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  1. #1
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    Default Advice needed, HH style QCTP

    Hi Forum members, I need your valued expertise! Apologies this is going to be long.

    I want to make a quick change toolpost, in looking at the various designs around I found the design of our own esteemed Harold Hall, (link here) I liked everything about his design except for two things, the amount of space taken up inside the toolholder by the dowels that pull it against the post and the fact that If i later replace the post with an Aloris when i become a millionaire I would be unable to use any of the toolholders.

    It needs to suit a 16" swing 5HP macson.

    This is my attempt to design one based on his, Im pretty new to technical drawing and CAD/CAM so excuse the terrible drawings, I have omitted measurements for the most part, but it is all imperial as that what tooling I have available/ It needs to suit.

    The only major limitation I have is that the cross-slide on my Macson has no t-slot only an integral post, I do not want to lose this (some other tooling is built around it) it also has a broad shallow taper and spring loaded indexing pin.

    Unfortunately I dont have a photo of the cross-slide with the turret off.

    20161002_184317.jpg

    Please any feedback would be welcome, more detailed drawings can be provided if needed (As soon as i learn to set solidworks drawing formats up properly) My major concerns are:
    • That the wall near the back of the bottom pin is quite thin (.100") although it doesn't bear the strain of the cam.
    • That the throw of the cam lobe is too short (0.010") Making the level of precision needed in the toolholder dovetail very high.
    • I dont really have room for a spring (other than maybe a belleville washer behind the pistons) to preload the cam
    • No locking mechanism for the cam


    I dont have a surface plate is my only tooling restriction so I try to keep layout to a minimum, I believe I have everything needed for this, although I am strong on enthusiasm and low on experience (maybe only 2 years or so fooling around)

    QCTP2Vertical.PNGQCTP2Vertical2.PNGSection.PNG
    Last edited by caskwarrior; 28th Oct 2016 at 12:30 AM. Reason: links

  2. #2
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    Default

    Firstly, with regards to the surface plate, Carbatec do these https://www.carbatec.com.au/sharpeni...-surface-plate which are not too bad for the price. If ever you buy something larger you can always use it as a cheese platter at parties...

    Addressing your points -

    • Do you even need a wall there? I can't see any great problem with it being open. If anything it may be better as there will not be crap building up behind the pin.
    • You won't need much throw provided that your holders are accurately made (btw, is the throw meant to be 0.100" and the wall thickness 0.010"?) 0.100" should be fine
    • I can't see why a spring is needed as the cam should push in both directions
    • With cam set ups like that, ideally the holders/ cams should be set up so that they stop just at TDC - that way the 'tapers' involved will lock. A stop just past TDC will stop things relaxing.


    The Aloris is nice and if you can cut a 3 start LH ACME thread there is no reason you can not make one. However, they did do a piston style post for a while. That used cams pushing out from a central eccentric. That style of design may be easier to make and as an added bonus if get the sizes right there is no reason that the tool holders will not be usable come lottery day.

    Michael

  3. #3
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    Default

    I do not have much to add other then it will need to be of quite a substantial size as you have a decent lathe. Otherwise it will not be rigid.

    I have a 17" swing lathe with a CA sized chinese QCTP. It is quite large.
    Gold, the colour of choice for the discerning person.

  4. #4
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    Default

    Thanks for the replies, Ill delete that wall.

    Just to confirm a couple of assumptions, standard toolpost dovetails are 30 degrees rather than 45 and does anyone have CXA dovetail dimensions they could send me, I found BXA on the web but no dice for the bigger sizes, I may have to order a sample. I guess one positive to having the wedge be a pull-in style is that I can make one for BXA and one for CXA. Ill go and get one of those carbatec plates this weekend.

    Ralph

  5. #5
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    Ralph,

    Could you get by with a single piston and its associated simplicity? I have a version of a Dickson on my bitty one horse Hercus and while it is the smallest post made by Boeni, they use the same single piston design on their posts to suit 26KW machines. The fit of the piston along with that of the eccentric is reasonably sloppy. Repeatability of less than 3um is guaranteed by the manufacturer so it's all in the fit of the vees.

    I popped the post apart...

    Bob.
    Attached Images Attached Images

  6. #6
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    Hi Bob, thanks so much for a look at that post, its not of a style i have seen before (Much simpler than the Dickson or Multifix) I might go for one piston if machining the cam proves difficult, Im still working out the order of operations for it, i think ill have to put an off-center centerdrill hole in for a center or run the far end in the steady. One piston would let me use a shorter, single cam.

    Have you made any toolholders for it?

    Ralph
    Last edited by caskwarrior; 30th Oct 2016 at 12:03 AM. Reason: more detail

  7. #7
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    Hi Ralph,

    I made a few eccentric cam locks for a swivel base a while back. This might give you an idea - //metalworkforums.com/f65/t1482...82#post1551982

    I should make a few more holders but I'm lazy. I have twelve, ten are Swiss. the other two are of a pretty agricultural appearance in comparison but they fit.

    Bob.

  8. #8
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    It's so annoying when you can't find the picture you want on the web -
    Piston style QCTP
    piston.jpg
    And for completeness, a wedge style

    wedge type.gif

    Michael

  9. #9
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    Note carefully the difference between the Dickson style piston and the Aloris style.

    The Dickson pulls the tool holder *in* tightly against its locating V ways.

    The Aloris pushes the tool holder *out* against the dovetail V ways.

    IMO (and experience) the Dickson method is a lot more rigid than the Aloris method. I'd not willingly have an Aloris piston type QCTP. I'd happily have a wedge type, though.

    My only objections to the Dickson style are the tool holders are a PITA to make and the tool block is pretty bulky. However, both things can be happily lived with, and if one makes a big pile of tool holders at one time, it's not so bad.

    I bought Ueee's big Dickson type to fit to my Monarch lathe and it's a hell of a lot better than the POS 4 way that came with the machine.

    PDW

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