48" Camelback Straightedge Pattern is in Process

I have started work on a new pattern for a 48" lightweight camelback straightedge.
I expect the casting to weigh something like 38 pounds.

The part I am making now is a router template for cutting the web of the straightedge. The template material is 1/4" MDF. I will use it to cut a 3/8 Baltic birch plywood web.

Here are all the corner holes for the cutouts bored.



Next the holes are connected with straight cuts.


More straight cuts and one complete cutout.


Once the boring and milling of flats is done, I will harden the edge of the MDF with polyurethane.

Denis

 

That is a nice piece.
I think I wold make a permanent pattern in aluminum from it.

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Whoa... I see the attraction to cast your own. Rock on!
https://www.ebay.com/i/173990540448...MIu_mx-aOE5AIVw5JbCh1-gAcGEAQYAyABEgJjGvD_BwE

 

That piece is the first faltering step to make a scaled-up version of this pattern.



I have used the above baltic birch plywood pattern many many times and it is holding up well. If I ever decided to have commercial foundry cast it for me, that is when I would hand them an aluminum pattern as they tend to be pretty ham-fisted with patterns. I would not mind having an aluminum patttern cast, but I would guess I'd have quite a few missteps learning the vagaries of aluminum before I could get a "perfect" casting to serve as a master. So, I continue to use the wood pattern...

Denis

 

Here are a few pics to serve as an update. The little ribs oriented vertical to the web of the pattern have tobe accurately located or they don't have proper draft. So I cut orientation slots for them. The mill makes accurate slot location easy.

 

I love your pattern, But how are you going to the machine and surface this? You could do this old-school with a lot of high spot blue and scrapers with a almost perfect reference. Or you could use a surface grinder or Blanchard grind with a few more set ups. I’m guessing you have a granite surface table and carbide scrapers?

My home shop is limited to 36 inch surface table(Starrett). And an 18 inch surface grinder(Churchill). Finishing something like this, is way out of my scope. I am a little bit envious.

You have a 60 inch milling machine? With 90° Head attachment in the home shop? I am following!

Lift up those garage doors and give us a shop tour! Your tables scrapings look mint on your milling machine. I would love to show my 1960’s Bridgeport (with my head in shame).

 

I may or may not ever scrape the sole of the new 48” casting but I hope to sell a few. I have scraped in my 36 and an 18 old school. I do not have a large surface grinder and even if I did, it would need to be in exceptionally fine condition to grind a straight edge to the required accuracy. I am unaware of such a reference straight edge being ground to final flatness. I think they are all scraped. (Beyond the scope of this thread ;-). )

I do not own a particularly special Bridgeport Mine is 60’s or 70’s vintage with a 48” table and a very nice DRO. Since it has a 48” table, it has about 24” of useful travel. So the slots were cut with two (careful) setups. It is a machine I treasure and use often, but nothing all that remarkable.

Mostly this work is just slow and somewhat careful but not anything likely to turn many heads. I am posting it just to share with kindred spirits and to show the utility of the milling machine as an aid to pattern-making accuracy. Likely most patterns don’t really require a high degree of precision. Some , like this pattern, do though.

Denis

 

A few more pics

Next step tomorrow is to cut a slot to set the web into the sole, but first roundover the cutout edges, make the draft cuts on the top of the sole and the bottom of the bow.

Once the sole and bow are attached to the web, I will then start gluing in the ribs. After they are all in it will be time to fill in all the inside corners of the rib-to- web joints and rib-to-sole and bow joints. That is a lot of bondo work and very time consuming!

Denis

 

If you ever considered putting a levelling glass ability into this, (and good levelling glass is about $40) this simple tool could shoot to $2-3000 in a finished state.

Yes I know… My service table is not completely level.

 

I want to go play at Denis' house!!

 

OMM, Here is a prism 18” SE that I make that has provision for a Geier &Bluhm precision vial.

This is the milling machine attachment device I dreamed up to drill and tap the three holes located under the overhanging upper surface.

Denis

 

Glued web to sole after rounding cutout edges and cutting draft into the sole. I used 2 degrees on the long side of the sole and 3 degrees on the short side.

Glued up:



End view:



I will hand plane draft into the bottom of the sole later. For now, I am preserving it as a reference.

