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Metal Top Pantograph kit available


#121

I’m waiting for a design thick enough to have bearings in the holes. My question how thin steel bolts can be to withstand the shearing force of a 15kg sled (+30% security) was never answered and I can’t calculate that.


#122

That would be extremely easy to do , instead of 1/2" metal use 1" or 1.5" metal bar
or you can just use a bigger bolt like 8mm instead of 6mm. The shear strength of bolts that size is 35,000 psi


#123

Thank you @aluminumwelder for the picture. I’m getting closer to what I have in mind.
Using bearings the inner diameter determines somehow outer diameter and I want to go as small/thin/light as possible. I have a bunch of bearing that fit a 3 mm screw, the shoulder bolt is a great find that I missed due to lack of technical English.
I fond this bolt https://www.mcmaster.com/#90265a112/=1e86j3n but I’m unsure about the shoulder length for 2mm thick steal arms. Still need washers I guess.
The shear strength is the same as for 8mm bolt so it’s the material strength that still needs to be applied to the diameter/ square-footage. I found this calculator https://www.engineersedge.com/material_science/bolt_single_shear_calcs.htm that also takes the arm thickness in to account.
I just can’t put it all together. Either I’m getting to old or I’m to burnt out from the job. I even fail of figuring if for the top-mount kit the shear force is divided by all bolts or just on the middle 2 attached to the sled. I need help or have to wait for a more sunny day :sun_with_face:


#124

You are over thinking it. The whole machine is being held up with tiny chain links

any bolt will be strong enough and not shear apart. 3mm will work fine


#125

for the four horizontal parts, you are correct, drill them together and as long
as the bit is straight, they will work. What matters is the holes are matching,
not the exact dimensions.

for the vertical parts, the key thing is that all three holes need to be inline,
but the two holes at the top (and the only two holes for the center support) can
all be drilled at the same time.

the notch diemensions aren’t critical, you just need the chain to clear.

the horizontal bars can be made any lenth, 5" clears the stock router, 7" clears
just about anything

David Lang


#126

I sent with stainless so that there was no rust to deal with that could cause
the arms to stick instead of rotate cleanly


#127

google ‘shear strength of bolts’, first link is

Using a .250-inch diameter grade 8 fastener gives you the following shear
capability:

A = Cross-sectional area of the fastener size (since bolt bodies/shanks have
circular cross-sections, use area of a circle) = Pi x r2 where R (radius) =
.250/2 = .125, therefore A = Pi x (.125)2 = .0491 square inches (in2)
Capability in shear = 91,000 lbs / in2 x .0491 in2 = 4468 lbs

Using the same .250-inch diameter grade 5 fastener results in the following:
Capability in shear = 75,000 lbs / in2 x .0491 in2 = 3683 lbs

66 pounds of tension from each motor just isn’t an issue.

1/2 diameter is 1/4 area, so a 1/16 bolt would have a sheer strenth of ~230
pounds

David Lang


#128

the pins in the chain are 2.1mm (3/32")


#129

Makes sense, and pretty easy to get the holes in line, just scratch a line and use a metal punch.


#130

drilling a small hole like the 3/32 hole through multiple layers is non-trivial.


#131

Just drill through one layer and use that as template to mark second bar. With the money you save over getting it laser cut. you could buy a cheap drill press. Not saying people should do it this way just another way to do it


#132

@dlang Thanks for the fast shipping on this. It would be helpful to ship the kit with some instructions or a link to some instructions, I just ended up here which is good enough. Anyways I’m impressed with the pieces and have started getting things together.

I like that the pantograph kit is positioned to the sled according to a center hole. I did find the instructions on this page difficult to follow and it’s probably my fault there. I found the Onshape drawing very useful and correct me if I’m wrong but doesn’t positioning of the pantograph 2x4 mounting holes just boil down to the following drawing?


#133

Yes, that looks right (and looks like the measurements I’ve been telling people have been wrong, oops)

It may be too many years of watching “this old house” and “new yankee workshop”, but I really try hard to use the story-stick approach of ‘put the small hole at the bit and drill through the part’ rather than measuring. That makes it so that people don’t end up translating from imperial to metric and if the parts are the wrong size (as long as they are consistent), everything will still work.