Why scrape when you can grind?

[EmGo]

I worked on a couple of Hor Bores that had the same system but we called it “ hydrostatic “.

I have a habit of getting this reversed because the terms seem backwards to me but hydrostatic is when the action of moving the bearing causes oil to be pushed into the free space. Hydrodynamic is when you use an external pump.

Unless I got it backwards again

 

[dcsipo]

I have a habit of getting this reversed because the terms seem backwards to me but hydrostatic is when the action of moving the bearing causes oil to be pushed into the free space. Hydrodynamic is when you use an external pump.

Unless I got it backwards again

Hydrostatic lubrication is essentially a form of hydrodynamic lubrication in which the metal surfaces are separated by a complete film of oil, but instead of being self-generated, the separating pressure is supplied by an external oil pump. Hydrostatic lubrication depends on the inlet pressure of lube oil and clearance between the metal surfaces, whereas in hydrodynamic lubrication it depends on the relative speed between the surfaces, oil viscosity, load on the surfaces, and clearance between the moving surfaces.

From:

Types of Lubrication - Boundary, Hydrostatic, and Hydrodynamic - Bright Hub Engineering.

 

[eKretz]

The Cincinnati HydroTel milling machines used hydrostatic ways as I recall also. Very stout machines. Wasn't real fond of the hydraulic positioning though. Could be a bit erratic at times on the ones I ran. May have been lack of maintenance though.

 

[Tyrone Shoelaces]

Hydrostatic lubrication is essentially a form of hydrodynamic lubrication in which the metal surfaces are separated by a complete film of oil, but instead of being self-generated, the separating pressure is supplied by an external oil pump. Hydrostatic lubrication depends on the inlet pressure of lube oil and clearance between the metal surfaces, whereas in hydrodynamic lubrication it depends on the relative speed between the surfaces, oil viscosity, load on the surfaces, and clearance between the moving surfaces.

From:

Types of Lubrication - Boundary, Hydrostatic, and Hydrodynamic - Bright Hub Engineering.

That’s the system I’m talking about. I remember the pump running at 650 psi. When it was running properly it worked a treat. The elements moved very easily, no “ stick - slip “ when moving, but very rigid under cutting forces.

When you changed axis to move the column it lifted 0.001 “ perfectly.

Regards Tyrone.

 

[5 axis Fidia guy]

I've watched scraping, I understand the idea with scraping, I understand the original question with scraping. I just don't personally get it. I have blued of shutoffs on hundreds of plastic injection molds in my day, so I understand a good true surface, it just seems to me you could surface grind something, add oil grooves or dimples and get the same results, only it just does not sound as cool as the scraping sales pitch. With todays modern equipment, I don't believe for an instant that a hand scraped surface is better than one that is properly surface ground.

 

[CarbideBob]

One reason could be that you need big grinders to grind ways.
Imagine grinding the bottom ways of a 18, 24 or 48 inch Blanchard.
Refitted a 10x24 surface grinder from hydraulic table feed to ball screw.
Needed a true surface to mount the screw's bearing block. Area in question cast with a 3 degree slope.
Option one. Pull the bottom table and take it to some with a nice mill and have it cut and hope it okay.
Option two. Grab the scraper blades and go at it.

Oil grooves and dimples do not work so well so I guess what would be called flaking is still needed if not a pressurized system.
A problem with pressurized is that you need this pressure on top and bottom. This is as much fun as designing air bearings with the added need to recirculate the fluid.
If only top that table floats at first and then drops during use once moving.

A mirror finish surface plate will not work very well.
If you have a grinder to handle the size grind in and then add the surface.
Bob

 

[PDW]

A mirror finish surface plate will not work very well.
If you have a grinder to handle the size grind in and then add the surface.
Bob

Very true. I had some used rectangular cast iron blocks precision ground back to dead nuts right angles to use as masters. They were shit for checking/transferring blue to the workpieces.

