A thought on lapping...

[Hamzter]

I've recently been looking at trying to reduce the friction in the ways of my CNC'd Sieg SX3, as the high force required to move the table with the gibs tightened is resulting in high backlash (~0.1mm / 0.004"), I ultimately intend on scraping the ways but for the time being I'm thinking about lapping them. Now I'm sure lots of people reading this are screaming "don't do it!", mostly down the understanding that the abrasive will become embedded in the ways, which is a fair point, but why don't we try and remove it? Most lapping compounds seem to be made of aluminium oxide, silicone carbide or diamond particles, the latter two seem pretty impervious but aluminium oxide is readily attacked by hydroflouric acid (nasty stuff) and phosphoric acid (not so nasty stuff), so why don't people lap with an aluminium oxide compound and clean it up with phosphoric acid?

Phosphoric acid is easy to get hold of and relatively safe, it's regularly used to remove oxide films too so shouldn't damage the base metal. I'm thinking of trying it out on some cast iron scraps, just to check there's no nasty chemical reactions that could damage it. Can anyone think why this might be a bad idea? It would seem to make sense for those who would rather lap their ways...

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[LongRat]

That's an interesting idea and I don't see why it shouldn't work, personally I wouldn't bother though as I think a thorough cleaning of the lapped parts should be sufficient without trying to react away the abrasive particles. But before any of that I'd try running the parts together totally dry. I've seen a number of comments from people who have tried this with success, resulting in the formation of easily removed cast iron powder and nothing else.

[awerby]

I don't think phosphoric acid will dissolve aluminum oxide particles, although it's good at cleaning microscopically thin oxide films off of metallic aluminum. And you definitely don't want to mess with hydroflouric acid. What's the matter with scraping? Lapping won't make things flatter unless you're lapping against something that's already flat, and harder than the thing you're flattening. Just introducing compound into sliding parts tends to make them even more banana-shaped.

[wizard]

I will say this old and clear don't do it!

Lapping is about the worst thing you can do for a plain bearing machine.

I've recently been looking at trying to reduce the friction in the ways of my CNC'd Sieg SX3, as the high force required to move the table with the gibs tightened is resulting in high backlash (~0.1mm / 0.004"),

that doesn't sound like backlash but rather gibs so tight you are twisting or distorting things. On these machine you should be able to disconnect the lead screw and push the table or saddle by hand. If you can't get the slide to do this without excessive effort while keeping the play minimized you will likely need to scrape the ways.

I ultimately intend on scraping the ways but for the time being I'm thinking about lapping them. Now I'm sure lots of people reading this are screaming "don't do it!",

Already screamed that. In this case more so as if I understand you correctly it sounds like you have the gibs way too tight. Of course I'm not sure what high force means to you but from the sound of your post it is advisable to try adjusting the gibs.

mostly down the understanding that the abrasive will become embedded in the ways, which is a fair point, but why don't we try and remove it?

Actually that isn't my primary concern though the embedded grit is a real problem. The problem is the uncontrolled way metal is removed trying to lap parts in like this. It really can't make the fit better though it can loosen up things in places. A loose fit isn't a substitute for a proper fit up.

Most lapping compounds seem to be made of aluminium oxide, silicone carbide or diamond particles, the latter two seem pretty impervious but aluminium oxide is readily attacked by hydroflouric acid (nasty stuff) and phosphoric acid (not so nasty stuff), so why don't people lap with an aluminium oxide compound and clean it up with phosphoric acid?

Phosphoric acid is easy to get hold of and relatively safe, it's regularly used to remove oxide films too so shouldn't damage the base metal. I'm thinking of trying it out on some cast iron scraps, just to check there's no nasty chemical reactions that could damage it. Can anyone think why this might be a bad idea? It would seem to make sense for those who would rather lap their ways...

If you are comfortable with acids go ahead. However just realize that lapping itself is a problem causer.

[Hamzter]

Hey, thanks for the replies, I'm not all that surprised everyone still thinks lapping is a terrible idea

That doesn't sound like backlash but rather gibs so tight you are twisting or distorting things. On these machine you should be able to disconnect the lead screw and push the table or saddle by hand. If you can't get the slide to do this without excessive effort while keeping the play minimized you will likely need to scrape the ways.

I can't move the table by hand at the moment, I've got the gibs at their absolute loosest without any slack in the table. You're right it's not really backlash I'm seeing but distortion, I spent quite a while probing the machine with a DTI, I've found that about 90% of it was coming directly from the ball screw. With the gibs loose I get about 30um of backlash, but 100um with the gibs tight, I suspect something in the nut is distorting slightly under pressure (the nuts are 'preloaded' but they're only rolled screws so preload will be minimal).

