Making a dob mount smoother

[Piero]

What do members use to make their mass produced Dobson mounts movements a bit smoother? Are there specific greases for teflon/wood which can be applied?  

[faulksy]

i found a good quality car wax realy works well grease or the like will attract dust and make a nice grinding paste

[scarp15]

I use McLube Sailkote, this was a tip provided by Mike73 sometime ago. It is very durable and has very good longevity and performs really well in terms of combining smoothness with stiction.

http://www.purplemarine.com/mclube-sailkote-dry-lubricant.html?gclid=CNGApsKQoM8CFQ26Gwodh9QBsg

 

 

[Guest Tuomo]

Ummm.....I would NOT use any kind of chemical products. Effects are most likely not what you are after and if you manage to find right chemical will it last...? I would use new teflon pads to improve your alt movement. People also use Star Ebony laminate in DOB rocker box to improve azimuth movement.

https://stellafane.org/tm/dob/resources/bearnings.html

[Piero]

Thanks everyone for your suggestions! Very useful! 

[brantuk]

I just made a new rocker box / turntable for my LB 16" - I've no intention of lubricating it right now and don't know if it will be needed in the future. But this is the material I used for the az bearing: "Formica Pattern range StarDust HGP Quarry F1782 CW". You can get it here:

http://www.idsurfaces.co.uk/c-3379-international-decorative-surfaces-f1782-stardust.aspx

And I got the PTFE blocks from AWR in this link: http://www.awrtech.co.uk/miscprod.htm#PTFE

The combination provides a smooth as silk gliding movement with just the right amount of "sticktion" to stay put once pointed at the target. It can be used for alt bearings too. Hth

[niallk]

I don't use any lubricant, but Armor All was the recommended lubricant for my Obsession dob, if I deemed necessary.

[Ben the Ignorant]

First, make the teflon pads as small as possible. I started with large round 30mm pads but seeing they had too much friction, I split them in four with a chisel. Four times less friction area means four times less friction, within reason, because if they are too small they tend to dent the wood. Now that I know I would buy the smaller 12mm or 15mm pads. Then add a couple CD's around the central axis, they slide like magic after you sand down the paint, and lube them with ordinary machine oil.

CD's have a small circular ridge on the mirror face, close to the center; these ridges must face each other to have the rocker box float like it has no weight. But now you have four contact points between the rocker box and the base, they won't match each other naturally like a tripod always matches the ground. That's why you see an adhesive strip acting as a shim under the right pad, it had to be raised a bit.

Be careful to remove all the deco on the CD, because that paint is like gum. Washers keep the CD's centered around the bolt, but the CD's holes needed to be enlarged a little to insert them.

[ollypenrice]

Turtle Wax works well for me. I once tried a different silicone based car wax and it was a disaster. The static and moving friction ceased to be the same so you got the push-overshoot business all the time. Turtle seems just right for mine. (Textured formica on teflon.)

Olly

[hughgilhespie]

Hi Ben,

On 9/21/2016 at 14:10, Ben the Ignorant said:

Four times less friction area means four times less friction,

A minor point, but in fact for most simple cases the friction is independent of the area of contact. Below is a quote from PhyLink.com.

But - if smaller pads work for you, then it's a good tip to pass on.

Although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. Since pressure equals force divided by the area of contact, it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the FORCE holding them together. 

If you were to increase the force as you increased the area to keep PRESSURE the same, then increasing the area WOULD increase the frictional force between the two surfaces.

Regards, Hugh

[Ben the Ignorant]

Yes, that's the general rule, but when I tried several combinations of pads in various numbers and sizes, with and without added CD's, I found smaller pads work better. Here I first rested the rocker box on the larger rectangular felt pads, rotation was stiff. I put the smaller round pads on top of the others, rotation became better, but still a bit stiff because felt is not the right material.

I then replaced them with full-size teflon pads, things improved but not enough. I rubbed my thumb against an entire 30mm pad, and compared the sensation with a quarter-size pad: I felt less friction. I rubbed a complete pad on the table, and a split pad, same feeling: the reduced pad slides better. The tactile sensation is confirmed by sight: the image starts moving with less stiction, and keeps moving without jerkiness (the rocker and scope weigh about 14 kilos).

[brantuk]

What's riding on your pads Ben? Is it formica or just laminated chipboard or something?

[Ben the Ignorant]

Just laminated chipboard, the same as on the picture above. This being my first DIY dob, I made it from shelves my former neighbors gave me for free when they moved to the countryside (lucky them), because I wasn't sure I would succeed.

The pinkish disk is an adhesive film thats slides very, very well, even when heavy things rest on the thing. It touches the CD's for a weightless effect. This was after the wear-in; the teflon pads have "scribed" the laminated board a little. Some 15W-40 oil poured on my thumb, and spread over the contact areas is still needed, teflon alone does not suffice with my materials. The larger dob in the background prefers light oiling, too.

[brantuk]

Yeah - teflon pads will mark laminated board - that's exactly what happened with my LB 16" which is just laminated chip board. I really can't recommend enough the material in my post above. It rides so smoothly on PTFE or Teflon pads and several folks here use it in rocker box builds. It's tough - won't scribe - and glides very sweetly - and has just the right texture to hold nicely for nudging. Closest thing to Ebony Star available (since ES was discontinued). Hth