Reduction gear for headstock?

[GrahamRounce]

Spectric: Yes, but I'd like it transparent. Admittedly, I didn't expect all this trouble!

 

[woodfarmer]

I use delron ok, but for speed reduction I think a layshaft might be a better way to go, It is more trouble to install but the bits are a lot cheaper. axle, pulleys. screw a big pulley to a small one. on a swinging layshaft.

 

[Inspector]

If the acrylic rod you are using is extruded it is quite soft, gummy and difficult to get a smooth surface when drilled. If you can get cast acrylic rod it works much nicer. The other thing you could try is put a cotton rag in the bottom of a big pot and boil the extruded rod for 20 minutes or longer. It is supposed to get better working properties after it cools down. Haven't tried it myself.

Pete

 

[J-G]

Although I've been following this thread I haven't commented before because as an engineer I couldn't imagine cobbling together some way to build a reduction gear but with further revalations about the material I'm surprised that the OP wouldn't just buy Acrylic Tube. 40mm Dia is available with 30,32,34 & 36mm bore 'of the shelf'

https://www.simplyplastics.com/catalog/acrylic-tube/clear-extruded-acrylic-tube/c-24/c-81/p-216

 

[chaoticbob]

I too have been following this thread from an engineering perspective. I work mostly with metal but sometimes turn plastics and even (shock horror!) wood on a metalworking lathe.
Graham - unless you need to reduce your minimum spindle speed for other reasons, this is not the way to go. 450 rpm is not fast for turning acrylic at these diameters. I have gone much faster using CCGT carbide inserts, which are intended for aluminium but perform well with other soft materials. I'm not sure how they could be used in a wood turning lathe, but the message is that they are sharp and have a lot of top rake.
From the pic of your 'side chisel' it looks like it might rub rather than cut, which might explain the molten mess. You need a sharp tool with top rake and an assertive feed - don't let the tool linger!
I'd be looking at tool geometry and feed rates before investing in spindle speed reduction.
I've never got a transparent finish straight off the lathe with acrylic, but I've turned it to polishable without recourse to flood coolant.
Bob.

 

[GrahamRounce]

Inspector: Interesting... thank-you.

 

[GrahamRounce]

30,32,34 & 36mm bore off the shelf

Because all I could find had a 2mm wall, much too thin. I'll check that link, thanks. If a 40 x 20-25mm bore was available, fine. I'm not mad, cough.
That said, I'd want one end sealed tight against pressure... I suppose a gasket and a ring of self-tappers, tho I've found that in acrylic self-tappers need a pilot hole hardly any smaller than the screw itself, so I'm less than confident, unless I use a lot of them in which case the wall would have to be thicker. It just seems not much less trouble than a nice clean bore, AS I THOUGHT, haha.

 

[J-G]

I work mostly with metal but sometimes turn plastics and even (shock horror!) wood on a metalworking lathe.

No 'Shock Horror' as far as I'm concerned - most hard woods can be treated just like metal or industrial plastics - the biggest problem is getting mixed swarf. Separating wood-dust from brass/alum/steel swarf can be a real pain!

Graham - unless you need to reduce your minimum spindle speed for other reasons, this is not the way to go. 450 rpm is not fast for turning acrylic at these diameters. I have gone much faster using CCGT carbide inserts, which are intended for aluminium but perform well with other soft materials. I'm not sure how they could be used in a wood turning lathe, but the message is that they are sharp and have a lot of top rake.

I suspect that the initial "drilling with a 25mm flat bit" was a major mistake. Flat bits are not known to be 'sharp' so the friction (and consequent heat) would be high. Personally - assuming I needed a smaller bore than the 30 available as Acrylic tube - I would have started with something like a 6mm HSS jobbers drill progressing in 1 or 2mm increments until I could get a boring bar in. My preference would then be to use HSS tools with a 30º+ top rake and take light cuts (<1mm) until within 1mm of the finished bore then <0.2mm at a higher speed.

As has been said, not letting the tool 'dwell' is important.

Just noticed that Graham has responded and refuted some of my reasons to use tube!

I wouldn't use 'self-tappers' - I would drill & tap for machine screws, but then I do have taps and dies from 12BA through to M12. To close off one end of a tube you could turn a piece of flat sheet with a tight fitting boss and glue it in ??

 

[nick61]

If it doesn't need to be acrylic, the easiest plastic to machine is acetal. Tricky to get nylon to size with a good finished. On a metal lathe it is low speed, deep cut and high feed for best finish.

