Gallery, Projects and General > How do I??

lapping plate?

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shipto:
I was watching a few youtube videos last night and saw one where a guy cut up a lump of cast iron and put some saw cuts in it for use as a lapping plate and thought surely for the price they charge for them there must be something... well more.
So while at work today I looked through my private stash of handy bits of metal and found a lump from a fire proof motor which had some holes I would perfer not to be there and thought I would give it a go and I have to say it seems to work well.
So this is more a am I missing something obvious here? below are pics of the plate which I would have liked to be bigger but its what I had and one of the lapping I did with it including some red marked areas where it wasnt doing anything.
Just want to make sure I am not making my mill head slide unusable really.

awemawson:
Lapping plate needs to be as flat as you want your  finished item to be. Google the 'three plate' method of lapping dead flat items such as surface plates. With only two the danger is of producing two identically dished items

shipto:
so I assume a shop brought one would need regular regrinds.

PekkaNF:
Therefore there are surface plates. You only need one that is prooven/known/believed accurate (usally way more accurate than needeed....) then you use that to spot the suspect.

Three plate method is known and it works, but it is slow and needs disipline. Having a surface plate as a reference is great. I bough my first surface plate from GB, it cost 29£ plus more to ship here....it is 250*300 mm wide and supricingly level. Important thing is to maintain it true, avoid abrasion (no abrassive paper/lapping/sin a top of surface plate), avoid denting and using it a general work top.

Laping plates needs truing every now and then...after you spot it using a true surface plate it is so much easies. Lapping plates do not need to über straigh just a straight enough for work. Every time you use lapping plate you change it - if you have a habit of using more front of it you wear front down. If you make efort of using alternative parts of it and turn it every now and then it thends to stay more stright.

Just don't get too paranoid about the accuracy with surface plate it is easy to spot innaccuracy tht has no consequence.

Pekka

shipto:
Yes I think paranoia is the biggest problem I have tested the work I did with this plate against the straightest thing I can find in my shop which is the slide on my decent vernier and even though it looks good to that I still can't bring myself to trust even though it probably better than I need for this.

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