Another light weight straight edge design

ewlsey · Aug 3, 2017

I've been playing around in Solidworks with light weight straight edge designs. This design is 52" long, has a 3.5" wide sole, and weighs just 37.5 lbs.

I know I saw a picture of a sort of similar design once, but I can't recall the manufacturer. Basically, the whole arch is angled at 50 degrees to sneak under a dovetail. Then you can flip it over to scrap the other dovetail surface.

Based on the center of gravity, it should sit on the sole without tipping over backward. Quick and dirty FEA suggests .0001 deflection under it's own weight.

I'm not a pattern maker, but I did work in a foundry for several years. I think it can be cast. I thought I would run it by the local pattern shop to see what they thought.

Zahnrad Kopf · Aug 3, 2017

Looks sexy. Numbers look good. Nice work.

D Dubeau · Aug 3, 2017

ewlsey said:
I've been playing around in Solidworks with light weight straight edge designs. This design is 52" long, has a 3.5" wide sole, and weighs just 37.5 lbs.

I know I saw a picture of a sort of similar design once, but I can't recall the manufacturer. Basically, the whole arch is angled at 50 degrees to sneak under a dovetail. Then you can flip it over to scrap the other dovetail surface.

Based on the center of gravity, it should sit on the sole without tipping over backward. Quick and dirty FEA suggests .0001 deflection under it's own weight.

I'm not a pattern maker, but I did work in a foundry for several years. I think it can be cast. I thought I would run it by the local pattern shop to see what they thought.

View attachment 204775

View attachment 204776

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Looks cool. Don't they need the resting pads so you can flip them upside down and blue the working surfaces? No idea, just replying because your sentence in bold made me think of this poem that I've got printed out above my desk.

THE DESIGNER

The designer bent across his board,
Wonderful things in his head were stored,
And he said as he rubbed his throbbing bean,
"How can I make this thing hard to machine?
If this part here were only straight,
I'm sure the thing would work first rate.
But would be so easy to turn and bore,
It never would make the machinist sore.
I better put in a right angle there,
Then watch them babies tear their hair.
And I'll put the holes that hold the cap,
Way down here where they're hard to tap.
Now this piece won't work, I'll bet a buck,
For it can't be held in a shoe or chuck,
It can't be drilled or it can't be ground,
In fact, the design is exceedingly sound. He look again and cried, "At last!
Success is mine, it can't even be cast."

Monarchist · Aug 3, 2017

D Dubeau said:
For it can't be held in a shoe or chuck,
It can't be drilled or it can't be ground,
In fact, the design is exceedingly sound. He look again and cried, "At last!
Success is mine, it can't even be cast."

Pretty well describes the Chicken dinner I just et....
Somewhere along the line, there had been nothing for it but to hatch it from an egg.

But they sure do SELL well!

So .. go for it Wes!

alskdjfhg · Aug 3, 2017

So is it canted over to allow flat and angle printing but cut down on the weight of the bottom shoe? No idea of the right terminology.

Only cursory knowledge of casting, I'm assuming it can be cast and I'm trying to see the best way to machine it. The traditional square ones would probably be a easier to set up in that regard, but I've got a couple of straight edges in this size, and one weighing only 40 lbs would be much nicer.

If you ever need a planer to machine these prior to scraping, I should have a 10'er running eventually.

ewlsey · Aug 3, 2017

You would only need to machine one surface. All you would need to do is clamp it on a 50 degree fixture plate and mill the surface. If you had a planer, you could just toss it on the table and angle the tool head at 50 degrees.

lucky7 · Aug 3, 2017

I think this is the thread the OP is refering to. And I saved the image that PB has now blocked. PM me your email and I'll email it to you.

http://www.practicalmachinist.com/vb/general/there-any-advantage-style-straight-edge-230643/

Regards,
Lucky7

ewlsey · Aug 7, 2017

I simplified the design. This should be a much easier pattern to make. I think I could cast it with the sole 3/4" or even 1" thick and machine it as thin as you prefer to go.

Hbjj · Aug 8, 2017

Could this pattern be 3d printed ?
Since you already have the solid model

WILLEO6709 · Aug 8, 2017

If you were just making onesy twosy why not machine from durabar then stress relieve?

ewlsey · Aug 8, 2017

That's a lot of hogging. Seems easier to just 3D mill a wood pattern and have it cast. But, I'm not the expert.

dcsipo · Aug 8, 2017

ewlsey said:
I've been playing around in Solidworks with light weight straight edge designs. This design is 52" long, has a 3.5" wide sole, and weighs just 37.5 lbs.