 

[J-G]

If it doesn't need to be acrylic, the easiest plastic to machine is acetal.

Acetal can't be polished to a clear finish.

Tricky to get nylon to size with a good finished.

I'll concur with that on both counts !

 

[GrahamRounce]

Good responses, thanks! It seems I've been a little er unknowledgeable on several counts, not for the first time
Very interesting discussions tho.

I think I'm going to go with J-G's "start with 6mm, progressing in 1 or 2mm increments until I can get a boring bar in".
I take it a boring bar is what I christened a "side-chisel"
"then use HSS tools with a 30º+ top rake"
? I'll look that up!
Thanks again.

PS: Of course the boring bar shouldn't be dull, hoho

 

[J-G]

I take it a boring bar is what I christened a "side-chisel"
"then use HSS tools with a 30º+ top rake"
? I'll look that up!

I'm making the assumption that you have a metal working lathe and/or that you can hold a boring bar in a firm position and control its progress by machine dial rather than just by hand.

Yes a 'Boring Bar' is probably what you called a 'side chisel'. Here's a photo of the 'business end' of three of my (many) boring bars :

They are all made from square stock steel, left to right Carbon steel, mild steel with carbide insert, HSS.

The first can get into a 6mm hole, middle - 15mm and right - 3mm.

The smallest is also a very high top rake ( > 45º) used for acetal/Delrin, Tufnol etc. the others more conventional with about 10-12º top rake.

The basic geometry that you should be aiming

for is something like :

... which shows your 40mm OD / 25mm ID, the 6mm start drilled hole and a 13mm hole which is often the largest drill in a Jobbers Set.

The blue lines are the shape of the cutting edge.

It might even be advantageous to have the cutting edge just marginally above the centre-line.

 

[nick61]

A little research on alternative material suggests that PET machines almost as well as Acetal. PET is the stuff of fizzy drinks bottles, so should be available in clear. The UK suppliers I've found only seem to keep it in white form, but there is a US listing of clear, so it must be around somewhere......

Acrylic isn't listed by 'engineering plastic' suppliers I've found, presumably because of difficulties in machining.... A few years ago I had a job I needed to make in 'plastic', and had a useful conversation with one of the suppliers which aidd selection, but sadly no recollection of their name. Might be worth calling one or two to get some advice.

 

[GrahamRounce]

Ok, thanks. I must say I've had success polishing acrylic by going down (up?) the grades, from maybe 120 to 1200 and finishing with Brasso. Fine for flat or even curved surfaces, but not for a hole only just large enough for a finger (if you want to hold on to all five).

 

[TheTiddles]

I’m a bit confused what you’re trying to make, but it sounds like some form of pressure vessel?

You can machine PMMA just as well as PET, neither is fabulous but they are often clear which others are not. So POM is lovely and slippery, but usually white or black. PA is tough but also usually beige, black or white too.

Screwing into PMMA is best done with tapped holes as it’s generally fairly brittle and self-tapping screws can cause stress raisers that later crack. Adhesives can be used but be careful you don’t create solvent stress fractures, especially on extruded not cast stock, so something like silicone works well.

There may be alternative solutions that save you trying to reinvent the lathe to do it

 

[J-G]

Acrylic isn't listed by 'engineering plastic' suppliers I've found

In that case you haven't found the right Engineering Plastic suppliers, try Direct Plastics | Engineering Plastics | Large Stocks In All Sizes or Cut to Size Acrylic (Perspex) Sheet, Tube & Rod - Simply Plastic both easily found 'Engineering Plastics' Suppliers.

 

[Inspector]

What will this plastic do when completed? How much pressure is involved and what is the pressure from? A liquid or a gas, air?

Have you considered making a mould to cast the part out of clear casting resin? There are polyester resins (same family as fibreglass resin) and Epoxy casting resins that are as clear as glass. If the mould is good enough you will only need a little polishing. It can be turned to size and polished too. Their suitability depends on what you need it for.

Pete

 

[Fergie 307]

To finish the inside of your bore you can try using a piece of bar with a slot down the centre the appropriate length. Then cut a strip of wet and dry, insert one end in the slot and wind it round the shaft in the appropriate direction. You want it wound quite loosely so when you insert it in the bore the paper will spring out and touch the sides. Go down to 2500 grit. Then repeat but with some fabric and t cut. Or use a bore hone, and at the end wrap it in some cloth over the stones. I have one where I have replaced the stones with narrow strips of leather for polishing the inside of tubes. Autoglym car polish also works well on acrylic.