Gallery, Projects and General > The Design Shop
Coax indicator design
BillTodd:
--- Quote ---will you be producing some drawings
--- End quote ---
Yes, assuming it works. ;)
I was expecting (still am) all sorts of criticisms of, and objections to, the ball angle changer
Hi John,
I don't like the idea of getting my hands anywhere near a spinning probe, so I'm wondering about a simple torque limit clutch between the drive and the rocker arm. Does your Blake device have anything like this?
I have also been thinking about a slow speed drive mechanism (which would also double as a clutch), but I'm not sure how to keep every thing concentric. Worth the effort?
Bill
Bogstandard:
Bill,
I wouldn't bother with the clutch idea, it has a sort of safety feature because the probe isn't mounted solidly, a bit like a probe on a DTI, if any force at all is put on it, it just moves away.
What isn't there can't go wrong.
I don't know how fast you expect to run it, as when I run mine, having electronic speed control, I run it at around 20-30 RPM, otherwise I find my eyes can't keep up with the bouncing needle. I suppose a sign of getting old.
So maybe a speed reducer would be advantageous to people who can't get their machines to go so low, but in all honesty, it would be a pig to do while retaining concentricity, plus also it would be just liable to increase the overall length, a thing your version scores on.
People have said that they aren't very accurate, but in all honesty, in the time I have been using mine, I could never get anywhere as near to true as the coax gets it, plus the ease and speed of use is fantastic. I used to hate setting up to find concentricity, and started to grow a 2 foot long rubber neck to prove it (just joking of course, it was only 18") but now I really enjoy the complete control I have over the process.
I think Stew has now invested in one, and I think he has the same sort of feedback as myself.
John
BillTodd:
--- Quote ---I don't know how fast you expect to run it, as when I run mine, having electronic speed control, I run it at around 20-30 RPM, otherwise I find my eyes can't keep up with the bouncing needle. I suppose a sign of getting old.
So maybe a speed reducer would be advantageous to people who can't get their machines to go so low, but in all honesty, it would be a pig to do while retaining concentricity,
--- End quote ---
All good points. My drill/mill requires a belt swap (a right PITA), but I can slow the Haighton with its VFD - you're right no slowmo drive.
--- Quote ---plus also it would be just liable to increase the overall length, a thing your version scores on.
--- End quote ---
Speaking of which... I've removed the unnecessary top cap (a legacy from an earlier design) which has shaved another 8mm off the height. I think I should save another couple of mm by re-thinking the rocker arm/probe holder.
RichardShute:
--- Quote from: BillTodd on November 04, 2010, 11:53:55 AM ---This coaxial indicator design is one I've been working on for a while. The images shown do not include fixings or screwed joints.
Before I commit it to metal, anyone have any comments on the ball angle changer ?
--- End quote ---
I can imagine a couple of aspects which might be less than ideal - I know, 'misery guts'.
The DTI is operated by riding up/down a conical surface so any radial play between the outer body/DTI mount and the inner shaft will give rise to a false reading. If you were to arrange the DTI to be actuated via a bell crank resting on a flat disc that potential error is removed and you may even be able shorten the whole assembly slightly.
Also, as shown, the DTI (obviously) does not rotate, but it does move up and down to give the reading. You will presumably hold the outer body by hand to stop it rotating and thereby have a very fair chance of inadvertantly raising or lowering it. By re-arranging it a little so that the DTI remained fixed vertically this potential error would be removed.
As it stands, I expect it will probably perform adequately, with care in use, but I think you can still improve the design. You have come this far, why not polish it a little more?
Richard
BillTodd:
--- Quote ---I can imagine a couple of aspects which might be less than ideal - I know, 'misery guts'.
--- End quote ---
Not at all Richard :)
The problem with pixel machining is that everything fits perfectly there's no wobble, no TIR or friction. Without someone to look over ones shoulder it is all too easy to miss the pit-falls.
Here's a perfect example:
--- Quote ---any radial play between the outer body/DTI mount and the inner shaft will give rise to a false reading.
--- End quote ---
I had considered radial run-out of the shaft, but assumed I could turn all the bearing surfaces at the same time, so keeping any run-out to a minimum.
I had however, completely ignored body-wobble as the upward force from the rocker moves around the bottom face (cheers Richard :))
I can see a couple of possible ways to minimise the effect without changing the design:
1)The bearing surfaces have to be a very good sliding fit, so I could make them tight then lap them to a good fit.
2)The stay arm (that prevents the body turning) could be hung off of the same side as the DTI thus moving the centre of gravity out towards the DTI. This should apply a (vertical) torque to the body, pre-loading the top and bottom bearings and keeping them snuggled up against the shaft.
--- Quote ---...arrange the DTI to be actuated via a bell crank resting on a flat disc
--- End quote ---
I looked at bell-cranks or levers (as used in Wes43's example above), but couldn't come up with a neat, compact arrangement. A lever would offer the possibility of some 'gain', to compensate for losses at the rocker/probe end. Wouldn't it need a gimbal of some sort to remove all radial errors ?
--- Quote ---By re-arranging it a little so that the DTI remained fixed vertically this potential error would be removed.
--- End quote ---
If the DTI/ body is fixed vertically then the problem of wobble & smooth parallel motion perpendicular to the shaft shifts to the rocker follower (IYSWIM) . I couldn't figure how to do fix that. :scratch: Had you something in mind?
Bill
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