Tool Holders for Dickson Clone

Whatever the system perfect mating is impossible without zero tolerances and absolute accuracy. Provided the tips of the Vees don't interfere, adequate stability from the Dickson system with random parts requires only that the Vees are parallel and side angle tolerances of opposite hand on the male and female parts. Obviously manufacture of the Vees has to be pretty precise but the tolerances needed are not unreasonable. Easiest to visualise if you consider the male Vee on the toolpost being toleranced on the smaller side of nominal whilst the females are toleranced wider. Imperfect angle match gives line contact rather than whole area but contact it will. The absolute magnitude of the relative varaitions from nominal simply cause the holder to sit either a little closer to or a little further away from the post centre. Small variations in Vee spacing don't matter either as this simply defines whether the line contacts are on the outside or inside of the male Vees. The bed and saddle of the common, Stark type, precision bench lathe exploits exactly the same principle albeit using only the outside of the male Vee and inside of the female one.

The really important thing is that the two Vees on each component be mutually parallel to each other. Something easily achieved by simutaneous finish grinding.

A bit of maths will show that tolerances on the internal face of the Tee slot don't have to be unreasonably tight by the standards of half decent grinding practice. Witness that there is essentially never any problem with professional market clones. I've yet to see a post - toolholder pair of random makes that fail to mate. The problem with amateur market clones is that the makers can't be bothered to do things right "customer paid, passed QC". That said the mating face of the Tee slot is where all the other tolerances build up so a certain amount of care and understanding of the design when setting the nominal dimension is needed. Blind copying of a sample will lead to trouble.

Although a Dickson like toolpost could be made with only single side male - female parts, like a precision bench lathe bed, there are advantages to the two Vee layout. As SouthBend well knew. In the tool post context the advantages are mostly concerned with being better able to cope with contaminated surfaces and improperly cleared swarf which can reduce line contacts to points. That said the Dickson isn't particularily tolerant of swarf. If properly made and kept clean, inside too, the grip is more than adequate and it won't come loose. If there were any real arguments against it the Dickson desing would not have become so widely used. Fact is the Dickson price performance ratio has always been about the best of any of the professional units. I'll bet that the profit margin for the makers was always very good too as its relatively easy to make if the factory is set up properly.

Clive.

Edited By Clive Foster on 15/07/2016 23:54:46

Edited By Clive Foster on 15/07/2016 23:56:22

I'm almost tempted to measure mine because i'm quite confident of it's grip and fit, It looks like a professionally made item. I frankly couldn't care how much they made out of me because i'm happy in the knowledge that it's an integral part of my methods of production. I don't buy the argument that the words cheap and copy should automatically lead to a bad association, an interesting one but not bad.

The only time when it ever looses it's grip is when it's subjected to heavy shock loads like a hammer blow. So under normal turning conditions, if it's coming loose 9/10 times it's because i'm doing something wrong rather than my equipment being at fault. If i set up the machine properly i can use it as a genuine "quick" change and go through 5 or 6 tool holder changes and confident, each and every time, that it will not loosen it's grip.

I don't know if it's genuine and i don't care, it serves my needs perfectly. If people have found another system that works for them, then best of luck to them but i don't want to fork out for something for where a need has already been served.

You ever get that feeling where things are constantly going wrong on one operation or part and think to yourself "is the machine trying to tell me something here?". Yes it probably is. 

Michael W

Edited By Michael Walters on 16/07/2016 00:36:36

Michael, what angle does your cam rotate from fully out to locked? Mine originally only turned about 90 degrees. I put a clock on the part that pulls the tool holder in and checked how much movement there was from the locked position to the fully retracted position and it was 1.25mm. I then made a new one of these parts to pull the holder that was 1mm longer from the hole to the working face. This allowed the cam to rotate close to 180 degrees and gives a much more positive clamping and locking action. This is possibly the problem with clones, what is probably a critical dimension is wrong on some of the clones. It is a relatively simple part to make so might be worth a try to see if it stops the holder coming loose in use.

Martin

I will mark the boss with sharpie and let you know how much it rotates.

Michael W

I've checked this and it only rotates 90 degrees, but thats what it takes to holder, if i remove the holder it will rotate almost 360, so there isn't anything wrong with it.

Michael W

I think the cam needs to rotate to near the top dead centre to give the best chance of locking the holder and for the best mechanical advantage in the pull up. If your cam is a long way off that best position then you do not have the best possible grip on the tool holder. The pull is off to one side and the forces acting on the cam will be able to rotate it to the release position.

Martin

Just looked at my Rapid and Dickson T2 size posts. Both go 90° (by visual estimate) between tool release and lock. Interestingly the Rapid only goes about 20° past lock if the mechanism is turned without a tool holder in place whilst the Dickson goes about 35° further. Travel is limited by the flange that goes in the tool holder Tee slot hitting the toolpost body.