Denis

 

Now the bow is glued to the web. I cut a groove into the bow to center it on the web and then cut draft on the grooved bottom of the bow. Now the trick was to glue the bow so that its undrafted (flat) top was very square to the web. Otherwise the draft would be increased on one side and possibly lost on the other. So I made the little brackets pictured to hold the bow square. To use them, I lightly clamped them in place and the used a plastic mallet to drive them toward the sole until the bow was solidly seated on the web and then snuggled up the clamp. That seemed to work well as checking the bow for square looks very good.



To give myself plenty of assembly time, I used a 12 hr epoxy rather than a 5min epoxy as I usually do. I was worried this somewhat springy limber piece might give trouble. But, it went on smoothly. Tomorrow will be the day for rib assembly.

Denis

 

Great work as usual! I'm enjoying following along.

 

Just a little time to work today. Starting to fit ribs. These are the rough backbones for ribs. They were fitted in two pieces as it makes fitting them easier—-the small 45-45-90 triangular part is cyano glued to the longer piece. At the present each glued up rib is just snapped into its groove. Next I’ll decide on the contour of the ribs and shape their profiles. Then each rib gets two thin “cheeks” glued on. The cheeks will have a couple degrees draft sanded into them so that the final 3-part rib will have proper draft. On my 36” camelback I made the ribs from a single piece of Baltic birch. Getting everything fit and correctly oriented using that method was a lot more time-consuming and subject to error.

Once all ribs are assembled, contoured, and glued into place, then all those inside corners
Will need to be radiused with Bondo. Not my favorite job.



Denis

 

Looking good.
That is going to be a big boy with a heavy mold.

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I am guessing 450 pound mold in the flask as present mold is is 350. ‘Course I flip it one-handed ;-)
It will be a challenge to roll it out for pouring. The present 350 makes me (160 pounds) grunt a bit on the slight uphill.

Denis

 

Okay so explain something to me. When you get to 48" you are building something like a miniature bridge. No steel or cast iron bridge has ever been built without the only shape that is rigid, a triangle. I used to work in the greenhouse industry. A customer had a greenhouse with a 21' span and trusses about 12" deep made from 1/2" sch 40 pipe and triangulated by 1/2" rods. A new greenhouse company buit them new greenhouses with the same span. They used 1" sch4o pipe spaced by 1/4" x 2" bars every 18" with no diagonals. I wondered how they compared and this greenhouse was low enough I could reach the trusses. No joke the owner is a dwarf, and very smart! So I hung on the old trusses and could not detect any deflection. Then I hung on the new trusses, and could easily deflect them an inch or more!

So why wouldn't you cast in diagonals and actually make this thing really stiff? In fact you can prove this with your MDF pattern.

 

If the webs were 90 degrees to the long axis, then you would get a lot of deflection, but with the webs parallel to the long axis, they act like diagonals.
Each one is a mini truss basically.

No need to add mass if it is not needed for rigidity.

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The proof is in the pudding. This straight edge edge in a 36” version will deflect about 7 hundred thousandths (.7 tenths) when supported at its ends only and stressed with a 30 pound weight placed midway between ends. The SE itself only weighs 25 to 19 pounds depending on how much metal is milled off the sole. The .00007” deflection was as tested on the thinnest-soled version (19 pound). This degree of stiffness is way more than adequate for the intended purpose.

Secondly, it is somewhat inappropriate to equate wood or metal truss construction with a cast iron structure like this casting. On a wood truss with mending plates joining the cross-members or relatively thin tubes welded at cross members, those junction act like hinges more or less as the have little resistance to rotation stress about the z axis. In the pattern shown the uprights are 1” wide cast iron. They have very large resistance to rotation about an axis vertical to the junction. (I’m talking about the Z axis rotation if we consider the sole to be x and the upright to be y.)

Every design is compromise. This pattern is complex enough as is so that it takes many many hours to produce. And then there are considerations of pouring success. It took probably six or eight attempts at molding and then pouring to suss out proper gating, runners etc. And the commercial foundry I originally hired to pour the pattern had about a 50% success rate and actually produced casting less refined than mine. So, sure, the optimal design for weight vs stiffness would approximate the trabecular pattern of bone.




But Mother Nature has 9 months to form this structure an about 20 years to continue refining and strengthening it. My time line and capabilities are somewhat limiting by comparison. ;-)

In summary, the design is not optimal, has not been subjected to FEA, has not been approved by ASTM or the FAA, but it works pretty well—-good enough for the girls I date

Denis