Mahogany, iroko, oak, beech, ash, (even lignum vitae) turn fine, but if you're new to woodturning, the main problem you'll experience with turning bowls in harder woods is 'tear out'. (The easiest wood to turn and finish is Sycamore, albeit it's a bit bland, which is why it's often used in segmented work, or for colouring and texturing . (London plane ('lacewood') looks a bit more interesting and turns well).
If you're new to woodturning, a few tips might help.
With a standard bowl blank (rather than 'branch-wood') as the wood rotates, the tool alternatively comes up against both end grain and side grain. The smaller the diameter of timber, the faster the lathe needs to spin at to get a good clean cut, or instead of the tool cutting the fibres, it tears them, then it’s necessary to resort to sanding by working through the grits for perhaps 80g to 600 grit for a good finish. The aim is to get the best finish 'off the tool' and to end up 'sanding the shape of the wood' - not 'sanding the wood to shape'. Until you become proficient with gouges, you might find it less challenging to use scrapers than gouges when finishing - a square-ended one for external use on bowls, a round-ended one for the inside.
One query that often crops up is what speed to run the lathe at?
The larger the diameter of the wood, the slower the speed needs to be. After truing up a blank till it rotates without vibration, a good 'rule of thumb' to use for the turning speed is thumb’ formula for calculating the speed at which a woodturning lathe should be run, is:
Diameter in inches x RPM = 6,000 – 9,000.
So for example, a 10 inch diameter bowl blank should be run at a speed of 600 – 900 RPM.
Initially, start off at a slower speed as when the blank is first mounted as it will be out of balance until trued with a roughing gouge. Using the above formula, it follows that a 2” diameter spindle would have a turning speed of 3,000 RPM, and a 1” one a theoretical speed of 6,000 RPM, well beyond the capacity of the lathe, so for spindle turning, most turners would run the lathe at perhaps 2,000 - 3,000 RPM.
At 3,000 RPM, a 1” diameter spindle would have about 3” of timber passing the tool for each rev, so that would be 9,000 inches of timber per minute passing the tool – about 750 feet. A 10” diameter bowl with the lathe running at 600 RPM will have 30 inches of timber per rev passing the tool, 18,000 inches – 1,500 feet per minute.
SANDING:
For sanding, too high a speed will generate heat, and risk cracking a thin walled bowl, so a lower speed - about 250RPM is sensible, moving the paper quickly back and forth to avoid score marks around the item being sanded, starting perhaps with 120 grit, then 180, 220, progressing to 320, 400, 600, depending on the degree of finish desired. When sanding, it’s hopeless going from say 120 – 400 – 1000 as the score marks left by the coarser grades will never come out.
I've attached a few pictures of items I've turned in hardwoods.
First pic is a mortar and pestle. Relevant to your query, the blank for the mortar was from an offcut of teak worktop fished out of a skip. I glued six pieces side by side. It turned well because it was all side grain. The pestle was a piece of African blackwood and the beech handle was spigoted into the blank.
Second and third pics are of a rosewood bowl.
Pics 4 & 5 are of a small bowl in iroko, which is similar to mahogany to turn. (It was an offcut from some dock gate repairs at Hull docks).
Lignum vitae bowling green 'woods' (sometimes referred to as 'bowls') are no longer acceptable for use competitively as they had to be weighed and certified periodically, adding or removing weight in the end, which no-one does any more. Hence, they sometimes turn up at car boot sales or on e-bay. Sometimes used by woodturners for making such things as bowls, clock mounts and woodcarving mallets. Pics 6 & 7 are of a mallet I turned, with an ash handle. (Lignum Vitae is now protected CITES timber:
Restricted and Endangered Wood Species | The Wood Database (wood-database.com)
It's one of the hardest timbers there is, but turns well with sharp tools and is saturated with natural oils, so if the lathe if run at a fairly high speed it can be polished with friction from a cloth to draw out the oils, which I did with the mallet.
Last two pics are of a small bowl in burr oak, which was a bit challenging to turn, which is why it's not very thin.
I'm just an averagely good hobby turner and I'm not holding any of these examples out to be competition winners, just to show that hard timbers aren't especially difficult to turn. (The mortar and pestle was the first item I turned as a novice turner).
Hope these ramblings might help and reassure you that your Mahogany worktop timber should turn fine.
Have fun!
David.
