Ideas On Correcting Fixture Index Marks.............

[JoeLee]

I finished squaring up the swivels or trunnions as I believed they are called. One was out of square by .003 top to bottom in 3"
I also noticed that the surface was sort of concave, I'm not sure if that was intentional or just error in the factory machining.
The fixture in the picture on the grinder took care of all that error. But what I've found and I knew this prior to grinding that the index marks are all slightly off. Since I can't grind them out or fill them in and re-scribe so the only way to bring them in would be to grind the horizontal surface of each swivel. By aligning the marks and indicating the error I found that it would involve grinding about .018 on one side of the horizontal surface, way too much to play with to get a mark to line up. Since the fixture is graduated in increments of 1 degree I figure it should at least be more accurate than it is. Any suggestions?????

JL....................

[Old Hat]

I'd shallow out the Target mark, and rivit in a peice of thick brass shim.
then re-mark the target.

[oldtiffie]

Just "draw" the index mark over - with a chisel - as you might draw over a drilled hole that had "wandered off". Its easily and quickly done and is quite effective.

I would certainly not use that index mark or the spaced "degree" mark with confidence in their accuracy at all.

I've checked mine and I'd be surprised if the degree marks were not "hand marked (chisel?)" - so it might be a good idea to take a comprehensive check of a good representative selection of those marks too.

I'd use them as a guide and make it more accurate by using a good analogue or digital protractor referenced to (ie :"on") the grinder table as they are calibrated to 0.10 degrees (ie 6 minutes of arc) = 1 in 570 (0.0017" per inches and can be read as fine as 0.05 degrees if needs be.

[JoeLee]

If what you mean by "draw the index mark over with a chisel" is to try to push the mark over would surely cause the area to crumble since that is the nature of cast iron.
I know that these marks should be thought of as only a rough reference and I do use a precision protractor and surface plate for almost all my set ups.
I've also found that even if the marks are lined up as close as the eye can tell there is still plenty of room for error.

JL...................

[caveBob]

I'd shallow out the Target mark, and rivit in a peice of thick brass shim.
then re-mark the target.

Ditto that... or maybe a small section of a hacksaw blade with the teeth ground off.

[RichR]

Hi JoeLee
I think Old Hat has the right idea but I'd use screws instead of rivets to ensure you can get the mark precisely aligned before locking it down.

[oldtiffie]

If I was that concerned about the "zero" I'd drill and ream both parts (for a sliding fit) and use a precision pin or dowel to locate the zero and if you are concerned for accuracy - leave the "punched" (I doubt that they are precision engraved) lines on the brackets alone when accuracy is required.

If I really needed "accuracy" from the "lines" I'd use this bracket which came with one of my grinders:

http://i200.photobucket.com/albums/aa294/oldtiffie/Grinder_work-head_demo/Grind_work-head_demo2.jpg

Or this:

http://i200.photobucket.com/albums/aa294/oldtiffie/Grinder_work-head_demo/Grind_work-head_demo10.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Grinder_work-head_demo/Grind_work-head_demo1.jpg

For "ordinary work" I'd use this and a good protractor.

http://i200.photobucket.com/albums/aa294/oldtiffie/Grinder_work-head_demo/Grind_work-head_demo3.jpg

[goodscrap]

If cosmetics bother you open out both of the hole up and across and re- position the castings relative to each other, then make an oversize stud with plain body for location, with the same size thread so the same spanner size is maintained.

or make an eccentric stud?

Brian

[Forrest Addy]

I like Old Hat's idea of relieving the index surface the thickness of an index plate and endorse Joe Lee's proposal to attaching the plate with screws. I amend their good suggestions by suggesting in turn the plate be made +/- 1/32" adjustable via elongated screw holes. You don't normally need all that adjustment but you may wish to re-orient for slightly out of square stock. The adjustable index allows accommodation.

Or heck, get fancy: make the plate a +/- 6 minute vernier. Calibrate it with a height gage and a good scribe.

[JoeLee]

If cosmetics bother you open out both of the hole up and across and re- position the castings relative to each other, then make an oversize stud with plain body for location, with the same size thread so the same spanner size is maintained.

or make an eccentric stud?

Brian

I was thinking along the same lines as you and Forrest about modifying the stud. My thought was to run a tap through the threaded hole in the one end of the fixture to make the hole a little over size and after snugging up the stud tap it with a brass hammer to slightly knock it in the direction of desired off set. This would shift the whole part over with out risking any damage. To re-bore the center hole and make an over size or eccentric stud would be a lot of extra work. Reaming the threaded hole a few thou over and turning the threaded part of the stud slightly may also give me the needed adjustment room. I think I'll try that.

JL..................

[Baz]

Out of interest please can you give the diameter of the part with the scale. It looks to be quite fine graduations. I find anything less than a mm between marks to be quite difficult as the lines get to be as wide as the spaces.

[Toolguy]

Just mill off the current index mark and remark it where you want it. It would only take a very shallow cut to remove the original, then you would have a nice background for the new mark.

[lane]

I have found over the many years ,of machine work.Most of the gradation on any but very top of the line tooling is not exact. Fix it are live with it. Grind out the witness mark and re mark.

[oldtiffie]

I agree absolutely with Lane.

Most of the applications for these types of set-up are OK if the angles are within say +/- 2 degrees - perhaps +/- 0.5 degrees for screwing tools.

I usually (always?) use off-hand grinding on a normal shop pedestal grinder with a protractor if needed pretty well all the time - and so my "good" tools remain in the drawer/s.

[JoeLee]

The diameter of the swivel is 3".
The graduations aren't as fine as you might think. Actually if compared to say the dials on a BP mill there are quite rough and crude.
As Lane and others have mentioned........ live with it. The witness mark is almost .030 deep so if I milled it out there would be a flat spot well below the graduated part and that would make aligning difficult as well. I always set this fixture up on the surface plate with the protractor if I want real accuracy.
Besides I've also noticed that I can adjust one side of the fixture .010 high and looking at the marks you can't even tell that they have moved. So striving for that kind of accuracy with this isn't worth the effort. The best option would be to make an off set stud to shift the entire part over and that is exactly what needs to be done as I can see it on the edge where the two pieces mate. KO must have been off on their center line when they bored the hole.

JL...................