Is your spindle bore large enough?

I've been looking around the forum and it seems to me most peoples lathes are 26mm or even less but you can't always tell what people use their lathes for so for those that use their lathes in connection with vehicle, tractor or stationary engine restoration what size spindle bore do you have and would a larger bore be more useful ?

Most of the older small used lathes I've seen have a 26mm bore but I see a lot of new far eastern lathes can be bought with a 38mm spindle bore but it is at a rather steep increase in price over a 26mm spindle bore lathe. Where a 38mm bore might be useful on occasion I'm wondering if it really warrants the extra money their asking ?.

Plus is the 38mm bore lathe offered at the expense of there being less material around the bore on the pulleys, cogs etc. that the bore passes through. As most of these lathes seem to be only available once imported actually getting to see and compare 26mm and 38mm bore lathes is hard.

Has anyone ever compared the internals of these lathes with the different bore sizes. Is a 38mm weaker than a 26mm ?

I haven't compared the internals of 38mm and 26mm spindle bore sizes, but I'm very glad I bought the 290 lathe with 38mm bore since I turn 30mm bars quite often. If there were room in my small workshop for a lathe with 50mm bore I would seriously considered such a lathe.

Thor

I've got a Crusader Deluxe with a 38mm bore, building a 6" traction engine and find the extra bore very useful; the extra length between centres is also really important (to me)

If you compare the gearing in the head with my previous Colchester Student it's not as good (generally not as wide gear teeth) but I've turned some big stuff on it with no problems and it makes less noise that the student (It is 40 years younger)

I spent months looking at tired Long Bed M300's and the like before buying Asian. I would say that the main part of the lathe is a bit lighter than old english, but still well enough made for large scale heavy hobby use. Some of the accessories were poorly made (steadys particulalry) but the bed and ways are still good after 4 years (and a lot of cast iron)

I have used a lathe with a 38mm bore for 16 years without any problems, swinging anything upto a 8 inch 4 jaw chuck, and taking cuts of 0.100 inch or more depth. Mandrels of this size seem to be supported in roller bearings, so there should be plenty of meat around the shaft..

When I had a smash up it was the pinion and rack that suffered, not the Mandrel!, or the bearings.

Howard

The Smart & Brown model A that I use has only got 25mm through the spindle, a nuisance at times.

The belt pulleys inside the Headstock on my lathe are pretty hefty. No sign of being frail there. Getting on for 125 or 150mm diameter

So far, I have not heard of anyone having a mandrel fail on the far eastern lathes sold in UK.

Howard

As rule of thumb, unless you're watchmaking, big lathes are always better than small ones. It's because they can accommodate bigger work without fuss. If you're likely to be turning long large diameter objects that have to pass through the spindle, then paying for 38mm or more will be well worth it. Threading the end of a 1" diameter pipe with a 28mm spindle is straightforward. Exactly the same job with 1¼" pipe is misery because it won't fit.

I suspect even 38mm is on the small side for full size vehicle work but much depends on what you need the lathe for. Machining the ends of an tractor axle would be challenging, but many parts, even large diameter objects like brake drums, don't need to go through the spindle.

The strength question is a red-herring I feel: I've never heard of a lathe spindle breaking whatever the bore. Severe accidents usually damage something else - sheer pins broken, teeth ripped off gears, snapped belts, bruised bearings and bent motor axles etc.

Dave

Edited By SillyOldDuffer on 09/08/2019 14:19:24

I would suggest that the extra price hike is due to the extra mass of metal required around the bore on the pulleys, cogs, etc. Every machining operation will entail more surface to be machined and machining costs are part of the product cost.

While a 50% increase in area machined, the volume of material being removed from solid items is over twice the amount for a 38mm hole over a 26mm one.. It all costs extra money as well as larger castings in the first place.

I would not expect either size to be weaker than necessary - if they are designed properly. Smaller, within reason, is always going to be cheaper. That applies to bearings, too. Then there may be larger gears required - more teeth for the same ratios? The price does not come down as the size increases.

Only when normal sized items are miniaturised, do prices start to rise again in the opposite direction, but now due to design problems and higher precision.

One might expect that a lathe designed around a 26mm hole might be about the optimum size for minimum production costs, but maybe 16mm would be cheaper - myfords and drummonds were cheap in their day, so perhaps they were right and the 26mm ‘standard’ has been adopted for reasons other than lowest cost?

My Craftsman lathe has a 38mm bore and roller bearings and seems quite happy about it.

Also, the larger bore does come in handy from time to time.

All things being equal ,I would go for the bigger bore.

Yes, Larry, but the prices cannot be equal for similar quality! I tend to agree that a 38mm bore is better - as long as the extra cost can be justified (without a reduction in quality) or a reduction of quality is acceptable. Purchasers choice.

Bending and torsional resistance or rigidity of spindles cannot be to high and is a power four thing of diameters.

Let us asume a Myford spindle is 30mm outside and 15 inside and let us calculate in centimeters.

Stiffness will be 3*3*3*3-1.5*1.5*1.5*1.5 eguals 81 minus 5(more or less) that is 76 something.

The same numbers for my proposed minilathe upgrade are 4cm outside and 3 cm inside gives 256 minus 81 is 175 in same units whatever they are.

To have the same softness as the myford my new spindle can have an hole diameter of 3.6cm or 36mm.

If the 62 mm bearing holes were 68mm ,spindle can be 45mm outside and hole almost 43mm

Edited By Niels Abildgaard on 09/08/2019 16:58:13

I will say that I have never wanted a smaller spindle bore!

Jim

Niels

How stiff is your headstock structure?

JA

The job will always turn up that is bigger than your headstock bore, it will be time to get the fixed steady out, the nice thing about a big bore is you won’t need the steady out so often. If the bore size you have is a regular nuisance then maybe a bigger lathe may be required. As a Myford 7 owner with the small bore which is widely derided I have not had to use the steady very much but it will all depend on what you do on your machine.

Mike

Very weak until I connect the unsupported corner of bed to Stone.

Am still undecided how to do it

For stationary engine work,and a lot of tractor work,plus fittings on full size steam I use a 6 1/2 c/h Colchester long bed master,spindle bore size ,it just takes nominal 1 5/8 dia bar,very useful. The advantage of Uk Built lathes is their weight ,foreign lathes are half the weight or less, I did have a Triumph with 2 inch bore and even then with some commercial work I borrowed a friends Mascot with 3 inch bore very usefull on pipe or tube work,and machining 2 ft plus flywheels. A Triump will not take 2 ft dia which was a nuisance as it would not hold a lot of Lister flywheels

I think the spindle bore is never large enough! but I usually make do with what I have, especially for a one off.

Can't remember exactly what I was doing here, I guess it was shortening a cardan shaft... oddly enough it didn't come out too bad!

Edited By ChrisB on 09/08/2019 19:21:39

Agree with ChrisB, however large the bore you will always want it to be a few millimetres larger.

My M300 has a 38mm bore and can swing an 11" 4 jaw BUT there are time when on small precision work and threading I prefer the Myford S7.

in the end it boils down to horses for courses and depends entirely on what you are making.