16 replies to this topic

[rfiol]

Kicking around this idea and would appreciate feedback from the community.  This is for a 12" dob.  Core idea is to place threaded collimation knobs under each mirror cog point of a 9-point plop.  I have a CNC router -- I know I can machine it.  Just not sure it's a good idea.  Talk me off the ledge (or nudge me over), please.

 

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[Jim45157]

long as it floats should work try it i would

[Jon Isaacs]

The flat disk would inhibit cooling

 

Jon

Edited by Jon Isaacs, 22 June 2025 - 03:41 PM.

[TOMDEY]

Interesting; Nice graphics! Well... I've often tapped wood to accept stainless or brass machine screws, and it works quite well. But wood on wood seems questionable for three potential reasons >>> 1) "stiction" which I guess wax or similar might address 2) the hole and insert will be with and cross grain (wcwc) likely to manifest as sinusoidally varying friction 3) wood changes anisotropically with time and humidity - shrinks, expands, contorts So threads that work today are likely/certain to loosen or bind up tomorrow or tomorrow...

 

Would make sense to make one specimen of the threaded interface and exercise it, expose it to anticipated environment (outside for a week?) and exercise it again.   Tom

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[TOMDEY]

long as it floats should work try it i would

Wood floats --- but avoid ironwood.    Tom

 

Ironwood is a common name for many woods that have a reputation for hardness, or specifically a wood density that is denser than water (approximately 1000 kg/m3, or 62 pounds per cubic foot), although usage of the name ironwood in English may or may not indicate a tree that yields such heavy wood. ~

[DeepSky Di]

I'd suggest plywood as it is less likely to expand and contract with humidity.

[Pinbout]

I like using nylon socket cap screws for the support points. And getting ones long enough the can pierce the base so to keep the triangle from spinning 

 

I think your triangle is too thick 

 

that’s my initial thoughts 

 

something like this, just using nylon hex head bolts for support points on a moving plate cell 

 

https://youtu.be/kdM...qhmLZj0HIf2aRki

Edited by Pinbout, 22 June 2025 - 05:20 PM.

[rfiol]

I like using nylon socket cap screws for the support points. And getting ones long enough the can pierce the base so to keep the triangle from spinning 

 

I think your triangle is too thick 

 

that’s my initial thoughts 

 

something like this, just using nylon hex head bolts for support points on a moving plate cell 

 

https://youtu.be/kdM...qhmLZj0HIf2aRki

"...pierce the base..."
Great idea.  Simple and smart.  Thank you.

[rfiol]

Interesting; Nice graphics! Well... I've often tapped wood to accept stainless or brass machine screws, and it works quite well. But wood on wood seems questionable for three potential reasons >>> 1) "stiction" which I guess wax or similar might address 2) the hole and insert will be with and cross grain (wcwc) likely to manifest as sinusoidally varying friction 3) wood changes anisotropically with time and humidity - shrinks, expands, contorts So threads that work today are likely/certain to loosen or bind up tomorrow or tomorrow...

 

Would make sense to make one specimen of the threaded interface and exercise it, expose it to anticipated environment (outside for a week?) and exercise it again.   Tom

Stiction is definitely a concern.  Thank you.

[triplemon]

These large diameter threads of wood on wood are unlikely to turn smoothly under some load. Wood has way too much roughness plus stiction and friction. You after all want to position (and hold for hours) to within a mil or two while load varies by a dozen pounds when the scope is repositioned.

 

I'd rather just put a steel bolt through a thread cut into the wooden baseplate and have it go into a oversized blind hole drilled into the  underside of the whiffle tree triangles to prevent them from sliding off position. If the thread in the baseplate ever wears out, add t-nut driven into the top of the baseplate or brass inserts. Grind the top of the bolts into a nice half-ball shape to allow the whiffle triangles to swivel freely.

Edited by triplemon, 23 June 2025 - 03:23 PM.

[brebisson]

Hello,

 

Nice clever and elegant design...

 

They are problems associated with wood on wood threads, they have been stated above... But they can be made to work (look at nut cracker for example). However, for it to work you need rather large threads. I do not know if you will have the space here for 10+mm pitch threads...

 

Question, how would you tap these threads? Do you have one of these "tap bits" that you make rounds with?

 

Lastly, for a 300mm scope, a 6 point cell works just as well and tend to be simpler to build..

 

Cyrille

[Jon Isaacs]

They are problems associated with wood on wood threads, they have been stated above... But they can be made to work (look at nut cracker for example). However, for it to work you need rather large threads. I do not know if you will have the space here for 10+mm pitch threads...

 

 

For collimation bolts, one wants fine adjustments which means fine pitch threads. 

 

I am not seeing the advantages of threading wood.  Oak and other hardwoods will take threads and hold reasonably well but T-nuts and threaded inserts are better choices. T-nuts can be glued in place if needed.

 

Jon

[whereIsIt]

Wood Threads will not turn smoothly enough for the Micro turning you may need to do for collimation...

Use Nylon/HDPE/other slippery material

 

I use candle-wax on wood, 3D-printed and hard-plastic threads

Edited by whereIsIt, 23 June 2025 - 08:37 AM.

[Oregon-raybender]

I like these, no guess work. Wood threaded inserts, brass, solid.

I use them on all types of wood. The sharp thread

cuts into hard wood easy. Be sure to get the kit

with the insert holder.  Available in different thread sizes.

Make sure you keep it straight 90 deg, when placing into

the wood. 

 

Starry Nights  

 

https://www.rockler....rass-pack-of-25

 

https://www.rockler....od-brass-3-8-16

[Bob4BVM]

As stated, a hundred ways to do this using standard hardware, all of which would work better and last longer.

As a lifelong woodworker, I have cut lots of threads in wood, both internal and external, for various projects. But this would not be one of them. 

Sometimes "just because it can be done" is not a good reason to do it.

CS

Bob

[rfiol]

I started this thread by asking you to "talk me off the ledge (or nudge me over)".  I have been talked off.  No wood threads.  Thank you for the great input.  Back to the drawing board for me.

[Starman1]

I started this thread by asking you to "talk me off the ledge (or nudge me over)".  I have been talked off.  No wood threads.  Thank you for the great input.  Back to the drawing board for me.

Also, I suggest NOT having the 3 triangles (or 6, since it's a 12" mirror).

Download and use PLOP GUI to figure out the shape and size of the triangles.

 

The reason not to have the triangles all move but not the lower contact points on the edge of the mirror is that the pressure on the bottom of the mirror will move away from the front-to-rear center of gravity of the mirror

as you adjust collimation, and this will induce astigmatism in the star images. (Use the Cruxis Mirror Edge online calculator to find the front-rear COG).

 

So, you have some alternatives: 

1) fix (basically don't turn) one of the 3 collimation bolts and only turn the other two to collimate so the mirror does not walk up and down the tube and change the contact points on the bottom edge of the mirror.

2) have the tilting mirror contact point triangles all attached to a plate to which the mirror edge supports are also attached and have the collimation bolts only move this plate.

3) use a Glatter wire sling where the hanging points for the wire sling move up and down with the mirror.  This keeps the wire in the COG and keeps the mass of mirror in front of the wire equal to the mass behind it.