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Making a straight edge

otter

Aluminum
Joined
Oct 17, 2008
Location
Sunnyvale, CA, USA
I'm going to make a straight edge to use (in time) to scrape my lathe cross slide. I'm going basically copy what Baxtool did here: http://www.practicalmachinist.com/vb/metrology/making-straight-edge-214001/

Looks like Baxtool's pictures have been mangled by Photobucket, so I am reposting them

prism1.jpg
prism2.jpg
Prism_CompoundSwivel.jpg

I got a piece of cast iron from McMaster years ago to do this and am finally getting around to it. It was a McMaster pn 8923K682. It's 2" x 1.5" x 24"

IMG_2124.jpg

I don't know how long it will take me to finish, but I will post progress here.

Last night I milled the sawed edges. Just rough machining to make things smoother.

IMG_2125.jpg

I looked at my lathe - the dovetails are 60 degrees. In retrospect, I should look at my mill's dovetails before I finalize the straightedge angle. I'm thinking 55 degrees, if the mill also has 60 degree dovetails. I don't have plans to scrape the mill, but I don't ever want to make this thing again.

I've taken Richard King's course and have scraped a bit, but never made a scraped part from scratch. Should be interesting...
 
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I'm going to make a straight edge to use (in time) to scrape my lathe cross slide. I'm going basically copy what Baxtool did here: http://www.practicalmachinist.com/vb/metrology/making-straight-edge-214001/

Looks like Baxtool's pictures have been mangled by Photobucket, so I am reposting them

View attachment 215311
View attachment 215312
View attachment 215313

I got a piece of cast iron from McMaster years ago to do this and am finally getting around to it. It was a McMaster pn 8923K682. It's 2" x 1.5" x 24"

View attachment 215314

I don't know how long it will take me to finish, but I will post progress here.

Last night I milled the sawed edges. Just rough machining to make things smoother.

View attachment 215315

I looked at my lathe - the dovetails are 60 degrees. In retrospect, I should look at my mill's dovetails before I finalize the straightedge angle. I'm thinking 55 degrees, if the mill also has 60 degree dovetails. I don't have plans to scrape the mill, but I don't ever want to make this thing again.

I've taken Richard King's course and have scraped a bit, but never make a scraped part from scratch. Should be interesting...

Do you really need a 2' prism? it will be quite floppy. i would make a slightly longer than 1' and a shorter one (15" and an 8"). Also if you just make a 1.5"x2" triangle out of it you will have a 37[SUP]o[/SUP] and 53[SUP]o[/SUP] useable angle i know that would make a long prism even more floppy, hence the suggestion of two shorter ones. I would look for a camelback in a 2' or larger size. Your project :), you decide.

dee
;-D
 
Those are fair points. I'm going to take a look at my mill's y axis tonight and see how long that is. If it is < 24", then maybe I will stick with 24". McMaster doesn't sell by the inch, so the 24" size was easy to decide on. I don't have plans to do tons of dovetail scraping, so I'm not looking for the tool to solve all dovetail projects.

I'm planning on making the angle as steep as I can (55 degrees), to keep it as rigid as possible. If I ended up with a 15" and 8", in reality I would end up with a 15" scraped straight edge and a machined 8" project that will never get done. Doesn't hurt to put it on the shelf waiting for a reason to finish it, though.
 
Those are fair points. I'm going to take a look at my mill's y axis tonight and see how long that is. If it is < 24", then maybe I will stick with 24". McMaster doesn't sell by the inch, so the 24" size was easy to decide on. I don't have plans to do tons of dovetail scraping, so I'm not looking for the tool to solve all dovetail projects.

I'm planning on making the angle as steep as I can (55 degrees), to keep it as rigid as possible. If I ended up with a 15" and 8", in reality I would end up with a 15" scraped straight edge and a machined 8" project that will never get done. Doesn't hurt to put it on the shelf waiting for a reason to finish it, though.

Don't store it on a shelf. Drill a hole in the end and hang it from an eyebolt. Shelf storage even on the 30% Airey Points will let is bend after a time.
Airy Points, Bessel Points, Minimum Gravity Sag, and Vibration Nodal Points of Uniform Beams | Mechanics and Machines
 
Good point, Richard. One of the features that I liked about Baxtool's design was the threaded holes for the handles. They should work well for an eyebolt (or even just a hole in the handle).
 
Put an hour in today. Two items:

- Considering I machined a rough sawn blank on a knee mill without a lot of care, I don't think anything is straight or square. So I roughly scraped one side into mostly taking blue. Not shooting for many spots / in^2, just coverage, end to end. Tough to get the spots on the picture, but it's sorta even coverage. Enough for further machining the other sides better.
IMG_2129.jpg

- I looked at my lathe and it has 60 degree dovetails that are 16" long. My mill has 50 degree dovetails that are 27.5" long. Considering that I don't plan on reconditioning that mill ever (a small VMC would a better fit, if I'm putting real effort in), this will become a specialized lathe straight edge. Having said that, I know the straight edge should be longer than the dovetail, but how much. I was thinking 3" longer? So a 19" straight edge. Thoughts? And going for 55 degrees to give me some room inside the dovetail, but leaving enough material to not weaken it more than needed.
 
