What have you invented?

[citybook]

Alright, the pictures didn't come out where I tried to put them - I am sure you will know which is which.

Ok RayG, how did you get your last pic in the middle of your text ?

Cheers,
Bob.

[RayG]

Hi Bob,

There's an image box in the editor, it's that square icon that has a mountain wierd yellow sky.. hang on... that's the one..

Before you click on that, you need the url for the image you want to insert. If you right click an image and choose view image info, you can cut and paste the url.

Click the image button and paste the url.

If the pictures are on your own computer, you'll need to upload them to somewhere, that's accessible by others.

That's it.

Regards
Ray

[citybook]

Hi Ray,
thanks for that - I am gradually learning my way around this thing.

I haven't got around to acknowledging your comments on magnetic and vacuum work holding - these are things I don't have any experience of - so I look forward to your gripping tales of suction.

I have used mag drills, and my main recollection of these exercises is the profanity associated with trying to do something that sounds deceptively simple.

Thanks for the tip on the image box,
cheers,
Bob.

[jhovel]

Maybe I misunderstood the function of the toe clamp?
I thought you would position it against the side of the workpiece - as Stuart did with his clamps. I may have misled Ray in the process as well.
Now that you expressed your view, Bob, I realise I may have been wrong. Do you suppose they are made to fit OVER the edge of just one size material?
I guess you could put packers under the little foot end....

[citybook]

Hi Jhovel,
I am not in the business of right and wrong.

The clamp you spoke of would be a handy thing in it's place - and if you look at shops selling to the F&T brigade, most of the clamps they offer are the compression type.

For me at any rate, I would say I spend more time at my mill setting up jobs than actually milling. And often I finish up making the fixture, which consumes even more time.

So this is why I asked the crew here what they thought about clamping a job by the sides.

Although I don't post much, I read a lot of what goes on here simply because people here have a lot of interersting ideas - and see, there was more than 3 pages of discussion on clamps.

All the best,
Bob.

[Stustoys]

The toe clamp (left pic) works by pressing down on the top of the work, and it's fixing is in tension - works well.

Tension?

The last bit - ie. shear load on the fixing - is the reason most of the pics above that are pinch clamps have two T bolts..

I can only see one picture that has two T bolts, the one you added making two.

Personally, I don't want a clamp that is "going to move somewhere"

Every clamp moves somewhere or something breaks. Adding another Tee nut may not stop them slipping(if in fact they are slipping) It will just require more force.
Most clamps are compression in one plane and in shear in the other two.

Its a question of how much clamp force is enough. This of course depends on what you are wanting to do. The clamp in the pictures exerts in excess of 120kgs of horizontal force(I would say far in excess but that's as high as my scales go). Force in the same ball park as my mill vice. I'm yet to think of a way to check its vertical force. Unless the work piece was very small there will be two of them. I can't image trying to take roughing cuts with the work piece held by these clamps alone.

Stuart

[citybook]

Hi Stuart,
yes, the toe clamp fixing is in tension - what's wrong with that ?

Yes, there aren't as many clamps there with two T bolts as I thought.

It's very rare that my clamps move - I want my job to stay put while I machine it - the job is in contact with the clamp - so if the clamp moves the job moves - not good.

Yes, it does depend what one wants to do - I want clamps that don't obstruct facing the top face of the job.

And yes, I have used pinch clamps to hold jobs for both roughing and finishing.

Cheers,
Bob.

[Stustoys]

Hi Bob,

yes, the toe clamp fixing is in tension

What part of a toe clamp is in tension apart from the bolt?

Stuart

[citybook]

Hi Stuart,
no part of a simple clamp is in tension other than the bolt. The bolt is the fixing. Take the bolt away and the clamp isn't fixed anymore.

When the tension in the fixing balances the compression in the work plus the packing then nothing will move.

My previous point is that when we choose to use something like a pinch clamp then there is a big issue with shear force - that shear has to be translated into compression of the work - otherwise something will slip.

This then comes back to the T bolt - really good in tension - pretty ordinary in shear - not usually used in compression.

As far as I am concerned, get this right before turning the machine on and the clamps (or job) won't move.

Cheers,
Bob.

[Stustoys]

Hi Bob,
I'd thought you were talking about the contact point between the clamp and the work. I'll have to go back and read your posts again.

Stuart

[RayG]

Hi Bob, Stuart,

I've been trying to work out how to clamp a 6mm thick bronze sheet about 400mm x 600mm, and ultimately came up with vacuum as the best solution, the clamping force for vacuum is going to be vertical, and the horizontal holding force is dependant on the friction between the table and the plate. At 15 psi ( ok 14.7) that works out to ~2500 kg vertical, and the cooefficient of friction for bronze on steel (oily surface, for worst case) is 0.16, so the horizontal holding force is ~400kg. I can't see how you would get that sort of holding force on a flat sheet any other way.

Back to moving clamps, I think there was some confusion over the clamps moving. In the following setup. What happens as the clamp is tightened? Either the clamp deforms or the work deforms (if the force is great enough) or, the fixed clamp moves.

The fact that the fixed clamp moved, is probably a good indication that the clamping force is sufficient to push the t-slot clamp. I can't see any difference between this clamping arrangement and a milling vise, provided the surface areas are equivalent.

