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View Full Version : Which way for dovetails?



form_change
09-29-2010, 09:07 PM
I've finally attacked the castings sitting in the shed for the last 5 years and am starting to machine up a Stent T&C grinder (for those not familiar, it's a English T&C grinder design based (I'm told) on a Clarkson T&C grinder, but small enough that it can be made in a HSM shop).
However, one thing that perplexed me was the way that the dovetails were dimensioned. I've always assumed that dovetails were dimensioned so that the slide was pulled down onto a large bearing area where possible (for least wear), and that the bearing areas were spread as far as possible (for best stability). This has been dimensioned in the opposite way.
It could be a historical carry over, as the design is reasonably old and done this way to avoid the need for equipment able to machine (comparatively) large flat areas as at the time of design a lot of model engineering milling was done on a lathe (that is, mills were not as common as they are today). However, what is the consensus on how dovetails should be set up?

Michael

J Tiers
09-29-2010, 10:03 PM
I'm not sure how you mean.

There are two ways it can be done. Assume you have a "keystone" shaped dovetailed piece sticking out from a surface or structure.

One method is that you have a bearing on the dovetail surfaces and the TOP of the "keystone".

The other is that you have a bearing on the dovetail surfaces and the area of the base surface adjacent to the "keystone" with NO bearing on top.

The first way is fairly common with things running on dovetails on a piece that is not much wider than the dovetails, like the knee on the face of a mill, or the mill saddle on top of the knee (although the second way is also used for mills)

The second way is common with lathe saddles, compounds, shaper rams, mill tables etc.

I really don't know a good reason for one over the other. The difference in width of base is maybe 20% in most cases. The second method is maybe more "enclosed" in the case of a short horizontal way, but the gib extends the length of the "enclosing" part. The first method requires less overall width, and the supporting structure can be narrower than the dovetails.

If the lower part is the "enclosing" part, as with a mill saddle and table, the gib can be short and the long moving part can have a wide bearing with the second method, and the bearing surfaces have no exposed parts facing "up". That isn't true of a lathe compound or crosslide, though, where there is exposure with either method. Normally, the enclosing part would have the gib.

For a grinder, preference is normally given to an "enclosed" design, where there is less exposed surface, and surfaces which are exposed are also set up so that nothing can fall onto them. Maybe that explains it, depending on the design.... or maybe the narrower base was the advantage.

form_change
09-30-2010, 12:31 AM
I have both types -
The type using the top of the key stone is for the main table slide and the top of the dove tail is only around an inch wide, although the table width is 2 1/2" wide. In this application I would have thought the other use preferable to give greater stability to the table.
The second type you describe is used to move the motor back and forth (perpendicular to the main table direction). Here though the top of the keystone/ dovetail is around 2" wide and the total width is around 2 1/2". As such there is not much material that the motor mount is bearing on - a strip say 3" long by 1/4" on one side and 3" by 1/8" on the other. Here I am thinking that the top of the dovetail should have been used to distribute the pressure over the interface and prevent localised wear on the slide.