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machine dovetails.

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old mart10/11/2019 16:48:58
802 forum posts
77 photos

The X axis of the Tom Senior light vertical at the museum has a bit of play. It has 18 5/8" travel and if the bed is pulled forward or pushed back, it can move up to 0.004", especially if it is to one extreme or the other. I have checked the knee and cross slide Y axis and when they are locked and they do not move, so the X is the culprit. The Ways on the cross slide are about 14" long, so I assume that the bed is pivoting very slightly as the ends are more worn than the middle. Now I have had a look at the design of the cross slide, it cannot have a 55 degree dovetail cutter run along it to true it up as there is nowhere enough room to get one in. A 20mm diameter one would just fit, but the length of the cutting edge would only reach half of the dovetail surface. The picture shows how close to the dovetail, the cast in bosses for the X axis leadscrew nuts come (top side)._igp2375.jpg

Edited By old mart on 10/11/2019 16:50:49

John Haine10/11/2019 16:56:50
2698 forum posts
138 photos

I assume that your picture got rotated to the right?

Do you need to machine both dovetails, could you get away with just the RH (bottom) one?

May be sacrilege, but could you take advantage of those meaty bosses by machining part of them away to make room for the cutter?

Moglice?

Nigel McBurney 110/11/2019 17:18:18
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623 forum posts
3 photos

If you look at the larger top surface in the photo , the frosting from the scraper is only visible on the centre section,so there is also wear on the flat top part so idealy both the dovetail and the flat both need attention,I visited Tom Seniors many years ago,at the time there was a mill table set up for machining on a vertical mill, they followed machine tool manufacturers usual practice of having a deep cast iron "box" on the mills table,the top of the box was machined flat and the box stayed in place ,and possibly never removed.so the mill table plus the fixture was solid and remained dead flat and could not be easily distorted when clamping the work the be machined,I do not remember seeing a large shaper in the works. so I should imagine they had special size dovetail cutters made to suit the job,there were lots of specialists making cutters in those days. A big vertical mill in good condition is required to machine this job as it is so easy to get the xy axis out of square, and keep the top of the table to the z axis square.

old mart10/11/2019 17:20:07
802 forum posts
77 photos

Its the narrower gap that would need machining, the other one incorporates a tapered jib. The minimum diameter of cutter which would cut the entire dovetail is 35mm, and that size would cut about 2/3 of the wall thickness of the bosses off. Yesterday, we removed the cross slide and I relieved the centre portion of the dovetail with a file. The area which was filed allowed for a minimum of 1 1/2" bearing surface remaining at each end at maximum travel of the X axis. The travel is asymmetric, being slightly further to the right than to the left, actually leaving about 4 1/2" at the left end, and 3 1/2" at the right, so something like 6" was relieved.

The mill was re assembled, but we did not have time to find out if there was any improvement. Next Wednesday we will know.

I wonder how Senior cut the dovetail in the first place?

Edited By old mart on 10/11/2019 17:23:13

Nick Hulme10/11/2019 17:49:06
711 forum posts
37 photos

If there isn't space for a dovetail cutter then it may have been done on a horizontal mill, I'll let you imagine the setup but it's not rocket surgery

old mart10/11/2019 17:51:35
802 forum posts
77 photos

I would guess that Senior used something like this, if they used a mill:_igp2482.jpg

Pete Rimmer10/11/2019 18:20:40
486 forum posts
24 photos
Posted by old mart on 10/11/2019 17:20:07:

Its the narrower gap that would need machining, the other one incorporates a tapered jib. The minimum diameter of cutter which would cut the entire dovetail is 35mm, and that size would cut about 2/3 of the wall thickness of the bosses off. Yesterday, we removed the cross slide and I relieved the centre portion of the dovetail with a file. The area which was filed allowed for a minimum of 1 1/2" bearing surface remaining at each end at maximum travel of the X axis. The travel is asymmetric, being slightly further to the right than to the left, actually leaving about 4 1/2" at the left end, and 3 1/2" at the right, so something like 6" was relieved.

The mill was re assembled, but we did not have time to find out if there was any improvement. Next Wednesday we will know.

I wonder how Senior cut the dovetail in the first place?

Edited By old mart on 10/11/2019 17:23:13

It was probably originally planed and then ground or scraped.

The wear is quite apparent on the face in view which is concentrated around the ends as you would expect. This will have the effect of 'loosening' the table at the travel extents in the way you describe and coupled to the inevitable wear on the angled vee ways will make the table rock the way it is.

The thing is you cannot fix that by just machining the one vee. You need to machine/grind and/or scrape the top flat AND parallel with the y-axis flat way first and then map the wear in the front and rear vee then figure out where it needs machining/scraping. If you just go at it you'll have no idea if the x travel is going to be perfectly perpendicular to the Y travel both horizontally and vertically.

old mart10/11/2019 18:25:38
802 forum posts
77 photos

You are dead right about my not knowing what is exactly happening, see my thread on 2-4-6 blocks.

Keith Anderson 123/11/2019 13:47:02
2 forum posts

What you have, is normal wear.

The saddle dovetails wear wider at the ends and the table dovetails wear so they're narrower in the middle.

The top surface of the saddle will also have worn to be convex upwards

And that will be matched by the underside of the table wearing more in its middle.

You will also have some degree of bending of the table, as a result of the peening it has received from repeated clamping of work and fixtures.

_______________________________________

please don't rush in to "machine" metal away. You may well create more geometric problems than you cure, and you will create surfaces which will wear into inaccuracy, far faster, and travel less smoothly than a correctly scraped surface.

The two references for understanding what is going on and how to return the machine to schlesinger limits, are; Connelly, machine tool reconditioning

And

Moore, foundations of mechanical accuracy.

There was a yahoo group on rescraping machine tools, and there is a currently very active forum on practical machinist forum, with participants organising scraping courses on both sides of the Atlantic.

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