Cutting a larger focuser hole in OTA

[DaveGibbons]

I just picked up a nice skywatcher 8" newt ota and wish to replace the 2" R&P focuser with a 2" Orion USA crayford I have spare. This will mean drilling four new mounting holes(no problem), but also as this has a much more substantial mounting plate internal diameter I will have to enlarge the focuser hole in the ota by about 3/4 inch diameter. Any advise would be welcome as to the best method for cutting a neat hole. I suppose something like a dremmel tool or even very careful use of a hacksaw would work. Has anyone done this?

Cheers

Dave

[Kaptain Klevtsov]

I'd go for a jigsaw, used with extreme delicacy. Cover the tube in masking tape to stop it from getting scratched.

Sorry, you knew this but, TAKE THE MIRRORS OUT FIRST.

Kaptain Klevtsov

[centroid]

The difficult bit will be making the enlarged hole assymetric to the existing one, as you won't have solid centre from which to 'scribe' a circle for the new one.

I reckon the best way would be to make lots of 'scribe' marks at the required distance around the existing hole, and closely spaced.

Cut just inside the 'scribed' circle, and then finish the last 2 or 3 mm with a 'half-round' file. That way you'll have more control over accuracy and finished edge of the hole. You can always take more metal off, but its not so easy to put it back on

Dave

[Kaptain Klevtsov]

The way to get the line even is by using a pair of compasses. Adjust the pencil bit to be much shorter than the point bit and let the side of the point ride along the inside of the hole. This way you can get an evenly spaced line around the existing hole as a cutting guide.

Kaptain Klevtsov

[barkis]

KK's suggestion is a good one as long as you use a blade witha narrow profile to allow navigating a fairly tight curve.

I would accurately drill the four holes for the Crayford first, then get a scrap piece of 1/8th aluminium, and using a panel beaters hammer, gently form it to the curve of the tube. Then place the ally inside the tube and accurately mark and drill the crayford holes into the ally and bolt it from the inside to the tube. Mark and centre punch the exact centre of the ally as seen the the original tube hole. Cover the area around the tube where you are going to drill your new hole. Place the tube on the froor and drill vertically at a slow speed using a metal hole saw with fine teeth of the size required. It will be easier to clean the metal swarf from your tube if you place newspaper beneath the drilling to catch it. And of cours as the Kaptain said, this work to be carried out with the optics removed and placed in a safe place.

It has took longer to describe this approach, than it will take to do it. But at least you have two methods to choose from now.

Ron.

[saturn5]

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[saturn5]

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[The Warthog]

I will want to do this if I ever get up the nerve to do it to my C6N. The plan I have is to draw my circle, and cut to it radially from the present circle, then cut around the circle, with curved tinner's snips. Then smooth with a rotary stone or file. Of course, take all the mirrors and everything else off the tube before you have even the slightest hint of a notion that you might want to start flinging tiny bits of metal around with tools.

[Keith2]

These all sound like very elegant solutions! I once did something much rougher and cruder, which has worked fine on my old newt.

1. Mark the circle you need, allowing a few extra mm

2. Drill lots and lots of little holes all along the circle, as close together as you can

3. Join up the holes by snipping with wire cutters

4. File down the rough edges.

You end up with something that looks untidy at first, but all the wavy edges are covered up by the focuser mounting itself and the end result is just right. It worked for me anyway.

Keith

[saturn5]

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[DaveGibbons]

Thanks for all the advice, I will let you know how it goes!

Dave

[jonifive]

I'm with WH on this one. The snips will give you a nice clean edge with just the burrs at the start and the edge of each cut to file down. If you do decide to file the edge down, can you PM me a warning so as I can wear my ear protectors....the noise will be horrendous.

BTW you are a braver man than I attempting this.

JV

[philsail1]

Dave,

As "Barkis" says - if you use a saw, make sure it has very, very fine teeth, otherwise it may "bite" into the thin metal of the tube and bend or kink the metal.

Best of luck,

Philsail1

[barkis]

Dave, I have another method for cutting your larger hole, that's if you have not decided which way to go.

