Camelback Straight Edge Scraping

[wcunning]

Hey Rich,

Any chance you could take a picture of where you had Andy put his eyebolt to hang for striking?

Thanks,
Will

 

[Bob Korves]

Are you joking about welding the eyebolt? THAT would cause it to move, surely!

I should probably have picked up a smaller eyebolt than quarter inch. Then I could have just drilled into the web. The hole I milled is more for appearance than anything (no way I’d have drilled exactly in the middle of the web without blowing out one side).

The nut is just to keep the bolt from spinning/rattling - not because I had any worry about threads in cast iron being strong enough. It was a very loose fit, however. I could have used loctite, but it came with a nut from the hardware store, so I used it.

I’ve not started serious scraping, I just took a few initial passes to remove the milling marks and break up the surface. I’ve not blued it up yet (I won’t have access to a large enough plate until the class).

I’ve got one more surface I plan to scrape (the 90 degree side). I’ll ring it a bit more after I take a couple passes along that side. I think removing metal unevenly will cause as much movement as anything (except maybe heat from spot welding!).

Some blue Loctite and it will not come apart until you take it apart. Room temperature welding...

 

[wcunning]

@Rex Walters -- I'd like to start a separate discussion of scraping work holding and spend some time talking about the woodworker's workbench for the purpose. Would you be interested in contributing to some of that? I'll be actually putting the wedges in the stretchers and pins to keep the top from sliding next week to finish my Moravian, and then I want to start figuring out how I want to setup to scrape some of the more complicated bits for my machines, even if I won't get to the actual scraping right away.

Also, side note, would a drop cloth with some holes to run the holdfast shanks through keep a sufficient quantity of the swarf off of the bench? Tarp maybe? Stefan Gotteswinter's scraping videos mention that he uses a vacuum every time instead of a brush to keep the scraping swarf from getting everywhere.

Thanks,
Will

 

[Rex Walters]

Would you be interested in contributing to some of that?

Sure. My posting here is a bit sporadic as I get sucked into other things, and I doubt I’d have much of interest to add, but happy to help.

A woodworkers workbench is basically just a large special purpose clamp — it’s whole purpose in life is workholding. No special techniques I can think of, but holdfasts clamping handscrews to a bench make for nice quick workholding for scraping and similar jobs.

Vacuuming regularly will definitely help with some of the mess (I suspect I have more iron dust in my lungs and nasal cavities than is healthy) but the bluing, solvents (windex), and lubricants for stoning (WD40) are just as messy. A tarp or paper/plastic will definitely help to protect the bench if you care about appearance. I’m considering making some semi permanent removable covers out of hardboard (since I do more metalworking than fine woodworking these days).

I’ve noticed that the machinists who do the best work tend to keep their shops and workspaces VERY clean and organized (constantly taking the time to clean as they go). My shop is currently about knee deep in accumulated swarf and other filth from the past umpteen projects, with barely any flat surfaces remaining to set something down. That should tell you all you need to know about the quality of my work!

 

[Rex Walters]

Some blue Loctite and it will not come apart until you take it apart. Room temperature welding...

Yup. As I said, I had a nut in hand so I opted to use it rather than loctite. That eyebolt isn’t going anywhere.

It doesn’t seem worth fretting much about a simple hanger — I just didn’t want it spinning freely. By drilling where I did (at the balance point) it hangs nice and vertical. If I’d drilled a bit lower as Rich suggested in his last message, I could have avoided cutting into the webbing or using a jam nut, but it wouldn’t hang vertically.

(Unrelated: A million years ago I studied statics and dynamics. I’ve forgotten 99.999% of what I learned about trusses, but I do remember enough that I’m not losing any sleep about that tiny little hole I made in the webbing having any measurable effect on stiffness.)

 

[Bob Korves]

Yup. As I said, I had a nut in hand so I opted to use it rather than loctite. That eyebolt isn’t going anywhere.

I meant for you to use Loctite on the bolt to nut interface, so it cannot rattle loose...

 

[Rex Walters]

I meant for you to use Loctite on the bolt to nut interface, so it cannot rattle loose...

Ah. I think my use of “jam” nut wasn’t clear. The hole in the straightedge is threaded - no point in a nut at all if I use loctite.

 

[Rex Walters]

Hey Rich,

Any chance you could take a picture of where you had Andy put his eyebolt to hang for striking?

Thanks,
Will

Pretty sure it was in roughly the same place as my photo. It’s possible he milled less off the bottom so the balance point was closer to the “meat” of the straightedge, but I think the point is just to put it where the straightedge will hang vertically.

This makes intuitive sense to me.

Same reasoning as building a vertically oriented rather than horizontally oriented box for a cylinder square or (rectangular) box square — gravity effects on the shorter sides will have less effect than on the longer.

In other words, while supporting a beam on Bessel points will minimize the average displacement, shorter beam lengths will minimize the TOTAL displacement regardless of the support points (and the long vertical scraped side should remain quite flat).

Hanging from an eyebolt is a lot easier than building a vertical box/stand to store a camelback on end, but I suspect either would accomplish the desired result of minimizing gradual deviation from flatness due to gravity effects.

I’m not much of an engineer, so I have to think in overly simplified, extreme analogies. Imagine you made a six foot “camelback” out of two inch strips of card stock (cereal boxes). The scraped “reference edge” is a 2” x 72” strip on the bottom, connected by a 2” by 73.33” arch on top (creating a six inch tall “camelback”). Maybe you even imagine some card stock truss work or paper webbing between the arch and the reference edge.

If you found the balance point on the end of the arch, near to the reference edge, such that you could hang hang the “camelback” on end, the reference edge would remain pretty flat/straight, even if the whole thing sagged and distorted significantly. If instead you rested it on the long reference edge, the whole works would sag and deform to match whatever it rested on. If you rested the reference edge on two cylinders about 22% of the way in from the ends (the Bessel points) you would average out the deflections, but the reference edge would be a long way from flat.

(I’m writing this after a healthy dose of good bourbon and after a long week, by the way. It’s possible it affects me differently than normal people. Apologies in advance if this comes off as at all preachy. I just enjoyed thinking it through and thought I’d share.)

 

[Richard King 2]

NO I didn't say to weld the hardware store bolt to the casting! I said to weld the eye-bolt as it is is a cheap hardware store bent bolt. I also have been thinking about the way you did it.I said to drill and tap a hole. I did say to put it on a pipe to find the balance point and drill and TAP a hole. Usually people drill the hole before they are machined.

You need to do that and don't do what you did as that little rib the weight on it will surely bend.

Here are some pictures of Andy scraping his HKA-36 straight-edge like yours. He worked on it at the Rockford Scraping class we just finished yesterday at Bourn & Koch Machine Tool Company.

 

[Richard King 2]

@Rex Walters -- I'd like to start a separate discussion of scraping work holding and spend some time talking about the woodworker's workbench for the purpose. Would you be interested in contributing to some of that? I'll be actually putting the wedges in the stretchers and pins to keep the top from sliding next week to finish my Moravian, and then I want to start figuring out how I want to setup to scrape some of the more complicated bits for my machines, even if I won't get to the actual scraping right away.

Also, side note, would a drop cloth with some holes to run the holdfast shanks through keep a sufficient quantity of the swarf off of the bench? Tarp maybe? Stefan Gotteswinter's scraping videos mention that he uses a vacuum every time instead of a brush to keep the scraping swarf from getting everywhere.

Thanks,
Will

Did you see Sefan was a 2 class student of mine? I also had him interpret for me on my last trip to Germany where he scraped and helped me.