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18th April 2013, 12:29 AM #1SENIOR MEMBER
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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?
IMG_1472.jpg
(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.
IMG_1475.jpg
(Partially ground plate)
DSCN2850.jpg
(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.
DSCN2838.jpg DSCN2837.jpgDSCN2839.jpgDSCN2842.jpg
(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.
DSCN2846.jpgDSCN2843.jpgDSCN2845.jpg
(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.
DSCN2848.jpg
(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.
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18th April 2013 12:29 AM # ADSGoogle Adsense Advertisement
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18th April 2013, 12:39 AM #2
Fantastic Josh
The tolerances you are working to are really something else.
One from left field though, with the little jointer near the rock, how do you go with keeping the dust down? I have had an old flannelette sheet over mine whilst doing some wood stuff but the dust is getting through. I'm thinking about putting it in the corner and building a dustproof box around it.....1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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18th April 2013, 12:49 AM #3SENIOR MEMBER
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The jointer is a bit further away than it looks and that plate can roll around the workshop. but I always have a problem with dust. It is normally clean before measure anyway. But if I leave it for a while 15-30 minutes where it is at I get a speckling of dust for several days after I do any woodwork Normally this plate is covered with a rubber mat and moved out of its little hiding spot behind the monster plate.
On the subject of dust proof box I have been temped more than once to build a little clean room box in one corner for that plate for when I'm scraping, the dust drives me crazy when it gets to the fine levels..
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18th April 2013, 12:55 AM #4Dave J Guest
Nice work
Dave
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18th April 2013, 04:11 AM #5SENIOR MEMBER
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I have been baby sitting the furnace shown earlier in this post all night. and it had been taking much longer to ramp to temperature than usual. It turned out to be a faulty output on the SSR which I only just checked (I thought it may have just been all the addition metal), with that fixed it ramped up straight away (it had only been running on half power) now I just have to wait out the two hour soak time at 955C.
anyway to pass the time here is a pic of the furnace at 955C.
Phil, if you look at the bottom you will be able to see the iron bar chopped in two being treated before being made into a couple of straight edges.
DSCN2852[1].jpg
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18th April 2013, 04:38 AM #6Intermediate Member
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What is your definition of a micron? Apparently there is more than one.
Gene
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18th April 2013, 07:35 AM #7SENIOR MEMBER
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Fantastic work Josh and thanks for the pic.
You have no idea how good it feels seeing that bar in there.
Phil
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18th April 2013, 08:47 AM #8
I had no idea that block was cut from those chunks of cast iron on your bench. Came out pretty cool!
Can the centre piece be reused and ground into a cylindrical square???
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18th April 2013, 10:00 AM #9SENIOR MEMBER
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Josh, given the amount the material moved, it would be an interesting exercise to run them through the furnace again and see if they moved more. My gut feeling is that if it moved that much already it will move more on another cycle. I understand multiple cryogenic (and presumably heat) cycles are used by some precision instrument and gauge manufacturers. Incidentally, it puts significant doubt on the "ageing" case put forth. Admittedly that was a heck of a casting that was chopped up, but it was hardly "green"!
I'm not sure if the centre would be much of a cylindrical square candidate, possibly the world's shortest if it was, but maybe it could serve as a crash test dummy to grind and see if it moved more next time you have something else cookin'.
Pete
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18th April 2013, 10:25 AM #10future machinist
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wow Josh that turned out really nicely, I just find it amazing the tolerances to your working to in your blows my mind I have enought trouble archiving accuracy of .01 let alone .002
BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE
Andre
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18th April 2013, 11:33 AM #11SENIOR MEMBER
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18th April 2013, 12:00 PM #12
Hi Pete,
Yes, the aging argument is a myth, I'd guess these tractor weights would be 30-40 years old at least, what seems to matter as far as stress relief is the time spent at high temperature.
There was a patch of white cast (chill) that showed up (when rough machining) on the shaper, that stuff is as hard as glass, and getting rid of that alone is worth the time and trouble to go through the heat treatment.
Still, I was surprised how much it moved 1/2 mm in 200 is pretty significant, also after a heating cycle converting carbides to graphite the colour was a distinctive gray, is that why they call it "gray cast iron"
Originally Posted by easymike29
Regards
Ray
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18th April 2013, 03:35 PM #13SENIOR MEMBER
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Gene, The miron I'm referring to is micrometer not a microinch.
TO clarify; 2 micron (µm) (0.000 002 meter(m)/ 0.002 millimeter (mm)) is around 0.000 079 inches("/in) or just over a 3/4 of a tenth of a thousandth of an inch and would be 79 millionths of an inch or 79 microinches (µin).
The 29th edition of the bible turned up a few days ago and I'm very pleased to see the quantity of metric information contained therein, enough that I think I will be using it over the black book as my shop reference. Incidentally Gene/and Others, there is a handy chart in the back under units that has conversion tables from micrometer to microinch and other way around (it is a well formatted chart and good to have blown up and laminated sitting close by the surface plate if you are like me a work in both scales)
-Josh
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18th April 2013, 03:36 PM #14SENIOR MEMBER
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18th April 2013, 03:48 PM #15SENIOR MEMBER
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Oh I like that idea, Dimension up the circular puck of iron and see how it behaves. I don't much like the notion of putting the square back in without good cause as I have to really pull my thumb out to get to where it is now.
From all that I have read and seen on the subject of ageing iron it would seem that the science suggests that leaving the casting for many years to season is nowhere near as effective as 1 day in a furnace, enough so to put it into the category of foundry myth. There is however a very strong case though for leaving it in the mold for as long as possible to allow it to cool as slow as possible.
-Josh
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