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Milling Machine Jig for Machining Masts, Spars etc


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In the past I have constructed many ad hoc fixtures to enable the accurate machining of masts, spars, yards, booms etc. I decided to have a go at making something more versatile that would work for items of different shapes and sizes. Having made it it seems to work well so I thought it would be worth sharing.

 

I started out with a set of design aspirations. For ease I will refer to "masts" rather than go through the full range of parts each time. 

 

1 Provide solid clamping along the length of the mast.

2 Locate / relocate on the milling table without the need for alignment / set up.

3 Positively locate and relocate the mast so that I can easily remove and replace it on the mill.

4 Clamping devices not to mark / damage the mast.

5 Clamp parallel and taper masts.

6 Clamps to be easy and quick to operate.

 

I started with a clamping concept based upon eccentric circular cams and the build started by cutting a piece of 3/4" MDF to sit on the milling table. I used the mill to accurately drill a series of holes along the length of the MDF to take the cams.

 

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The cams themselves were turned (circular) from hardwood. An eccentric hole was drilled along the axis of the cams before they were separated.

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The cams are mounted on the MDF using a pin. The pin protrudes below the bottom surface of the MDF and the protruding part is cut to a diameter .001" smaller than the slot in the milling table.

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Once the pins are pushed into the MDF they give positive and repeatable location on the milling bed.

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The top of the pin locates the cam which is locked by a wing nut. The additional holes allow the cam positions to be varied to suit the mast being worked on.

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Holes at either end of the MDF take the "T" nut bolts which attach the MDF to the milling table.

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The MDF was then placed on the milling table (located by the pins) and a row of 4 holes were drilled parallel to the pin holes. Into these holes were placed accurately made dowels. These dowels provide the "fixed" support against which the cams clamp. I think this will become clearer in later photos.

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A simple piece of wood is then placed up against the dowels. This forms the face against which the mast is clamped. In the following picture a mast is clamped in place. Because the cams act as a finely tapered wedge hand rotation is enough to very rigidly hold the mast.

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The cams give a lot of flexibility on the diameter of mast that can be held - .200" to .700". But larger is possible by using a narrower wooden strip.

 

At this stage I checked the alignment of the mast to the axis of the mill. The run out was .0015" over a 12" length. Much better than I expected.

 

I did however need an end stop to control the position of the end of the mast. This was relatively easily achieved and for good measure I included an option for 3 positions. See Photos:-

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The solution to dealing with taper masts is straightforward but does require a bit of trigonometry. The taper is achieved by changing the diameter of one of the fixed dowels. This is done by making a collar to fit over it. This gives a triangle the base of which is the distance between the first and last dowels and the "opposite side" is the thickness of the collar wall = (outside diameter - inside diameter)/2.

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In the last picture I replaced the wood strip by a steel bar - but this proved to be un-necessary. I still have a few bits to develop but I think thats enough for now - except for the mystery of the missing wing nut!!!!!!

 

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  • 5 months later...

Thanks Keith for your original post on this, and thanks Bob for the link to the Cam clamps.  I've combined these two ideas to make my own holding jig for the mill table.

 

The base is 19mm MDF - some scrap I had lying around.  I modified Ketih's design slightly by using 1/4" aluminium rod instead of wooden dowels for the rear support. These are a nice fit into the T-slot channel in the Sherline Mill Table, so I've kept the original design idea of having these go right through the bed of the jig and into the Mill Table T-slot channel (for the same reasons that Keith gave). Two Sherline T-nuts/bolts are recessed into the table surface and secure the jig in the other T-slot channel.

 

Then I incorporated the ideas in the video link provided by Bob, to create a series of "dog holes" on the jig base, to provide a range of options for positioning the cam clamps. In the video, he uses 18mm copper pipe - just because he had some lying around. Instead, I used some 3/8" aluminium bar stock - just because I had some lying around.....

 

In the picture below you can see the layout. The 1/4" pins at the back are 1 1/2" long, while the 3/8" "dogs" are 1" long. The "dog holes" do not go all the way through the base - they bottom out at a depth of about 12mm.

 

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Then I made some cam clamps from the downloaded patterns provided as a link from the video. I made these in two different thicknesses in case I needed something thinner for small stock. Here's a pic of one of each. Again, these were made from some scrap plywood I had lying around.

 

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And finally, here's a shot showing the jig in action. The cam levers work surprisingly well. They are quick and easy to use, and the work piece is held extremely securely. The cam clamps show no inclination to back themselves off. In the picture, I've shown five clamps in use - in reality, two or three would likely be sufficient.

 

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The use of spacers, as shown in Keith's original post, is still a good idea. I may get around to making the upgraded cam clamps with built-in adjustable spacers - see the link following the video above if you're interested in this.

 

Thanks again to Keith and Bob for showing the way here. This is going to be a really useful addition to the workshop.

 

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