Thread: Surface Plate Substitutes

The Cross Slide scraping thread from Stuart is going strong and I have spent some of today browsing other forums and individual sites just to pass the time as well as providing info for my bandsaw update post. I came across this post What is flat in the home shop? - The Home Shop Machinist & Machinist's Workshop Magazine's BBSregarding what is flat in the home shop. I found it an interesting read as I have issues regarding my cross slide and wear. I am not sure if my lathe is able to be fixed or not (by me at least due to the amount of wear. This thread is on the issue of what can be used instead of forking out the money for a surface plate. The engineering firm I used to work for had an inspection table I took for granted initially until I was told they were very lucky to pick it up cheap (relatively). I have no recollection of price but from memory it was a surface table about 4 ft X 8 ft. Seriously heavy with massive cast legs etc.

Anyway if you have read my post on http://www.woodworkforums.com/f65/mi...6/#post1493766 one of the things Wilf told me was that mirrors are made very flat and if they are bedded carefully in plaster they will provide a useful surface plate. I would just like to throw this out there for comment. This was quite a long time ago.

Dean

This topic has been discussed in many forums to exhaustion.
Glass is not suitable for marking from - I've tried and gave up long ago. Secondly, glass is not generally flat enough for the purpose.
A certified small granite plate is $52 at Carbatech at the moment - thick enough to be useful glass would not be much cheaper without any indication of flatness. Bigger than 300 x 250mm would not be considered in glass anyway.
Cheers,
Joe

I would like to know the answer to this one also.................................
Matt

It all depends what you are trying to do.
The problem with glass is that it may well be flat.......... but then again it might not be. generally it is very flat locally.* I dont know why mirrors would be any flatter.. its made the same way and I think you'll find its the top that it silvered which is the flatter side.
If its a choice between glass and the concert floor I'd go with the glass.

As Joe says $52 is pretty damn cheap.

Stuart

*I assume we are talking about float glass here. You can get glass surface plates. and then there are optical flats which are another level of flat.

I've got a booklet called Around Wilf's Workshop I picked up years ago at an engine rally. It's a collection of Wilf Manning's old school tips, tricks and techniques for general restoration and engineering and I reckon I have read through it dozens of times. Among the many gold nuggets of information is his plans for building a surface plate from glass.

Now I know as well as the next bloke that your run of the mill piece of glass can not be considered truly flat and would not think to compare it to a granite surface plate, but I think sometimes people get a little carried away when it comes to what is ideal and what is practical.

For myself, I have used an old piece of mirror with wet & dry paper as a flat for hand lapping mating surfaces on old engines for years. This was a tip given to me by an oldtimer when I was a kid building go karts using old lawn mower engines and it has served me very well ever since.

Now I might be shooting wide of the mark here and sticking my head up for a flaming but my guess would be that only a minority of people using a surface plate really need the level of precision offered by a quality granite plate. Don't get me wrong here, if you are restoring a machine tool such as is being done by a few blokes on here then yes that level of precision can be essential in order to produce a quality output when the finished machine is being used. But if you are restoring say an engine built in 1920 with tolerances when new of +/- a few thou at best then poo pooing the idea of using glass for hand lapping mating surfaces is to me a bit misguided and misleading.

Aiming for as close to perfection as possible is very admirable but I think it is easy to get carried away and loose sight of what is not only practical but realistic. A mate of mine is an oldtime mechanic and he has restored engines that have spent fifty years at the bottom of a river to pristine running condition. His main workhorse is a flogged out lathe from the 1950's but his most powerful tool is his engineering aptitude and yes, he too uses an old mirror for lapping.

I love precision tools as much as the next bloke and freely admit to being somewhat of a tooling snob but all I would say when considering something like using glass as a surface plate is think more about how you intend to use it before you think about how precise it really needs to be.

Cheers,
Greg.

Originally Posted by kwijibo99

I've got a booklet called Around Wilf's Workshop I picked up years ago at an engine rally. It's a collection of Wilf Manning's old school tips, tricks and techniques for general restoration and engineering and I reckon I have read through it dozens of times. Among the many gold nuggets of information is his plans for building a surface plate from glass.

Now I know as well as the next bloke that your run of the mill piece of glass can not be considered truly flat and would not think to compare it to a granite surface plate, but I think sometimes people get a little carried away when it comes to what is ideal and what is practical.

For myself, I have used an old piece of mirror with wet & dry paper as a flat for hand lapping mating surfaces on old engines for years. This was a tip given to me by an oldtimer when I was a kid building go karts using old lawn mower engines and it has served me very well ever since.

