Dovetail Measurement?

[Anorak Bob]

My little ball turner will utilise a dovetailed slide to carry a tool post, a more simplified version of the design favoured by Steve Bedair et al. Today I braved the oven like heat in the shed and cut the dovetail in the round base. I milled a 1" x 3/8" slot then cut the dovetail with a 1 5/16" x 60 degree cutter, cutting one side then the other. And that is where I encountered difficulty. Naturally enough, I wished to have the dovetail centred. Measuring the distance between the dovetail and the outside of the round base proved troublesome.

Initially I used a micrometer and a dowel pin but found it difficult to ensure that the mike spindle was located exactly at right angles to the base centreline and the mike anvil was sitting square to the ground dowel. My small dial verniers have thin jaws and proved pretty useless in measuring this dimension but my 300mm vernier is a touch more robust and the jaws sat along both the dowel and the outside of the base but proved a touch unwieldy. After a number of passes with the cutter creating dust I ended up throwing in the towel with an error of about 0.0003" out of whack. I'll live with that.

How would you blokes tackle the measurement? No avoidance by saying you'd use the DRO on the mill.:roll: I haven't got one.

After all the rooting around nibbling with the cutter, the base of the slot didn't look as pristine as when I started. A nicely trammed spindle results in a not so nice finish anyway but I thought I'd blue the base of a sine bar I have ( thanks to Alan "C-47") as a master and check it out for flatness. Not bad.

BT

[Stustoys]

Hi Bob,
I think you might have added an extra 0 this time?

Does saying "I'd do it on a rotary table" count?

Was a great job for your shaper ;)

Looking good so far.

Do you have some packing under the clamp piece?

Stuart

[pipeclay]

Did you consider after centreing your initial cutter, setting up a dial indicator and just moving your cutting tool the desired amount to cut the slot for the dovetail,then using the dovetail cutter to finish.

[krisfarm]

Hi Bob,
How about you place a single large V block vertically up against round face opposite the dovetail and then use your nice big set of Swiss verniers to measure over the outside. The flat ground base of the v block and the side of the round bar should give you two good long flat surfaces. The results you achieved are very good, I know the feeling you get when you are measuring against a round surface you are never sure you are at the same spot each time. Those dovetail cutters are not in my experience the most accurate cutting tools as well, every pass seems to cut a little bit more off- lots of spring cuts and then they just rub.
Bob

[Anorak Bob]

Stu,

While maths isn't my forte nor is typing, I did mean 3 tenths. I had contemplated the rotary table but direct mounting seemed easiest and also avoided any clearance issues.
Bruce and I had a discussion earlier today regarding using the shaper for dovetail cutting. Now if I reckon I've had a challenge cutting this on the 13, how do you reckon I'd fare with the Douglas? Not well.:no:
I will cutaway most of the underside of the clamp tomorrow leaving a heel for it to pivot on.


Peter,

I just read your comment. In hindsight, I should have done as you suggest. Would have saved a lot of nibbling. I do have an indicator mounted on the mill's Y but I had scribed a pair of lines on the base with a height gauge thinking it was easier to cut most of the iron away with a smaller 1/2" cutter. Then I used 31/32" slot drill, moving the cutter to finish to size. Whilst I have an inch slot drill, you can bet your bottom dollar the slot would have ended up wider than an inch if I'd used it.

Bob.

[Anorak Bob]

Hi Bob,
How about you place a single large V block vertically up against round face opposite the dovetail and then use your nice big set of Swiss verniers to measure over the outside. The flat ground base of the v block and the side of the round bar should give you two good long flat surfaces. The results you achieved are very good, I know the feeling you get when you are measuring against a round surface you are never sure you are at the same spot each time. Those dovetail cutters are not in my experience the most accurate cutting tools as well, every pass seems to cut a little bit more off- lots of spring cuts and then they just rub.
Bob

Thanks for an obvious to everyone but me solution Bob. Never thought of the humble block.:doh:

Towards the end of the nibbling the amount removed didn't tally up with the displacement of the cutter hence me jumping ship when I did.

BT

[localele]

Are you going to use a jib strip in there as the clamping method Bob or use a bolt to pull the dovetail up as a clamp?
I would use a strip with grub-screws from the side to lock it in position as the top half is then pulled down against the base which would be more solid than having the dovetail pulled up.This will also take care of the minuscule error you have reported.Given the difficulty in measuring it you may actually be spot on already.

[cba_melbourne]

I too recommend using a gib strip:
- you will not even have to measure the dovetail, its dimension becomes irrelevant (as long as you are not >1mm off :D)
- you can adjust the gib to be tight (stiff), so it won't move during light cuts even if not locked
- you can add a an extra lock screw (or T-handle) if needed for heavier cuts
- you can easily loosen up the slide to remove chips and dust from the cutting that takes place right above it. I could imagine the slide could otherwise quickly lock up itself.
- and you can correct for wear (unlikely to matter in your case, but still a plus being able to correct for)
Chris

[Abratool]

Bob
I sent you an email on this only a few mins ago before reading the post on the Forum.
Mentioned that I woke up through the night thinking of the problem (perhaps I have a problem) & it would seem "Krisfarms" solution & what I was thinking were the same. The Vee Block idea would be good.
However, I offered another alternative of machining a small flat diametrically opposite on each side of the cylinder parallel to the dovetail ( on the same set up).....
To allow the flat of the micrometer to seat easier.
This idea would best be done in your, you beaut indexing head, so its too late to use this method.
In any case I think for a Ball Turner 3 tenths or .0003" is plenty good enough.
regards
Bruce
ps Please dont let the heat in Perth at the moment get to you.