Clocking and milling an existing dovetail

[Antonyong]

Yeh, i'm in a bit of a dilemma. I recently noticed that the crosslide ways on my 9x20 lathe weren't parallel. After measuring either end of the dovetail i found that there is a taper of around .4mm! Unfortunatly my warranty has expired and re-machining the dovetail on my mill we be a last ditch attempt at saving money on having to buy a new part.

I put the saddle of the lathe onto my tilting vice so that i could try to match the angle of my cutter to the angle of the dovetail. Then i tried clocking the other side of the dovetail with my dial indicator without much luck. I can't get both planes of the dovetail to line up close enough to take the cut.

Basically, is there a way i can cut one side of a dovetail so that both sides become parallel?

[HuFlungDung]

I'm not sure why you would set the part in a tilting vise. By setting the part up askew, you cannot take advantage of the common plane shared by both sides of the dovetail.

I would get the proper angled dovetail cutter and set the bottom plane of the dovetail perfectly parallel to the table. Set this up carefully and do not spring the part in clamping. Use 3 point support for clamps and indicating level. Then add the 4th clamp and shim or jack the part so that tightening the 4th clamp down does not spring the part.

After that, then you also need to indicate that the carriage surface is square to the dovetail. Otherwise, you might machine both sides of the dovetail parallel, but out of square to the machine. You'll have a most unusual lathe after that

When you've got that square, then you can start to check the angled surface of the dovetail and find out which, if either, surface is correctly cut. You might need to trim both sides.

[lerman]

Adding to what Hu... said, the easiest way to indicate a dovetail is to put an appropriately sized ground dowel in the groove and indicate the edge of the dowel. Be sure to set the indicator at the proper height.

Ken

[Antonyong]

Umm... i was kinda wondering if it was possible to do without a correctly angled dovetail cutter?

[HuFlungDung]

Tilt the head on your mill, or use a shaper.

[handlewanker]

Hi, scrape it!
I took .013" off of the Vee slide ways on the bed of my 10" X 30" lathe by hand.
Make sure which slideway is square to the axis of the lathe centres and scrape the other to it using a dovetail shaped gauge or you'll cut concave or convex when facing.
Don't touch the bottom of the dovetail.
Use two round steel rods, one on each side and the length of the slideways, and having a diam to touch the middle of the dovetail face and bottom.
Measure over the outside of the steel bars to check taper and squareness to the lathe centres.
You only need a steel block, about 2" long with a dovetail on one side and some mechanics bearing blue to check the vee of the dovetail while you progressively scrape it, and the round bars to check parrallelism and squareness.
The dovetail block should be made first to the shape of the dovetail at the narrowest end.
If you don't want to go the "scrape it" path, and a dovetail cutter is too expensive, then if you can get the saddle on the mill set-up correctly as noted before, and clocked true, just make up a flycutter from High Speed Steel, to the dovetail shape, and as you've only got to take out a bit is probably the simplest and least expensive way.
The fly cutter is set to touch the dovetail side and NOT the bottom.
The fly cutter will probably have a swept diameter of 2" and so should rotate at about 100 rpm for cast iron, faster than this will dull the cutter and work harden the dovetail face, apart from leaving chatter marks.
It is most imperative to make sure that either side of the dovetail is square to the lathe centres, and to cut the opposite side.
You can finish off the dovetail face by making a dovetail shaped block of wood and carefully lapping the face of the dovetail with some wet and dry emery paper glued to it.
I can just see the purists frothing at the gills at the thought of using emery paper to finish a machine part, but if you're not up to scraping it will do.
The final test is to blue the crosslide and see where it fits on the saddle.
Don't be tempted to use grinding paste between the slides as you'll never get rid of it and it gets into everything.
Ian.

[lerman]

http://www.gadgetbuilder.com/ToolHolders.html shows a DIY dovetail cutter.

Ken

[handlewanker]

Hi Ken, thanks for the info and website on the DIY dovetail cutter.
I like the designs of the Quick Change Tool Holders, and the tangential tool holder is very interesting.
I used to go to a model club in Melbourne in the mid 80's and Ken Burke, who made and sold the Burke type Tangential tool holder, was a member there too, always seen with a frown on his face and a cap pulled down to eye level.
I've been using a QCTP of my own design for the last 30 years or so and wouldn't be without it.
It's a home built one with a single dovetail clamp type and was made for my 2-1/2" centre height lathe that I made in 1966.
Sometime ago I was doing a job on my 10" swing Colchester Bantam, 1920/30 vintage, and used this although small, tool holder set-up as it had 8 tool holders, and am now making a 4 sided QCTP with 8 tool holders to suit the bigger lathe.
Ian.