2" shaft mounting Pt.53: Base fork and sawing aluminium.

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Progress has been rather slow of late due to low temperatures. I spent a couple of hours playing with cardboard patterns for a new fork without coming to any firm conclusions. The temperature remained fixed on 33F in my workshop. About +1C in new money. Fortunately I had a warm duvet jacket but my feet began to feel the cold after a while. I was working with the polar axis laying flat on the bench to avoid having to support any heavy weights. It also saves having to properly support the wormwheel when checking clearances. It just need a short length of 2" OD pipe to center the wormwheel relative to the PA, the support fork and the base plate.

Cutting the thick aluminium, for the new support fork, was a major hurdle to further progress. Despite using lamp oil [kerosene] with my jigsaw, even 10mm aluminium was rather a slow process. It was then that the subject of circular saws for metal cutting arose. Quite amazingly the tungsten carbide teeth can manage aluminium.

My problem was owning an old, cheapo, 7" supermarket bought, circular saw. It runs at much higher speeds [4500rpm] than is desirable for cutting aluminium.  YouTube has a number of useful videos about metal cutting with chop saws and miter saws. One chap was cutting huge sections without any apparent effort at all. It was then that the mention of aluminium's toxicity was mentioned. I had been using an angle grinder and orbital sander and producing masses of dust! I could often taste it and it was almost certainly getting into my chest at times. Not a good idea at all! 

I really need to build a raised, heavy wooden platform, to do any serious observing or imaging, from our semi-wooded locality. Which means I badly need a much better saw to work with such heavy timbers. You have to be young and fit to use a handsaw for extended periods on heavy timbers. Fortunately my wife persuaded me that I really should avoid the nasty aluminium dust in future. Which meant I needed accurate cuts with a decent finish straight off the saw. There followed intensive, online research into likely saw candidates including watching YT video reviews. 

Eventually I settled on a DeWalt, bar slide, miter saw. Not only could it manage huge sections of wood but it could cut metal as well. The 12" model also had speed adjustment. Once I had found the cheapest stockist with a decent reputation I placed an order for the DWS780. Not only will it manage large section timbers for the platform but it will make quick work when I board indoor ceilings. It looks as if I will have my work "cut out" for this saw.

A local DIY store has specialist alu. cutting blades in the correct size for very little money compared with many others. So I can save the woodcutting blade for its intended purpose. I even have a large stick of beeswax which makes an excellent metal cutting "lubricant" apparently. Oil slows cutting by being too slippery and paraffin [kerosene] is not ideal.

The images show potential forms for a wider support fork made from 8" wide stock compared with the original [half thickness] 6". A deeper base helps the fork to "get under" the loads it carries. I do wonder whether a half circle is the most desirable form in such an overall, sharp-cornered, all rectangular mounting design. Only my inability to quickly cut such thick aluminium was holding me back from diving straight in. So I have had time to seriously ponder the aesthetics too.

It will also be much easier to use a decent saw to reinforce the saddle. I need to cut a 38" long strip of 10mm alu. to fit inside the channel section. One which [ideally] has perfectly straight edges for neatness and hopefully increased stiffness for a really snug fit. So denying the strip any lateral movement. Not one requiring endless filing or abrasion to make it straight and just fit so-and-so. The saving in time, noise and effort with a new saw should be considerable. I have spent many, noisy hours with an angle grinder trying to tidy aluminium edges from the jigsaw.

I intend to increase the radius of the top part of the fork where it camps to the polar axis housing. Two full-sized plywood patterns will help me to check how the mounting balances overall. With the present fork depth the sides want to fold away from the polar axis when resting on a flat surface. This unwanted rotation would be resisted by the altitude adjustment turnbuckle. Though I would still prefer that the mounting does not place heavy loads on the turnbuckle. Not just for long life, or even ease of adjustment, but to reduce its sensitivity to adjustment. Having the mounting slowly 'sag' in altitude over time would be worse than not setting it correctly in the first place. I would have no idea that anything was changing without constant monitoring.

For simplicity I have been assuming that the center of gravity of the complete mounting will lie at the center of the declination T.  It must be remembered that the OTA and counterweights will add at least 100lbs to the declination axis. Fortunately the loading must be symmetrical or they would not balance each other.

Update: While browsing a large DIY superstore I discovered aluminium tube in round, square and rectangular form @ 2 meters [6'6"] long. I had previously drawn a complete blank, as a private customer, in trying to find aluminium tube for sale in Denmark. No shortage of wholesalers but they won't deal with the public. It is certainly not cheap but not quite enough to "break the bank." It certainly opens up some further options for a truss or skeleton tube design for my 10" f/8 Newtonian. The round tube was in size steps up to 30mm diameter which is really quite sturdy. I could have ordered tube from eBay Germany or UK but international postage costs made rather a dent.

Click on any image for an enlargement.

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