Now I might be shooting wide of the mark here and sticking my head up for a flaming but my guess would be that only a minority of people using a surface plate really need the level of precision offered by a quality granite plate. Don't get me wrong here, if you are restoring a machine tool such as is being done by a few blokes on here then yes that level of precision can be essential in order to produce a quality output when the finished machine is being used. But if you are restoring say an engine built in 1920 with tolerances when new of +/- a few thou at best then poo pooing the idea of using glass for hand lapping mating surfaces is to me a bit misguided and misleading.

Aiming for as close to perfection as possible is very admirable but I think it is easy to get carried away and loose sight of what is not only practical but realistic. A mate of mine is an oldtime mechanic and he has restored engines that have spent fifty years at the bottom of a river to pristine running condition. His main workhorse is a flogged out lathe from the 1950's but his most powerful tool is his engineering aptitude and yes, he too uses an old mirror for lapping.

I love precision tools as much as the next bloke and freely admit to being somewhat of a tooling snob but all I would say when considering something like using glass as a surface plate is think more about how you intend to use it before you think about how precise it really needs to be.

Cheers,
Greg.

Thanks Greg,
I was about to have my little rant about glass, but hadn't even considered uses outside of machine re-building.

The only thing i can say about glass, is that (and yes please tell me if i have been miss-informed) it is a solid but it acts like a liquid and slowly "flows" over time. A once "flat" piece off of float glass may not be flat after many years of use.

When i was cabinetmaking, we always used glass with silicon carbide powder and kero to re-face our oilstones when they became hollow or just clogged. For hand planes we always used a cast iron machine table and sticky backed paper, but i have heard of glass being used instead.

If i tried glass in my shed i'd probably break it pretty quick.

Ewan

Hi greg, your comments are both useful and agreeable. I've used plate glass and abtasive paper to level out may motorcycle cases and rocker covers in my time as motorcycle mechanic.
I also agree that a nice thick piece of ground glass charged with an abrasive (I used to use valve grinding paste) makes quite a reasonable lapping tool for hardenend surfaces.
The original poster however asked about a surface plate for scraping parts of his Mitsubishi lathe. That is a very different purpose: you need a referece surface that allows the application of marking blue which transfers reliably to a scraped surface for measuring or judging the depth of flatness errors - and measuring parallelity errors etc. Glass is simply not reliable enough for that, nor does it hold marking blue predictably. I'm convinced it would lead to enormous frustrations and eventually giving up or reducing the expectations of a reliable restoration of his lathe.
The only realistic alternatives are a cast iron surface plate or a granite surface plate - both with known flatness and precision .
Cheers,
Joe

For nearly 30 years now I have used a piece of glass to lap in the valves on boilers. In fact I will be doing just that today. What used to be a nice piece of glass is now a well frosted piece that I am sure has a nice hollow in it. Using a figure 8 pattern and lifting the valve and rotating 120 degrees ad nauseum works well and my valves have never leaked. What I do use glass for a lot is a marking out table and the accuracy is really quite astonishing. All I do is lay a towel on my new optically flat work bench and put the glass on that. When I went to the local glass bloke for a piece of the thickest glass he had about 2 foot square the bloke looked at me and enquired if it was for a marking out table. Seems I was the second person that day to come in for a piece. As a surface plate for blue transfer I am not so sure and agree with Joe on this point.

Phil

The key point for any reference surface is whether it is flat. That is influenced by whether it starts flat, how rigid it is and how well it resists wear.
Granite and cast iron surface plates start out as flat to a known specification, is thick enough to be pretty darn stable and usually wear very slowly. By comparison, glass is not supplied to known flatness and in the thicknesses readily available may flex enough to produce measurable distortion. (Phil's towel trick will help) I would guess mirrors are mentioned because to get a good image the glass needs to be flat.
Glass would be better than many things for a work surface but as a reference surface introduces too much uncertainty.

Michael

Originally Posted by kwijibo99

I've got a booklet called Around Wilf's Workshop I picked up years ago at an engine rally. It's a collection of Wilf Manning's old school tips, tricks and techniques for general restoration and engineering and I reckon I have read through it dozens of times. Among the many gold nuggets of information is his plans for building a surface plate from glass.

Now I know as well as the next bloke that your run of the mill piece of glass can not be considered truly flat and would not think to compare it to a granite surface plate, but I think sometimes people get a little carried away when it comes to what is ideal and what is practical.

For myself, I have used an old piece of mirror with wet & dry paper as a flat for hand lapping mating surfaces on old engines for years. This was a tip given to me by an oldtimer when I was a kid building go karts using old lawn mower engines and it has served me very well ever since.

