Dovetail Tolerance

I machined and scraped a 60 deg. straight edge from a 26" long piece of Durabar. It started out as the oversized 1' x 2" and I hollowed it out to reduce weight.
If the ends are put on parallels it sags three tents in the middle . A camel back wasn't a reasonable option for me either but it would be a much better tool.
 
Kernbigo said:
They way you check doveltails is with 2 dowl pins ,and mike accross
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Using two dowel pins is a good start. But by no means will it give you the true representation of the dovetails overall dimensional landscape. And this is an interesting topic. I have spent days scraping in dovetails for that perfect fit. Only to find that after a few months of use and re-adjustment of the gibbs that the ends of travel tighten up again. Maybe that perfect fit is not possible and just live with it…Dave.
 
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I thought about that, but have read that it would not be stable and would sag too much. Great to hear that someone has had success doing it. I was thinking that it would help to put it through a self-clean cycle on the oven (~800F) between rough and final machining to help get some of the stresses out. I will see if anyone in central Florida has a real 18" straight edge when my diamond wheels finally get here. If not, then I'll do the cast bar route.
 
The piece was very unstable. I would get it close and it would warp.. Finally had to put it through several sets of heat soaks in the kitchen oven at 550F.
That seems to have given it some stability but it will need to be checked prior to each use.
Was surprised when it showed only three tenths sag over 26". Could have lightened it more. A shorter bar should have far less problems.
The bar had a tendency to bow down on the ends and go high in the middle of the 12" long cross-slide. Got better readings when the cross-slide was on edge
so the 2" width of the bar supported itself.
Was using dowel pins in the dovetails to measure them but have yet to build a indicator jig and my mic wouldn't lay in there.
So, had to use the dial calipers. Poor option but it worked. Machining a good gib was also a chore.
 
Crossfeed.jpg Back way.jpg Front way.jpg Way measure.jpg Mic.jpg I would use a variation of a king way alignment tool and modify it to you application, sue durabar of different diameters and use the locks and rods that are adequately heavy enough to not flex which keeps the measurements accurate to .0001
 
I bought an 18" camelback casting from greg dermer I think is his name, 75 bucks plus 18 for shipping. It's styled like a brown and sharpe, except it has a thick flat that's cast with the ability for a dovetail angle. I'm really pleased with it. I have to machine the dovetail edge on it still then will start scraping. It comes stress relieved, it's recommended to rough machine then stress relieve again, then finish machine, the scrape. I'm going to try it without the additional stress relieve we'll see what happens.

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astjp2 said:
View attachment 122577 View attachment 122578 View attachment 122579 View attachment 122580 View attachment 122581 I would use a variation of a king way alignment tool and modify it to you application, sue durabar of different diameters and use the locks and rods that are adequately heavy enough to not flex which keeps the measurements accurate to .0001
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The King Way alignment fixture is a nice tool to have. Rather it is original or home made. But the way you are using it in your pictures, using the tailstock ways as your datum plane, you may/can get false readings, and you probably are. Not trying to create an argument here, just saying. How do you know that the tailstock ways are straight without any wear? Did you confirm they were straight? or just assume they are thinking they did not appear worn as the carriage ways were? If you did confirm they were good and straight, how did you verify this?

Ken
 
After all the work and problems I had to deal with, using durabar, I would suggest that if there is any reasonable way to get a camel-back, do so!
Machining and scraping in a 60 deg straight edge is a big investment in time and labor and a camel-back will be more rigid and better serve you.
Something I had forgotten is the last time I put it in the oven I decided it wasn't hot enough so I turned on the broiler and let it heat for 1 1/2 hrs before
turning it off and letting it cool in the oven.
Where did this script come from? I don't know how it happened. It just started. Musta hit a wrong buttun
 
I have a 36" straight edge made out of a piece of DuraBar that I bought back in 1997. It sat outside in the hot South Texas sun for at least ten years before I started machining on it. Scraped it flat on the two edges two years ago. Hung it from one end and beat on it like Richard King said to do with a rubber mallet. Checked it again, still good. I had a local shop I deal with machine the straight edge under my supervision so it would come out straight without too much warpage. I don't recall the amount I had to scrape to get it straight, but was less than .002" in 36". I thought that was darn good. They got it parallel end to end, but missed it side to side, its out about .005". I can live with that since I'm not depending on it for anything that would require measurement in that plane. I guess from my experience about cast iron being left out in the sun for a year or so to "season" the iron is true!
 
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