C-Bag]

I used Oxtools and Robin Rinzetti as guides for lapping. Two of my lapping projects were old Starrett and Brown &Sharpe height gauges that were no longer flat. Both guides used small 9x12 granite surface plates with quality sandpaper stuck to them. I ended up getting a couple of rolls, 180grit and 600grit stick on to put on my cheap 9x12 plate. When I bought the plate it was like $35 delivered. Don’t know what they are now but worth every penny IMHO. Rubbing a small surface on a larger work is a guaranteed fail if you truly want it flat. And I don’t believe you learn anything if it’s not flat except bad habits.

The stick on sandpaper made by Norton left no residue and has proven to be very effective.

I did like Tom(Oxtools) using the whole surface and turning the work constantly. Started with cheap HF diamond hones(with a special liquid I found that keeps the hones clear and cutting), 180, then 600 sandpaper and finished with a 12” round cast iron Shamrock lapping plate that I found cheap on eBay with 2000grit to get the final mirror finish.

I‘m glad I had watched Tom’s short series on the 3 plate method as it takes a lot of concentration not make things worse. The height gauges both came out great, far better than I would have ever thought.

 

[homebrewed]

I did like Tom(Oxtools) using the whole surface and turning the work constantly.

We did a lot of grinding/polishing in the lab where I worked. Turning parts while doing it now is an almost unconsciously-performed part of the process. It's almost impossible to manually apply even force across the extent of your workpiece so it's necessary to randomize the process by turning the part. You also want to spread out the wear on your sandpaper. If you end up with a more-worn region in the center, the outside edges of your part can be exposed to more-aggressive, less worn grit surface as you move the part around.

 

[C-Bag]

If you end up with a more-worn region in the center, the outside edges of your part can be exposed to more-aggressive, less worn grit surface as you move the part around.

The whole process of lapping just gets more and more involved and the scale gets more and more focused as the process progresses. I felt like one of the problems I was constantly dealing with was the fact the friction caused by the abrasive diamond or paper would cause more pressure on the leading edge so I seemed to be having the opposite problem Tom and Robin talked about where the middle of the work was getting dished out. I had great flat area out to the last 1/8” from the edge that would be slightly relieved and the only way to get rid of it was by extreme lapping as there was less friction. The Shamrock 12” cast iron lapping plate I have came charged with really fine diamond dust so is wonderful for final finish but literally impossible to really remove much material. If i was going to be doing that for any larger area I’d use my shaper(closest to ground finish), then surface grinder and finish with lapping. Lapping by hand is a HUGE time sink and can be frustrating to say the least.

 

[homebrewed]

Some time back I built an attachment for one of our polishing machines. The machine used a small spinning disk to do the polishing while the sample was moved in an X-Y fashion underneath it. The XY table used a simple mechanical arrangement so there was a fixed relationship between X and Y -- it basically described a Lisajous figure. The only "programmable" aspect was adjustments to set the extent of the X and Y motion. This had all sorts of polishing-uniformity problems because the table speed varied (it was sinusoidal) so the center of the sample got less polishing time than the edges. The machine was an earlier version of this.

I replaced that XY table with a programmable XY stage made with two RC servos, controlled by an Arduino. I converted the rotary motion into linear using a rack and pinion design. The idea was to enable me to selectively polish areas that needed to be polished down more. It worked, but the key was figuring out how much time to spend on the high areas relative to the low ones. That took some time to figure out, and was based on the operator's experience because we didn't have the right kind of metrology to formalize the procedure. It also was _really_ slow. But I also have to point out that the tool was meant to de-layer integrated circuits, where the difference between "good enough" and "too much" was less than 1 micron. So slow was good

I got the idea from a polishing technology used to make highly accurate optics, called "deterministic polishing".

 

[C-Bag]

a Lisajous figure.

The word of the day I guess Had to look that one up.

Not being as highly technical as you I think would employ a DA sander somehow to agitate the work on a XY table seeing how it’s supposedly random orbital. Probably wouldn’t work but it does a nice job of smoothing paint, metal, bondo etc.

 

[MrWhoopee]

For what you are trying to accomplish, a few sheets of wet-or-dry paper on a plate glass table top will work great.

 

[compact8]

Tough plastics like HDPE actually will wear less, and, in the end, be less messy.

Have never imagined that plastic will work ! I have a piece of ABS block collecting dust. Will it be good for the purpose ? ABS is known to be not very resistant to chemicals so I am wondering if stuffs in lapping compounds will attack it.