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Les

John,

i'm going on theory, not practice, so I will stand correction , but I think you can get a true arc at 37.5 rad if you set your boring tool to 37.5 (70 dia.) and do a compound incline of the cutter head. The inclinations should be equal but to save any complications its probably easiest to do both at 45 degrees, sins and cosine being equal. As Clive Foster suggested early on, your cutter describes a spherical path which forms a gutter (what your after) as it progresses along the X axis.

Bob D

If you have CNC why not program a cherrying operation in Y Z and peck along the X axis ? I have used a boring head as you show and had good results also used a Cherry head on the Bridgeport.

Les, that set up is how I used to produce the dies for oil and fuel filter housings. The dies I made had a round form but the principal was the same just rotated on the rotary table. Providing the wall section was OK no one measured the end result ! CNC now does it better and with great accuracy. Chris.

Hi Chris,
When I saw the first post of this thread I could see that the result would not be perfect but could also see that over the small width of the rack the error would be quite small. I could not see a way to work out the exact shape that would be cut by this method. When I saw the link that Michael G. posted I thought it would be helpfull to extract the page and add it to the thread.

Les.

Les - the exact shape is an ellipse. But for ellipses with small eccentricity, the curvature is close to circular.

Ideally you would try to ensure that the major radius of the ellipse is smaller than the radius of the tube it is intended to mate with. Certainly, given that the two surfaces are not both cylindrical, you want them to have two lines of contact rather than one. If you don't manage that, you will have a problem.

MichaelG, thanks for the link. Pity about the single-page restriction. I set up a Uni-Michigan 'Friend' account, logged into their library, and called up the book. Still no-go, Friend accounts are specifically excluded. Damn!

Roger

John,

Had a play on Solidworks this morning and came up with this sketch.

Basically, a Ø22 cutter inclined at 15° will give a profile that matches to within 0.03mm the high spots at either end and a gap midway.

Trev.

PS. you may remember me coming to see you about the laser engraver, well, thanks to your help I had the confidence to buy my own.....and very pleased with it I am thanks. One question; do you put antifreeze in the coolant? Am getting a bit nervous as the machine is in the unheated shed.

Hello John,

You may have the answers you need already, but a possible alternative has come to mind

I don't know how you plan to cut the rack teeth, but if you are using an angled drive to hold the cutter at right angles to the rack as might be the case for a short rack, would that be man enough to hold a side and face cutter of suitable diameter and mill the curved face from the side with the rack held teeth up as a second operation?

Regards

Brian

OK now sorted.

Rack cutter ground up, as it's brass it's just a single lip engraving type cutter with 40 degrees included angle.

Found a jig to hold it, not the same one I used last time but similar. Nice sharp shell end mill sorted and the angle needed.

Brass bar got delivered this morning in 3M length, trying to find the best way to get this in the safe without cutting it too short.

Should get round to this later this week, need to have a word with purchasing to see if they don't want to buy a spare.

Glad you have got this done. I have had a thought for the next time this needs doing though. If the required diameter is always the same at Ø75 then why not get something like a T slot cutter ground to the required radius so that it is a straight forward machining job? If you have a CNC lathe you could make your own tool since you are cutting brass, some silver steel shaped, hardened, tempered and sharpened would do it.

Martin