Make it 45 degrees then it will be good for both machines. You don't want is the straight edge angle to fit the ways exactly.
 
I second a 45* angle. You don't even have to scrape the angled part if you don't want, all it is is clearance. Just scrape the flat and flip it upside down to print the dovetails if your looking to save some time.
 
I second a 45* angle. You don't even have to scrape the angled part if you don't want, all it is is clearance. Just scrape the flat and flip it upside down to print the dovetails if your looking to save some time.
That’s a really interesting point about scraping one side. I’ve never heard of that, but it makes sense. I’ll still scrape two side, but keep that in mind for the future

Enough votes for 45. I’ll do that.
 
Might want to consider stress relieving after the machining is done.

Yes indeed or at the very least check it if it's ever sitting any length of time after you finish it. I made one from cast iron bar and after several months it spun like a top. I had to scrape it a few passes to bring it back into usable condition.
 
Might want to consider stress relieving after the machining is done.

Obviously we live in different areas, but how much would you expect stress relieving this to cost? I have a much too small heat treat oven and don't feel like pretending to be a heat treatment pro with my BBQ.
 
Obviously we live in different areas, but how much would you expect stress relieving this to cost? I have a much too small heat treat oven and don't feel like pretending to be a heat treatment pro with my BBQ.

The most common heat treatments applied to gray cast irons are stress relief because of nonuniform cooling of castings and annealing to improve machinability. Subcritical heating is used for both. Stress relief is done at temperatures between 1020 and 1200˚F (550 and 650˚C) without significantly lowering strength and hardness. Heating at temperatures between 1290 and 1400˚F (700 and 760˚C) lowers the hardness for improved machinability. Nodular iron, also known as ductile iron or spheroidal graphite iron, is cast iron in which the graphite is present as tiny balls, or spherulites, instead of graphite flakes (as in gray iron), or compacted aggregates (as in malleable iron). The nodular irons typically contain from 3.2-4.1% C, 1.8-2.8% Si and up to 0.80% Mn as major constituents. Several types of matrix structures (including ferritic and pearlitic) can be developed by alloying and heat treatment. The various grades of regular, unalloyed ductile iron are designated by their tensile properties

Heat treatment of ductile cast iron includes stress relief and annealing, as well as heat treatments used for steels including normalize and temper (for higher strength and wear-resistance), quench and temper (for the highest strength), and austempering. Ferritizing (for the most ductile microstructure) is done by austenitizing at 1650˚F (900˚C), followed by holding at 1290˚F (700˚C) to completely transform austenite to ferrite and graphite​

I am not metallurgist, just stole the above from one :). As a practical advice, all heat treatment has to take into consideration the material thickness. As a general recommendation is keeping the soaking temp at least 1h/1" thickness and not lowering the temperature on the cool down faster than 50F/h

dee
;-D
 
Obviously we live in different areas, but how much would you expect stress relieving this to cost? I have a much too small heat treat oven and don't feel like pretending to be a heat treatment pro with my BBQ.

Over here in RSA I pay around 50 cents/kg. During stress relieving the material loses 3/4 of it's strength and that means the manner of supporting it becomes very critical. Making any gage w/o stress relieving is a waste of time. Stresses can survive indefinitely inside ferrous materials.
 
Dura-Bar tells me that their extruded Gray Iron does not need stress relieving. I have done both. Do you know if MacMaster sold you Dura-Bar? I use a company here in MN that puts my small order castings in the furnace with a large order and he charges me around $1.00 a pound and a case of beer. You should call around and see who can do it and if they would charge you a minimum.

Member Steve Watkins (The Beast thread in Vintage forum) bought a furnace that could do it and I bet he would do it cheaper then the UPS charges. If I were you try it with out doing it and see what happens. As what Dee says hang it up on a rope and use a chunk of oak 2 x 4 or a dead blow hammer and hit it several times and vibrate or ring stress relieve it. Also do it 3 or 4 times while your scraping it. Hang from the end on eye bolt and rope.
 
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From what I can tell, McMaster sells Durabar as their grey iron. Im pretty sure the cast iron Alro sells is definitely durabar. Check them out, they often have remnant/drops for sale of nice sizes. Unless your a professional and you'll be using your straight edge alot, IMO id say dont worry about the heat treat if it isint readily accessible/affordable to you. Ring it out when your making it, worse case, it moves over time. If your making it specialized for your lathe, once you scrape in your machine, you may never even use it again anyway. And if you do, maybe months/years later, checking and touching up your straight edge before you start is good practice anyway. Save some time and money and start making chips.
 
From what I can tell, McMaster sells Durabar as their grey iron. Im pretty sure the cast iron Alro sells is definitely durabar. Check them out, they often have remnant/drops for sale of nice sizes. Unless your a professional and you'll be using your straight edge alot, IMO id say dont worry about the heat treat if it isint readily accessible/affordable to you. Ring it out when your making it, worse case, it moves over time. If your making it specialized for your lathe, once you scrape in your machine, you may never even use it again anyway. And if you do, maybe months/years later, checking and touching up your straight edge before you start is good practice anyway. Save some time and money and start making chips.

This is grossly incompetent advice - the kind we could do without here. Without proper stress relieving the cast iron will keep moving while you scrape at it. Or mill it, or turn it.
 








 
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