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Machining a spacer shim

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Machining a spacer shim

Postby Guineafowl21 » 11 Sep 2021, 10:51

I’m new to the lathe, as some will know. I have a mystery Frezite cabinet door cutter set for the spindle moulder, where you space the cutters according to the thickness of tenon required.

For some reason, the tenon ends up 0.3mm smaller than the spacers you put in, so to get a 6.3mm tenon, you need 6.6mm spacers (I use 6.5mm). This is not very consistent, I think because the spacers I have are small, so there must be some wobble.

So, what I needed was a shim, OD about 60mm, ID 30mm, T about 6.6mm but may need adjusting. Obviously the faces need to be dead parallel.

I merrily chucked in a slug of 306 (stainless?) that came with the lathe, faced and centred it, and machined the OD and ID. I didn’t forsee the difficulty of getting the thickness exactly right, and being able to finish both flat faces neatly and exactly parallel. The parting tool has left a rough and inconsistent finish on one face, while the other is neat.

Now it’s parted off, how do I hold it to machine the parted face, being only 6.6mm thick? Should I have machined the majority of both faces before parting off?
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Re: Machining a spacer shim

Postby Woodster » 11 Sep 2021, 11:07

Place the remains of the bar you cut your shim from back in the lathe. Face it off then cut a tenon just under 30mm diameter for your shim. Make sure the face of the step is nice and flat. You may need a slight undercut at the corner to ensure the shim sits flat. Superglue your shim to the part and turn the face to size. Heat will loosen the glue bond when you’ve finished.

Edit: Here you go, same idea.
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Re: Machining a spacer shim

Postby novocaine » 11 Sep 2021, 12:23

What he said.
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Re: Machining a spacer shim

Postby Trevanion » 11 Sep 2021, 12:37

Wot they said!

Measure your part to find out its thickest point if it isn't parallel first (it should be pretty good off a parting tool but best to check in a few places anyway), I like to set my digital caliper to the size I want which would be 6.60mm and then zero the display, when you go to measure the part it will tell you exactly how much over the measurement the part is (I tend to switch it over to the imperial thousands setting now to convert the measurements to suit my machine, but it doesn't matter if your machine is metric). Glue the part to your mandrel as above using the centre to apply pressure and keep it centred on the mandrel (having it centred isn't too critical, it's more the clamping pressure and the even amount of glue that will keep the part flat and accurate on the machine) until the super glue goes off, then set the compound slide so that the tool is perpendicular to the face that needs to be parted, run the machine and gently turn the handwheel until the tool is just barely scraping the work, set your compound slide's dial to zero and then advance the tool gradually until you're at the measurement you need to remove.
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Re: Machining a spacer shim

Postby Guineafowl21 » 11 Sep 2021, 14:23

Thanks! Easy when you know how... :eusa-clap:

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Re: Machining a spacer shim

Postby Woodster » 11 Sep 2021, 16:49

If this requirement comes up a lot, some machinists will make up one or more spiders to do the job.
Google Chuck Spider to see the type of thing. I should make one myself just in case but I haven’t got round to it.
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Re: Machining a spacer shim

Postby Guineafowl21 » 12 Sep 2021, 11:14

I’ll look into chuck spiders, thanks.

I discovered yet another thread type yesterday. A mate wanted some little brass adapters for a chandelier-type light fitting, which appeared to be 1/2” nominal diameter, but 26 TPI. It’s not Whit, BSF, UNF, BA, but apparently BSC. British Standard Cycle thread. I might have a go at machining my own taps out of some reasonable steel.

Tapping drill is 11.7mm, and I’m too tight to buy those big sets of 0.1mm drills. Does anyone else drill out to the nearest 0.5mm size and cut again while pulling the drill bit off to one side, or is this beyond the pale?

I’m also looking for a good, comprehensive thread chart to stick on the wall behind the lathe. Plenty available, but none that cover all the Imperial sizes, as far as I can find.
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Re: Machining a spacer shim

Postby RogerS » 12 Sep 2021, 11:37

Guineafowl21 wrote:.....
I’m also looking for a good, comprehensive thread chart to stick on the wall behind the lathe. Plenty available, but none that cover all the Imperial sizes, as far as I can find.


Try Googling imperial thread chart and then each thread type BA, Whitworth etc and look at the images. I don't think you will find a chart that covers them all in one chart.
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Re: Machining a spacer shim

Postby AndyT » 12 Sep 2021, 11:46

If you want something really comprehensive, you can read an old edition of Machinery's Handbook here and download all of it if you want.

https://archive.org/details/machinerysh ... 6/mode/2up

That's the start of the section on threads, which goes on rather a long time.

For the rest of us, Roger's advice is spot on!

And your brass fittings are probably British Standard Brass - still available -

https://britishfasteners.com/threads-bsb
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Re: Machining a spacer shim

Postby Trevanion » 12 Sep 2021, 12:03

Guineafowl21 wrote:I discovered yet another thread type yesterday. A mate wanted some little brass adapters for a chandelier-type light fitting, which appeared to be 1/2” nominal diameter, but 26 TPI. It’s not Whit, BSF, UNF, BA, but apparently BSC. British Standard Cycle thread. I might have a go at machining my own taps out of some reasonable steel.


That'll be British Brass Thread/British Standard Brass, you can still pick up taps and dies for it:

https://www.tracytools.com/taps-and-dies/26-TPI-BRASS?product_id=1029

Edit: Andy beat me to it :lol:

The #181 chart from here seems to be what you're looking for, not cheap though!
https://www.ebay.co.uk/itm/203264779665?hash=item2f53866591:g:GR8AAOSwRhVgHzae
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Re: Machining a spacer shim

Postby Guineafowl21 » 12 Sep 2021, 17:43

Gah! Yet another thread type. BSB sounds more likely than BSC, although I did cut the threads at the BSC 60* angle and they fitted the BSB 55* one quite nicely. A tightish fit, but that’s what I was going for.
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Re: Machining a spacer shim

Postby Trevanion » 12 Sep 2021, 17:56

Guineafowl21 wrote:Gah! Yet another thread type. BSB sounds more likely than BSC, although I did cut the threads at the BSC 60* angle and they fitted the BSB 55* one quite nicely. A tightish fit, but that’s what I was going for.


Wait until you get into "Sewing Machine Thread" :lol:
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