Improving precesion of old summa cutters

[Peter Martins]

What things wear and reduce the accuracy of summa cutters?

We look after a summa cutter. It's generally cutting lots of circular stickers about 2 or 3 inches diameter.

I'm told when it was new was accurate on large sheets, and would cut at high speed.

As its worn the speed has to be reduced and the sheets reduced in size to produce acceptable work.

The media twists so the the top right cut is perfect but the buttom left cuts an eccentric circle or off set circle to the print

I'm puzzled why this is.

We have replaced the rollers, though we used cheap Chinese ones which worked much better after shimming to stop them flopping in the jaws.

But that other factors Could there be?

The drive roller is titanium coated for hardness and to make the knurled finish wear resistant, and keep the sharpness of the roller.

This can go blunt / smooth / polished with use.

Would replacing this help?

Replacing them is costly and long winded.

Belts may wear and stretch. Would this reduce accuracy ?

Bearing wear but surely this is not the cause.

Pressure springs may relax a bit but unlikely.

The machines look simple but finding the cause of the issues seems not.

any comments or wisdom are very welcome.

 

[DL Signs]

If you had to shim the rollers you put in, they're not right. That could eventually wear other parts, not clamp properly, cause twisting in the roller or holder which will cause media to not track straight... (If one roller is crooked, it's like driving a car with one wheel not pointing straight). The weight of longer material with bad or dirty rollers will cause slippage, or if one of those aftermarkets isn't tracking straight, your media won't track straight, and get worse with distance.

Always use OEM parts for stuff like this, aftermarket parts use cheap bearings, cheaper materials, don't last as long, so you just burn through them (and money). They're cheaper for a reason, and while there are some instances where aftermarket parts will do, moving parts that need to match tolerances of a machine aren't usually one of them. Also check how much slop there is in the roller frames on the bar they ride on, if it uses nylon bushings, they can also wear out and cause slippage, as can the springs. I'd start with a fresh set of correct OEM rollers.

The grit rollers don't usually wear out, they can get buildup of dirt and debris in the knurling, just make sure they're clean. Metal parts like that you can use a little alcohol and a stiff toothbrush style cleaning brush on.

If it's a feed problem, always suspect rollers, roller holders first/ if it's a cut quality issue the blade, blade setup, or holder/ scan problems belt, dirty rail, bad rail bearings, these are the most common problems... You can usually find most other mechanical issues by looking for slop or binding in components, every thing should move without slop or binding.

 

[unmateria]

Hi! Never had any problem with rollers. In my case is always the same, the 2 little prisoner screws inside the drive belt pulleys (in the left tap). Just tighten them every 6 months and you can cut thousands of circles at one time

 

[Saturn]

Hi! Never had any problem with rollers. In my case is always the same, the 2 little prisoner screws inside the drive belt pulleys (in the left tap). Just tighten them every 6 months and you can cut thousands of circles at one time

Can you take a quick photo or even make a simple diagram showing their location?

 

[unmateria]

Hi! Just tighten the 3 pulleys with an allen key

 

[FrankW]

You should replace the belts too. The small belts on the motor, and the large belt driving the cutting head (a little bit expensive).

 

[Peter Martins]

There are some interesting suggestions here.
The shims are spot on I think - no play apparent.
I'll check the screws and then do the small belt. I've got belt material with the right tooth profile so I think I can make one up.

I agree it better to use oem parts, but cost is also a factor.

My suspicion is that the design is flawed.

The drive roller has knurled sections.
Most are about 15mm wide, but the rhs one is 100mm or so
This 100mm section must grip the media better than the little rollers.

Less slippage on one edge means it will twist, as it does.

 

[jfiscus]

We have one here, and while it is still technically "operating", it has been an extreme POS ever since we purchased it. Not really reliable for much besides larger decals without fine details.

 

[unmateria]

Mine has 16 years. I wish all machines in the shop had the maintenance and reliability of this machine hahahaha it was very expensive (you could buy a new car with that money 16 years ago) but it has payed itself dozens of times