Curious threads

[Allen Smithee]

One of the first things I want to do when Workshop Storm is finished is to make some new collets for my cheap balsa knives. These are knives which have good handles, excellent blades, but are let down by having injection-moulded nylon bladeholder/collet units. I have quite a few of them because Lidle keep offering them for peanuts, and while the blades fit my old X-Atco handles it would be nice to make replacement alloy collets so that I can have a dozen different blades "ready for use".

The bladeholder is essentially just a small tapered collet (looking a bit like a miniature R8 format) with a slit to take the blade and a threaded spiggot at the back. I thought that making replacements would be a simple bit of turning followed by a tapping operation and a cut with a slitting saw - just the sort of thing to start me back into this machining malarky. But I've hit a snag - I had assumed the threads on the spiggots would be standard metric ones, but last night I measured some (there are five different sizes). The thread pitches appear to be imperial, because they don't fit any of my metric thread gauges, and the ODs (bearing in mind these injection mouldings aren't exactly that precise) are as follows:

OD(mm)           Pitch(TPI)
7.5                     26
5.6                     26
5.6                     25
4.9                     32

[Yes, I am certain that there are two different pitches in the 5.6mm ones]

Can anyone give any hints as to what these threads might be? I have a vague memory about 26tpi threads being "cycling threads", but that's not really gointg to help me much! I'd prefer to use a die, but if I have to I can "augment the relearning experience" by screwcutting them.

Any thoughts appreciated,

AS

[Arbalest]

There have been a number of complaints about these knives not holding the blades securely so I quite understand why you want to replace the plastic parts.

Made in the Far East I expect them to have any old thread and may not comply with any normal standard but can you not drill out and re-tap the handle with a suitable metric thread?

[Allen Smithee]

I could, or I could (more easily) just screcut threads to match what's there. But I can't believe they aren't *something*, because in this case it would take the manufacturer more effort to be lazy than to do it right!

AS

[Arbalest]

Your "4.9 x 32" thread is most likely M5. Both of the 5.6 are so close to M6 it makes little difference, just run a M6 tap down the handle. The 7.5 is a bit odd but could be meant to be M8 fine. One again, just run an M8 X 1 tap down the handle.

[sshire]

Allen
I checked both of my knives.
10-24.

[Allen Smithee]

Well I've just checked the smaller threads by running taps into the (metal) handles and you are indeed right, the threads are M5 and M6 

For once in my life I had tried to check threads the "proper" way (ie by measurement rather than just trial fitting) but the male threads on the moulded plastic collets are so badly formed that my metric thread gauge wouldn't fit them. But laying the relevant tap into the thread clearly shows them for what they are. The M6 tap also matches the pitch on the largest (7.5mm OD) collet, so I'd guess you're right there as well and it's an M8x1 thread (I don't have any metric fine taps so I can't check it). That means that this job can go ahead as soon as I get the lathe installed. I've also established that the knife blades are all 0.42mm thick, so I giess a 0.5mm slitting saw will do the job nicely. There are some rod-based chisel blades which are 2.3mm dia, so some of the collets may get drilled 2.5mm and slit into four jaws rather than two. And the razor saw blades have 5mm circular shanks, but I think I have enough of the pukka X-Atco handles for those.

AS

[sshire]

I'm curious as to why Xacto would make a metric and an Imperial version. Or,is an M6 the same as 10-24?

[Allen Smithee]

M5 and 10-24 are fairly close, although the M5 is closer to 32tpi than 24tpi - it's possible that the smaller size of the 10-24 male thread allows it to go quite a long way into an M5 female thread?

PDR

[Arbalest]

I'm curious as to why Xacto would make a metric and an Imperial version. Or,is an M6 the same as 10-24?

The world and his wife make these type of knives these days.
I added to my collection of X-acto knives some time back and the threads aren't metric.

I suspect the cheaper ones with the plastic collets are made in the Far East hence the metric thread.
As mentioned earlier there have been more than a few warnings about the cheap ones not gripping the blade properly due to the plastic parts.

Having said all that I've no idea where the current X-acto knives are made. The larger ones are not as well made as the old ones.

[fcheslop]

If its of any help.I bought some from Aldi in a kit with 4 handles and a shed load of blades.
The small handles are M6x1 , the large are M8x1.25
They are very poorly made as in the threads are rather crappy and loose?
Another set are like Stans with the alloy colletty thing and they are Imperial 10-26.
cheers
frazer

[Bluechip]

Swann-Morton make a similar thing ...

http://www.amazon.co.uk/Swann-Morton-handle-number-blade/dp/B004UJT7R4/ref=sr_1_28?ie=UTF8&qid=1415375373&sr=8-28&keywords=swann+%26+morton

Fairly robust, mine is about 10 yrs. old and still ekkling ...

They also do a kit with 3 handles I think ..

Dave

Found it
http://www.amazon.co.uk/Swann-Morton-Scalpel-Blades-Price/dp/B008B5DAFS/ref=sr_1_56?ie=UTF8&qid=1415375788&sr=8-56&keywords=swann+%26+morton