PDW

 

[MCritchley]

I've watched scraping, I understand the idea with scraping, I understand the original question with scraping. I just don't personally get it. I have blued of shutoffs on hundreds of plastic injection molds in my day, so I understand a good true surface, it just seems to me you could surface grind something, add oil grooves or dimples and get the same results, only it just does not sound as cool as the scraping sales pitch. With todays modern equipment, I don't believe for an instant that a hand scraped surface is better than one that is properly surface ground.

The answer is simple- wringing.

No scraped surface is remotely flat, it’s thousands of facets that’s high points are rounded off with a stone. Eventually the surfaces do wear in and stick slip (wringing) increases. Look at animations of a kingsbury bearing and the wedge of oil, it’s the same principle just thousands of wedges.

I don’t know if you’ve had the opportunity to run a tenths indicator over a well scraped surface, not many machinists and mold makers have. It’s fascinating to say the least. I had no concept of the art until I spent ten years messing around with scraping.

A ground surface even with grooves or flaked will still have a lot of wringing potential. Let’s say if you tried to remove scraping and try a ground interface on slide would it stick? Yes it would, oil a surface plate and do a comparison with a pull scale between a ground and grooved surface and one that’s scraped. A 1-2-3 block hardly breaks free and a five pound 5x8” scraped reference moves with ounces of force.

And let’s talk about a very common machine in mold shop, the Mitsui grinder. The cross feed ways are scraped and wear well and will not lift while making small movements. If any company could grind a machine component it would be a grinder manufacture. And as said before metal is really flexible, if you were to grind the surface grinders cross slide it would warp when you flipped it over to match the machine base. So if you have this warped part you will have to put it back on a grinder to try and tweak it. Or you can just scrape the geometry and get perfect contact and adjust a tenth where you need to. Mitsui’s are high precision machines and scraped for a reason.

 

[MCritchley]

Very true. I had some used rectangular cast iron blocks precision ground back to dead nuts right angles to use as masters. They were shit for checking/transferring blue to the workpieces.

PDW

When I scrape a reference surface, I’ll scrape deep so I have a reservoir for blue to wick onto the part being scraped. The depth also cuts down on wringing.

 

[EPAIII]

You say, "A mirror finish surface plate will not work very well."

Care to elaborate on that? Why not? Optical flats are used. They are even better than most surface plates. If this is due to stiction, then why does a surface plate need to be free of stiction? It is a reference surface, not a working surface, like a way.

One reason could be that you need big grinders to grind ways.
Imagine grinding the bottom ways of a 18, 24 or 48 inch Blanchard.
Refitted a 10x24 surface grinder from hydraulic table feed to ball screw.
Needed a true surface to mount the screw's bearing block. Area in question cast with a 3 degree slope.
Option one. Pull the bottom table and take it to some with a nice mill and have it cut and hope it okay.
Option two. Grab the scraper blades and go at it.

Oil grooves and dimples do not work so well so I guess what would be called flaking is still needed if not a pressurized system.
A problem with pressurized is that you need this pressure on top and bottom. This is as much fun as designing air bearings with the added need to recirculate the fluid.
If only top that table floats at first and then drops during use once moving.

A mirror finish surface plate will not work very well.
If you have a grinder to handle the size grind in and then add the surface.
Bob

 

[PDW]

When I scrape a reference surface, I’ll scrape deep so I have a reservoir for blue to wick onto the part being scraped. The depth also cuts down on wringing.

Yeah - what I did. PITA it was too, the scraper tends to skate on the ground surface.

PDW

 

[EmGo]

A problem with pressurized is that you need this pressure on top and bottom. This is as much fun as designing air bearings with the added need to recirculate the fluid.

They are an absolute pain in the ass ... but they sure do work good

Do you know Andrews, C-Bob ? Your neighborhood. If you can finagle a visit, check out his cam grinders. Of course they are boring machines-inna-box but watch them grind. Or listen, you can't see much through the chaos inside. Awesome.

