Comments on frosting of machine ways

From the Chaski HSM forum-

Bump method of frosting ?

Posted By: MDM February 2002

Anyone care to explain the bump method of frosting a lathe bed as shown in Brett's pictorial on his Southbend lathe? Thanks!


Reply By: Forrest Addy

Here's one of the best kept secrets of the machine tool rebuilding racket: that pretty frosting pattern so much admired on exposed hand scraped ways is strictly for appearance and it works against the owner shortening the life of the machine.

I've heard old hands sincerely speak in glowing terms of a "proper frosting job" as though it was both the crowning glory of a machine's appearance and a conscienciously applied means of retraining oil and reducing friction. Given a free hand a master scraper would apply an artistic scraping pattern to all exposed bare metal surfaces mush as a prideful bosun's mate applies fancy ropework to all quarterdeck fittings and appurtinances.

The master surface plates at Hunter's Point Shop 31 scraping bench had a handsome logos scraped in the center of their reference faces and the logos and all had near perfect bearing.

I've seen case haedened machine tool purchaser of vast experience be so takes with a beautifully frosted way surface they completely ignore the hazard it represents to machine tool longevity. They admire its glistening irredescent surface, run their hands over it'shypnotic indentations and breathe "It takes a master srraping hand to do this>"

I think BS, I used to do that by the acre with a Biax half moon power scraper and the brilliance of the scraping came from lapping the scraping edge with 9 micron diamond compound to a mirror finish. You can cover up a multitude of machine discrepancies with pretty frosting. It's eyecatching and beautiflu. It's also the used machine tool sales equivalent of putting a banana in the rear axle of a used car to silence failing gears.

Most machine tools feature way wipers either of felt or sophisticated molder rubber designed to exclude grit and chips plus retain oil at the ended of the ways. Frosting doen with a heavy hand may go as deep as 0.002 below the scraped surface and if done for maximum effect the frosted depressions are abrupt intersections ideally configured to trap dirt.

I'm a firm believer of frosting as much as 5% of the area in any fully housed way bearings as a means of lubricant retention and reduceing "stiction" but never in ways intended to be exposed to chipflow or airborne dust. There the ways should remain scraped smooth so the way wipers can better conform to the surface, excluding dirt and retaining oil. When the hand scraping pattern fades from the exposed ways in a few years, it's time for a re-scrape.

My suggestion to MDM is to consider my above remarks with his application.

Push frosting is easy to describe but it requires a great deal of practice if a good appearance is desired. You select a sharp scraper with only a little crown, apply the edge firmly to the work, and give the heel of the scraper handle a sharp bump eoght with the heel of the hand or a rubber mallet. Ideally the frosted depressions should be oriented alternately 90 degrees and be near perfect squares.

Acvocates of hand frosting frequently make a "mushroom" consisting of a knurled aluminum handle of normal proportions having a 1 3/4 dia mushroom knob on the heel. There is a #1 Morse internal taper in the business end designed to take the welded on shanks of a selection of frosting scrapers having end radii suited for each of a variety of frosting paterns.

Masters of the skill can produce a perfect checker board in alternating bearing points where the frosting is of uniform size and its orientation is in alternating direction. If done correctly any percentage of original bearing can be attained.

For half moon frosting, imagine the mushroom end of the scraper handle as a clock dial. The tool is used by gently striking the clock dial with a soft rubber maul or the heel of the hand. A right hander firmly grasps the scraper by the snak of the blade (not the handle) and applies it to the work with the left edge held firmly down. He strikes the mushroom at about 4 o'clock with a repeated slightly down handed blows. A series of half moons progress with each blow. The blows have to be absolutely consistant in force and direction.

You have to experiment. Individual anatomy has a pronounced effect. I have a muashroom handle and some odds and ends of frosting scraper. While I've acquired the knack many time it has to be maintiained through frequent practice.


Reply By: John White
I am surely no expert on the subject of either scraping or frosting let alone the longevity of machine tools. But It has been my opinion all along that a smooth, unscraped surface has got to be longer-lived than a scraped one. As the poster above mentioned, it must surely be tough on wipers. But, I agree, it is pretty. Sort of like engine turning on a rifle bolt.






[This message has been edited by Admin5 (edited 02-15-2002).]

Just imagine an oil supply only 1/8 inch away.
Move your machine 1/8 inch and she's floating on oil from capilary action.
Ground, on ground surface you may have to move a foot, or a yard to redistribute the oil.
Any good wiper will reach into a frosted surface to wipe it clean. They are rubber after all.
You are right, a Devlieg Jig Mill is a thing of beauty right from a rebuild. Functional too.
I'll take mine frosted, thank you
mite

Forrest is right. All frosting does on ways exposed to chips and dust is decrease the bearing surface, increasing the rate of wear and create grit traps that the wipers can't follow. If there is oil on the ways, it'll stay there, frosting or not. I've never been a fan of felt wipers, they make good oil wicks, and even better dirt and grit wicks, and most if not all rubber ones are stiff to the point of not being able to follow and kind of small surface irregularity.

