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Poor Man’s Reverse Chucking System

Discussion in 'Tutorials and Tips' started by Dennis J Gooding, Aug 22, 2020.

  1. Dennis J Gooding

    Dennis J Gooding

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    Location (City & State):
    Grants Pass Oregon
    This is not for production bowl turners who merely want to neaten up the bottoms of bowls and platters, but is an essentially no-cost system for turners who want to decorate the bottoms of turnings without being hampered by a live center in the way. Some alternatives to this method include vacuum chucks and special chucks such as the donut chuck, the Longworth chuck and Cole jaws. These are either expensive or time consuming to make and in some cases, cumbersome to use. I have a fully plumbed, remotely switched vacuum system at my lathe, but quite often, I still use the method described here to reverse turn pieces. I find that it often takes more time to set up the vacuum system and center a work piece than to drive a few screws with a cordless screw gun.

    Fig1.jpg

    Figure 1



    Fig2.jpg

    Figure2



    Figure 1 shows an example of the system that I have used for countless turnings. It consists of a disk of ½ inch or thicker plywood or particle board with a diameter within the swing capacity of the lathe. Ideally a faceplate would be dedicated to the system. If that is not practical, then a tenon can be turned from a piece of durable wood to fit an available chuck (preferably large-jawed) and screwed and/or glued to the disk. (Note that the disk shown in Figure 1 has been used many times.) The only other components are a few sheet metal screws and some small pieces of ¼ to ½ inch plywood which generally can be reused many times. (I did not have a work in progress at this writing, so the illustration uses a previously completed bowl.) Figure 2 shows the disk that I used to turn the bottom of the 20-inch walnut tray shown my members gallery on this site. Note the 7/8-inch holes; they were bored to allow checking the thickness of the tray as the work proceeds.

    The system is used as follows: If a means is available for quickly centering the turning and holding it against the disk temporarily , then the lateral restraint blocks (I generally use four) can be placed snugly against the outside of the rim and fastened with two screws each. If there is no quick reliable way to hold the bowl in a centered position, then pencil several concentric circles on the disk and choose one just smaller than the bowl. Now mount the lateral restraint blocks to just touch the selected circle. Then, with the lathe spinning, use a parting tool to nip away the inner edges of the blocks until the bowl will fit in place. Finally, For plates and bowls with outside edge steepness less than about 60 degrees, install the hold-down strips as shown with a piece of tape or other cushion under them to prevent marring the bowl. If the sides of the bowl are too steep to accommodate the hold-down strips, one can use clear strapping tape wound around the bowl and the disk, while avoiding the area to be turned, to hold everything in place.

    The same basic approach and the same components can be used for hollow forms. In these cases, the lateral restraint blocks would be installed on the inside the form. The blocks would be mounted on a circle a bit larger than the opening and then trimmed to fit using a parting tool. Again, clear strapping tape can be used to hold the form snug against the disk. There is another hold-down method that I generally prefer for hollow forms and that ilk. It is described in a companion posting “Simple Hold-down Method for Reverse Turning Hollow Forms and Such”.

    In addition to being cheap and fairly fast to use once it has been set up for the first time, this system has other advantages compared to some of the alternatives. They include:

    It can be used with porous or holed turnings.

    It can be used in some cases with turnings that have fairly large gaps in the rim by positioning the blocks to avoid the gaps.

    It can be used for turnings that have gone oval or otherwise distorted, by positioning the blocks appropriately.
     
    Dennis Weiner likes this.
  2. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    Thanks for sharing Dennis. How do you remotely turn on your vacuum pump? I was told you have to bleed it before shutting it down, or you could damage something. I have to be part contortionist in order to remove my pieces, a remote switch would be nice. I deleted your double thread.
     
  3. Dennis Weiner

    Dennis Weiner

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    Hey, Dennis J. And Emiliano
    The Links are not working as I can not view the attachments.
    Dennis W.
     
  4. Dennis J Gooding

    Dennis J Gooding

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    Emiliano, I never heard of the need to bleed a vacuum pump and I can't imagine why. I built my system around a surplus Gast pump about 15 years ago and have had no problem. It is noisy though so I put it in separate storage room and plumbed it to the headstock of my lathe where I have a bleed valve to adjust the vacuum. Just opening the bleed valve or dismounting the turning will end the vacuum when the pump stops. I turn the pump on and off using a Long Ranger remote controller.

    Separate subject: The duplicate posting that you deleted must have been the good one. The survivor does not display the pictures. Can you delete this and start over?
     
    Last edited: Aug 24, 2020
    Emiliano Achaval likes this.
  5. Tom Gall

    Tom Gall

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    I’ve heard the same thing about “bleeding” the system (don’t know if that is an accurate term) many years ago. I believe it only refers to rotary vane pumps ..... something about the vanes slightly rotating backwards after shutting off under vacuum and perhaps getting hung up. Don’t know if that is true or not. I’ve been using vacuum chucking since 1994 and have been “releasing” the vacuum all those years. I close the ball valve (pump on) and remove the work piece – open the ball valve and then turn off the pump. I also have an inline blowgun valve installed for a quick release if needed. All valves & switches are close to the headstock.

    “I find that it often takes more time to set up the vacuum system and center a work piece than to drive a few screws with a cordless screw gun”.

    I have to disagree here. Don’t know how your system is set up – but on mine….I just slide my rotary union into the hand wheel side of the spindle (2 seconds) ….mount a vacuum chuck on the spindle (10 seconds) …. mount & center the workpiece and turn on the pump (15-20 seconds max.). It would take me longer than that to find my screw gun. :D
     
  6. Dennis J Gooding

    Dennis J Gooding

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    Well, I guess you are a speedy guy Tom. I would add another zero to these numbers in practice, particularly the centering part. Unless the piece has a center mark to guide a live center or is already mounted on chuck and you have means of mounting the chuck directly on the tailstock or on the live center, it can take a long time to accurately center a piece on a vacuum chuck. Also I find that sometimes because of lack of uniformity of the vacuum chuck seal, the turning can move slightly when the tail support is removed. Lastly, if you should need to check bottom thickness during the base turning, you will no longer have the above aids to remount. Slow Guy :(
     
  7. Tom Gall

    Tom Gall

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    Dennis, I always have a center mark (well 99.9% of the time :)) and when I forget there are methods to get very close to the exact center. And, as you explained in your post #1 you could / would have the same problem with your method…and then have to add pieces of wood around the perimeter and others for securing the piece against the disc. That has to take longer.

    Re: possible slight movement when tailstock support is removed. If you leave the live center engaged until the last 1/8” or less any possible movement is negligible at this point – only sanding remains ….. and even sanding can be done before removing the support to turn off and sand the tiny nub.

    Re: checking bottom thickness, etc. I try to account for this during the turning process. I always decorate the bottom (foot) so I usually include a new (small) center mark and a few rings and some texturing near the outer edge. This makes it easy to re-mount on the vacuum chuck in case there are any mishaps in the future.

    Not trying to be difficult here, Dennis ….. just disagree with the premise that a vacuum chuck would take longer than your method. You’re not a slow guy ☹ ….. I’m just a “half-fast” guy! :rolleyes:
     
  8. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Dennis, do you plan on editing your first post to add the photos and attachment? It seems sort of useless without those important pieces of information. As to deleting the thread and starting over, I don't like that solution because it would delete all of the comments which isn't fair to everybody else.
     
    Emiliano Achaval likes this.

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