Cutting long racks

You may be struggling with the RA head and a 2-1/2" or 3" cutter to get clearance.

Work from a spreadsheet in Absolute mode not incremental or you get aculmative errors.

-- Regards,

John Stevenson Nottingham, England.

If this for your profile cutter, I suggest you do what some manufacturers have done, make them in several pieces. Some only used a rack on one side, with the driven side being the crossbar of a tee shaped frame.

Tom

Andrew, If this is the case have you thought about using long strips of tooth belt super glued to a steel bar for a rack. We did a tracer machine some years ago by this method and it worked very well.

-- Regards,

John Stevenson Nottingham, England.

I wonder whether you'd be better relying on your leadscrew than the DRO over the full travel of the table, although you may get greater local errors where there's a bit of wear. At least do some sort of comparison between the two over the length of travel before starting. I've got a good solid r/a attachment which might do the job well if your cutters are big enough diameter, trouble is it's got a BT50 shank & is an awfully big lump to hang on an R8 spindle. You're welcome to borrow it if you want to play at making a new shank for it, I'll need to modify it before I can use it anyway. Let me know if you're interested, I'll send you a pic. What about grinding up a form endmill for the job? You'd probably need to rough out with something else first. If supreme accuracy isn't important, how about buying some long timing belts, cutting them & fixing them flat instead of your racks? They do have a tiny amount of stretch possible, which could be a bl**dy nuisance or could possibly be used to advantage. Depends what it's for, obviously.

Cheers Tim

Measure each cut from the starting datum and not from the previous cut.

I think I might approach things in a slightly different direction to the others...

I would make fixtures, one that would fit into a rack tooth/slot and one that would hold the yet to be machined stock on the mill table. The first fixture would initially be movable along a slot on the mill table and the second would be fixed. There would be flat faces (touching each other for the first tooth) on the facing sides of the fixtures so that I could use slip gauges or setting rods to space them accurately. Using the setting rods or slip gauges, I would cut the first twenty or so inches of rack, being as paranoid as I could to get the spacing accurate. Thereafter I would have the 'movable' fixture fixed and index the remaining teeth against this. All this grief and effort would give me racks that had very small cumulative errors and inter-tooth errors measured in low numbers of tenths.

Don't know if that makes any sense, but that's what I'd do.

Mark Rand RTFM

Mark,

I came up with a 'cheating' version of your idea - to buy a short length of commercial rack - use it as you propose, and then bootstrap the rest of the rack from that !

Yesterday I got a quote from Halifax Rack and Screw which doesn't seen too bad, 8 foot 6 inch length of 16 DP rack cut on the shorter face of a 1" x

AWEM

That variant hadn't even crossed my mind. It would certainly eliminate the grief of messing about with slip gauges. Having said that, it's just crossed my mind that the obvious way to get DP indexing is to combine an ordinary indexer with a drum turned to an appropriate pitch circle diameter minus half the thickness of some wire.

Their name is definitely one for the note-book. What sort of tolerances do they quote? They don't specifically mention any on their web site.

Mark Rand RTFM