Centec MIll

[duncan webster 1Participant @duncanwebster1]

At the risk of another smartass comment, 20t 0.2″ CP has almost the same pcd as 22t 1.5 mod, and 1.5 mod gears and racks are readily available on ebay.

A car on a breakdown truck is not ‘useable’ in a meaningful way, fitting a DRO to a milling machine is an improvement.

[duncan webster 1]

On duncan webster 1 Said:

…20t 0.2″ CP has almost the same pcd as 22t 1.5 mod

We are now back on track – thinking of creative (traditional, model engineering) ways to mend his machine instead of cheque book solutions.

The circular pitch of a 1.5 MOD gear is 0.1855″ so over the full 22t circumference of the gear, it would move the knee 4.081″ instead of the required 4.000″.

The 16DP/20t suggested above is a tiny bit better as its CP is 0.1963″, resulting in a knee movement of 3.926″.

Whatever size and tooth count we come up with, it has to result in a pitch circle circumference (a slightly non-standard term, I know) very close to 4.000″.

I think it could be done with a pitch shifted gear – making the blank a non-standard size for the number of teeth.

However, the bit I am struggling with is whether the standard D+f written on the cutter would still be correct when used with an incorrect blank.

Another solution might be to do as Graham Meek did with his Myford carriage handwheel dial and put an intermediate set of gears between worm wheel and rack pinion, so 49/50 would work well with 1.5 MOD and 54/53 with 16DP.

[Dave HalfordParticipant @davehalford22513]

The 20 tooth gear size is constrained by shaft and rack relative positions within the knee. If the worm end of the shaft has the .074″ correction built into it changing the 20T will throw the worm out, which is probably why Centec didn’t turn the spur gear drive 90deg via a pair of bevel gears instead of the worm gear arrangement.

[martin cross 1Participant @martincross1]

I will count the teeth on the worm wheel and dismantle the other Centec and check the pinion gear. This knee moves properly and smoothly and is a 2A, although for some reason the racks are not interchangeable as I checked that out a few years ago when I got number two. I seem to remember that the rack on number two has some sideways adjustment allowance (slots) so that correct meshing of the pinion and the rack can be set up, unlike the one with the problem.

For info, I have checked with a dial gauge that the divisions on the handle are accurate, so the basics are correct it’s the lack of adjustment which is the issue. Maybe I just machine some slots in the rack and set it up that way. Although the teeth on the rack are clearly not that good anymore.

 

 

[Dave Halford]

On Dave Halford Said:

If the worm end of the shaft has the .074″ correction built into it changing the 20T will throw the worm out

I am not sure that is so. You build the correction (translation gears) downstream of the worm wheel and upstream of the rack pinion gear. Effectively, you cut the shaft between the worm wheel and rack pinion, install a 54t on one end of it, a 53t on the other end of it (both gears cut on the same pitch diameter so they mesh at the same centres) and a double width idler pinion to mesh the two and maintain the direction. It would be very similar to what goes on in a set of Gamet dual dials.

The existing 20t 0.200″ CP gear is 1.400″ OD. A 22t 1.5 MOD gear is 1.417″ OD. A 20t 16DP gear is 1.375″ OD. This means the space requirements of any change are 17 thou increase or 25 thou decrease. I would find it hard to believe things are so tight there that these requirements are unmanagable.

[martin cross 1Participant @martincross1]

I’ve counted the teeth on the worm gear and it’s 40, also I’ve checked by turning the handle and 40 turns gives 1 revolution of the gear.

I’ve re-measured the pinion and it’s 1.4″+/- a thou or so due being a bit abused. I’ve also posted a few pics of the rack. 

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