EDIT: By that I mean you could easily take the same idea and make the clamps (say) 100mm wide, covering multiple t-slots with say two t-nuts on each fixed point.



In fact this setup would be better than a milling vise, if you could get clamps on multiple sides. (like the 4 sides of a square part)

Interesting question, work holding is almost always the trickiest part of a lot of jobs.

Regards
Ray

[citybook]

Hi Ray,
of course I am curious about why you are setting up a thin(ish) sheet of bronze for machining ?

Yes, I agree with you that setting up jobs can be tricky - could it be that home machinists set themselves (!) up with tricky jobs ?

Got a pic of this vacuum rig of yours ?

Cheers,
Bob.

[RayG]

Hi Ray,
of course I am curious about why you are setting up a thin(ish) sheet of bronze for machining ?

Yes, I agree with you that setting up jobs can be tricky - could it be that home machinists set themselves (!) up with tricky jobs ?

Got a pic of this vacuum rig of yours ?

Cheers,
Bob.

Hi Bob,

The bronze sheet, is a simple enough job, it's some scrap bronze that I'm finishing for a friend who makes in-fill planes, the source material is some unused bronze cast plaques. I just need to skim the top and bottom flat. But, holding it flat has proven to be a tricky problem. The vacuum table hasn't arrived yet, I'll post some pictures when it does, but there are some videos of how they are set up on their website, that will give you the general idea. Vacuum Tables

Regards
Ray

[BobL]

Back in April Last year I proposed this idea


Well after a lot of cussin, and metal dust here it is and it works a treat.


The steel was all bought at scrap prices. The pedestal base uses a 20 mm x 350 mm diam steel floor plate and a gusset reinforced 100 x 5 mm SHS column.
I have set it up temporarily on some timber cross pieces so that it wont tip over while I am building it and while I work out where I will eventually put it. Eventually the timber will be removed and it will be anchored to the concrete floor using concrete anchor bolts.

The turntable base uses a 16 mm thick x 315 mm diam steel plate and works just like a giant lazy susan bearing using 25 mm diam roller balls.

In the middle there is another bearing and a shaft goes through the central bearing to stop the top plate sliding around.

The turntable toplate is 12 mm thick steel plate turned to be the same diameter as the turntable base.

The top plate is bolted centrally to the shaft that passes through the central bearing which is in turn bolted to the plate underneath. Unlike a lazy susan the top plate does not go round and round but only rotates 240º one way or the other.

Those bits of angle hanging out from the sides of the base plate support a 5 mm thick steel band that wraps around the edges of both the top and bottom plates and acts like a brake. It effectively locks the movable turntable top plate to the fixed bottom plate together, using that massive swivel locking handle - this works really well.

A round galvanized steel plate then covers the top plate and the locking band to prevent grinding dust from falling down inside the turntable.

The 75 x 50 x 3 mm RHS arms that support the grinders are then bolted to the top plate using 10 mm tensile bolts. 25 mm diam x 3 mm thick tubular spreaders are welded vertically inside the arms to prevent the tightening bolts from crushing the RHS.

The large slots cut in the RHS are so that the grinders (which all have an 8 mm thick steel base plate bolted to them and a 200 mm long 5/8" all thread rod welded to the underneath of the middle of the baseplate) can be positioned more or less anywhere along the length of the arms, as will be shown later.

The grinders are locked into position using these chunky handles.

The round locking nuts (A and B) have been turned from 38 mm diam steel rod. The handles attached to the locking nuts are turned from 12 mm rod.
I made several locking nuts like type A before I realized that the relatively shallow angle of the handle whilst easy to rotate made it difficult to push the grinder on its base plate out of the way up against the turntable. I have since made two locking nuts like B which work a lot better. Handle C is the turntable locking handle. This swivel type handle, even though it is relatively short, works really well.

Here is what it looks like from underneath.


The 4 bolts that hold the turntable base to the pedestal are long enough so that double nuts can be employed to raise and level the turntable after the pedestal has been bolted to the floor. I will use these once I bolt it down.

As well as moving the grinders in out out from the turntable, the grinders can also be rotated on their baseplate like this.

This provides for much better access than even is possible with a single grinder up against a wall - this feature is especially useful for things like the buff. This works really well.

Finally a view from above.

Once I get that old bench frame behind the pedestal out of the way I will be able to push the whole thing back some 350 mm so it will take up relatively little space up against a wall.

Still to sort out are a coolant tray/tank and the power cables . One has to remember to make sure a cable is not draped in the wrong place or the rotation of the turntable might guillotine it.

I will use it like this for the time being and iron out any bugs and then I will take it apart and pretty it up a bit. Meanwhile I have been using it all weekend and I love it already. The thing is so heavy that even though it is not yet bolted to the floor it is moderately smooth and already much better than any of my previous arrangements.

Making this has been a real learning exercise for me. I have not welded 20 mm steel plate before and making metal things fit on a slightly larger scale than I usually do has not been as forgiving as I thought it would be.

[jhovel]

sorry mate, but only your first photo is visible... something gone astray. You should be able to edit the message and add the missing photos.
Joe