Simple scale error made by me

[old martParticipant @oldmart]

I am in the process of making a basic 8″ rotary table out of odd bits that I have accumulated at the museum over the years. We already have a 6″ and a baby 100mm one, but making an 8″ one without paying a lot of money for a proper one appeals to me. The base is from some kind of mill which has a 150mm round top with 90-0-90 markings around it and a square base/ The top is an 8″ tee slotted round with a lower part which just happens to be about 120mm diameter There were some thick steel round plates of exactly 150mm diameter with a central axel welded in place of about 50mm diameter, far too well made to be commercial, possibly something from a helicopter. The original intention was to make a simple locking top for the RT, but I chanced a source of single start steel worms with Mod 1.5 45 teeth worm gear as sets. The worm has 18mm shaft diameter and the gear is about 72mm x 17 thick which can fit in the base casting.

Much milling later, I wanted a scale made to suit the 45:1 gearing which would give 8 degrees per turn and we had some very nice black anodised alumium bars of about 50mm diameter, which would drag engrave nicely. Wth the 6″ RT with a Pratt chuck in it I used the mill quill to cut lines about 0.1mm deep with a very sharp carbide lathe tool every 4 1/2 degrees for 80 divisions. The long marks were 10mm long and the short ones were 5mm long. The result was very nice until I noticed my mistake, ( for want of much ruder language). I had put a long stroke every 36 degrees around and got ten divisions with eight spaces instead of eight divisions with ten sub divisions. Now I will have to make another with the eight degrees subdivided into 1/10 degrees. Fortunately, there is enough anodised bar to make hundreds of them. Repeat after me long stroke every 45 degrees.

[Nigel Graham 2Participant @nigelgraham2]

For engracing such a scale, would it be easier and safer to engrave all of them at the short length, go round with a felt-tip pen marking what will be the longer lines (so verifying no mis-count somewhere) then lengthening those?

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A pity I’d not known you are doing this – I forget when I last used my 10″ rotary-table and I’m sure the museum would be a better home for it!

 

 

[old martParticipant @oldmart]

Our mills are just big enough for the 8″ RT, I shudder to think how heavy a 10″ one would be. Nice thought, though. It takes two of us just to lift the 6″ vise that we were given by Rolls Royce, it has been fully restored minus paint and I have used it to hold parts of the RT I’m making and with some 10″ long jaws I made for it, makes a useful fixture for bending sheet metal for restoring the helicopters, more a size for a Bridgport.

[old martParticipant @oldmart]

For the replacement scale, I will be using Nigel’s method of engraving all at 5mm long at 4.5 degrees apart and then changing the stop to 10mm long and engraving at 45 degrees apart for the eight degree marks. It will be easier and quicker.

On the first one, I tried knurling with a cheap tool unsuccessfully, and then tried fine diamond knurling with a good tool made in Yorkshire and got perfect results.Because of the diameter I had to use a push knurler, the clamp one only goes to about 1 1/4″.

The worm and worm gear were delivered direct to the museum on friday, but I’m not in till wednesday. Gary has already bored some soft jaws to 72mm to fit the worm gear for boring to match the table shaft. I bought a 6 3/4″ Tudor chuck with serrated jaws which was hardly used and at the time, job lots of 1/16″ serration jaws were cheap to buy on ebay. This chuck is not used often on the Smart & Brown model A as it is rather big for the 9 x 20 lathe size.

When the replacement has been made I will post a picture.

[old martParticipant @oldmart]

I have started making the replacement scale, it is to be bored out bigger and shortened to be close to the knurling. The plain part between the scale and the knurling is for the numbers 0 to 7 to be stamped on and I didn’t think of making the 1/2 degree lines a little longer until it was too late.

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