The Student Room Group
Reply 1
What job are you working on exactly and in what material?

Filing generally leaves you with a rough finish but is useful for removing a lot of material quickly

Scraping goes one step further than filing. Essentually a scraper is a blade with the edge folded over and will give a finish not too dissimilar to wet n dry paper.

Lapping is essentially a very fine surface finishing process. your goal would be to remove any machine marks or, in the case of something like a valve, creating a close as possible fit between two surfaces eg valve head and seat.
It's so fine that, if done propperly, will actually make a perfect seal.

Anyway, as I said, what's your project?
Reply 2
JC's your man :P
Reply 3
Cheers,

Im doing a engineering skills course which invloves making various things generally with mild steel by machining and by hand. The questions I were answering were all based on general hand fitting skills.

I was not sure what it meant really by produce a sliding or mating fit - can you do this by filing.

Cheers
Reply 4
Think back to the example I gave you of the valve in a cylinder head.

Once it's all assembled then the polished surface between valve head and seat IS a mating fit.

You would use all three techniques turning raw MS into the finished product if you had to start from scratch.

Have a word with your tutor. Find out the sort of thing he wants you to produce.
Reply 5
Ah I see what you mean now. I was not thinking along the lines of doing one after another to produce the finish.

Cheers
Reply 6
No probs.

I often wish I'd done an engineering degree instead of a music degree now.

-JC
Reply 7
mr cool... by any chance do you have a computer copy of your peo 006 section questions? that you could email me?
i am running behind on the last section and would very much appreciate it.
Reply 8
your lucky being able to do your corse on the pc i have to write everything up old school style :frown: so if you make a mistake the wholelpage has to be re written :frown:
and yes im probly doing the same corse as you (MME3 060)
No probs.

I often wish I'd done an engineering degree instead of a music degree now.

-JC


except bench fighting is a tiny part of Degree level Engineering courses ...

I can't do degree level engineering maths but I can make a reason go of bench fighting and other such tasks based on what I learned at my dad's knee as a young'un .... now your time served fitter with a HNC or HND would be an expert bench fighter...
Reply 10
What job are you working on exactly and in what material?

Filing generally leaves you with a rough finish but is useful for removing a lot of material quickly

Scraping goes one step further than filing. Essentually a scraper is a blade with the edge folded over and will give a finish not too dissimilar to wet n dry paper.

Lapping is essentially a very fine surface finishing process. your goal would be to remove any machine marks or, in the case of something like a valve, creating a close as possible fit between two surfaces eg valve head and seat.
It's so fine that, if done propperly, will actually make a perfect seal.

Anyway, as I said, what's your project?


Hi there JC,
I needed to know some similar information for a project I am working on, and I used a bit of what you said here to get me going. I really appreciate it. Here is what I wrote:

It is often necessary for two fabricated components to fit together nearly perfectly. A near perfect, mating fit between two metal components provides a number of benefits in working applications including sealing surfaces to prevent the passage of air or other fluids. When fabricating components with mating surfaces it is best to work the material into a state that matches as closely as possible the outer tolerances of the drawing.

Filing generally leaves you with a rough finish but is useful for removing a lot of material quickly. When using filing techniques to bring the dimensions into fitment, you must stop short of the outer tolerance in order to leave enough material to work out the cut marks and gouges left by the filing. Files should be used in progressively smoother grades which results in removing larger cut marks and leaving finer ones.

Scraping goes one step further than filing. The fine cut marks left behind from a smooth file may be worked out further with a metal scraper. A scraper is a blade with the edge folded over and will give a finish very similar to the use of wet and dry emery paper (the use of which is not recommended for mating surfaces because of the tendency for the finishing pressure to be uneven across mating surfaces).

For the final phase and the closest mating fit possible, the two mating surfaces (or similar sacrificial surface of the opposing part) may be used to “lap” the surfaces into mate. Lapping is essentially a very fine surface finishing process. This is done by applying a lapping compound in progressively finer grits and then applying a mechanical force, moving the two surfaces in opposing directions effectively “wearing” in an average surface between them. The goal is to remove any machine marks. For example a piston engine valve requires a perfect seal between the valve head and valve seat whenever the valve is closed. Lapping techniques allow for creating as close as possible fit between two surfaces. It's so fine that, if done properly the lapping compound and the mechanical movement will create a perfect mating fit.
There are 3 types of fit; clearance, transition and interference. For filing, the piece of metal would need to be made of bar stock and you would work to tolerances that are found in a zeus book (every engineer should have one of these). There are 4 grades of file; rough, *******, second cut and smooth. The general rule is, the more material that is required to be removed, the rougher the grade of file you would use. The rougher grades remove metal faster, where surface finish and accuracy is unimportant. You would finish your datum with a smooth file, first by cross filing, then by draw filing (this provides a better surface finish and removes bumps caused by cross filing). If you require a better surface finish, you would lap the face using a fine grit lapping compound, which removes any high spots caused from filing. This creates a flat, smooth finish.Scraping is a process usually used for finishing machine ways. Parts of the ways are scraped away to allow oil retention which helps to prevent seizure on moving mating parts.
Reply 12
Original post by Joe85pink
There are 3 types of fit; clearance, transition and interference. For filing, the piece of metal would need to be made of bar stock and you would work to tolerances that are found in a zeus book (every engineer should have one of these). There are 4 grades of file; rough, *******, second cut and smooth. The general rule is, the more material that is required to be removed, the rougher the grade of file you would use. The rougher grades remove metal faster, where surface finish and accuracy is unimportant. You would finish your datum with a smooth file, first by cross filing, then by draw filing (this provides a better surface finish and removes bumps caused by cross filing). If you require a better surface finish, you would lap the face using a fine grit lapping compound, which removes any high spots caused from filing. This creates a flat, smooth finish.Scraping is a process usually used for finishing machine ways. Parts of the ways are scraped away to allow oil retention which helps to prevent seizure on moving mating parts.

Why have you posted this?

@RoyalSheepy