polishing gibs

Polishing guides is a useless procedure. They have to be a bit rough to build oil pockets. So aligning them and adjusting them is what you want.

You also should check the spindle stock's alignment first.

Nick

see

Lapping sliding surfaces on your machine is generally considered to be bad practice as it does nothing to correct the geometry there--it is essentially accelerating the natural wear process. A process which leads to inaccuracy.

It sounds like your problem is pretty severe. 20 thousandths of taper is quite a lot. You need to check your workpiece and tool. It sounds like you're probably getting some serious deflection in your workpiece, because if you're getting that much just from your machine, you may as well weld a piece of chain to it, because it's a boat anchor. You could check if your bed is twisted (do a search on that procedure in this newsgroup), but i think that's unlikely to cause such a large error on such a short lathe.

Bruce Barnett wrote:

What do you mean by "top or bottom"?

Hul

snipped-for-privacy@bellsouth.net wrote:

According to Justin :

Speaking of which -- since it appears to be a lathe (not specified in the "Subject: " header), let's clear a couple of things:

1) What is the diameter of this workpiece which is sticking out at least two inches? 2) Are you using a live center to support the free end, or just letting it hang there?

Essentially, workpieces flex while you are cutting on them. If the extension is perhaps four times the diameter (thus 1/2" diameter with your two inches of extension) it *will* flex enough so the free end will measure larger than the end in the chuck -- and depending on *what* the workpiece material is, it may flex quite a bit.

If it is even longer -- you will want either a steady rest clamped to the bed close to where you are working, or a traveling steady rest on the carriage to support the workpiece near where the cut is being made -- or perhaps even both.

Agreed. I think that workpiece deflection from lack of support at the free end may be the problem.

Enjoy, DoN.

One should be weary of the lapping technique mentioned in the above article.

The author suggests pushing forward and back until the desired finish is obtained. This may be acceptable for purely cosmetic surfaces, but it is entirely incorrect for bearing surfaces, esspecially those used as masters for machining operations.

Hand finishing using an abrasive stone or paper should be done in a criss-cross pattern. Going in only one direciton will only serve to magnify already present errors, and will cause the lapped item to inherit any errors from the master surface (if there are any).

When using sandpaper on a flat surface, a figure of eight patter may be used. The operator must be cautious so as to not tilt the workpiece, causing excessive wear around the perimeter of the surface. Once there is a high spot in the middle of the part (such that the surface is convex), it is unlikely that the error can be removed by further lapping.

For steel items, I would recommend starting with at least 120 grit sand paper. As long as the technique is correct, starting with finner grits (220 is too fine) will only serve to make the process take longer to complete.

Ideally, spotting blue/prussian blue should be used to check the flatness of the piece against a master (granite/cast iron surface plate). At least the use of a straight edge would be necessary to make sure the part is indeed flat.

Regards,

Robin

Good advice. You've learned a lot in you job. You must have some good experienced people you are working with that take the time to teach you.

John

>

Spotting on a flat surface and scraping is the way to go.

Nick

I don't understand this. How does one spot and scrape? What sort of tools are needed?

=============== Several very expensive books available or see

videos also available If your lathe is this much out, easiest thing is to sell it and buy a new one.

Unka George (George McDuffee)

...and at the end of the fight is a tombstone white with the name of the late deceased, and the epitaph drear: ?A Fool lies here, who tried to hustle the East.?

Rudyard Kipling The Naulahka, ch. 5, heading (1892).

True indeed. If one gold plates a turd..its still a turd

Gunner

"I think this is because of your belief in biological Marxism. As a genetic communist you feel that noticing behavioural patterns relating to race would cause a conflict with your belief in biological Marxism." Big Pete, famous Usenet Racist