 

[ss_user]

Scraping uses an optical flat for reference. good or bad depending on the skill of the scraper but measurable by the number of fringes. It provides space for oil but also reduces the contact area for friction and therefore stick-slip.

 

[Shuttle Guy]

That's some pretty work.

 

[Devmeister]

Scraping

Scraping is old school and a skill in itself. Old machines had iron way systems that were not hardened. No hardened steel ways like my leblond. No flame hardened ways like my 10EE. Ways like you find on my old Myford ML7. Many old machines off eBay are done! I get one every once in a while knowing I bought a casting kit.

Hardened ways you grind. Old ways you scrape. I am working on an old south bend 9. What can I say I love lathes!!!! it will be scraped and the other parts as appropriate either turcited or brass-ed. Then scraped to fit. I learned brassing from an old machine guy in England. I actually prefer it to turcite. Either way this toy will be tight as a vestal virgin and outlast my days I have left.

My HLV is ground tool steel. Can’t scrape that. My Wadkin PK table saw ways are cool. The sliding table has its own ways on which the bearings run. Wadkin was BAD ASS at green lane works! They attached hardened steel strips with special steel bolts. They break off at a certain torque. Then the whole thing was precision ground. If you shine a glancing light on the ways you can barely see the remains of the bolt circles. Hairline circles. Dead flat!

Hardinge did the same. The T 10 lathe had these type of ways as did the TM/UM mill. These will need to be ground if needed.

Oil pockets are not easy with grinding. The frosted appearance of scraping is just cool.

I have a beat up Oliver shaper. Table needs work from the YawWhos at Boeing. Originally it was ground. I might just send it to nortfield for planning. Planned tops on wood shapers are nice to work with.

So there you go…. Lots of info and no one right answer.

 

[EdDanzer]

When scraping is used for retaining lubrication it will also retain abrasives when they go past the way wipers or way covers. When scraping is being used for alignment, there is a chance if an overload condition happens there will be permanent deformation and things will come loose or deflect. Having had several machines with Turcite and still owning a few with Turcite I will take ball or roller linear guides if possible. We must adjust the gibs on all the Turcite machine regularly to make good parts.
How many machine tools use plain bearing headstocks? Almost none because there must be running clearance in all plain bearing systems. That means box way machines cannot be as stiff as a linear guide machine if properly designed. Properly designing and fastening linear guides to makes them more rigid is challenging and expensive.
With today’s CNC controls it is easier to map out small inaccuracies than physically take them out of the parts used in a machine tool.

 

[dian]

three things:

1) some schaublin beds were ground
2) components are often scraped out of flat (to compensate for sag and load), e.g 20µ on a reiden beam
3) grinding introduces an awfull lot of energy (be it with coolant or without) into the part with uncertain consequences for long term stability

 

[J.S.H. Products]

I would like to scrape in my large lathe bed to repair damaged areas. Also the cost of shipping the lathe to American so it could get done professionally is more than the lathe is worth. In my opinion this is one reason why scraping would be done rather than grinding. The biggest problem I have is the lathe bed is 6m long so getting a straight edge and a reference plate that long is 1) hard to find 2) the cost is scary if buying new


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[Demon69]

I would like to scrape in my large lathe bed to repair damaged areas. Also the cost of shipping the lathe to American so it could get done professionally is more than the lathe is worth. In my opinion this is one reason why scraping would be done rather than grinding. The biggest problem I have is the lathe bed is 6m long so getting a straight edge and a reference plate that long is 1) hard to find 2) the cost is scary if buying new


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Thankfully you wont need a 6m straight edge (do they even exist?), just a reasonable understanding of what you're doing will be enough.

 

[ValkmanFP4]

Thankfully you wont need a 6m straight edge (do they even exist?), just a reasonable understanding of what you're doing will be enough.

I was about to write this. Richard King has written about this problem earlier.