You could trepan the hole, using a piece of metal tubing the same diameter as the hole you require. This is the method commonly used for cutting out the hole in a main mirror for a cassegrain mirror. Since the wall of your tube is quite thin, it would not take long to cut through it via this method, and it is probably the cleanest and safest way to do it. You have probably seen, or know of how a motor mechanic ground in new valves into a cylinder head. Well, the same action applies here.A piece of 18 mm plywood, with a hole cut in it, the size you want in the tube.

This will act as a guide for the metallic tube that is going to cut through your scope tube. The wood guide prevents wandering off line.

A wooden dowel attached to the metal tube somehow, and easy way is to stick a disc of wood in one end, and glue in the dowel, a piece of dowel half an inch diameter and about 8" long will suffice. Serrate the other end of the metal tube with a hacksaw, this is just to allow the cutting compound to be evenly spread whilst moving the cutter back aand forth in the palm of your hands. No presure need be applied.just keep the tool charged with cutting paste. Mirror makers usually use a grit size of 400 which is very fine, but it cuts rapidly, and very smoothly.

You could use a coarser grit of course, but as I said, it won't take long to cut through the thin wall of your tube.

I hope you get the gist of what I have written. You probably have got the job done by now anyway, but never mind someone else might want to try this method. If they do, and require any grit to do the job, they can let me know, as I have loads of it left from grinding glass.

hope you have not fallen asleep reading this.

Ron.

[omaroo]

Sorry to interject so late... but I'd like to offer another alternative so that you could avoid using jigsaws or rotary hole saws. Maybe use a nibbler instead. You'll have the job done in a few minutes without any risk of damage. The rate at which you "nibble" is too slow to have it careening off course, and the finish can be simply cleaned up with a fine file afterwards. no stress - no massive vibration and no ripping and tearing!

I know it's not a telescope - but here are a few images of the hole I recently cut in our 4WD to fit a snorkel (breather) into similarly-thick steel.

The nibbler cost me $14 from a local electronics store.

Cheers

Chris

[barkis]

I think that one might get the vote Chris. Pretty nifty tool that nibbler, I think i will get one for my toolbox, never know when it I might need one.

Ron.

[iancandler]

We had to do this to my brothers 10" ota lst christmas so he could fit his new focuser.

In the end we marked up as per KK's post, then got out the dremel and fitted a small cutting disc to it.. than anchoring the heel of the palm in the centre carefully followed the line round.

We finished the cut hole off with fine emery paper and just drilled the new mount holes.

Another thing we did before assembling was to paint any bare surfaces with matt acrylic paint.

[Nibor]

A very interesting topic!! - especially as I have just received the speculative purchase of a 2" Skywatcher crayford focuser to upgrade the 2" rack and pinion one on my 10" Orion Optics GX250 newtonian.

I found it on Astroboot and felt that at the £20 asking price for a good used crayford from a scrappped 10" or 12" size scope it had to be worth a try.

It arrived today and I lined it up against the edge of the GX250's actual tube to see what the fit looks like. - It seems pretty close but there is probably a maximum gap of 2mm. Is this indicitave that the focuser base is not a 10" one??

It does seem a very small difference and it also seems that focuser bases are designed to fit a range of tube diameters so that there must inevitably be some discrepancy:-

See this one from TS:- Optimised for 230mm tubes:-

http://www.teleskop-...-205-260mm.html

1. Will the 2mm gap be OK and what will be the best way to fill it?

2. OR will the aluminium tube conform to the focuser base as the fixing nuts are tightened?

As the Skywatcher fitting requires a larger hole than the current one the focuser hole enlarging ideas in this thread have been most helpful but I would be grateful If anyone can update me on any new methods since the last post in 2007.

My current thinking is either chain drilling or nibbling.

Many thanks.

EDIT - Just been doing some more research it seems that I may have one of these:-

http://www.ebay.com/itm/New-Orion-2-Crayford-Style-Telescope-Focuser-/160596873879?pt=LH_DefaultDomain_0&hash=item256451d297

So perhaps it will be OK after all?

[kirkster501]

Always make a template and if needs be, practice with that template on a tube of paper/card the same diameter as your scope to get it right...

Measure three times and cut once is the chippies number one adage....