Why are straight edges for dovetail scraping 45 degrees?

The Bridgeport ways from my understanding are 50 degrees, so why not have a mating 50-degree surface for your referance edge? That 5 degree delta should mean they won't match up. Unless I'm missing something.

I know V blocks are 45 degrees and that might be the reason why. You can bolt your stright edge on vblocks and mill it that way. Otherwise, it's a pain in the ass to set up a 50 degree cut. But that's the only reason I can think of for using 45 degree, because you can just use vblocks to machine the angle.

You're only spotting one side of the dovetail at a time, so the straightedge has to be acute enough to fit and no so large as to contact both sides at a time.

The straight edge is not to be confused with a master angle. It is simply a tool to transfer the marking medium and act as a flat reference surface that happens to be angled at 45 degrees so it will fit down in most dovetails and have clearance. While 45 degrees may in fact be the dovetail angle on some things then your straight edge bevel most likely should be less than that angle, once again for clearance to get all the way down in with out interference.

You scrape a mating surface to the angle of the mating part or master. For example if I was scraping a Bridgeport column way I would scrape in both sides nice & even. Getting my flats to my liking. Then work on the dovetail angles following whats there. If I thought for some reason I needed to check my angle I would use a small machined and scraped setting master or a simple protractor. (one usually follows what is already there as closely as possible) Once I was satisfied with that angle and everything else is parallel and straight I would then scrape the mating part(knee) using the column ways as my master. Fitting my mating sliding angle and gib to my perviously scraped column ways. Even if that angle turned out to be 55 1/2 or some other number that was close. It is not that important as long as everything lines up and works correctly in the end.

45 degrees is just a nice number that fits down in most machine ways and provides clearance.

Pete and Warren are correct. When "Rebuilding" a machine we could careless if the dovetail angle is 44, 45, 46 or what ever degree it was when new. One can screw around with a protractor and measure the angle but as Warren said you follow what is there and you need not worry if it is a perfect 45 deg. As Warren uses the Bridgeport Column as an example it is made at the factory 50 degree's or close to 50 deg.

They probably use 50 deg. cutters or set the planner or way grinder at 50 with a planner gage (or just eyeballed it on the machine angle setting) so when they scrape using a "Dummy Master" or Template s shown in the Moore Book and Connelly Book which is an exact copy to speed up scraping the flats parallel at the same time or blue both sides up at the same time. They don't have the column sitting on the back and scrape the flats and dovetails at same time as they would have to flip the column over to each side or bend over and scrape the dovetail upside down.

We scrape the flats first and get them flat and parallel to each other and both sides are Panar to each other, not worrying about the dovetail then. On a worn column the "rebuilder" knows both ends on the ways are not worn as the travel of the knee is in the center of the ways where it wears, so they use a straight-edge that fits close to the bottom vertex and need not fit it exactly. Then once the flats are scraped to original angle and touching in the worn area you flip the column over and scrape one side of the dovetail the same way, bringing the angle ends down using a straight-edge (angle camelback preferred to a none camelback prism) and then set the column on back and measure the worn in-scraped dovetail and using the same process scrape the 2nd dovetail parallel and flat to the first finished dovetail.

When I scrape a Bridgeport column I scrape the back of the knee flat or Planar to a surface plate first and then use it alternatively with a camelback straight-edge to get the flats on each side planar sideways to each other as they were when new. If a "rebuilder made a exact angle for each machine he rebuilt he would need dozens or an unlimited number as not all machines are 45 or 50 or 60 degree's.

Many Rookie Rebuilders make that mistake and scrape Dovetails or V ways exactly the angle is. A few years ago a rookie wrote in here about he started scraping his lathe bed in and discovered the V's measured 43 1/2 degrees and not 45 and scraped the angle to an exact 45. It worked, but he also wasted many hours doing that because he as Warren and Peter point out only needed to follow the original angle and "match" fit the riding part to the original angle.

I have discovered that on Monarch Lathes they make the dovetail angles 50 degree's on the cross-slide and compound and using a regular 45 degree camelback to pre-qualify the shorter sides one of my normal 45 degree camelbacks angle vertex base side is to long and I will be making new castings with a 50 deg. instead of 45 deg. So for Monarchs I will have a new 12" (1/2 size of one of my 24") straight-edge with 50 deg angle. I have only seen this issue on Monarchs.

When we use to rebuild and scrape 2 or 3 Bridgeport's a month we made special cradle skids that the column sat in and they had pivots on each end so we could pull a pin and rotate the column faster then using straps to rig flip it from back to dovetails.

New machine builders use a dummy or template plate or combo straight-edge that had an exact copy of the riding part was scraped in the inspection area and the Techs used it to speed up the process. You can see them in the books mentioned above. You saw them especially on 2 Grinder V ways or Grinder flat and V ways that are used instead of using a flat camelback and level sled like a King-Way or level sled to check parallelism.