Think I'll skip the lapping anyway, though if anyone reading this is insistent on lapping, maybe try cleaning it up with phosphoric and feedback the results

[hanermo]

Lapping might work fine.
How will you make the brass or cast iron laps and control them ?

Lapping is NOT sanding.

[LeeWay]

Feeler gauges can initially help show you where the ends of the ways are the tightest. Get a bit of the blue machinist dye as well. That will help show you the high spots or rub points on contact surfaces. Then at least you will see what needs to be done where.

[arizonavideo]

You can also lap without any compound.

Remove all the oil and blue the surfaces and run the ways dry to see the contact area. If the dovetails have poor contact then don't oil them but oil the flats. By hand run the ways full travel with the dovetail dry. The high spots will grind away fairly fast. The flat will help to keep things straight.

After a 50 or 100 strokes re blue and see what you have and remove the dust. Repeat as needed leaving only one mating pair dry for each lap.

The main problem with lapping is it will remove too much material. You don't want to change the shape just remove high spots that cause binding. If the shape is wrong it is going to be a real PIA to get things straight.

[SCzEngrgGroup]

All this talk about the evils of lapping is really fascinating. Obviously, there are no amateur astronomers in this bunch. Reflecting telescopes use parabolic mirrors that are shaped to incredible optical precision by.... abrasive lapping! Amateurs achieve excellent results by doing the lapping by hand. I've done it myself. Even the 200-inch mirror for the Hale telescope at Mount Palomar was made this way (though, obviously not by hand, due to its size). Valves in IC engines are abrasive lapped to achieve a perfect seal. Contrary to what's been stated here, lapping will NOT remove material "everywhere", unless you're using a really coarse abrasive. It will remove material where there is close contact, and the other areas will remains essentially untouched, as the abrasive will simple slide, with little or no force, across the surfaces, doing no real harm.

Regards,
Ray L.

[Wizbaa]

A very aggressive lapping may cause you problems, but Id say its easier to mess up a machine scraping than lapping. As pointed out in here the ways will naturally rub and form to each other with use. In essence the machine will lap itself over time. IMHO and especially so with chinese Iron, a careful lapping is the same thing as breaking the machine in.


If you havent seen his thread, Ryan both scrapes and laps his G0704 and recommend lapping if you have fitment issues.
http://www.cnczone.com/forums/bencht...ans-g0704.html

[Gregj888]

Ray, I make my own telescope mirrors and scraped the ways on my mill. Lapping is capable of making an air tight seal or an optical surface to a sub-micron level. Unfortunately there's a lot of technique in lapping optical surfaces that doesn't apply to a mill. Optical surfaces self-control their surface shape, the mill ways do not.

If you do need a "quick touch up" you can look at this- Page 7 Using the sand paper as described will at least let you control things to some extent. This is especially useful if you know which surface is the flattest and most square. After doing my S2, I will say there's not much chance "sanding" will get you good results in a "real way." My X table was bowed- well, really thicker at both ends, so needed scraping on the ends both top and bottom to control the Z direction. Dovetails are still to be done but appear to be reasonable ( <0.002" in across the full length in X measured against the front of the table , most in the actual front its self. Z is <0.0015, most or all in deviation of the top surface (cupped)).

So to borrow a saying from the ATM world, there's a 0.001" tool in there someplace, your job (in scraping) is to find it. Start wit a copy of the Home Shop Machinist series on scraping, it will save you a lot of time.

Greg

[Winnfield]

All this talk about the evils of lapping is really fascinating. Obviously, there are no amateur astronomers in this bunch. Reflecting telescopes use parabolic mirrors that are shaped to incredible optical precision by.... abrasive lapping! Amateurs achieve excellent results by doing the lapping by hand. I've done it myself. Even the 200-inch mirror for the Hale telescope at Mount Palomar was made this way (though, obviously not by hand, due to its size). Valves in IC engines are abrasive lapped to achieve a perfect seal. Contrary to what's been stated here, lapping will NOT remove material "everywhere", unless you're using a really coarse abrasive. It will remove material where there is close contact, and the other areas will remains essentially untouched, as the abrasive will simple slide, with little or no force, across the surfaces, doing no real harm.

Regards,
Ray L.

That is completely ridiculous to compare the two because when lapping a parabolic shape with a concave lap, the lap will slide over all of the surface of the part. it isn't a a controlled back-and-forth motion where the abrasive is piling up at the leading edge of the part, and thinning out between the sliding surfaces, causing more wear at the outside edges. Add to that the fact that the sliding isn't controlled by perfect rigidity, you will have one part tipping as the direction changes, adding to the problem.