How to Recondition a Lap

Hi All,

I just bought a 10" X 14" surface plate lap. I don't know
if it's flat yet. When I get it I will throw it on my surface
plate and see how it measures up. It's a two sided lap, and I
would expect the two sides to be parallel with each other.

If it does not check out I'll machine it and then scrape it
to 40 ppi. Then I'll try to use it as a lap so it will wear
flat with no scrape marks.

Or I could put it on my 6"X18" surface grinder and make two passes and
get it to about .0001"

Does this sound like a workable plan or have I missed the
boat somewhere?

Best Regards,

Paul Hoffman

You are way over thinking this thing.

What do you plan to lap that needs a perfect surface to work against?
I don't know of any application that expects the lap to start "perfectly". but lot's need to END at perfection.

Lap surfaces are constantly in movement first flat, then concave , then flat, then convex etc. ad nausium...;-)

A spherometer, a ring lap corrector and a plug lap corrector along with a selection of lapping medium will produce and maintain your small lapping plate until it is worn to nothing. (And it starts to wear and go away from "perfect" with the first stroke.

(The three plate method is useful for those needs when you MUST start with a flattened lapping plate.) (What would that be? repairing gauge blocks??? IDK)

You WILL need something to check your work , an optical flat or an interferometer if you are working to that precision.

Expecting two sides parallel is a bit much, but you might get lucky, And the only way to maintain it would be to not use the lap. Why would you want that? 'Can't see the reason.

I am thinking about lapping my surface plate.

toolnuts said:

I am thinking about lapping my surface plate.

Click to expand...

Two flat surfaces rubbed one against the other with abrasive between make a curved surface....if not corrected.

A Flat lapping plate is not a requirement to flatten your surface plate*. It's how you manipulate the lap that achieves the proper results. And when you are finished, both pieces will be very flat. But if you choose to continue the process, the lap and the work will only get "less flat'. It's just the way it works.

*reasonably flat is expected.

toolnuts said:

I am thinking about lapping my surface plate.

Click to expand...

how big is your surface plate? How much did you pay for the lap? How much is your time worth? Do you have collimator? If the total you need to expend on lapping your surface plate exceeds about 30 bucks/sqft of surface plate, you better off hiring a pro to lap your plate.

just saying...

unless the cost is significantly less or you really want to know how to do it it is a waste of time . Just said just saying....



dee
;-D

dcsipo said:

how big is your surface plate? How much did you pay for the lap? How much is your time worth? Do you have collimator? If the total you need to expend on lapping your surface plate exceeds about 30 bucks/sqft of surface plate, you better off hiring a pro to lap your plate.

just saying...

unless the cost is significantly less or you really want to know how to do it it is a waste of time . Just said just saying....



dee
;-D

Click to expand...

I do have an autocollimator. My plate is 2' x 3'.

I am retired so it's a hobby.

When is learning ever a waste of time, unless you have mouths to feed,
and you need the time to make money to feed those mouths.

Paul

CalG said:

Two flat surfaces rubbed one against the other with abrasive between make a curved surface....if not corrected.

A Flat lapping plate is not a requirement to flatten your surface plate*. It's how you manipulate the lap that achieves the proper results. And when you are finished, both pieces will be very flat. But if you choose to continue the process, the lap and the work will only get "less flat'. It's just the way it works.

*reasonably flat is expected.

Click to expand...

we're into muck though.....it doesn't need to be flat, just reasonably flat What's flat and what's reasonable?

I get what you are saying. Seems to me lapping a surface plate is an exercise in zone work vs points or spots like scraping. So I think you would need a lap that is close to the same sort of flatness you're trying to achieve - ie if you are trying to lower 4 sq in buy 3 tenths you're going to need a flat cutting tool (and some skill). In my (albeit limited) experience properly charged laps hold there form and don't wear much - its the cutting edges that wear with the base material being largely untouched. but thats experience lapping metal not granite.

Obviously with scraping you can create flatness from the three plate method - can you do so with lapping? OR did you mean scraping three plate method (UGH!!)

It's a two sided lap, and I
would expect the two sides to be parallel with each other.

Click to expand...

Why? and what's parallel? (to what tolerance)

Or I could put it on my 6"X12' surface grinder and make two passes and
get it to about .0001"

Click to expand...

how do you grind a 10x14 lap on a 6x12? I'd suggest even if it was 10x14 grinder, flat to a .0001 might be optimistic. flat to .0001 is hard to get on a large surface (why quality plates are expensive). Machine wear, clamping forces and so on. I'd grind as best you can (to get under the embedded layer) then scrape to your plate. I'd also think, because the sampling that goes on - large lap surface cutting a large work surface, you'd not need 40ppi

My grinder is a 6"X18" (I corrected the error in my post)

The only reason for 40 ppi is because I'm self taught and still
have a long way to go - I need the practice.

If I really want to check for flatness I have an autocollimator.

I just thought it might be nice to have the two surfaces parallel so
I could throw it on a surface plate to quickly check flatness later,
down the road.

I could put it on my 6"X18" surface grinder and make two passes and
get it to about .0001"

SG grind it to .0001 to .0003 and then use your lapping parts to lap it to closer.
think a lap should be dead flat but every time you use it you make it flatter or more out of flat as where you lap.

or take it to a Blanchard shop for a skim

Three points determine a plane. One would be hard pressed to improve on that with any sort of grinding machine.

Just as a mental exercise in lapping, consider what effect running a 1/2 sized concave lap plate full length over a concave surface plate.

Do the same mental image for a lapping plate 1/4 the surface plate area. ~~~~
Now do the same mental image with a perfectly flat lapping plate on a concave surface plate, again with the 4:1 surface ratio.
Recall that three points determine a plane.

It' show the lap is manipulated that is important to the end result.

Where is that thread/video/posting by a member here who recently had surface plates redone? The error was easy to see during the correction process. The manipulation of the lapping plate was easily understood, and the results were rapid and sufficient.

Consider also the requires aspect ratio to hold the lapping plate flat 10:1 is not sufficient for careful work, but quite thin tools will produce acceptable results

toolnuts said:

Hi All,

I just bought a 10" X 14" surface plate lap. I don't know
if it's flat yet. When I get it I will throw it on my surface
plate and see how it measures up. It's a two sided lap, and I
would expect the two sides to be parallel with each other.

If it does not check out I'll machine it and then scrape it
to 40 ppi. Then I'll try to use it as a lap so it will wear
flat with no scrape marks.

Or I could put it on my 6"X18" surface grinder and make two passes and
get it to about .0001"

Does this sound like a workable plan or have I missed the
boat somewhere?

Best Regards,

Paul Hoffman

Click to expand...

Id suggest checking to see if the lap is charged with diamonds before you do anything. If you want to check it mount it on 3 jacks or similar and indicate the top surface. If it isnt full of diamond go ahead and scrape both sides flat, doesnt have to be parallel, 5ppi is fine.
When youre done charge one side with 40-60microns and the other with 10-20microns. Ie one to rough with and one to finish up.

Heres the video i think Cal was suggesting. If youve half an idea theres a lot of great info in that video.