11 Attachment(s)
From Tractor Weight to Inspection Square
Here is a little project I have been working on, on and off in between other things since Joe dropped by. Joe was kind enough to give Ray and I some cast iron blocks from a chopped up tractor weight. The weight is labelled IHI?
Attachment 263751
(IHI tractor weight section approx 240mm x 350mm x 39/49mm)
EDIT** (Ray tells me it should be IH for International Harvester not IHI)
Not having regular access to nice chunks of iron I decided one piece would be set aside for a Repeat'O'Meter and another one to be made into a handy 200mmx200mmx35mm Inspection square. The treatment of the inspection square will be the topic of this post.
First off was to get the block down to rough dimensions, this was done on the shaper with a mind that it would be a good test to see what the old girls dynamic geometry is like over a reasonable work envelope. I was very interested in how this would go since I have only just recently rebuilt the vice. I did not get any photos of this as the camera went missing for a few days. The result were ok. After spending the time to precisely tram the vice to the ram (to within 5-10 or so micron vertical and horizontally) the finish cuts were straight to and parallel to within 50 micron (I conclude that the ram is in good nick) but it was bowed in the centre by about 200 micron (saddle need a little tickle with the scraper).
After that I put it on the mill to quickly square up the sides and get it ready for the surface grinders. During this process and on the shaper there was a hard skin that I though may have been case hardening. It cut fine on the shaper as long as the cut was bellow the skin. The Mill carried on a little more but got there in the end without too much complaint once again as long as the cut was bellow the skin.
Then onto the surface grinder for final clean up of the block before went into the heat treatment oven for a graphitizing annealing cycle. I wanted to conduct another experiment on this piece as it is a similar size to the Master Squares that I'm getting cast. The experiment is to see how much dimensional change there is before and after a heat treatment cycle. So I ground the block square and parallel on all six faces to better than 5
micron.
Attachment 263752
(Partially ground plate)
Attachment 263749
(here is some iron in the furnace being heat treated current temp around 750C)
The graphitizing annealing cycle is at a high enough temperature (870-955C) that it breaks down the carbide in the iron to graphite (or pearlite). Once the iron has been held at 955C for a hour per inch of thickness what you can do with it then is a matter of choice. I preferred to go for a stress relief cycle with a balance between wear resistance and machine-ability. This means a slow controlled cool in the furnace down through the transition range at no more than 100C/hr to 290C. I ended up leaving it in the furnace till it was bellow 100C and then air cooled it on the workshop floor sitting on a couple of ceramic spacers. When it was cool enough to handle many hours latter is was surprised to see how much it had moved in the furnace. Some of the dimensions had moved as much as 500 microns with 70-200 microns out of square being more typical and it had also warped by 600 micron, This is much more that I though it would have moved. This was final proof that the hard skin was not a case hardening but rather it was chill iron.
After heat treatment it is onto the mill to cut out some excess weight. A 150mm dia circle. Small tabs were left on the block so that it would not drop down over the small rotary table before the cut was finished.
Attachment 263741 Attachment 263740Attachment 263742Attachment 263743
(Milling out the 150mm dia excess (looks like pulley stock to me)
Then some more weight taken out in the form of a 12.5mm square pocket on each side.
Attachment 263746Attachment 263744Attachment 263745
(Pocket milled both sides)
Next it was over to the surface grinder to take out the bow. My target was to have the two large sides flat and parallel to better than 5 micron, in the end it turned out a little better at 2 micron, but I did have to take a lick off the chuck to get that level and most of it is actually better with just one corner sitting a little proud, but I was not wearing gloves when I was measuring it so that might just be a little body heat. Either way it is close enough to what i was aiming for.
Attachment 263748
(measuring for flatness)
The next step will be to grind the edges, I'll be looking to get less than 2 micron in square both ways and 2 micron in parallel. This would put it well into the A grade status on a 200mm x 200mmm x 35mm box square.
I have also come up with a colour scheme for my shop made measuring gear and fixtures.
White for master standards
Yellow for inspection fixtures
Red for Precision fixtures
Black for General purpose fixtures.