Dovetails

Hi chaps I've been having a few problems with my lathe not being able to part of very well I've found the problem, or at least on of them anyway it's the top slide that's badly warn, but not one to panic I removed it from the lathe (Colchester bantam) took some measurement to find the wear and measured the dovetail angle at 50degrees, the plan is to set it up in a sine vice and skim of just enough on the surface to make it well again, the problem is after setting the sine vice to 50degrees the dovetails don't match the angle so the question is does any one know what the angle is supposed to be? My sine vice rolls are 100mm apart so I have used gauge a gauge block stack of 3.016 inch and checked with an electronic angle gauge which also reads 50degrees, please help if you can it's bending me head so to speak.

Many thanks Alan.

How much don't they match by? could it be the complementry angle of 40deg that you need?

I've had another look Jason and 40 degrees looks about right, could you explain the complementary angle thing as I don't under stand what you mean.

Alan.

I thought standard dovetails were 60 degrees (complementary angle 30 degrees)?

It really comes down to the surface you measure from or the angle of the part in relation to the cutter.

For a triangle with one corner at 90degrees, the other two angles must also add up to 90. If one know one angle in this case your measured 50degrees then the remaining angle is called the complementry angle so 90-50 =40.

You can see here I have drawn out the triangle, the long hypotinuse is 100mm as per your sin bar and angled at 50deg. The measuring box is measuring the angle between the two black lines and shows 40deg

It probably comes down to measuring the angle from a horizontal surface but setting it up in the mill with a vertical cutting edge.

Another time this often crops up is where people try thread cutting and set the topslide to 29.5 deg from the lateh axis when it should be 29.5 degrees from a line at right angle sto it so the angle scale on teh topslide should really show the complementry angle of 60.5deg (90-29.5 = 60.5)

That's what you can buy dovetail cutters for, but it's not used by some machine tool manufacturers. My lathe (Harrison) and horizontal mill (Adcock & Shipley) are 55º, and the Bridgeport is 50º.

Andrew

Resurrect this one just for confirmation, theres no set standard.

Harrison users group state lathes dovetails are 60 degrees, have measured 55 degrees on last 140 scrapped 7yrs ago and 100% 55 degrees on current which will share some parts with your Bantam made under the 600Group banner.

Hi Jon it's 50 no mistake but trying to find a dovetail cutter to match is a PITA it needs to be a 30mm cutter also, Jason was right the with complementary thing.

Alan.

The math way will be accurate but the lazy person stuffs some milliput (epoxy putty) surrounded by clingfilm into the dovetail and then uses a protractor..

Hi Gary,
I've neither seen that method before or thought of it. I will have to remember it for future use. I like it.

Les.

Edited By Les Jones 1 on 23/12/2016 18:52:12

I've been having a few problems with my lathe not being able to part of very well I've found the problem, or at least on of them anyway it's the top slide that's badly warn

The simple approach to your problem is to lock the top slide with the gib and use the cross slide for cutter advancement, surely? Repairing the top slide is a completely different matter - unless you were looking for an excuse to do it? Or is your cross slide in need of repair, too?

Hi Alan,

I have a dovetail cutter here, that I had re-ground to 50 deg, when I made a new topslide for my mates bantam a few years ago.(Hacked out of an old B.R. rail chair)

If you can wait a few days you are welcome to borrow it.

Nick.

PGK thanks for the hint, dove tails coming up in My future also.

Nice one Gary for bringing back and showing the correct way to measure the dovetail angle. Trouble is, basic trigonometry and English spulling have suffered thanks to electronic calculators( inability to actually USE trig never mind how many gizmos included) and spell check for spilling.

Surprising how many were away from school the day when maffs woz tort let alone inglish. - thERE are and not thEIR are and part ofF not of when cutting a bit OFF.

Petty and Pedantic? maybe but it sure makes reading posts hard at times trying to decipher the intention but as pgk pgk has already inferred "the lazy person . . . . . . "

Regards Ian.

.

... I think perhaps you meant 'implied' [or perhaps maybe even a simple 'stated'].

MichaelG.

.

Edited to correct my sloppy use of the same word twice in a short sentence

R.I.P. my old English teacher [J.P. Mulholland]; you may stop turning in your grave now.

Edited By Michael Gilligan on 24/12/2016 16:19:53

He may well have used heuristic reasoning based on observations to reach that conclusion though, so both may be correct

- Nick

**LINK**

ALLWAYS some smart***e tries to correct the corrector. Probably accounts for the difference in post counts.

Seasons Regards Ian.

Tut Tut, Ian ... There is really no need to be so unpleasant.

Surely you could tell; from the language used, and the 'devilish smiley' included; that my post was light-hearted.

MichaelG.