Expectations Of Accuracy?

savarin

savarin

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Do I have unrealistic expectations?
I'm trying to machine a 100x50x20 mm block of steel out of a slightly larger piece using my 4 jaw chuck to hold it and registering of the face of the chuck.
Its to make a vertical slide so I can do this kind of work easier than at present.
The 50mm face is 0.2mm wider at one end and the 20mm face is 0.06 wider at one end.
I'm agonising over getting it exact and even over the both ends.
Looking at what I've written I just realised I havnt checked the chuck face to see if its flat so that may be one area of contention.
Of course if I had a mill I wouldnt have to worry about this as it wouldnt be needed.
Any suggestions that could help?
 
You're right about checking the face of the chuck. also, once you've done that, verify that the part doesn't move when it is clamped/chucked.
 
The face of most chucks I know of, are NOT a precision surface. If I wanted to do the work, I would check/true the chuck's mounting plate, strip the jaws out of the chuck, bolt it up and true the face of the chuck. Then I would mark the chuck and mounting plate for reregistering after disassembly. Not perfect, but it should get you in the ballpark.

I would do this on my mill. Do you know anyone with one from whom you could 'borrow' some shop time? :)
 
savarin said:
Do I have unrealistic expectations?
Click to expand...

This what grinding is for, send it to a grinding shop, if they do not meet your standards reject the part and have them do it again and if they can not do it send it to another grinder.
Eventually you will find a company that will do the job that you require.

Or just buy a l grinder for home use.
 
as a retired hobbiest there aint no way no how the minister for war and finance will release those kinds of funds.:laughing:
Plus I want to be able to say "I made it myself"
I think I must also check the jaws to see if they sit out when tight.
 
Well, will it work as a vertical slide with those dimensions? If so, you done good. That's how you can tell if your expectations are reasonable or not. You are doing pretty good considering you are using a tool that's less than optimal for that job.
 
Even if the face of the chuck is true, the jaws will have an effect on the position of the workpiece. If they are bell mouthed for instance, they will have a tendency to push the work away from the chuck face, but not likely to be even so you won't be able to cut two faces dead parallel.

If it's a 4 jaw, many can be used as faceplates with work clamped to them for facing. Naturally, regular clamping methods like used on the mill table won't allow you to face 100% of the surface, so you'll have to work around some limitations.

0.2mm is not much, and I presume you stayed Metric when you said 0.06. Again, not much. 0.2mm is about 0.008" 0.02mm is ~0.002". If you have a flat surface you can use to back up some relatively coarse wet or dry paper, it would lap down pretty quick. This may be a viable option, provided you are not restricted tightly to a finished size. With a little effort, less than fussing with the lathe, IMO, you should be able to hit flat, parallel and size.
 
A couple of things found out today.
1, the chuck face is out of square by approx 2 thou"
2, one of the jaw faces is approx 5 thou" proud, the other three being equal.
3, my face plate is out of square by approx 3 thou"
I think I can square off the face plate reasonably easy being cast iron then all the parts I want square and parallel can be screwed from behind onto the face plate for surfacing.
I take your point on the wet and dry Tony and will have to resort to that as well.
Its all the joy of learning, I cant believe I never thought of checking the chucks first :bang head:
 
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