Gallery, Projects and General > How do I??

How to measure "square" for scraping? Angle plate

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PekkaNF:
That is very correct. Sometimes I'll get too much into the process itself, because I want to learn stuff before I actually (and often desparately) need it.

Angular error is a killer, as you note it's possible to reach good accuracy, but it is much of a mental process as well. I had very small accurate square and all shop squares tested differently. It was a pain to work with them and I know that there are ways to true them, but they were different sizes and different models. I needed a standard I could trust to some degree. Threrefore I bought two bigger and identical pretty accurate squares to DIN standard and class #1. I tried them and they are nearly identical

I can tested this angle plate with them and surface plate using feeler blades. I can "feel" the hills/valleys, but can't really put number on it. Here it is not much, but for rudimentary "mapping" it would be nice to get a value.

My lathe is not that accurate, it needs adjustment during turning, therefore I need to figure out which parts to make and in which order to get acceptable results. I have noticed that as soon as I move tail stock on the ways, it moves sideways too a little unpredictable way. When I know more about this subject I will fix the actual problem, but I'm not too keen on much of the quick/dirty methods (like much of the "in situ"  lapping on internet).

Pekka

 

PekkaNF:
I digged out my souvenirs from last trip to USA: Brayer and Dykem spotting blue.

Dykem tube had seeped out a little even before I opened it. It was a bit lumpy and harder to work that Stuart mictometer blue. Then again I don't know much this stuff.

This time I used cast iron surface plate, because:
1) Now I can keep it clean and I know it is good
2) Cast iron palte is bigger and it apears to be good, but I haven't check it all over yet.

The suspect angle plate does not rock, but it does not feel "tight" against the surface plate.

I checked it with with square and feeler blade, can feel it is not all straight and it feels like it has a dip in the middle both sides.

Then I spotted it twice and got similar results, although I'm not confident about it, I clearly can feel and see that spotting needs some practice. I'll lower it down on it's fulcrum then lay it flat and move 3" back and fro.

I'm not sure how much pressure to apply.

Also I need to think which high spot to start. Not onlyt to get it flat, but also square.

Pekka

chipenter:
It's own weight will be enugh press to hard and you can bend it , to get it square a cylindrical square and a back light will work .

sparky961:

--- Quote from: PekkaNF on October 21, 2015, 02:48:32 AM ---<snip> I have noticed that as soon as I move tail stock on the ways, it moves sideways too a little unpredictable way. <snip>

--- End quote ---

You should be able to turn the outside of a cylinder square without moving the tailstock on the ways.  Set it up with a live center in the tailstock, or slow speed with a greased dead center if that's all you have.  You could set it up between centers but you still need to drive it somehow.  Better would be to hold some extra length in a 3 or 4-jaw that you'll discard later.  Then take successive cuts dialing in your tailstock alignment using the adjustment screws your lathe hopefully has. 

You may have to loosen the chuck jaws while dialing in the tailstock to let the work move around.  But once you get the diameter consistent along the length you don't want to do anything except take your finish cut.  I also suggest letting it cool to room temperature and measuring before your finish cut and before you take it off the machine.

Depending on what material you have access to, starting with CRS (cold rolled steel) or TGP (turned, ground & polished) will let you get things really close with an indicator before even taking the first cut.

Pete.:
You don't need a large micrometer to measure the cylinder either if you're making a larger diameter one. Take a normal scribing block/surface gauge and loosen the clamp, then push the rod through so that it projects downwards instead of up. Now set up a tenths-reading DTI on that rod to read across the diameter of your part with both the base and the rod touching on the OD and slide it along to check for taper.

Don't forget to break the corners of the cylinder to help prevent burrs being raised in handling.

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