I am on the lookout for a 38dp gear hob suitable for cutting a rack that's about an inch long, in one pass. I'm not overly conversant with gear or rack cutting, so please excuse any obvious error in my search. The suggestion of what I need came from a customer. The plan is to mount the cutter in a vertical mill. Bob
In the words of Al Read . . . . "You'll be lucky, I say, You'll be lucky".
You could have one made of course but I doubt that you really want to spend the amount of money necessary.
38 DP is going to be a 'Special' whoever you try.
If my calculations are correct then 38DP is 0.668 MOD so if you can get away with 0.7 MOD . . . . and you want 20° PA . . . . . . and you can make two passes . . . . . . ArcEuro have a 25mm dia Hob with 8mm bore which would cut about 15mm wide.
Many thanks for the info, shows I wasn't fibbing about knowing little when it comes to gear cutting. I'll check with my "man" about the alternatives, and confirm the original info is correct. 2 passes is not a problem as the job is to be on the CNC. As the job is an ongoing one, I may have to consider having one made if the alternative is not suitable, but dread how much that will be, all guesses welcome !!! Bob
A hob has spiral teeth and as it rotates it expects the blank to move with it, Ok for a gear as it round and the start point will come round again but a rack won't.
Imagine a worm and wheel, that's the action.
It could be done with a non spiral hob but chances of finding one of these is as great as getting Gordon Brown to admit he's wrong.
I see from a later post you will be doing this on CNC so I's advise getting a normal Brown and Sharp gear cutter either 38 DP, or 0.7 mod in a No1 , that does from 135 teeth to a rack, cutting one tooth, raise or lower the cutter pi / DP or in this case 83 thou and cutting another tooth etc
Sounds long winded but not too bad with CNC
Depending on material another way is to grind a Vee shaped D bit up with a included angle of 40 degree and use this vertically, again cut one tooth and index along.
I hope you don't mind me adding onto this, for the sake of clarification. I had to read it twice to understand.
The first paragraph of John's suggestion above has the stock that you are cutting mounted _vertically_. Hence, as he says, the spacing between adjacent teeth is 0.083" (raise or lower the cutter/quill/knee). The depth of cut in this case would be put on using either the X- or Y- axis. Depth of cut is normally denoted D+f and should be noted on the cutter (should be about 0.057").
In the second paragraph, as in the photo, the stock is mounted _horizontally_. Hence, you move the X- or Y-axis 0.083" for each tooth, and use the Z-axis to put on the cut.
If you work on a 52% space and 48% tooth, the end of your D-bit needs to be 0.021" (21 thou') diameter, again for a 0.057" (57 thou') depth of cut. Hopefully John will chime in and check this calculation or advise otherwise as these numbers are from my understanding of Ivan Law's book, not from actually doing this.
I forgot on the first post to add that the hob had to be a "straight" one. I was told to ask for one of them. The samples we made were indeed done on the mill one tooth at a time, vertically though. The previous manufacturer of the item gang milled the teeth on the rack in one pass, trouble is, you can see the error in the teeth, even I can see it, which says something !! the many cutters that were ganged must have been spaced wrong, or slightly different diameter, as flats on some parts of the rack are more evident in places. I hasten to add that this job is not some intricate mechanism. There are two racks in this product, with a pinion in between them, so when the pinion is turned, it advances or retards the racks The plan of a straight hob, or something similar, was to do it in one or two passes, time being the essence in an ongoing production job........ Thanks Bob
Appologies Bob. I had considered this issue but with only a short time available this morning (I had to be at a concert rehearsal) I didn't think the issue through correctly. My thought process had the workpiece mounted at the helix angle of the hob but in fact you would either have to have the hob (and therefore the milling spindle) or the milling table turned to the helix angle.
It might be worth talking to Dathan as I suggested in my second post. They may well be able to provide an annular 'hob' - essentially a 1" + wide milling cutter with the rack form or a multi N° 1 B&S Gear cutter. This would circumvent all the valid issues JS raises.
Unless this is Bob's retirement job I'm guessing that the money Dathan want for making a special cutter will not be acceptable.
38 Dp will be easy to cut in one pass even in steel but hard on homemade cutters unless you have plenty of time to mess about making, hardening and grinding form cutters, something I'm guessing Bob hasn't got.
Two ways I can see to address this cheaply and commercially, one is to but enough new No 1 cutters that you can surface grind to thickness and stack on an arbor. Downside to this is I can't find any 38DP or
-0.7 mod cutters new so they all match, forget Tracy and the like as they will be all mismatched.
Other way is to buy two hobs from Arc, grind the ends so they match the pitch and then grind all the rows of teeth off except one row so you now have an annular hob or more to the point, flycutter.
Sounds drastic but it will work for Bob's application.
Smallest I have heard of for a Sunderland is 24 DP.
HPC does a 400mm length of 32 DP rack in EN8 for about 25 quid. That could make a cutter or you could cut it up have it hardened and make a holder to use the rack as an insert ? or even multi inserts ?