What's new
What's new

Acro Laps not working as intended, Searching for explanation

R-Engineering

Plastic
Joined
Mar 14, 2018
I have been using 3d printed lapping bits to improve the rugosity and visual appearance of the bore in Airsoft Barrels. Improvements in accuracy can be dramatic :). Below is a picture of Steel barrels before and after lapping:

Left is an unmodified Steel barrel, Center is Steel & lapped a few thousand passes using a 3d printed lap, Right was lapped by a Taiwanese company using unknown methods. All Steel barrels appear similar to the barrel pictured left before being lapped. The unmodified barrel is able to reflect the camera flash back to the lens and you can see the shadow of the leading ridge. On the lapped barrels no light is reflected back to the camera and the shadow of the leading edge is not visible. This is because all light entering the bore is reflected down the bore as a mirror would, the sign of a very fine surface finish.




I decided to produce a hand driven rig to speed up the process. After completing the device I found that the 3d printed bits are less than ideal. I decided to seek out metal adjustable laps and found the Acro brand. They produce Brass Laps. Unfortunately they do not provide the same effect. Instead the surface finish has been made worse & I do not know why:


Lapping steel Before & After




Lapping brass Before & After



Using 3d printed laps we can achieve a visually improved surface finish. However the brass Acro lap has worsened the appearance of the surface. I read online that the lap must be made of a softer material than the surface that is being lapped & this could explain why the Brass Acro lap with a Brass barrel did not work. However it does not explain the Steel barrel having the same effect. I realize now I don't fully understand the lapping process. Can anyone explain why this happened?

Brass Acro lap next to a 3d printed lap coated in lapping paste and cut metal.

 
What grit compound did you use. If that X400 is the grit, that is the answer. For polished surface 1000 plus material is the ticket.
400 grit is to make size, much finer needed to polish.
 
What grit compound did you use. If that X400 is the grit, that is the answer. For polished surface 1000 plus material is the ticket.
400 grit is to make size, much finer needed to polish.

I used 28 micron Diamond grit paste. The x400 means 400 passes through the barrel were done. I should have been more specific, good catch there!

I coat the lap in Diamond paste and some silicone oil and push it through the barrel maybe 15 or 20 times. Until I can see the entire inside of the barrel is coated in paste. Then I will add a very small amount of paste to the lap and from there on I begin counting the number of passes completed towards the final count, stopping every 50 passes to check if more paste or oil needs to be added to the lap.

Ive tried lapping without using any silicone oil before and the results are not as good and take far longer to achieve. Ive found the oil prevents the lap from getting clogged with cut metal.

I used this method with the Acro lap and with the 3d printed laps Ive been using for years.


The Acro Lap Company suggested I use silicone carbide grease mix instead of diamond compound. However I am unsure if this will solve the issue.
 
Lapping is a two action process. Rotation and reciprocation. Both items wear, the lap and the workpiece. You need to keep an interferance fit when lapping or there is no lapping action. Tom Wheels hit the nail on the head regarding grit size, 28 micron equals about 400 grit. Also lapping almost always produces a mat finish like your first set of pictures.

Also what is rugosity? Oh I looked it up, it is the the measurement of surface roughness for Coral Reefs??

Reccomendations are use an expandable lap and keep an interferance fit. Change to a higher grit Aluminum Oxide lapping compound. Maybe 600 grit and 1000 grit. There are companies that specialize in internal lapping, if you google it you may find one.
 
For doing the brass barrels you may look into casting laps of lead or bismuth. Start looking through articles on benchrest shooters and lapping barrels. They don't go to a polish but the concept is there you'll just need to go to a higher grit.

Also for final finishing you may try something of a springier material. I have used felt bobs and brushes for polishing bores to great success.

Have you looked into Flex hones from brush research?
 
From the looks of the pictures, ur lap is not rotating?

I agree, it looks like you are not spinning the lap.

A brass lap will cut more aggressively than a plastic one because the abrasive will not embed into the brass as much as the plastic.

It is common to use bamboo stick (kabob skewers) for final diamond polishing of molds.
 
Lapping is a two action process. Rotation and reciprocation. Both items wear, the lap and the workpiece. You need to keep an interferance fit when lapping or there is no lapping action. Tom Wheels hit the nail on the head regarding grit size, 28 micron equals about 400 grit. Also lapping almost always produces a mat finish like your first set of pictures.

Also what is rugosity? Oh I looked it up, it is the the measurement of surface roughness for Coral Reefs??

Reccomendations are use an expandable lap and keep an interferance fit. Change to a higher grit Aluminum Oxide lapping compound. Maybe 600 grit and 1000 grit. There are companies that specialize in internal lapping, if you google it you may find one.

"Rugosity, fr, is a measure of small-scale variations of amplitude in the height of a surface.

f[SUB]r[/SUB]=A[SUB]r[/SUB]/A[SUB]g[/SUB]
where A[SUB]r[/SUB] is the real (true, actual) surface area and A[SUB]g[/SUB] is the geometric surface area"

Apparently it is used for marine applications as well. Had no idea!

My method of lapping uses linear strokes only. The lap is rotated roughly 15 degrees every 50 to 100 passes to randomize any effects caused by imperfections in the lap itself. The 1st stage of lapping I use 20 to 28 micron paste, and rarely 40 micron if the bore has deep areas. Brass gets 300-400 passes per stage and Steel gets 600-800 passes per stage, and on the 1st stage it is imperative to continue lapping until the bottom of any low areas is reached. From there I will work down through 10, 5, 1.5, .5 micron diamond paste, and lastly 1.0 micron CrOX which has a different crystalline structure yielding an improved finish despite its larger grain size.

In the first picture the barrel pictured center is the one I did. It had over 2,000 passes total spread over 28, 20, 10, 5, $ 1.5 micron Diamond paste.

I will try again with a looser tension. I don't think I was any where near an interference fit.

For doing the brass barrels you may look into casting laps of lead or bismuth. Start looking through articles on benchrest shooters and lapping barrels. They don't go to a polish but the concept is there you'll just need to go to a higher grit.

Also for final finishing you may try something of a springier material. I have used felt bobs and brushes for polishing bores to great success.

Have you looked into Flex hones from brush research?

I have not looked into Flex Hones. I will now though, thanks



From the looks of the pictures, ur lap is not rotating?

This is correct. The idea is to ensure all remaining scratches & imperfections are parallel to the BB's path of travel.



I agree, it looks like you are not spinning the lap.

A brass lap will cut more aggressively than a plastic one because the abrasive will not embed into the brass as much as the plastic.

It is common to use bamboo stick (kabob skewers) for final diamond polishing of molds.


How can I make a lap with Bamboo sticks? It would need to be expandable to suit a range of 6.00mm to 6.1mm
 
How can I make a lap with Bamboo sticks? It would need to be expandable to suit a range of 6.00mm to 6.1mm[/QUOTE]

Sorry, I wasn't saying they would work for you. They are used in a pneumatic die filer. Your parts probably a bit long and I think some sort of expanding plastic lap is actually a good approach.

UHT Ushio TurboLap Air Filers
 








 
Back
Top