MAKING A PRISM STRAIGHT-EDGE OUT OF DURA-BAR

[dgfoster]

Denis, do you have your own heat treat oven now? If yes, what type did you get?

I got a “Foster Featherweight 48"



That is, I made my own oven for annealing and stress relief about a year ago, That saves trips to a heat treater 60 miles away. The actual charges he made were small---it was the time to drop off and a day later pick up that was the problem. My oven is not a full-on heat treat oven capable of going through a complex and rapid up and down cycle like that needed for various high speed steels and the like. It is a simple fabricated steel box insulated with three inches of ceramic wool and heated with a resistance wire coil I wound. The coil is controlled with a very nice Bartlett (made in Iowa) programmable kiln control unit and fairly good-sized relay. It is perfectly suited to my needs as it will accommodate my 48" camel back and my precision 30x12x4" square and anything smaller. I just load it, program the control, and push "Start." It takes a few hours to reach 1150, holds for 1.5 hours, and then shuts itself off. 12 hours after shutoff it has cooled and the part is done. I really like it. I also have a smaller commercial kiln for 18's and 8's (my most popular seller BTW).



To anneal cast iron the part must be wrapped in a SS envelope or placed in a SS cylinder to prevent oxidation. Stress relief causes discoloration, but the temps are low enough that there is no problem with oxidation.



Denis

 

[Richard King]

KEITH RUCKER STRAIGHT EDGES - He has some really nice SE's. He is really cheap too. Check them out.

 

[lucky7]

Denis, sounds more sophisticated than my previous chuck it in the bonfire method ;-)

Now I have occasional access to a commercial shop’s oven.

 

[Richard King]

King-Way Straight-edges. By Keith Rucker

 

[MCritchley]

Here are a few Dura bar and Versa bar scraping masters I’ve made over the years. I like using barstock iron as it’s easy to source and I can machine the shape I need in a few hours. Castings need machining so why not cut out a step and just hog out of a bar? I have two prisms that were lightened by 3-d machining in about 20 minutes in cycle time. I regret doing the 3-d cut outs as the edge is very temperature sensitive. I filled one of the cavities with thermally conductive epoxy as an attempt to get the temperature from my hands to spread more evenly. It didn’t have the effect I was hoping for, it’s still much more temperature sensitive than a solid edge. So going forward I don’t feel the need to mill out a cavity to lighten a prism.

I do have cast camelback straight edges that I use first to establish absolute flatness, they are stiff and out perform a prism. One I have a flat surface I’ll use a floppy temperature sensitive barstock “prism” for convenience. I will go back and forth between the two styles just to audit what I am doing.

 

[dcsipo]

Stock removal isn’t as directly measured as on a surface grinder (or other machine tool). There isn’t a uniform flat surface from scraping- there’s points ie hills and valleys. I average measuring with a dti reading off an old gauge block run along the surface. I’ve measured depth of cut to try to learn consistency and can reliably rough with a wide radius carbide by hand at 3-4 tenths depth- in the individual divot. But the overall surface goes down a variable amount- until you get to (guessing here) 10-ish points per inch. Gentle picking of polished high spots on finish passes is something I haven’t measured (not sure I could with my gear) but guessing one tenth depth? Not sure if this helps or confuses?

....best way to assess stock removal in hand scraping is by counting the passes during step scraping. Let's say you need to lower a side by 0.003" for simplicity's sake divide the surface into 4 bands. The first one is left alone the second gets 1 pass the third 2 and the fourth 3 passes between blue ups and height checks. if I reach my 0 height in 4 checks that means 0.003" were removed in 12 passes. which means I lowered the entire surface by 0.00025" each pass. That does not mean that all my scrape marks are that deep, it just means on average the overlapping passes created the lowering of the peaks by 0.003". Depending on the hardness of the CI, your bearing down, the sharpening angle, and the polish of the carbide all contribute to the depth and quality of the individual marks. A scrape mark can be somewhere between 0.0005" and 0.0015" deep. One could get really aggressive with roughing at the expense of creating a very uneven surface, not worth the hassle. It is better to have two lighter passes than one gorilla pass.

When finishing, i.e. going from 10 PPI to 40+ PPI the surface hardly drops, since this process is mostly designed to divide the larger contact points and only remove the very highest contact points.

Keith just posted this

very good example of step scraping

 

[dgfoster]

I prefer Dura-Bar - It doesn't have a rough finish like a sand casting. It doesn't have hard spots especially on this corners from heat treating. Here is the last order I made 2 years ago. I use these plates for my practice plates when teaching. I have the host mill them.

I said yesterday that I did not believe that heat treating cast iron for stress relief could cause "hard spots and hard corners." To back that up I said I would try water quenching (no one in their right mind would qater quench cast iron from its stress relief temperature) a piece of cast iron to prove the point.