Now I might be shooting wide of the mark here and sticking my head up for a flaming but my guess would be that only a minority of people using a surface plate really need the level of precision offered by a quality granite plate. Don't get me wrong here, if you are restoring a machine tool such as is being done by a few blokes on here then yes that level of precision can be essential in order to produce a quality output when the finished machine is being used. But if you are restoring say an engine built in 1920 with tolerances when new of +/- a few thou at best then poo pooing the idea of using glass for hand lapping mating surfaces is to me a bit misguided and misleading.

Aiming for as close to perfection as possible is very admirable but I think it is easy to get carried away and loose sight of what is not only practical but realistic. A mate of mine is an oldtime mechanic and he has restored engines that have spent fifty years at the bottom of a river to pristine running condition. His main workhorse is a flogged out lathe from the 1950's but his most powerful tool is his engineering aptitude and yes, he too uses an old mirror for lapping.

I love precision tools as much as the next bloke and freely admit to being somewhat of a tooling snob but all I would say when considering something like using glass as a surface plate is think more about how you intend to use it before you think about how precise it really needs to be.

Cheers,
Greg.

I agree whole-heartedly with everthing you stated here.
It boils down to the amount of precision that is required, and to put it bluntly, the amount of precision a good piece of glass affords with do what the average bloke is his backyard is trying to achieve.

Bruce must have been distracting me when the trick was demonstrated.
Any chance of enlightenment Michael?

Bob.

Phil's trick is in the post before my previous one.
All the towel will do is provide a uniform support under the glass. Provided the bench is reasonably flat the towel should allow the stiffness of the glass to exert itself and give a surface more likely to be flat (as that's how float glass is (or was) made).
If the bench has a dip or a bump on it and the glass is on top with no padding it will bend, lessening the flatness of the surface the glass presents.
Anything of a uniform thickness that has a small amount of spring should do something similar (thick felt, firm carpet, stiff foam rubber). Again, it comes back to the uncertainty of the surface - that is, how confident are you that the surface is 100% flat within the limits you want.

Michael

Before I became engrossed in metal working I was solely a woodie. Hand planes, chisels, etc etc. My planes were all flea market finds, lovingly lapped on a 20mm thick piece of float glass. Imagine my dismay when I spotted them on a granite surface plate. They were canoes. Bananas. Three dimensional irregular convexities. Crap, in other words.

I think that the act of lapping an object opens the door to inconsistencies owing to the simple mechanics of human motion. We are hardly jig-like in our swaying, stroking, oscillating attempts at transferring flatnes from one object to another.

Having said all that, I have come to regard glass as useless for serious accuracy with regard to the foundation quality that we call " flat". A kitchen granite benchtop is probably flatter. Certainly more rigid.

The big problem with glass is its flexibility. As I write I am sitting in a hotel in Hong Kong, watching the lobby windows distort in the brisk but not storm winds. We all have that experience of watching reflections in windows distort with ease. That should tell you all that you need to know about plate glass and the need for a dead flat support for it. (And if you have a dead flat support surface, why use glass on top?)

GQ

Originally Posted by Ueee

Thanks Greg,
I was about to have my little rant about glass, but hadn't even considered uses outside of machine re-building.

The only thing i can say about glass, is that (and yes please tell me if i have been miss-informed) it is a solid but it acts like a liquid and slowly "flows" over time. A once "flat" piece off of float glass may not be flat after many years of use.

When i was cabinetmaking, we always used glass with silicon carbide powder and kero to re-face our oilstones when they became hollow or just clogged. For hand planes we always used a cast iron machine table and sticky backed paper, but i have heard of glass being used instead.

If i tried glass in my shed i'd probably break it pretty quick.

Ewan

Hi all. Im not experienced or qualified enough to make comment on suitability of glass but i can say that glass is considered a liquid. A supercooled liquid to be precise. It does slowly flow over many years. In fact if you were to measure the thickness of a glass window in a very old house, it would be thicker at the bottom. However it is so slow that i don't think this property would be a factor in its use in this thread.

Cheers

Simon

Originally Posted by Greg Q

Before I became engrossed in metal working I was solely a woodie. Hand planes, chisels, etc etc. My planes were all flea market finds, lovingly lapped on a 20mm thick piece of float glass. Imagine my dismay when I spotted them on a granite surface plate. They were canoes. Bananas. Three dimensional irregular convexities. Crap, in other words.

GQ

I question the pursuit of sole flatness perfection GQ. The World is full of beautiful examples of the cabinetmaker's skill, executed with a wooden soled plane.

BT