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View Full Version : Dovetail Scraping....Corner Relief Needed?



gzig5
12-20-2012, 01:03 PM
I'm going to re-scrape the compound and cross slide on the SB 10L I'm rebuilding before putting it back together. The wear isn't too bad but I'd like to have it "right" from the get-go. It's a hard bed with no wear that I can determine and the under side of the carriage looks pretty good for now. The inner corners on the female dovetails don't have a relief groove, they are essentially sharp. My understanding is that I will want to mill a 1/8" or so groove down in those corners to provide some space for the scraper. Is this correct?

I'm in the process of making a 60 degree prism out of a hunk of cast iron to use for spotting. I have a heavy duty angle table to set the bar up on the mil l for the rough cut. Should I strive to make that angle 60 degrees exactly, or a few degrees shallower? I'm confident I can get the rough cut to whatever angle I'm shooting for, but once I start scaping the prism faces I'll want to check that the angle is correct and isn't changing over the length. I don't have a good protractor to measure with, so I figure I'll have to check the slope with an indicator with the prism fixtured on the mill table. Any better suggestions for getting that angle right and keeping it right across a 13" length?

Before anyone suggests it, I have Connelly's book, I just haven't read it all through and the questions popped into my head today. Done a bit of scraping on flats but this will be my first dovetails. Will practice on an Atlas lathe compound I have extra.
Greg

Dave S.
12-20-2012, 01:08 PM
I built a corner relief plane that works well. My Horz mill has the same problem and needs scraping.

Dave

http://i132.photobucket.com/albums/q3/Dave_Sohlstrom/000_0511.jpg

http://i132.photobucket.com/albums/q3/Dave_Sohlstrom/000_0512.jpg

Mcgyver
12-20-2012, 01:49 PM
the plane is very neat.

imo the corners do need to be relieved. mill or I think Connely even shows using a hacksaw blade....a section of a thick one from a power hacksaw blade might be just the thing....or make one of Dave's planes. While you can't scrape into a sharp corner, you can file away the corner of the mate so the corner isn't bearing...but I think relief is the correct way and has been there on all the projects I've done

Your reference flat only needs one scraped surface and it needs to be acute to the angle you want to scrape. to get a precise angle, you make a small angle gauge of two surfaces. scrape it precisely using sine and indicator. if you get the two surfaces of the perfectly flat the angle will be same anywhere along them so the angle gauge need not be the length of the work.

I've made lots of small scrapers over the years for getting into tiny dovetails. A thin piece of narrow carbide is what you want....will let you get into the corners. Also, this is an instance where the forged file works very well as you can taper to a very thin edge.

Furthermore, the angles of the dovetail need not be exact. What matters is the horizontal elements are parallel and the two dovetails are parallel (ie the imaginary apex's of the four surfaces are parallel). Frankly, I wouldn't worry about the angles. Get the horizontals on the long part parallel (ideally coplanar). Scrape one dovetail flat and the correct angle to the lathe axis (not perfectly perpendicular). scrape the other parallel (across pins mics the same). Use that piece to spot its mate.

TGTool
12-20-2012, 03:24 PM
Another tool for relieving the inside corner is a cutoff wheel on a grinder - air or electric. Don't forget the safety glasses and don't let it get away from you.

daryl bane
12-20-2012, 05:36 PM
I've used a large dovetail cutter set at a angle to mill a relief. Getting the scraper in a small dovetail can be a problem.

TexasTurnado
12-20-2012, 07:04 PM
I milled mine, since it seemed easier to do and more precise:

http://i288.photobucket.com/albums/ll168/TexasTurnado/P7070077_zpsb0746711.jpg

Make sure to realign to the dovetail if one side is tapered for a gib - I used a piece of drill rod held in the dovetail to align with a test indicator clamped to the spindle. I believe this is a .062 thick cutter....

J Tiers
12-20-2012, 08:30 PM
A piece of hacksaw blade works about as well as anything short of the super-cool relief plane. I like the corner rounded, so I have a piece of HSS with a small radius on it to clean up the hacksaw cut.

I like that thing, partly because it seems like a very good way to get the corner done the same depth everywhere, but also because it's just cool. With a rounded cutter, you could get some really nice looking corners.

part of scraping is actually the details and finish... not really adding any function, but why should a quality job be ugly?

gzig5
12-21-2012, 06:18 PM
Thanks for the responses. The relief scraper is a neat idea. I had hoped to mill the groove like Turnado shows, but I can't nod the mill head and the carriage may not fit on the angle table I borrowed for this so I may end up making something like that. I got an 18" piece of 1 1/2" square cast iron and will make the prism 13" long which will cover the 12" surfaces on the carriage down to the 6" long compound slide. I will have enough left over to make a couple short test gauges. I have also borrowed a 5" sine table which will be handy for setup on the mill and for checking as I go on the granite.
The angle table I borrowed from a friend is a Beast! It will bolt to the table and the vise will bolt to it's top.
http://i584.photobucket.com/albums/ss287/gzig5/P1000042_1_zps0cd8ef1a.jpg

TexasTurnado
12-21-2012, 11:21 PM
Thanks for the responses. The relief scraper is a neat idea. I had hoped to mill the groove like Turnado shows, but I can't nod the mill head and the carriage may not fit on the angle table I borrowed for this so I may end up making something like that. I got an 18" piece of 1 1/2" square cast iron and will make the prism 13" long which will cover the 12" surfaces on the carriage down to the 6" long compound slide. I will have enough left over to make a couple short test gauges. I have also borrowed a 5" sine table which will be handy for setup on the mill and for checking as I go on the granite.
The angle table I borrowed from a friend is a Beast! It will bolt to the table and the vise will bolt to it's top.


Here are a couple of other ways to solve the problem:

http://i288.photobucket.com/albums/ll168/TexasTurnado/P5300078_zps41e04276.jpg

http://i288.photobucket.com/albums/ll168/TexasTurnado/PC200108_zps92e7b8e0.jpg

In the first case, I milled the aluminum blocks at the needed angle and bored them for 1.000 W-1 drill rod. The grade 8 bolt to hold the saddle on is threaded into the drill rod (rather than the aluminum).

In the second case, the plates are drilled to align with the screws that hold the way scrapers on - the angle plates at either end allow the angle to be adjusted to what is needed as the clamps are tightened.....

Note I was doing rather light milling as shown (removing a few thou to straighten and make room for Turcite or milling the Turcite itself for oil grooves) - a more robust arrangement would be needed for heavy cuts. I also used spherical washers on the clamp straps to prevent binding when tightening the nuts.....