So, today I was stress relieving six 8" parallel/prism/level castings and my new staight parallel 18" casting and I stuck in a piece of scrap---actually a riser cut off one of the 8's. When it had been soaking for an hour at 1150F I opened my oven and snatched the glowing test piece and immediately dunked it in cold water. Then I ground off the skin and tested it three times on my calibrated Wilson hardness tester. It tested less than zero on the C hardness scale. That is as soft as you will see cast iron. So, doing an extreme test which would never happen in practice failed to induce any hardness whatsoever. That is what one should expect given the fact that the accepted stress relief temperature of 1150 is at least 200 degrees below the lower critical temp for cast iron.

In the pic below the fairly deep (due to the softness) indentations from the hardness tester diamond are clearly visible in the upper right hand portion of the test piece.



I apologize for not having any scale in the photo. The piece is about 1.5" high and similar in diameter.
Denis

 

[Richard King]

We wrote about heat treating on PM before. Nothing new....The best Heat treater I have ever used "cooks" his castings for 18 to 24 hours at 1170 F. Then turns off his 24" walled furnace and it takes 8 to10 hours to cool down The company does work for Boeing. Engineered Foundry Solutions in Winona, MN.
Here is a thread we wrote about in 2016 https://www.practicalmachinist.com/forum/threads/casting-a-straight-edge.318261/

 

[Mark Rand]

I think that demonstrated proof trumps appeal to authority.

Denis was showing that stress relief heat treating cannot cause hard spots to form. He wasn't addressing retained stresses, which are the reason for (reasonably) slow cooling after heat treatment.

 

[dgfoster]

I think that demonstrated proof trumps appeal to authority.

Denis was showing that stress relief heat treating cannot cause hard spots to form. He wasn't addressing retained stresses, which are the reason for (reasonably) slow cooling after heat treatment.

Mark,

I am glad somebody gets it.

And, Mark, you have something to look forward to. I am told that once you become, at least in your own mind, an absolute authority, life gets easier. That's because you can just sit back and

1) Pontificate without real concern for whether what you are saying is factual or whether you can cite anything besides your own authority to back it up.

2) Ignore anything contrary to your pontification.

3) Fortify your authoritatrian position by saying you have been associated with or even worked along side other impressive authorites.

4) And, best of all, never retract or apologize for erroneous pontification. That's because "erroneous" and you simply don't compute.

5) Never worry about muddying up the already murky enough waters by making incorrect assertions. Just bring up some unrelated notion to deflect attention. Let peons figure it out eventually, maybe.

As attractive as the state of authority sounds, you may ask when it occurs. I am told (on considerable authority) it is magic, really. You just wake up one morning and you have somehow emerged into that bright and shiny world. I am certain that will never happen for me, sadly. Oh, well.

Denis

 

[Richard King]

I know what I am talking about.

 

[dcsipo]

Mark,

I am glad somebody gets it.

And, Mark, you have something to look forward to. I am told that once you become, at least in your own mind, an absolute authority, life gets easier. That's because you can just sit back and

1) Pontificate without real concern for whether what you are saying is factual or whether you can cite anything besides your own authority to back it up.

2) Ignore anything contrary to your pontification.

3) Fortify your authoritatrian position by saying you have been associated with or even worked along side other impressive authorites.

4) And, best of all, never retract or apologize for erroneous pontification. That's because "erroneous" and you simply don't compute.

5) Never worry about muddying up the already murky enough waters by making incorrect assertions. Just bring up some unrelated notion to deflect attention. Let peons figure it out eventually, maybe.

As attractive as the state of authority sounds, you may ask when it occurs. I am told (on considerable authority) it is magic, really. You just wake up one morning and you have somehow emerged into that bright and shiny world. I am certain that will never happen for me, sadly. Oh, well.

Denis

https://safetyrisk.net/the-dangers-of-being-an-expert/

 

[Garwood]

No experience making a straightedge here, but I would like to point out that anyone of millions of people with a commodity CNC mill could skeletonize even a large straightedge from Durabar in a very short time.

Scraping may be a lot of work, but milling cast iron is like cutting Styrofoam.

 

[Richard King]

No experience making a straightedge here, but I would like to point out that anyone of millions of people with a commodity CNC mill could skeletonize even a large straightedge from Durabar in a very short time.

Scraping may be a lot of work, but milling cast iron is like cutting Styrofoam.

I have to point out Garwood knows to scrape. He volunteered his shop when Hunter Hightower asked Make a Wish to have me teach him to scrape. Garwood and his daughter hosted the class.

 

[CBlair]

I think everyone has had a chance to voice their opinion, it has been a good conversation but I think we can end it here for the time being.

Charles