I know I saw a picture of a sort of similar design once, but I can't recall the manufacturer. Basically, the whole arch is angled at 50 degrees to sneak under a dovetail. Then you can flip it over to scrap the other dovetail surface.

Based on the center of gravity, it should sit on the sole without tipping over backward. Quick and dirty FEA suggests .0001 deflection under it's own weight.

I'm not a pattern maker, but I did work in a foundry for several years. I think it can be cast. I thought I would run it by the local pattern shop to see what they thought.

View attachment 204775

View attachment 204776

View attachment 204777

if i would make anything like this i would reverse the webbing. get the closed parts on the 50 degree part and you have a wide dovetail, whatever the height of the bottom webbing is. Put all the flats on the 50 degree side. In other words on the first image everything that is in the back, move it to the front, and make it flat on the 50 degree side.

dee
;-D

ewlsey · Aug 8, 2017

Two issues with that.

First, you can't cast it unless you use a core or make the sole triangular and much thicker.

Second, the arch is meant to give stiffness to the sole. If you move the reference surface to be on the "front", you lose all of the strength.

There's no need for there to be two reference surfaces. In fact, the whole point is that you only have one.

dcsipo · Aug 8, 2017

ewlsey said:
Two issues with that.

First, you can't cast it unless you use a core or make the sole triangular and much thicker.

Second, the arch is meant to give stiffness to the sole. If you move the reference surface to be on the "front", you lose all of the strength.

There's no need for there to be two reference surfaces. In fact, the whole point is that you only have one.

You could always do it in investment casting instead of sand (more $$). You could offset the horizontal lower webbing, the whole point of having a dovetail is to have nice and wide, so big dovetails can be easily spotted. I am not certain what you would have with a 52" dovetail, but i would thing it would be pretty substantial

. I am not dictating anything, just thinking out loud, since you already have that wide 50 degree surface on the camelback, why not use it to full advantage. I would think the deflection of the sole between the vertical webs would be very minimal moving the horizontal webbing from one side of the camelback to the other would not significantly change the static behavior of the whole structure. But you are doing the analysis. You can prove me wrong in a few minutes

.

dee
;-D

ewlsey · Aug 8, 2017

The sole is 3.5" wide. How wide are you wanting it?

dcsipo · Aug 8, 2017

ewlsey said:
The sole is 3.5" wide. How wide are you wanting it?

Duh..I think i am looking at this the wrong way. I might be confused, but are you proposing to use the sole for all purposes and rely on the fact that the webbing allows you to get into the dovetail?

dee
;-D

ewlsey · Aug 8, 2017

Yes, more or less. That's how a prism type straight edge is used.

Halcohead · Aug 9, 2017

I like the design and would love to hear what it takes to have it made. A few small comments:

Can you increase the fillet sizes where the flanges meet the webs? Seems like a small weight penalty to reduce probability of casting cracking at sudden changes in section (I could be wrong, but would appreciate hearing why).

How are you simulating the deflection under its own weight? FEA can give some odd results without careful setup.

I don't do a lot of castings, but I do lots of very weight-critical designs. I usually plan on ~10-15% mass growth when going from the CAD model to real life. But maybe you already know this or that's a small change in this application.

Is it a pain (or huge weight penalty) to add cast-in bosses or flanges to allow easy fixturing for machining?

dcsipo · Aug 9, 2017

ewlsey said:
Yes, more or less. That's how a prism type straight edge is used.

I wold still scrape at least two sides of a prism

...maybe i am strange...but i get it. flip it if you need to spot the dovetail.

dee
;-D

Paolo_MD · Aug 9, 2017

Although I like more the prototype that Wes posted earlier in a different thread, I think he is right with the fact that, in principle, you need only one scraped face. True, on Connelly you see the suggestion that "the experienced scraper" would spot both sides of the dovetail with the same bluing. However, this is tricky and, at least in my little experience, the flat and angled faces of the dovetail are scraped at different times, sometimes alternating master and "template".

A couple of quick comments on the design: First, I second Halcohead's suggestion of beefing-up the edge of the casting.
Second, considering how a dovetail straightedge is used, the main danger is twisting the casting. Therefore, wasting a couple more pounds of cast iron by beefing-up the "backstrap" mostly in width, it's worth the trade-off in weight.
Third, I believe that some machines have 50° dovetails. Therefore, it is worth designing the ribs so that the angle with the scraped sole is 45°.
Lastly, although it is calculated to be standing (at least with the current 50° design), the distribution of weight is extremely uneven. Therefore, I wouldn't care about that aspect and either store it in a shoe with built-in complementary angle, or I would store it flat on one side (in this case, it could be handy to have a boss on one side in the middle of the arc, which could also help positioning it while machining the casting.

Paolo

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