Another grinding question.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Clive,

I agree you need three rough surface plates to finish up with 3 excellent finished ones. However I have one good and one rough plate. So you just scrape the second plate using the good one as a reference.

The method of doing the three plates is told in the excellent little book from Tee publishing, it is called Elements of Fitting, or some similar title.

Andrew.

[SteveIParticipant @stevei]

Andrew,

You have received a lot of good advice in this thread.

You don't need 3 surface plates to make an excellent result provided that you have a master plate that is good enough. You need 3 plates and a lot of time and effort if you have no confidence in your plates.

You say you have a plate that is good? Is it? How do you know it is good? Have you measured it? In my limited experience if you don't store your cast iron plates at a consistent temperature, (same temperature it was ground or scrapped at) and store it on 3 points, (same three points that it was scraped or ground resting on) you are very unlikely to have a "good" plate. Furthermore did the previous owner of the plate do that?

For the purpose of this thread I define "good" as a surface plate that is flat enough to be used as a reference master plate to scrape another item to. I.e. it needs to repeat so well, that when you blue up you get consistent results. Otherwise you are in for a long and frustrating process.

I have found that to make the process efficient the very best quality granite plate is needed. I suggest granite as you have not confirmed you have a temperature stable workshop to store a large cast iron plate at consistent temperature. Granite is more stable than cast iron. Having said that you should try and keep the room and hence the granite temperature consistent as well and scrape your parts at the same temperature as they will be used. 19 deg or so 24/7 365 is recommended, and keep all direct sun light off the equipment and parts.

If indeed you have a "good" plate stored correctly then just get on with it. If you are roughing in with push scraping you should be scraping to a depth of 0.0002” – 0.0004”. Any shallower and you need to push harder. If you measure your dings, rust pits and any low spots you can estimate how many passes you will need to scrape it out. Shims are your friend and hinging the part will tell you how flat it is and how you are progressing.

If you are just getting started in scraping then just get on with it. Practice practice practice. You have commented on your arm strength. Many professionals and enthusiastic amateurs will place the handle of the scraper in to their stomach/chest and hold the end with 2 hands when roughing and scraping lines. They push with their bodies by rocking the legs. As well as being less tiring this also has the advantage of more control. You also want a bit of spring in your scraper otherwise it is much harder work. The sandvik scrapers are too stiff in many peoples opinion. Much to hard work. If you have one consider to get it thinned out so it is more springy. A friend with a mill can do that for you.

If you are competent and trying to get the plate finished then get it milled, planned, shaped or ground, why scrape for the sake of it? "If it is more than a mm back to the mill!" If ground ask the grinder for a rough dressing of the wheel. and you'll need to rough scrape initially to break up the surface. Ground surfaces do not blue up as nicely as scraped. Having said that on such a small plate why waste a grinding wheel. As has been said already it will be your scraping that determines the final surface result and nothing to do with the initial machining operation.

>>

In case you are just getting started:

1. Scrape it flat within 0.001" minimum. 2 tenths min depth of scrape. Before you move on to improving the PPI and % bearing. Since this is a surface plate I would scrape it flat. within 0.0005". That means no holes! Most likely by that point you are at 10-20 PPI and 50% bearing.

2. Scrape it to a higher PPI and 50% bearing if you so wish. In days of old a surface plate would have been scraped to 40-60 PPI and 50% bearing. Even on a small plate like yours that is a fair bit of work.

These days granite is the material of choice for many good reasons. Although for the hobbyist it is not inexpensive, if you want to scrape more I strongly recommend looking for a good granite surface plate. My piece of granite came out of a scrapped CMM. There are deals out there and granite plates can be reconditioned by the enthusiastic amateur and professionally.

Good luck

Steve

[Andrew TinsleyParticipant @andrewtinsley63637]

Steve,

Thank you for taking such time and trouble in answering my queries. I think it a model reply and only wish there were more like you!

The good plate, I am confident has been stored as you suggest. I will be keeping it in a highly insulated garage that is part of the house. It is heated and has a thermostat that maintains temperature to within 2 degrees centigrade of nominal even in the hottest of weather. I have spent some time at getting this correct, as it is essential for some plating work that I do, I prefer ambient to a heated system. I take on board your warning of temperature stability. Getting a granite surface plate is beyond my financial means, they seem to be in the thousands of pounds range! Just as an aside, would granite that is used for kitchen surfaces be a cheaper way into getting such a surface plate? If so, how would one scrape it? Or more likely what other method would you use?

Initially I would like to get the plate as flat as possible, before scraping. This to reduce the work load because of my medical condition. Thanks for the detailed instructions, I have learnt something new from them. I have scraped before and I have some specially made Sheffield scraping tools (not old files ground to suit!). I have been practicing to hone (!) what skill I previously had. I find it to be quite exhausting, but well worth the effort!

Scraping is rapidly becoming a lost art and I would like to refurbish my old plate and keep the skills alive. So I have an ulterior motive here! I have plenty of time and aim to produce a finish that is the best I am capable of. The initial query about surface grinding was simply to get rid of the imperfections thus allowing me to have to scrape less for a given finish

Thanks once again for such an excellent reply.

Andrew.

Edited By Andrew Tinsley on 09/05/2017 13:07:16

[Martin KyteParticipant @martinkyte99762]

Granite surface plates seem to be a little cheaper than you think.

**LINK**

**LINK**

rotagrip show a 3 foot by 2 foot by 4 inches plate for a lttle under £300

regards Martin

[SteveIParticipant @stevei]

There is a very nice video explaining how to lap in a worn granite plate on youtube. oxtools if I remember correctly. I know someone that has done it. He made a lap by scraping in an old cast iron surface plate to 40PPI 50% bearing from a good granite master plate. He measured the local variance of the worn plate before he started and yes that needs not inexpensive equipment to do properly. I am not sure what rating the plate was afterwards but he made it better than it was.

I only scraped cast iron so far. I use a carbide blade. 5deg neg rake. with a nice radius to avoid scratches. If you are new to it make it round like the end of your finger nail. I use a 260 grit wheel to rough it then a 600 grit wheel and finally a 1200 grit diamond wheel. All going slowly maybe 500 rpm.

[Andrew TinsleyParticipant @andrewtinsley63637]

Thanks Martin,

Last time I looked 2 or 3 years ago, they were a lot more expensive. Maybe I will finish up with a granite one in the long term.

I once surveyed the Northern Exploration granite quarry on Spitzbergen. It folded in the early 1920s when they found that the beautiful granite blocks they excavated were actually fissured all over the place. It was only held together because it was frozen solid. When they tried to unload it in the UK it all fell apart!

If only they had gone down another 3 feet. That stuff was fine! It was the site of the most northerly railway in the world (not the coal railway at Ny Alesund). It was built in the mid 1890s and was to seven foot gauge. There are some choice pieces of rolling stock still left there.

Andrew.

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