I've never got my head around how these "rotate to jam" cam locking systems actually manage to generate locking forces. Clearly with solid to solid contact there can be no overcentre effect as with sprung systems. Probably some sort of effective contact angle involved whise sine has a similar relationship to the coefficient of friction between the cotacting materials in the sane manner as for the taper angle of self holding. If this is the case one would expect locking to occur once the angle of turn gets close enough to 90°. Which seems to be the case.

Really must find the time to do a proper analysis of the Dickson system to put numbers on the effects of dimensional tolerances.

Clive

Edited By Clive Foster on 16/07/2016 22:26:33

Clive, if you do the calculations of forces on a morse taper you find that when the tan of the angle (I can't remember the exact details) is less than the coefficient of friction the taper is classed as self holding. However the self holding force is easily overcome by additional force such as used to eject the tapered item from the socket. I think there are similar forces involved with these tool holders and they are relying on friction. Given sufficient force this friction will be overcome as well. That is why I made a new plunger to take the holder closer to a mechanical lock. True mechanical lock would require the cam to be at tdc but would also mean perfectly sized parts all round. That is why I aimed for close to tdc. At this point the pressure point on the plunger will be close to the centre line and the force required from the plunger to turn the cam will be very high. With only 90 degrees of motion to the pull up point the contact point between cam and plunger will be off the centre axis and the force required to overcome friction and turn the cam much lower. If I remember correctly it only took a couple of hours to drill and turn the new plunger so I think it was a small investment of time to get a better tool holder clamping action.

I think the off centre pressure point means that the plunger may twist anticlockwise when viewed from above. This would put all the pull up force on the tool holder on one lip and that is the one furthest from the workpiece. This is the worst possible result mechanically which is why the idea of the pressure point close to the centre line of the plunger seems much better to me.

Martin

A heads up for anyone tempted to buy dickson toolholders cheap on ebay from india.

I just bought a couple for my boxford from "globaltools2016" because UK suppliers appeared to be out of stock. They don't have a chance of fitting without significant modification.

My post is an original Boxford badged one and (from the look of them) I have a mix of original holders plus later clones which all fit fine so I am pretty sure it isn't the post. However the globaltools2016 ones need a significant amount removing from the Vs to fit.

I also agree with the comments about the difficulty of making them accurately, having milled the Vs deeper I am going to have to resort to hand finishing to get them to fit without rocking.

When I purchased my S7 lathe it came with a Dickson QCTP but only a couple of tool holder so I purchased a couple from the Myford stand at the Midland Model Exb 2 years back although quite well made they did not fit correctly so a friend eased them on his surface grinder. I do not know if the problem still persists or if it was just a 'batch' problem I have not purchased any more fortunately I was able to do a trade off for four originals

H

Toby, I just had the same experience buying a couple of Indian made toolholders for my Boxford badged toolpost. The claw on the back of the holders, which are supposed to engage with the piston head of the toolpost were too thick, compared with a genuine Dickson, by between 0.74 and 0.65mm, (both were different), The Ebay seller, techtoolsuk told me that the holders were manually ground, and compared against a master copy of a Dickson toolpost. I suspect that the master copy was of the 'Dickson style' toolpost imported by the same company. Since then I've been unable to find an importer who will guarantee compatibility with an English made toolpost. I'll just have to keep looking for secondhand and hope I don't get trampled in the rush.

I'm using two clones from different suppliers on my Super 7, the RDG at the front and a Chronos at the rear, this is a test with a 30mm 314 SS bar to see if the front setup is too flexible as all the "Experts" will tell you it should be -

https://youtu.be/pUI2GIGTz_c

Edited By Nick Hulme on 19/08/2017 09:01:35

I got a few genuine toolholders literally "off the back of a lorry" at a steam fair last year, scattered through a box of junk. I gathered together all I could find (5 or so IIRC) and asked the guy how much - he said a fiver - I said "the lot?" - he said yes. I paid and ran before he changed his mind!

I've bought 3 from RDG in T1 size last month for the genuine Colchester 3 sided post on my Chipmaster and they all fit perfectly on all 3 positions, could be luck I suppose but not bad for £22.50! The screws aren't as soft as usual either!

After my bad experience with a couple of Indian made toolholders, (Got my money back). I bit the bullet and ordered two from Rotagrip. Made in Poland. The finish is streets ahead of the Indian clones, and they fit my Boxford toolpost, smooth as silk. Twice the price of the Indians, but you get what you pay for. Hats off to Ian (Rotagrip/Fordeight) who even managed to get them to me from inside France, saving a few £'s on the postage. Highly recommended.

Just finished another 3 toolholders in alloy for my Bison clone for my Boxford.

Have 3 original, hardened, ground Bisons.

Have 11 home made, non hardened, non ground steel holders. Work a treat, couple of years of abuse, zero problems.

Have another 5 in alloy, ditto above.

Makes 22 in total.

The last 3, I made a cock up on the machining of the small outer vees, going a few thou too deep, daylight on one side, and a couple of cigarette papers clearance on the other when clamped to the toolpost. ie they are located and clamped only on the two large inner vees. Well, finished them off, and they repeat and they turn and face and bore and do NOT move. Put all my low power, small DOC boring tools in them.

Waste not want not etc...