Dickson type. RDG may stock?

I have made some of these holders with one V. I think two Vs is over constraining and as a result allows the holders to take up different positions when put on the mount. Due to machining tolerances the two Vs can never be identically spaced from one holder to the next or to the mounting block. This means that either V may be the one that controls the position of the holder when it is mounted or it may be the two outer faces or the two inner faces and it is impossible to predict which it will be. The single V works as well as the twin V for rigidity and has better repeatability of position.

Martin C

Martin

Despite appearances the Dickson 2 see system is never over constrained. The system will always take up a properly spaced 3 point location, or rather 2 lines and maybe a point even if there are dimensional errors. The only constraint is that the Vees be parallel and of sufficiently similar angle to achieve line contact on at least one face. Which makes manufacture much easier as parallelism is easy to achieve in any competent jigging and set up for final grinding.

Really accurate depth setting, as needed with one V and a point so that the male and female Vees achieve full line contact on both sides, is not so easy. Tolerance on the point or flat height relative to the Vee is very small. Theoretically zero if the male and female Vees are exactly the same angle. Tightening the female angle a bit probably adds tolerance but reduces stability if taken too far. Had some optical mounts in the lab using a version of the single Vee and flat system. Using in an interferometer set up showed repeatability to be insufficient for the task.

If you analyse the Dickson setup it's clear that one side of one pair of male and female Vees produces the line contact needed for vertical and tilt stability. The minimum point contact needed for rotational stability could be anywhere on the other Vees and the system must work. In practice scuff marks show that any professional standard Dickson set gives line contact on two opposing sides of two Vees. I suspect that the female Vee angles may actually be a touch different to the male ones.

Tool tip positional variation depending on which side of the Vees do the guiding is immaterial out in the real world. It must clearly be very small

Interestingly scuff mark positions differ depending on who made the toolpost and holder. I have a Rapid Original post on one lathe and a real Dickson on the other with industrial quality tool holders from a variety of makers. I blued things up when I was having rigidity issues to see what was going on and it was quite clear that different holder / tool post pairings had different contact line positions.

Eventually I twigged that my rigidity issues were due to the thickness of the bottom collar on the height setting device varying between holder makes. If the collar is too thick it cannot properly enter the snail like slot below the hex of the locking device. So the locking force acts to bend the setting screw rather than pull the holder tightly agains the toolpost. It didn't help that several of my holders had acquired bent setting screws in a previous life. Testimony to the attempts of unthinking gorilliods to substitute brute force for careful examination and sorting of the problem.

Appropriate adjustment to collar thicknesses and new screws where needed sorted the problem. As I recall matters real Dickson collars are thinnest, Rapid maybe 15 thou thicker and the worst "unknown" make almost 40 thou thicker. By the time that one arrived and failed to fit anything I was aware of the problem so rectification was swift. The screw would have failed the EU cucumber specification!

One thing I've never understood about Dicksons is how they manage to accumulate fine swarf inside so quickly. Popping apart to clean is dead easy so its no great chore to do regularly but where does it all come from?

Clive

Edited By Clive Foster on 06/09/2023 10:08:57

Well Richard, that's a relief. I'm obviously not the only one to experience such a problem. I have a couple of Warco tool holders, and a couple of "T51" tool holders from Amazon where the cam either doesn't engage, or is only just in effect.

The solution was some strips of metal about 0.3mm thick and 4mm wide, stuck into the slots with JB Weld. Some tight packing was required until set, to ensure a minimal adhesive thickness.

A small dab of paint reminds me which tool holders have the mod, so that I fit them to the tool post with a little more care.

A quick blip on the googlenet revealed we've been here before:

Dickson Tool holders

and also that Dickson were part of the same group as Colchester so it would be pretty natural for a simple re-badge exercise.

Rgds Simon

Clive, doesn't your third paragraph prove my point? The two lines of contact on opposing sides of two Vs is the same as two lines of contact on one V. By having them separated by a relatively large distance and clearances between the tool holder and central block the tool holder can take up different positions, much like putting a part in a lathe chuck and not getting repeatability. Using one V and a positive fixed stop at the other end gives better repeatability. Even with one V the weak point in the setup is likely to be the small cross section of the piston pulling in the tool holder either side of the cam hole. Since my stop on the unused V is a Loktited grub screw I can adjust the position of the holder so that the pull in cam over-centres (toggles) to lock the holder in place and does not just rely on friction of the cam on the plunger. It always bothered me that the pull in cam on the various bought tool holders I have all ended up with the cam in a different position when tightened up and only one or two over-centred the cam.

The only good reason for two Vs on a tool holder is so that it can be used on multiple faces of the centre block without changing the basic geometry. Manufacturers do not need to make left and right handed holders and stockists do not need to hold two "hands" with the chance of people ordering the wrong one.

I agree that the error from slightly different mounting positions is going to be small but if you are using CNC that small error is unwanted as tool tables become less accurate mainly along the Z axis. Also for someone making their own holders the process of making two accurate parallel Vs is the hardest part. If it can be reduced to just making one V it is much easier.

Martin C

you could try the second hand machinery dealers,used genuine Dickson toolholders are not cheap as they are always in demand,

Unless I am mistaken, that appears to be for a four-way indexing toolpost. I would be interested to see a patent for the quick change post.

Emgee, the problem with two Vs is poor repeatability of positioning when fitting the tool holder to the block. It's a bit like the boat in a lantern tool post. It doesn't self level under the pressure of the clamping screw due to friction. The same is true for refitting something in a 3 jaw chuck. Friction stops the part from fully self aligning in the jaws. A tool holder mounted on two Vs is the same, pulling the holder into the block with the piston does not cause the holder to always take up the same position because of friction and because absolute perfection in manufacturing cannot be achieved, there are manufacturing tolerances in the holders and the mounting block. It is interesting to look at the design Harold Hall came up with for a home made QCT. It follows similar lines to what I am suggesting regarding a single V. Looking from above it follows the similar tooling practice for locating parts with two holes that use a round dowel for position and a diamond dowel for rotation. In the case of a tool holder as it is pulled in it rotates round the pivoting point of the V (or a dowel in HH's design) and up against a hard stop.

Martin C

If you would like to see what is involved measuring and making Dickson tool holders have a look at Jon’s Workshop on youtube he goes through measuring and manufacture, might be of some interest.

Type in “Jon’s workshop tool post” in Youtube

Note— did try to used the Youtube embed button but the instructions shown do not match my Youtube.