Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

New alt bearings on homemade dob mount


Rusty Strings
 Share

Recommended Posts

It's been 3 years since I built the dob mount for my 200p and decided to do those summer mods.

I'd read on various ATM sites that the optimum size for the alt bearing is 1.8 x the OTA diameter so the 4.5 inch diameter PVC drain stops looked to be well under size. Balance had aways been an issue so a magnet moved around the tube helped there, also friction brakes were needed to keep the tube steady.

The replacements are fashioned from 18mm birch ply with laminate bearing surfaces and PTFE blocks and what a difference!!!

Perfect balance can now be achieved, there's no stiction, and the whole thing feels so much smoother.

The azimuth bearing was a 12" vinyl record and PTFE, again, it worked ok but stiction was a problem. So I've replaced the record with a 40cm ebony star ring and that feels much better.

post-13958-133877618795_thumb.jpg

post-13958-1338776188_thumb.jpg

Link to comment
Share on other sites

  • Replies 28
  • Created
  • Last Reply

Top Posters In This Topic

Great looking Dob ,new Bearings look superb , i am glad you posted this as i have changed mine from 6.5" to 12" diameter but haven't put teflon and formica on yet, but it so much better already as is ( wood to wood ) can't wait to do mine now,

Link to comment
Share on other sites

superb job John

coincidentally, I have just rough cut a template for doing my 16" dob bearings. see my last post at http://stargazerslounge.com/diy-astronomer/140533-16-f4-orion-optics-uk-dobsonian-project-3.html

they will be quite like yours but a full ring 'cartwheel', 22" in diameter and 30mm thick.

I will also be painting mine black as it happens to match the usual OOUK dob base (broadly anyway).

Link to comment
Share on other sites

ply would be better but I'd make them from mdf if you have it obviously needs to be well sealed (grannie eggs etc). make a template to keep and then when you have spare ply in the future replace them!

I know what you mean about projects!

Link to comment
Share on other sites

I have access to a workshop with a table router so I could cut an accurate circle from which I made a template. This then meant I could cut two identical trunnions (is that the right word?). This could be done with a hand router if you make a replacement base which extends in one direction so that a pin can be fitted that enables the router to pivot in a circle on the ply sheet.

George, I don't have any drawings as I first cut the ply circle out and drew the final shape on the blank, I could post a photo of the template with a few dimensions if that helps.

Link to comment
Share on other sites

Just give the laminate surfaces a polish with turtle wax as I've seen recommended here and on the cloudy nights forum and what a difference that makes. I was already pleased with the low stiction and balance I got with the new bearings, now it is buttery smooth without becoming too slippery.

Dead chuffed

Link to comment
Share on other sites

George, I don't have any drawings as I first cut the ply circle out and drew the final shape on the blank, I could post a photo of the template with a few dimensions if that helps.

That would be most helpful :)

Link to comment
Share on other sites

  • 2 weeks later...

Cheers. I figured you would've used two but I thought I'd ask because I've been trying for weeks now to get my alt bearings to sit square on my scope and I hoped you'd worked out a reliable way to do it with just one.

Back to the old drawing board I suppose.:hello2:

Link to comment
Share on other sites

When you say square, do you mean the faces of the bearings being in line with the sides of the mount? If so, then mine are very slightly out and it doesn't appear to be a problem. I may try to improve it by placing shims between the tube rings and the bearings.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.