slitting saws

I am thinking to buy a slitting saw, about 65mm dia with 20 hole and about 3 thick. I will need to make a mandrel for it. I want some slots in brass, about 20 deep and 25 long and 4 wide.Question- what RPM to run, can I cut 20 deep in one pass and can I offset by 1mm to widen the slot ? Sounds simple but never used a saw before so need some answers before I buy one. PS will be used on 42 ch lathe with the brass mounted on crosslide.and fed underneath the saw.

For that depth and width I would by a cutter of the correct width, for a single pass.

Beware of your lathe cross cutting. They usually face cut as a slight concave, if anything and that may either cause problems or produce a wider than expected cut.

If it has a 20mm hole and you wish to cut 20 deep you will need a larger diameter saw than 65mm as that only leaves 2.5mm for gripping the saw to the mandrel.

20mm deep with a 65mm saw blade won't leave much to clamp, probably be 22mm bore if a metric one and I would say 30mm flange on the arbor would be minimum so only aaround 16mm depth available.

Saws do flex and will try and wander into the first cut, if you must do it with a saw then 1mm down each side of the 4mm slot and then take out the waste with more passes of the same saw or a 3mm one down the middle

Depends how rigid your lathe setup will be, with a coarse tooth blade that has about 24teeth and using the 1mm option you may be OK but the load from a 3mm cut will be similar to parting off 65mm stock so chatter will be likely..

Thanks for that. BTW lathe is 4" CH. The slot width is not critical as side play taken by shim. I've seen some saws with 3/8" and 1/2" bores but I thought that seemed a bit spindly. Maybe buy a bigger dia. and 4mm thick ? Low revs 100rpm at 4" dia.? Lathe is pretty good for rigidity, have parted off with 3mm blade in steel. Only about 55mm center ht. over cross slide so will have to check figures.

I would not expect chatter, like possible when parting off. After all it is not like cutting on centre height (and a reducing diameter) with a parting tool, when using a slitting saw with a large diameter.

My favourite rule of thumb is 10000/diameter in mm, which agrees about 100rpm at 4" dia.

The calculation doesn't take everything into account (like single point vs multi-points cutters, carbide vs HSS, or different metals), but it's a pretty good approximation. Try a shallow cut at 100rpm and see if anything complains before going deep. Brass will probably cut somewhat faster, but play it safe at first. If it's important to get the cut right first time, experiment with scrap first!

Dave

The old rules of thumb don't always work with modern variable speed machines which start to loose power at lower revs. I have posted this before, 100rpm text book speed on a direct drive brushless machine and it just stalls, 200rpm and it will cut.

Having checked things out, trying different mounting for the work etc. and saw diams. I've decided it's a no-go. Will have to get the old hacksaw out. I just fancied a slitting saw but have an excuse to buy a new file. Thanks to all.