We have always had the policy that the two elemnts of a slide that are scraped to each other the one that gets the flaking is the top element. In fact on many jobs we intentional did not scrape the bottom element so as to keep as much contamination out as possible. Frosting is just that, frosting on the cake. A properly scraped surface depending in the width of the blade has a beauty all its own

I prefer to way grind the top of the bed, then scrape the underside of the mating slide, no valleys means no swarf gets into the ways. Also I install brass wipers over the rubber ones, this keeps the hot chips from ruining the wipers.

It isn't a question of top or bottom, although it usually works out that way.
Customarily, the longer member gets ground and the shorter, mating member gets scraped.
Arno

So are you telling me that the person who did this 'attractive' job on my lathe bed has stuffed it?? If so is there any remedy??
Merry Christmas all.

Banana? I thought it was sawdust!

Speedy, not at all.

First of all I don't see any undelying scraping. Has it been hand scraped to a surface plate? If the way surfaces are straight and your OK. If not perfect but pretty good you still might be OK. I'd still like to see how they blued in on a good surface plate with a thin layer of blue on it. You lathe bed looks small enough to tuck under your arm. Got a buddy with a granite surface plate or a local small shop you could impose on?

Second, the frosting on your lathe bed doesn't look very deep anyway. It look barely deep enough to go beneath the machine marks. I suggest you give it a light stoning with a worn fine India slip stone and let it go.

Third. Does the carriage work off a handwheel and rack? If so the backlask between handweel pinion and the bed rack has been opened up by the amount of the total stock removed between saddle and bed. Thus you have to move the rack down to compensate so you get 0.003 to 0.005" backlash.

You also have to move the leadscrew and all other shafting down my the same amount. This is usually compensated by shimming the Q/C box and brackets down and elongating the screw holes accordingly.

There's more but I'm sure yo've looked into this

Thank you for your comments Forest. Yes, you are correct, it is an ML7 lathe bed.
The factory specs for the shears is 0.500", they are now 0.492" after the surface and shears were ground;( 0.005" off the top surface). I have no concerns as to the accuracy of the work. My concern after reading the comments regarding frosting was the affect that was imposed on the lathe bed. The machinist assured me it would reduce stiction, no mention was made about increased rates of wear.
I have access to the surface plate in my previous posting on HSM.I will measure the depth of those "frosting" marks and post them later today( it's now 12.40am zzzzzz ) With regard to your advice to move the rack and feedscrew down by the amount removed, could I remove this amount from the top of the apron to achieve the same result??

here is the 'before'condition of the lathe bed.

Merry Christmas,,Ken

If your ML-7 has power feed to the cross slide (I can't recall) there's a gear that pokes up from the apron that engages a gear on the cross-feed screw. If you remove material from the top of the apron casting to compensate for the carriage drop you'll close up the gear's center distance so it can't be brought into mesh.

If there's no power crossfeed or other working part affected by skimming off the apron top my remarks are moot.

As for frosting on the exposed ways "causing" wear, that's not the concept I was trying to convey. The frosting, if done with a heavy hand, frustrates the function of way wipers allowing dirt to slip past them into the bearing surfaces to cause wear. Deep frosting protected from dirt with efficient way covers has little effect on the axis' longevity.

By the way, when was the last time that handsome little surface plate was checked against a master flat? It's not fair to judge from a photo but I can see no remaining traces of the hand scraping used to make it flat, suited for a reference tool.

No, the lathe does not have power cross feed, Forest; it is a 1951 model ML7. So I guess my option of removing material from the top surface of the apron could be a go??
I do intend to add an independant variable speed drive of some sort to the cross slide at a later date. One that can also be used with the feedscrew as well
Thank you for the clarification on frosting. I have measured the depth of frosting to the best of my ability with a Mitutoyo DTI ( 0.01mm ) and the average depth seems to be in the range of 0.00mm -- 0.02mm / 0.035mm max.
I have blued the surface plate and checked it against the bed and it "shows" on 99% of the surface, but I will take your advice and take the bed and surface plate to E.G Whiters in Otahuhu for them to double check. I will post an image of the blueing shortly.
Forest, I have no idea when the surface plate was verified . With the surface plate inclined a hatch in the manner of a chess board can be seen, it is faintly discernable in the image; is this the affect that you mentioned??
It is Summer?? here and we are getting fronts driving up from the antarctic; hailstorms and big winds; strange....
Seasons Greetings, Ken
Sorry, the image is very amateur, 'cos I am