General questions about machine dovetails

[KKrum]

I may simply not have seen enough machines to make these generalizations, but...

1. Why are dovetail bearing surfaces like A instead of like B?


2. Lathe cross slides and compounds are dovetails. All three axes of a mill are often dovetails. Why aren't lathe beds dovetails?

 

[benmychree]

Some old toolroom lathes had dovetail beds. Also watchmaker's lathes.

 

[homebrewed]

I may simply not have seen enough machines to make these generalizations, but...

1. Why are dovetail bearing surfaces like A instead of like B?

View attachment 476493

You need matching dovetails so for every "A" you have a "B". It's what makes the world go 'round .

 

[benmychree]

I suppose the bottom line is that dovetail beds on lathes is not the most practical form of guidance for a lathe bed, subject to more wear and difficult to keep adjusted and free from slop, a vee and flat being much better adapted for long wear without slop, the best being all V ways, such as the American High Duty and Pacemaker lathes.

 

[jwmay]

This might help.
I think that fixed your drawings. You’ll never have one without the other. So the premise is incorrect.

Lathe beds are usually prismatic. I’m taking a WAG that this was the best compromise on cost/performance as determined by the people doing the designing.

 

[KKrum]

@jwmay @homebrewed I made a more complete drawing to better explain what I mean. Contact surfaces red, non-contact surfaces black. I don't think I've ever seen a dovetail made like B, and I'm curious why.

 

[markba633csi]

I think you have more bearing surface the original way- good question tho

 

[Shotgun]

When you go to scape in the top portion of B, that overhang from the dovetail will be a pia. You have to get under the dovetail, but stay over the opposite side dovetail. Whereas, in the bottom portion of A, you scraping from the outside where you have free clearance.

 

[RaisedByWolves]

I think you have more bearing surface the original way- good question tho

This, more wear surface.

 

[jwmay]

How would it be possible that both A and B are not bearing exactly the same? What sort of anti gravity is being employed that the black portions of B labeled as non contact are not contacting?
You’ve drawn two identical things. All those surfaces are bearing surfaces. A sliding dovetail is a sliding dovetail is a sliding dovetail. They all work exactly the same, and the only difference is the shape of the gib strip.
Or…just ignore me cause I’m totally missing something. That happens.