Tape Strip Holders

[mrandt]

Sorry, my bad. I misunderstood this:

How about: Put the indexing on the bottom plate.

I thought of indexing = fiducials, while Juha meant the grid to align the feeders... But what I wrote with regards to fiducials still applies. These should be on top of the feeder and the pin "poking through" top plate would give precision and cotrast.

I also like the idea of a fixed bottom rail and sliding top with thumb screws

[JuKu]

Is there a reason you're thinking of M4 instead of M3? Or would M3 work just as well.

No reason. I suppose 2mm is enough for M3? But I'm not a mechanical engineer.

[mrandt]

I am no mechanical engineer either, but I know there are norms for screws and nuts. For example ISO 4032 "Metric Hex Nuts Dimensions"

A standard M3 hex nut is 2.4mm thick; gives you a ballpark figure. So 2.5mm should be good, 2mm might still work as we probably apply a lot less force than the ~2 NM torque which would be allowed for a regular M3 nut.

[mrandt]

Now that I just wrote that, I thought of something else:

Instead of screwing thumb screws into tapped holes in the bottom plate, you could insert the screws from the bottom (countersunk from bottom) and use a thumb nut to fix the sliding top rail.

[JohnSL]

How about: Put the indexing on the bottom plate. That way, the holders will always sit precisely in the same place, the software does not need to measure the location and the placement is faster. The bottom plate then has holes for screws to fix it on the table. The top plate has openings for these screw heads, so we don't need countersunking in production. The top plate has oval holes so it moves; the bottom plate also has tapped holes for thumb screws that hold the top plate down. And therefore, the plates are steel, so the threads in the bottom plate are solid.

I've done some work on making that change. The plates are currently 2 mm thick because that's what I had before, and I haven't changed the thickness yet. I was working mainly on the location of the holes. Below are some different views. The first one shows the top plate in the locked position, which will hold the feeders in place.

Hold Down Bar 2a.PNG (66.7 KiB) Viewed 16821 times

This next image shows the top plate in the retracted position, which allows adding and removing feeders.

Hold Down Bar 2b.PNG (72.29 KiB) Viewed 16821 times

The final image shows the slots a little better. The top, thicker slot is to clear the head of the screw that will hold the bottom plate to the machine.

Hold Down Bar 2c.PNG (84.48 KiB) Viewed 16821 times

In looking into thumb screws, I found the following part: https://www.mcmaster.com/#91175a061/=159vvhb

These are thumb screw heads that will work with any socket head cap screw, which means I can use screws of just the right length.

Juha, I had another idea, which is to have the bottom plate 3 mm thick and the top plate 2 mm thick. Doing this would provide more thickness for the screw threads in the bottom plate. But the thinner top plate would work well with my current geometry (which is actually designed for a top plate of 1.8 mm at the moment). Do you see any problem with having the top and bottom plates different thicknesses?

-- John

[mrandt]

Hey John,

looks great - I like it.

May I suggest once more to insert the screw from the bottom and use a thumb nut instead?

See this part:
http://uk.misumi-ec.com/vona2/detail/22 ... LLNN-M3-NI

A 2mm bottom plate would allow counter-sinking an M3 screw on bottom side of rail - would need a bit of glue to keep it fixed.

Threading the hole would also work, but 2mm plate is a bit thin for that. So if you want to either follow my suggestion and insert the screw from the bottom side or want to use a screw from top, the bottom rail should be 3mm.

The top plate however would be fine in 1 or 1.5mm.

Cheers
Malte

[JuKu]

John: Juha, I had another idea, which is to have the bottom plate 3 mm thick and the top plate 2 mm thick. Doing this would provide more thickness for the screw threads in the bottom plate. But the thinner top plate would work well with my current geometry (which is actually designed for a top plate of 1.8 mm at the moment). Do you see any problem with having the top and bottom plates different thicknesses?

No big deal, one way or another. Different thicknesses are insignificantly more expensive, as the factory needs to change stock plate and more steel costs more. As Malte pointed out, a M3 nut is 2.5mm, and with thumb screws, we are not going anywhere near the maximum strength, 2mm or 3mm. A much bigger issue is the 1.8mm: Make it 2.0mm (stock thickness), please!

May I suggest once more to insert the screw from the bottom and use a thumb nut instead?

Is there a reason? Countersinking increases cost, and glueing might be unreliable.

[mrandt]

Is there a reason? Countersinking increases cost, and glueing might be unreliable.

Tapping also increases cost and you might run into problems with screw length:

Let's say the top plate is 2mm, bottom plate is 3mm. Then you would need an M3x4.5mm thumb screw to fix the top plate. There is no such thing so you will also need a custom made thumb screw. Or add many washers.

Also, the tap in the bottom plate needs to be long enough to allow for some screw travel distance and reliable clamping of top bar - assuming you don't want to drill holes into your table top. 3mm might not even be sufficient, so this increases cost for bottom plate.

If you invert the thing and use the thumb screw I suggested, all these issues disappear. You could even get away with both plates being 2mm thick.

I agree that glueing of screw had *might* not be reliable, on the other hand thumb screw will only require very little torque to tighten and there are many metal-metal bonding solutions which are quite strong.

[GaryM]

If you are having these strips of steel laser cut even in small quantities you will be able to have your manufacturer press a 3mm threaded stud into the bottom rail so it sticks up and will take a "thumb-nut" from above. This is the right way to do it, and would be little different cost from having to countersink holes on the bottom side. Also can I offer a link to a company that may be of use to you for the manufacture? I have had some interaction with them, and they are quite professional. Based in China but owned and operated by westerners... I have no connection to the company, I just have them bookmarked for some possible projects for my own business. http://www.star-prototype.com/

[mrandt]

Thanks Gary.

I agree, that would be the perfection solution.

Unfortunately I have no idea what the cost would be and which shops in Europe would do such work. I enquired with my preferred laser cutting companies and they said they could not press fit or weld threaded rods onto the plates.

Similar Designs are used in many commercial products though, so it can't be too difficult or expensive.

Offshoring to China might also be an option.

Juha, do you have a source?